EFFECTS OF SHADE ON MAIZE AND SOYBEAN PRODUCTiVITY IN A

BASED INTERCROP SYSTEM

A Thesis

Presented to

The Faculty of Graduate Studies

of

The University of Guelph

by

JAMES A- SIMPSON

In partial fulfilment of requirernents

for the degree of

Master of Science

July, 1999

O James A. Simpson, 1999 National Library Bibliothèque nationale 1*1 of Canada du Canada Acquisitions and Acquisitions et Bibliographie Services services bibliographiques 395 Wellington Street 395, rue Wellington Ottawa ON KI A ON4 OttawaON K1AON4 Canada Canada

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The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantid extracts fkom it Ni la thèse ni des extraits substantiels may be printed or otherwïse de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation. ABSTRACT

EFFECTS OF SHADE OB MAIZE AND SOYBEAI

TREE BASED INTERCROP SYSTEM

James Simpson Advisor: University of Guelph, 1999 Professor AM-Gordon

Maize and soybeans were intercropped with hybrid poplar (Po@us delroides x nieDN-177) and silver maple (Acer sacharriltum) at a within row spacing of 5 m and between row spacing of 12.5 or 15 m. In 1997, overall yield of maize was reduced to

7 1% and 8 1% of a control in the poplar and maple intercrops respectively. In 1998, yields were 61% and 85%, respectively. Soybean yields were 71% and 75% (1997) and

70% and 101% (1998) ofa control in the poplar and maple intercrops, respectively.

Daily rates of carbon assimilation were generally lower near the trees where competition for Iight was the greatest and coincided with the lowest yield. Maize appears to mmpete effectively with the trees for soi1 moisture and benefits fkom microclimate changes.

Soybean receiving moming shade and aftemoon sun did not fùlly respond to improved light conditions. The soybean crop was susceptible to moisture competition, likely a result of a relatively small root system and inherently low water use efficiency. ACKNOWLEDGMENTS

"Anintelligent man is sometimes forced to be drnnk to spend time with his fools." - Ernest Hemmingway

1wish to thank the many people that have assisteci me through the stages of

developing this thesis For theu assistance in collecting the field data I wish to thank

Nancy Luckai, Ion Short, John McEwan, Trevor Cwy, Ping Zhang, Tim Bohn, Ralf

Kuessner. Special thanks go to Naresh Thevathasan, Rick Gray and Gordon Price for theû help in al1 aspects of my work, especially for theu insightfüi questions and comments. 1 wish to thank my committee members, Dr. Thijs Tolienaar and especially

Dr. Phi1 Reynolds for bis tireless efforts in collecting the field data. Many thanks go to my Supe~sor,Dr. Andrew Gordon, for the years of encouragement and for creating this oppodty for me.

To my parents, thank you for al1 your support and encouragement- 1especially wish to thaak my wife, Anne-Me for her strength and support throughout this ordeal.

Funding for this project was provided by the Ontario Ministry of Agriculture,

Food and Rural Mairs. TABLE OF CONTENTS

Abstracî

Acknowledgements

Table of contents

List of tables iii

List of figures

1-0 Introduction

2.0 Literature review

3.0 Sitedescription

4.0 Hypothesis development

4.1 Materials and methods

4.2 Results

4.3 Discussion

4.4 Conclusion

5 .O Main study

5.1 Introduction

5.2 Materials and methods

5.2.1 Basic design

5.2.2 Site selection

5 -2.3 Sample location

5.2.4 Yield

5.2-3 Gas exchange measurements

5.2-6 Plant water deficit and water use efficiency 5-3 Results

5.3-1 YieId

5.3 -2 Photosynthesis

5.3 -3 Water use efficiency and plant water deficit

5-3 -4 Soi1 moisture

5 -3-5 Land equivalent ratio

5.4 Discussion

5.4.1 Maize intercrop

5.4.2 Soybean intercrop

6.0 Conclusions

7.0 Literature cited

Appendices

iii Page Table 3-1 Climatic summaries for the Agroforestry Research Station 12 (ARS), Guelph, Ontario, 1996 - 1998.

Table 4.2 Yield of saybean aaoss the crop area (excluding land area lost 20 to meproduction) intercropped with white ash or black walnut.

Table 4.3 Mid-day rates ofphotosynthesis for soybeans intercropped with 20 white ash (August 29, 1996) and black walnut (August 30, 1996)-

Table 4.4 Mid-day rates of photosynthetically active radiation (PAR) for 21 soybeans intercropped with white ash (August 29, 1996) and black walnut (August 30, 1996).

Table 4.5 Water use efficiency of soybeans intercropped with white ash 21 (August 29, 1996) and black walnut (August 30, 1996).

Table 4.6 Soi1 moisture content detennined at 15 and 30 cm below the 23 surface under soybean intercropped with white ash or black walnut, August 30, 1996.

Table 5.1 Characteristics of trees intercropped with maize for the years 30 1997 and 1998.

Table 5.2 Characteristics of trees intercropped with soybeans for the years 30 1997 and 1998.

Table 5.3 Mean yield of maize as a hnction of distance nom the tree row. 36

Table 5.4 Mean yield of soybean as a function of distance fiom the tree 38 row.

Table 5.5 Rates of actual water use eficiency (AWUE)for maize on July 58 30, 1997.

Table 5.6 Rates of achial water use eniciency (AWUE)for maize on 58 August 28, 1997.

Table 5.7 Rates of actual water use efficiency (Am)for maize on July 59 Table 5.8 Rates of actual water use eficiency (AWUE) for maize on July 18, 1997-

Table 5.9 Rates of amai water use efficiency (Am)for maize on August 19, 1998,

Table 5-10 Plant water deficit for maize on July 29, 1997.

Table 5.1 1 Plant water deficit for maize on August 29, 1997.

Table 5.12 Plant water deficit for maize on July 16, 1998.

Table 5.13 Soil moisture wntent (%) at a depth of5 cm and 15 cm for maite on July 16, 1998-

Table 5-14 Soil moisture wntent (%) at a depth of 5 cm and 15 cm for soybeans on hly 16,1998.

Table 5.15 Land equivalent ratios (LER) determineci for rnaize and soybean intercropped with hybrid poplar and silver rnaple in two successive years at the Agroforestry Research Station, Guelph, Ontario. WST OF FIGURES

Page Figure 3.1 Intercropping maize and silver maple (top) and soybeans and 14 hybrid poplar (bottom) at the Agroforestry Research Station, Guelpb, Ontario.

Figure 4.1 Sampling locations for gas exchange and yield measurements of 17 soybean intercropped with white ash or black walnut. Locations 1 and 5 are 2 m fkom the tree row, 3 and 4 are 4 rn fiom the tree mw and 3 is 6.25 m (under ash) or 7.5 m (under walnut) fiom the tree rows.

Figure 5.1 Sampling locations for gas archange and yield measurements. Three replicated measurements were taken fiom each distance dong the east - West axis.

Figure 5.2 Relative yield of maize in the two intercrops as a percentage of the control plot.

Figure 5.3 Relative yield of soybean in the two intercrops as a percentage of the control plot.

Figure 5.4 Rate of photosynthesis measured at the top of the maize canopy, July 29, 1997.

Figure 5.5 Rate ofphotosynthesis measured at the top of the maize canopy, August 29, 1997.

Figure 5.6 Rate of photosynthesis measured at the top of the maize canopy, July 1, 1998.

Figure 5.7 Rate of photosynthesis measured at the top of the maize canopy, July 16, 1998.

Figure 5 -8 Rate ofphotosynthesis measured at the top of the maize canopy, August 19,1998.

Figure 5 -9 Photosynthetic photon flux density (PPFD) received at the top of the maize canopy, July 29, 1997.

Figure 5.10 Photosynthetic photon flux density VPFD) received at the top of the maize canopy, August 29,1997. Figure 5.1 1 Photosynthetic photon flux density (PPFD) rezeived at the top 48 of the maize canopy, July 1, 1998-

Figure 5 - 12 Photosynthetic photon flux density (PPFD) received at the top of the maize canopy, July 16, 1998-

Figure 5-13 Photosynthetic photon flux density (PPFD) received at the top ofthe make canopy, August 19,1998.

Figure 5.14 Rate of photosynthesis measured at the top of the soybean canopy, Jdy 29, 1997.

Figure 5.15 Rate of photosynthesis measured at the top ofthe soybean canopy, August 29,1997.

Figure 5.14 Rate of photosynthesis measured at the top ofthe soybean canopy, Juiy 3, 1998,

Figure 5.17 Rate of photosynthesis measured at the top of the soybean canopy, July 15, 1998.

Figure 5.18 Rate of photosynthesis measured at the top ofthe soybean canopy, August 19,1998.

Figure 5-19 Photosynthetic photon flux density (PPFD) received at the top of the soybean canopy, July 15, 1998.

vii 1.0 Introduction

Agroforestry has been used both as a general and a specific term. In its mon general sense it refers to the use oftrees in an @cultural setting. This would include managed woodlots, plantations, windbreaks, orchards, etc. It is aiso used to describe the specinc system ofintercropping where the cultivation of woody and non-woody plant species occurs on the same unit of land at the somc the. This agronomic system may take on many different buf for now, this de--on provides a reamnabfe starting point for discussion.

Intercropping has been employed for centuries around the world. References date back as far as the fist century B.C. where ColumeUa, author of Dere mstica', described intercropping wheat (Tririnm, aesthm) and olive trees (Olea europea) (Dupraz and

Newman, 1997). In Ontario, intercropping likely dates back to the establishment of the first fiuit orchards by European smlers, although agroforestry systems had been employed by the aboriginal population long before the conquest of the New World. (See Wiams et al. 1997 for a bnef review.)

There are numerous benefits that may be derived from intercropping. Total resource capture by plants (light, water and nutrients) can be enhanced through the use of multiple crops on the same unit of land (On& 1991) . Soils may be conserved via several mechanisms: reduced kinetic energy of rainfàll under the tree canopy; interruption of surnice flow canying sedùnait (sheet erosion) (Young, 1989); additional inputs of organic carbon (through litterfâii and root turnover); and increased nutrient retention and cycling via root interception. The relatively undisturbed tree rows may also provide habitat for beneficial species incIud'ig predatory insects and mycomhizae. However, these rows may

dso provide protection for h-1 pest species.

Of particular interest in these systems are the potential interactions inciuding those

of a positive nature (Le. Newman et a%, 1998; Thevathasan, 1998; Thevathasan and

Gordon, 1995; Wfiarns and Gordon, 1995; Ranganathan et al. 199 1). However, there is

greater potentiai for negative interactions, especially in a tre!e based intercrop 0.e.

- Newman et al., 1998; Rao et al., 1998; Williams and Gordon, 1995). The presence of

trees will create cornpetition for both above- and below- ground resources. This study

was designed to explore the effect ofshade on the understory crop and the resulting effect

on economic yield- Although intercropping may provide a method of rnaximking the available resources of a site, this is not typically the primary design criteria. While it is desirable to create axienWonment for good growth of the perenniai overstory, it is generaiiy a higher priority to produce an economicaüy viable aonual (understocy) crop. The potential

ümitations to productivty in the understory due to sbading rnay be addressed through appropriate overstory species selection and plantation design. Given that a C3 species becomes light saturated at approxhately 50% of fuli sunlight, an overstory species which reduces light levels by this amount or Iws should allow an understory C3species to operate at full photosynthetic potential. This of course, does not apply to understory C4 species.

A simple equation to descnbe the treg - crop interaction has been proposed by Ong

(1 996).

I=F-CkM+P+L (1) where F is the benefit derived fiom tree prunings on soi1 fertility and microclimate changes at the soil surface, C is the intercrop yield reduction due to direct cornpetition nom the trees, M is the above-ground microclimate change, P is the benefit derived firom soil physicd and chemicai changes, and L is the benefit of nutrient and/or water conservation.

To quant@ ali of these components is a monumental task due to the integrated nature of each cumponent. However, ongoing research at the Agroforestry Research Station (ARS) addresses several ofthese wmponents, at least in part (Thevathasan, 1998; Thevathasan and Gordon, 1995; Williams and Gordon, 1995; Ntayombya and Gordon, 1995; Gray, unpublished data). The F component cm be ignored for the current management system

as prunings are typidy ofa size that would intderewith the mechanized nature of

agriculture in the area. Yield reduction wiii be addressed Iater in this thesis. M is

addressed by WiIIiams and Gordon (1995) for young trees during the establishment phase.

However, more microclimate information needs to be coilected throughout the Life cycle

of the tree crop for proper integration Limited data related to P and L has been coiiected

by several researcbers (Thevathasan, 1998; Thevathasan and Gordon, 1995; Ntayornbya

and Gordon, 1995 ), however, these works relate to nitrogen dynarnics oniy. The above mentioned equation has recently been expanded upon to include allelopathy (Rao et al.

1998).

Rao et al. (1998), in a review of hedgerow intercropping in the humid and sub- humid tropics, summarize the results of severai studies investigating competition. Of particular note is a long-tenn study conducted in Lampung, Indonesia which attempted to differentiate between diflierent levels of competition (ICRAF, 1995). Three contrasting tree species were employai - glincidia (Glircidia sepium), leucaena (Leucaena leucocephala) and peltophorum (Peitophonn ckuyrruchis), a slow growing species with a small compact canopy. Resource competition berneen each species and an annual maize crop were partitioned into above- and below-gromd effécts. Long term efEects of rnulching (prunings fkom the hedgerow) and residual fertility were also assessed.

Peltophorum was the sole species to show a net positive effect on maize yield. Yield effects by rnulching and root competition were found to be minimal. AU three tree species exhibiteci a positive effect due to increases in residuai fenilty and yield reductions due to shade. The net positive effiof peltophorum was amibuted to its relatively smail shading

effect.

The use ofdeciduous trees in the temperate zone dows for early season soü warrning and drying to occur. Prior to bud break and leafexpansion shade production is minimal. Thus, germination and early growth of the annual crop should be comparable to a sole cmp situation. Haggar and Beer (1993) observed this response in maize intercropped with Gffricidiasepiunr and Eiythrfmpoqpigicau~in Costa Rica The trees were pollarded prior to seeding virtuaiiy eliminating any shade. A reduction in biomass accumulation of the annuai intercrop, as wmpared to a sole crop, was observed after 60 days. The authors concluded that light was likeiy the most Iuniting factor to crop growth.

They went on to suggest that a short season crop, such as beans, may complete more of its Life cycle during the period where shade is less likely to limit potential biornass accumulation,

Shade tolerant crops have also been shown to benefit from interaopping. Ginger

(ZNlgiber oficiinole) intercropped with paulownia (Paulownia elongota) grown at 15 x 5 m spacing yielded 34% more than open grown ginger (Newman et al., 1998). In the same senes of experiments, intercropped maize and field bans (Pbeolusspp.) exhibited overail yields of 63 and 68% of open grown wntrd plots and exhibited significant morphological diierences. The intercrops were typicaiiy shorter, had a smaüer stem diameter and intemode distance and lower leafspecific weight as compared to the open grown control crop. These dinerences were attributed to shade. Similady, yield reduction was aiso attributed to shade although the shidy does not appear to account for below ground cornpetition.

5 In addition to the morphological differences observed for understory species in an intercrop, Newman et al. (1998) report a fparaboliceffect' in tems of yield and height growth with the apex of the parabola occurring in the middle ofthe crop stop. Intemode distance and specific Ieafweight were dso highest in the middle of the crop sirip.

Newman (1987) observed increased leafgrowth and a dishedswoilen root harvest of radish (wks&s) htercropped in a pear orchard. Plants adapted to fidi Sun rnay docate more resources to leaf area in response to rduced light conditions. However, they will still be Limiteci by a net reduction in carbon assimilation.

For shade tolerant or short season crops the effect of shading may be neutral or positive, while for shade intolerant or long season crops the effect may be negative.

However, the overail productivity ofthe system may be greater than that of either species grown as a monocrop. This is most kely where the understory crop is a shade tolerant C3 species. Marshd and Wdey (1980) report a 28% increase in biomass accumulation in a groundnut (shade tolerant) millet (Setaria itaIicc~)intercrop.

Orientation of the tree rows may signincantly affect the amount and duration of shading on the annual crop. With tree rows oriented north - south shading at noon fds primarily on the tree row. This becornes more prominent in higher latitudes. Typical rates of carbon assimilation by crop plants are maximum around won where the solar angle is at or near its azimutb At the ARS the tree rows are oriented roughly north - south which results in the minimum shading ofcrops during the maximum potential carbon assimilation period, except for the crop immediately under the tree canopy. However, yield reductions that may be attributable to this shading is confounded by edge effécts. These can be typicdy observed dong field edges where there are no trees (Le. fencerows, streams and ditches) and is due to unmoderateci microchnate conditions. This is mimicked in the ARS intercropping shidy where the tree rows are crop free and most evident in the corn plots where weeds, ifany, are too short to provide any si@cant horizontal protection (or above ground cornpetition).

Shade may aIso increase the humidity under the trees and reduce the rate of dew evaporation. This may, in tum, lead to an increase in fingai diseases (Kater et al., 1992) and a reductïon in yield and harvest ~ality. Information fkom temperate regions is lacking. Although intercropping is not an . uncornmon practice, it has ody been recently that forma1 research has been conduaed in an organized fashion, with one ofthe leadiig countnes in this regard being China. For more than 20 years Chuiese scientists have been conducting systematic research on intercropping crops with PuuIawnia trees. PmZowniu is an ideal tree for agroforestry. It is deep rooted, possesses a sparse crown, is fast growing and Ieafemergence and senescence occur late relative to the understory crop development There are also numerous products that can be denved f?iom the tree (Wu and Zhu, 1997). Trees are typically spaced at 5 m within the tree row while inter-row spacing depends on the relative value of the crop and tree. Where wood is the most important comrnodity rows are 5 - 10 rn apart, 15 - 20 m where the value is the same for both the wood and understory crop and

30 - 50 m where the crop is most valuable.

A greater understanding of the antagonistic and synergistic effects of growing widely spaced trees with annual crops in temperate North America is not iikely to be accomplished through reviewing the current fitenture. There are many questions that must be answered before appropriate pdelscan be made betwan the current literature, much of which originates in the tropics or semi-arid regions of the world, and the

intercrop system present at the Agroforestry Research Station in Guelph, Ontario.

Given the collective shortcomhgs of the current state of research in temperate zones, perhaps a step backwards is more appropriate. A simpler assessrnent of the effectiveness of intercroppiqg, relative to monocropping or plantation management is the land equivalent ratio GER) (Wüley, 1979; Rao et al., 1990, 199 1). This is simply the sum of the productivity of each intermp relative to its associated monocrop. EIowever, this simple ratio relies on the assumption of linearity between crop yield and planting density.

This is certainiy not the case for the annual crops planted at the ARS beyond a vely restricted range. However, research to date has not attempted to alter the planting density of the annual crop. The planting density of the tree crop is signincantly less than that of a plantation. However, the growth curve (measured in terms of volume) of the intercropped tree is ükely iinearly associated with that of a plantation grown tree @upraz and Newman, 1997) aithough the morphology may be different. Without the direct competition nom surrounding trees the open grown intercrop trees may devote more resources to diameter growth than plantation grown trees. To date, there has been no research conducted on this topic.

In a biological system, where two or more individuais occupy the same space at the sarne tirne, competition for resowces may ensue. Currently, there is Little in the literature that wiU help us to understand the nature and degree of competition within a tree based intercropped situation. Van Noordwijk et al. (1 9%) indicate that much of the available literature on intercropping, or dey cropping, may be diaicult to properly evaluate due to the lack of proper controls in the studies. liituitively, we know that a tree, being tdler and possessing a large canopy, dlprovide some degree of competition for

Iight over an annual crop. We also know that the root system ofa perennial tree will

occupy a much larger area than any single annual plant, eqloiting the soi1 for nutrients

and moisture weii beyond the hunediate proximity of the tmnk and crown.

Although many questions exkt. this treatise will provide insight into only a féw, specifidy, the effêcts that shading may have on the response of a growiag crop. The degree to which a crop is innuend by a tree depends on many factors. Ignorùig below ground effects, at least for the tirne being, the height of the tree, the crown architecture and the leafarrangement are most iikely the most important factors. The extent of shading is dependent on these factors as weil as the latitude, which controls the angle of incidence of solar radiation. Additionaiiy, the rnahod of pruning wilî infiuence the development of the crown. Beyond this, there are factors over which we have no control, prixnarily weat her,

Attempts to estabiîsh a relationship between single leafphotosynthetic rates and yield have been unsuccessful (Evans, 1975). Measurements of whole canopy photosynthesis taken at several times through the Eeason have produced a high correlation with crop growth and yieid (WeUs et al., 1982; Ashley and Boernia, 1989). However, such techniques are expensive and theconsuming. Detecting gross physiological dserences in a single crop over short distances are possible by measuring gas exchange of a single leafif temporal daerences are smali. However, such rneasurements provide merely a glimpse into the workhgs of the individual plant.

Severai tropical studies have ïndicated that competition for soi1 resources is high in acid infertile soils, This has been attributed to the concentration of roots of both intercrops in the same shailow zone (O - 20 an) (Rao et al., 1998). For this reason it is recommended that deeper footing species, such as peltophonun and Dactyiadeniu bwten' be used as the interaop on these soils (Ruhinga et al., 1992; van Noordwijk et al.. 1995).

At the ARS the soils are of recent origin and cdcareous parent material and, as such, they possess a relatively high ùiherent fertility. Root growth would be restricted prirnarily by p hysical characteristics of the profile- 3.0 Site Description

The Agroforestry Research Station (ARS) is located within the city limits of

Guelph, Ontario. The landforni is a drumlin oriented approxhately north - south with the lowest point approxhately 334 m above mean sea level. The soi1 is fiom the Guelph

Loam senes and the texture ranges nom silt loam to loam. Drainage is naturaiiy imperfect to moderately weli drained, although, much of the site is now tiie drained-

A variety of tree species were planted in 1988 on the west side ofthe drurnh.

Tree rows were onented dong the long axis of the dnimlin with each species planted in groups of eight. Tree rows were either 12.5 or 15 m apart. Mtiaiiy, this resulted in approximately 1 m strips being removed nom crop production. This represented 8 and

6.7 % of the avdable land area Current management practices combined with larger tree crowns has approximately doubled the width of the tree rows or fdow strips.

The relatively widely spaced trees at the ARS Uely results in little intraspecinc competition between the trees dthough no direct empiricai evidence has been colected with respect to tree - tree cornpetition. Currently, the canopies of only a few species have begun to overlap. However, there is only limited information on the extent of the rooting of the tree crop. There are many factors here that may infiuence root growth, including annual cultivation and fertiiizer additions near the trees. These factors, and others, may drarnatically alter the genetidiy-programmeci root architecture.

The average fiost fiee period is 136 days (May 15 - September 28). hnuai precipitation averages 833 mm and approximately 334 mm fiills during the growing season. The mean annuai corn heat units received is 2740. Clirnate summaries are presented in Table 3.1. In July, 1996 three major rain events accounted for 85% of the Table 3.1 Chatic summaries for the Agroforestry Research Station (ARS), Guelph, Ontario, 1996 - 1998,

Mean temperature Total precipitation Total shortwave cc> (mm) radiation

May June July August September

1997 May June Juiy August September

1998 May 15-6 34.2 665,866 June 27-1 126-0 639,611 Ju ly 19-3 34-4 697,493 August 19.2 34-4 591,150 September 15-9 33 -4 490,280

1 Recorded at the Elora Research Station, approximateiy 30 km nonh of the Agroforestry Research Station, monthly precipitation. Cool spriag temperatures in f 997 delayed planthg by approximately two weeks. August was generdy cooler and overcast resulting in delayed maturation of the crops. May 1998 was very warm and dry resultuig in excellent early growth of the crops. However, low rainfd du~gIuly and August had a sigdicant impact on yields.

Maize (Zea mqs), soybean (GQcine nim) and winter wheat (TM- aesfivum) are grown in rotation between tree rows. After harvest, the wheat stubble is disc ploughed to promote germination of voluntea plants. Al1 crops are planted using a no-tül planter.

Two examples of intercrops being snidied et the ARS are presented in Figure 3.1. Figure 3.1 Intercropping maize and silver maple (top) and soybeans and hybrid popk (bottom) at the Agroforestry Research Station, Guelph, Ontario. 4.0 Hypothesis Development

Plants adapted to fidl sunlight dl,when shaded, fk less carbon. This is dependent on the timing and extent ofshadùig as well as the physiological status of the plant. To varying degrees the effects of other factors, both directly and indirectly a result of the presence of trees in intercropping sysîems, wiU infiuence plant development-

SpecincaiIy, trees msy compte directly for moisture and nutrients. They will also create a windbreak effect, potentidy reduciag the rate of evapotranspiration.

In this study it was not possible to examine ali mors simultaneousiy, nor was it possible to isolate single factors in ternis ofthe potential effect on the crop. Given the potential shortcornings ofthe avdable resources it was necessary to fist test the methodology and generate testable hypotheses. Samplhg for yield differences across the crop strip provided the empirical evidence of the effkcts ofthe trees without providing insight as to the response of the plants growing in conditions of varying shade. Initially, it was proposed to measure the gas exchange ofa single leafofmany plants at several locations across the crop strip. DifEerences in individual plant response would then be evaluated in terms of the proximity to a tree.

4.1 Materials and Methods

Two areas were selected on the basis of the composition of tree species across parallel rows. Specifidy, the rows must have bem of the same species and the trees should also have been relatively uniforrn in terms of sue (height and crown dimensions).

Subsequently, white ash (Frtuhs meri~4rmr)and black walnut (JugIkms ni,)were chosen with soybeans being the intercrop in 1996. Ten pairs of trees were selected for each tree species. Sampling locations for the

crops were estabtished at 2 and 4 m fiom each tree row aod in the exact middle (6.25 and

7.5 m for the ash and wafnut, respectively) of thc iateraop arip (Figure 4.1) for a total of

five crop plants sampled between each pair of trees. The within row spacing of the trees

was 5 m. The mean height of the ash was 5.4 rn (* 0.9) and the walnut was 2.6 m (r 0.7).

Gas exchange, ushg the LiCor 6200 (LiCor, Lincoln, Nebraska), was measured on

a single leaflet fiom the upper canopy of the crop. A terminal (sun) leafwas chosen fkom a randomly selected plant at each sample location A total of 50 plants were sampled in

each intercrop with 10 repiicated measurements at each sample location.

The LiCor 6200 simultaneously measures Ca concentration, leaf and air temperature, relative humidity and photosynthetic photon flux density VPFD). From these measured values estimates of rate of photosynthesis stomatd conductance and transpiration are calculated.

Sarnpling on August 29& occurred in the ash plots, starting at 11 :43 AM and concluding at 5:30 PM. Temperature increased fiom 20' - 23" C and the relative humidity dropped from 57% - 43% over the time of sarnphg. Sampling on August 30~occurred in the walnut plots, starting at 11 : 17 AM and concluding at 2:06 PM. Temperature increased from 22" - 25' C and humidity dropped from 54% - 40% over the period of sampling. The total incident solar hadiance was 20.3 MJ mW2and 22.5 UT m-2 on the 29& and 3O", respectively.

Soi1 samples at each location were collected fiom depths of 15 and 30 cm for gravimetric moisture anaiysis. Samples were stored in sealed polyethylene bags and Distance between tree rows was 12.5 m under ash, 15 m under wainut 4

Figure 4.1 Sarnphg locations Br gas exchange and yield measmments of soybean int-opped with white as6 or bkk wainut* Locations 1 and 5 are 2 m hm the tree row, 2 and 4 are 4 m hmthe tree row and 3 is 6.25 m (under ash) or 7.5 m (under -ut) hmthe tree mws. irnmediately chiiied and retumed to the lab. Subsamples weighing appro>

At the end ofthe growing season, 0.5 m2 quadrats were harvested at each previously identified crop sample location. The samples were air dned, processed by hand and economic yield determined. Mass was correctecl to a constant moisture content

(15 %). Reported yield does not hclude land area lost to tree production.

4.2 Results

Yield pattern for both intercrops may be described as parabolic, wïth yield increasing towards the middle ofthe crop row. Soybean yield (Table 4.2) under white ash did not significantly dEer @ < 0.05) between sample Locations. DXerences did exist under black walnut with the crops closest to the tree row (positions '1' and '5' yielding approximately 23% less than that from the exact middle (position '3').

Yeld of soybeans intercropped with bIack walnut closely folow the midday rate of photosynthesis as measured on August 30, 1996 (Table 4.3). Measurements were taken around noon (dayüght savings tirne), however, significant shading was observed around location '1' (2 m east ofthe westerly tree; see Figure 4.1) due to the orientation of the tree rows and the solar angle. Three replicates within the white ash intercrop were sampled after 4 PM and were removed fkom the analysis. In the white ash intercrop there was a significant difference in the rate of photosynthesis beh~eenplants at positions '5' (17.12

@molni2 il) and '2' (12.97 molm-2 s-'1). There were no sisnifiant merences in PPFD

(Table 4.4) between these two locations. Ifthese rates ofphot~synth~sare indicative of seesonal rates, the dierences in yieid in the black walnut intercrop are obviously explaineci. However, for both days,

PPFD received at this time was out oforder with the rates ofphotosynthesis (1 < 2 = 3 =

4 = 5) (Table 4.4).

Water use efficiency (Table 4.5) was calculated as rate of photosynthesis / rate of transpiration @mol Cm-2 s- L f mm01 HBm -2 s-1 ) (Larcher, 1995). No significant Table 4.2. Yield of soybean across the crop area (excluduig land area lost to tree produaion) intercropped with white ash or black walnut.

Yield Mgha3

White ash 3.60 a2 3-64 a 3 -94 a 3.57 a 3.12 a Black walnut 3.28 a 3-86 ab 4.29 b 3.80 ab 3-24 a

L Locations I and 5 are 2 m east and wesf respectively, fkom parallel tree rows. Similarly, locations 2 and 4 are 4 m fiom the tree rows, Location 3 is the middle of the crop are* 6.25 m from either tree row for the ash intercrop and 7.5 m fiom either tree row for the walnut intercrop.

'values within a row folIowed by the same letter are not signjficantly difEerent @CO-05, Tukey's HSD)-

Table 4.3. Mid-day rates of photosynthesis for soybeans intercropped with white ash (August 29, 1996) and black walnut (August 30, 1996).

Rate of photosynthesis - &mol CO2 m-2 s-') Location Species -lL -2 -3 -4 -5

White ash 13.3 1 ab2 12-97 a 16.83 ab 16.67 ab 17.12 b Black walnut 10.17 a 13.45 b 13.79 b 14.39 b 10.52 a

1 Locations 1 and 5 are 2 m east and west, respectively, nom pdeltree rows. Sunilarly, locations 2 and 4 are 4 m Corn the tree rows. Location 3 is the rniddle of the crop area, 6.25 m fiom either tree row for the ash Uiteraop and 7.5 m fiom either tree row for the walnut intercrop.

values within a row followed by the s*e letter are not signincantly dserent @

PPFD &mol m-' ssL) Location S~ecies -1' -2 -3 4 -5

White ash 983 a2 1480 b 1677 b 1571 b 1571 b Black walnut 1585 a 1957 b 1838 b 1902 b 1924 b

Locations 1 and 5 are 2 m east and West, mpectively, fiom paralle1 tree rows. Similady, locations 2 and 4 are 4 rn fiom the tree rows. Location 3 is the middle of the crop area, 6.25 m fiom either tree row for the ash intercrop and 7.5 m fiom either tree row for the walnut intercrop

Values within a row followed by the same letter are not significantly diaerent @

Table 4.5. Water use efficiency of soybeans intercropped with white ash (August 29, 1996) and black walnut (August 30, 1996)-

Water use efficiency &mol Came2 s-' / mm01 Hz0 m-2 s-1 ) Location S~ecies -lL -2 -3 -4 -5

White ash 3.20 a2 2.89 a 3.11 a 3.36 a 3.37 a Black walnut 2.95 ab 2.61 a 3-40 b 2.52 a 2.36 a

L Locations 1 and 5 are 2 m east and West, respectively, f?om parallel tree rows. Similarly, locations 2 and 4 are 4 m fiom the tree rows. Location 3 is the middle of the crop area, 6.25 m fiom either tree row for the ash intercrop and 7.5 rn fiom either tree row for the wainut intercrop.

2 Values within a row foiiowed by the sarne letîer are not significantly dxerent @<0.05, Tukey's HSD). dserences were observed in the ash intercrop. The middle of the walnut intercrop had a significantly higher WUE than all other locations*

Soi1 moisaire content was determined for the date of sampling (Table 4.6). There were no signifiant differences (p < 0.05) in moisture content across sampling locations under the black walnut. Under white ash mil moisture at a depth of 15 cm in the westerly tree row was significantiy @ < 0.05) higher than that at position 5,2 m fkom the easterly tree row. In general, the Iowest moisture contents were observed UnmediateIy adjacent to the easterly tree row. It was expected that the soi1 near the tree rows wodd be more depleted as both the trees and crops would be wmpeting for this resource. This appears to be the case where the ground was shaded in the moming and exposed in the aftemoon.

Relatively higher moisture content was observed where the ground was more shaded in the aRemoon.

4.3 Discussion

Despite the similarity in chatic conditions on these two days, the rates of photosynthesis in the ash intercrop was generdy higher than that measured in the walnut intercrop. Similarly, WUE was generdy higher for the ash intercrop. An explmation for this difference relates to the presence ofjuglone, a phytohormone secreted fkom the roots of Jughspecies. However, Thevathasan (1998) has suggested that young black walnut does not release signincant concentrations of juglone to the soi1 environment. Also, yield near the tree (locations 1 and S), where juglone concentrations would likely be greatest, under both species is comparable. Thus, there is no evidence of any phytotoxic enect at this tree age. Table 4-6. Soil moisture content determined at 15 and 30 cm below the surface under soybean intermpped with white ash or black walnut, August 30, 1996-

Soil moisture (%) Location White ash Black walnut 15 cm 30 cm -1s cm 30 cm West Tree row' 13.7 b2 12-9 a 11-2 a 11.2 a 1 12.3 ab L3-0a 11.2 a 10-8 a 2 13-1 ab 12.0 a 13.0 a 15-0 a 3 12-7 ab 12.4 a 13-4 a 11-8 a 4 12.0 ab 11.5 a 12-1 a 9.7 a 5 10.9 a 9-6 a 9.7 a 9-7 a East Tree row 13.4 a 12-2 a

1 Locations 1 and 5 are 2 m east and west, respectively, nom parallel tree rows. Similarly, locations 2 and 4 are 4 m fkom the tree rows. Location 3 is the middle of the crop area, 6.25 m eom either tree row for the ash intercrop and 7.5 rn nom either tree row for the wainut intercrop.

2 Values within a row followed by the sarne letter are not significantly different @

The height Merential between the two tree species, 5.4 m for ash vs 2.6 m for walnut, is a signincant factor in the extent and duration of shade on the crop. It appears likely that the greater extent to which yield was reduced under the ash is related to the height of the tree and, thus, the extent of the shade. As the trees were aü of the same age and received the same level of management, differences of height and crown architecture within a species were negiigible. Without a greater range of heights within eech species it is impossible to explore this relationship freely.

4.4 Conclusions

This preliminary research resulted in two major findings. Fust, the thelapse between the first and last measurement was too grrat to rnake any concrete cornparisons using gas exchange data. The design would have to be changed to mlliiMze this tirne lapse. Additionally, increasirig the number of measurements through the day may have provided some insight into the diumal response of the crop plants. Second, spatial differences also appear to dst, although, variability across the landscape may obscure statisticdy significant differences. Reduchg the area sampled would ducethe error associatecl with the physid and chernical fatures ofthe site. Without a variety of height classes to choose fkom for each tree species it is unlikely that accuracy of any estimates would be improved by extensive sampling. Thus, it was concluded that intensive sarnpling would be more valuabIe, sacrificing accuracy for preciseness.

The generated hypothesis are presented in the next section. 5.0 Main study

The preliminary study conducteci in 1996 resulted in the development of two hypotheses. First, ifüght is the predomuiant iimiting factor to understory crop growth then the extent and duration of shade created by the trees should be wrrelated to economic yield and rates of photosynthesis (net assimilation). This effect would be most pronounced in a C species as a C3 species typically becomes üght saturated at radiation levels much less than fUsuniïght Provided that the sbading does wt limit the light levels beyond the threshold oflight saturation., no reduction in net assunilation should occur.

Second, if direct competition for soii moisture is Iimiting to growth and yieid of the understory crop, empirid measurements of plant water use shouid indicate difïaerences based on relative location and proximity to the tree.

Kt is assumed that soi1 nutrients are not Limiting based on the addition of fertilizer as part ofthe normal cropping practices and the additional nutnent inputs from litterfa1

(Thevathasan, 1998). Thus, the nuii hypotheses are stated as 1: light levels in intercropped maUe and soybean are sufficient to allow for full rates ofphotosynthesis (net assimilation) and, subsequently, no effect on economic yield (prorsimity and aspect to the tree has no intluence on yield). 2: Competition for soi1 moisture does not ktcrop growth.

5.1 Introduction

The yield loss observed near the tree row appears to be largely a factor of the arnount and duration of shade created. Thus it wodd be advantageous to explore the eEect that diffierent crown architectures would have on yield. As previously discussed, Wuand Zhu (1997) have suggested that Paulownia is an iddspecies for intercroppuig

due to its growth habit and crown structure. There is no comparable species native to

Ontario that possesses the desired morphologid characteristics and has a high potentiai

value that is required to offieconomic 10s ofthe reduced cropphg ara However, the

hybrid poplar (PopIlus&lfoi&s x nipa DN177) planted at the ARS possess some

morphological characteristics that suggest that it may be weii suited to intercropping

systerns. Specifidy, the crown is typicaliy only moderately dense with few interior

leaves relative to many other species. This suggests that a signincant arnount ofiight may penetrate through the crown to the understory crop. As well, senescent leaves contain high levels ofnïtrogen (Thevathasan, 1998) and are thus an organic fertilizer source.

Poplar also possess an extensive root system which may act to cycle nutrients and water fkom soi1 depths mavailable to the annual crop, aithough, empirical evidence of this is

Whially lacking in the current fiterature.

Silver rnaple vividiy wntrasts with the poplar at the ARS. Silver maple has a broad dense crown with many interior leaves which allows very linle light to pass directly through the canopy. It typicaily has an extensive, but shaüow, root system which may compete directly with the understory crops for soi1 resources.

5.2 Materials and Metbods

5.2.1 Basic Design

The main treatment of interest is the yield response of the growing crop to difEerent levels of cornpetition. The trees growing at the ARS are essentidy even-aged and there is iittle variation in the physical characteristics of individual trees within a species. nius, it was necessary to select two species possessiag different size and morphological characteristics. Silver maple was selected and is characterized by a dense, broad crown and a proEc, but shallow, fibrous root system (Gabriel, 1990). The other species was a hybrid popiar. This Ewumericma type hybrid is characterized by a more open arrangement of the leaves and a columnar shaped crown. Strong lateral roots are

Wcely near the surface with secondary roots plunging vertically (Demeritt, 1990).

Although rows of trees have been established over the entire ARS there are several stretches of tree row that were sparsely popufated with trees under 1 m in height. In these areas it was assumed that there was no direct wmpetition between the trees and crops for

Light, water or nutrients. Therefore, a areasonable estimate of productivity and crop response within a monocrop management system can be assumed. It was not possible, however, to remove the indirect microclimate changes that may be iduenced by the other trees growing in close proximity to the study area.

5.2.2 Site Selection

Two locations at the ARS were selected for this study based on the relative unifomiity of soi1 conditions, proximity of the tree species of interest and the two desired crop species. Three crops are grown in rotation at the ARS: maize, soybeans and winter wheat. Previous measurements on winter wheat have indicated that the trees had Little or no negative impact on economic yield (Thevathasan, 1998). As such, it was not included as a treatment in the current study. Both maize and soybean had been observed to negatively respond to the presence of trees in terms of plant heaith, biomass accumulation and yield (Williams, unpublished data). Additionally, these two crops possess the desired

modes of Cafixation, C3 (soybean) and Cd (maize).

Each tree species had been planted in groups of eight Height, crown fonn and

density and overd vigour of the 6 interior trees were visudy assesseci. Trees that

deviated remarkably fiom the average were not considered for inclusion- One tree fiom the remahhg population was randomly selected for tbis study. In 1998, the crops planted to each area were rotateci. Trees were again selected as outhed above. Characteristics of the individual trees are presented in Tables 5.1 and 5.2.

5.2.3 Sample Location

The initial study completed in 1996 indicated that a relatively smaü set of meanirements could be made in a timely fahion. Therefore, 12 locations around each tree or plot center were selected. The tree rows were onented approximately north - south. Twelve locations around the tree, at 2 and 6 m east and West of the tree (primary ruris perpendicular to the tree row) and at 2 m no& and south ofeach location of the primary axis, were identifid as sampling points (Figure 5.1). At each theof sampling a single leafiet fiom the upper canopy was selected within a 0.5 m radius of the identified sample point-

5.2.4 Yieid

In September and October of 1997 and 1998 samples for yield determination were couected fiom the soybean and make plots, respectively. An 18 x 14 m grid was laid out Table 5.1 Characteristics ofselected individual trees intercropped with maize for the years 1997 and 1998. Poplus @N177) Acer sacharrr-mmi

-1997 -1998 1997 1998 Height (m) 12.3 13 -3 10-1 7-8 DBH (cm) 25 -3 24.7 15-5 17-6 Depth of live crown (m) 10-0 10-9 13.5 5 -9 Mean radius of crown (m) 2.9 2-7 2-7 3 -2

Table 5.2 Characteristics of trees intercropped with soybeans for the years 1997 and 1998.

-. Poplus (DNl77) Acer sachrrrn'mm

-1997 -1998 1997 1998 Height (in) 12.1 11-1 7-6 8.5 DBH (cm) 22.3 21-5 15-6 17.6 Depth of Live crown (m) 9-9 8 -9 13-6 6.0 Mean radius of crown (m) 3-1 2-1 3 -2 3 .O 1 Leaf Area Index &AI') was determineci with a LiCor LAI 2000 Plant Canopy Analyzer &Cor, Lincoln, Nebraska). East - west axis (m) Figure 5.1 Sampiing locations for gas exchange measurements conducted on soybean and maize during the 1997 and 1998 growing seasoas. Three replicated mwwnements were taken at each distaoce dong the east - west ais. with the long axis perpendidar to the tree rows. Subsarnples (1 x 1 m), were collected

fkom approxhately one third of the ara Yied samples were storeci in paper bags and

either kiln-dried (1997) or Wied(1 998). The grain or oilseed were mechanicdy

separated nom the non-economic plant parts. Samples were weighed and mass per unit

area was corrected to a constant moisaire content. The yield values are reported on a per

hectare basis and do not represent land lost due to tree production-

5.2.5 Gas exchange measurements

Gas exchange, ushg the LiCor 6200 (LiCor, Lincoln, Nebraska), was measured on

a fixed area of a single leaffiom the upper canopy ofthe crop usïng a 1 L chamber.

Measurements were repeated at three locations at each distance fiom the tree row (2 and

6 m nom the tree in both the east and west direction). The device was set to complete a

measurement when the Caconcentration in the chamber changed by 10 pol. The

LiCor 6200 simultaneously measures CO2concentration (LI-6250infÎared gas analyzer),

leaf and air temperature (bi-metal thermocouple), relative humidity and photosynthetic

photon flux density (PPFD) (LL-190SA point source quantum sensor). From these

measured values eshates of rate ofphotosynthesis, stomatai conductance and

transpiration are caIculated.

The spring of 1997 was unusuaily cool and wet, delaying spring cultivation and

planting by approximately two weeks. It had been initiaiiy proposed to measure gas

exchange on the crops during the last week of June- However, at this time the crops had not yet developed to the point that sampling using the existing equipment was possible. The 6rst samplhg was delayed until July 2gUL(soybeans) and 30* (maize). The second sample period was subsequently delayed until August 28' (both crops). However, at this time the nights were very cool and humid resuIting in heavy dew covering the crops through the early part of the day and well into mid-moniing. As the idtared gas anaiyzer

(IRGA) component of the LiCor 6200 is extremely sensitive to moisture it was impossible to sample until mid-day.

By the end of August, severaï leaves in the soybean plots were beginning to yellow. The continuing cool nights resulted in the soybeans rapidly senescing- By the end of the second week of September the soybeans had completely senesced and sampling was impossible. No measurements on the corn were taken at this tirne.

In 1998 gas exchange in the maize was measured on July 1, 16, 18 and August 19.

In the soybean plots, gas exchange was measured on July 3, 15 and August 19. As in

1997, monhg measurements could not be taken on August 19.

5.2.6 Plant Water Deficit and Water Use Efliciency

A single Ieaffiom each soybean plant sampled for gas exchange was excised, stored in a polyethylene bag and chiiied. A single tassle (1997) or a single leaf(1998) fiom each maize plant sampled was similady wliected. Samples were analyzed for water potentid using a Soi1 Moisture Corp. pressure bomb in the field.

Water use efficiency was detemiined nom measurements obtained by the LiCor

6200. Actual water use efficiency (AMTUE) was calculated as rate of photosynthesis divided by rate of transpiration (po1 Ca rn -2 s 1 I molH20 m-2 s") (Larcher, 1995). Intrïnsic water use efnciency (WIE) was caldateci as rate ofphotosynthesis divided by stomatai conductance (polC& m-2 s-1 I mm01 Hani2 s") &archer, 1995). 5.3 Resde

5.3.1 Yield

For both crops, in both years, yield as a fiinction of orientation to, and distance fiom the tree row (est versus west) was analyzed. No significant differences (Tukey's

HSD, p < 0.05) were observed that could be attributed to distance or orientation.

Therefore, yield values fkorn the east and west side of the tree row were combined, but, distance intervals were rnaintained separate.

In 1997, average maize yields (Table 5.3) under poplar and maple were 71% and

8 1% of the control plot, respectively. In 1998, yields were 6 1% and 85%, respectively.

Obviously, the presence oftrees significantly reduced yield near the tree rows. However, yield in the intercropped areas increases drarnatically towards the center of the cropped area. Indeed, under the maple, yield at a distance of approximately 5 m fiom the tree row and beyond is equal to, or greater than, the control area at an equal distance from the fallow strip. Under the taller poplar trees, yield approaches that of the control at approximately 7 m fkom the tree row. A relatively high incidence of mould was observed on the ears of plants growing in close proximity to the trees in 1997. This resulted in fewer kemels of low weight thus reducing yields withh 2 to 3 m of the tree row. Figure

5.2 shows the yield of the maize htercrops as a percentage of the control.

In 1997, overd soybean yield in the control plot was significantly greater than under both poplar (71% of the control) and maple (75% of the control) (Taôle 5.4). In Table 5.3 Mean yield of maize os a bction of distance fiom the tree row.

Yield (Mg hiL) @y treatment) Distance f?om tree row (rn) 1997 1998 controi Made Po~Iar control Ma~ie Po~lar 1 4.48 b' 3.12 ab 2.05 a 2-44 b 1-57 b 0-00 a 2 4.21 b 2-07 a 2-89 ab 5-70 b 3-79 b 0-69 a 3 5-84b 3-84 a 2-65 a 6-27b 5-71 b 1-33 a 4 5-53 c 4-41 b 3.19 a 5.32 b 5-76 b 2-00a 5 4.99 b 5-84 b 3-33 a 6-33 b 6-86 b 3-53 a 6 4.83 a 4-64 a 4-61 a 5.88 a 7.07 b 5-29 a 7 7.22 a 5.81 a 5-92 a 6-50 a 6-73 a 6-59 a 8 5.20 a 5-61 a 5.56 a 6-69 a 6-58 a 6-77 a 9 7-41 b 5.16 a 5.32 a 7-78 b 6.06 ab 5-90 a

Overail average 5.53 4.48 3 -92 5-88 5.0 1 3 -57

1 Within each year, mean values within a row foliowed by the same letter are not significantly different (p < 0.05, Tukey's HSD). \I\Fest Distance from tree row (m) East

-10 -8 -6 4 -2 O 2 4 6 8 10 Wst Distance from tree row (m) €as t

Figure 5.2 Relatiw yield of mai= in the bno inkrcrops as a percentage of the conml plot Table 5.4. Mean yield of soybean as a hdon of distance fiom the tree row-

Yield (Mg ha-3 (by treatment) Distance fiom tree row (m) 1997 1998 control Ma~le Poplar Control Ma~ie Po~iar 1 1.98 bL 0.98 a 0.93 a 1-72c 1A7b 0-80a 2 2-51b 1-29a 1-04a 2-24c 1S5b 1Ma 3 2.43 b 1-75a 1-42 a 2-46c 2-12b L49a 4 2-31b 1-75a 1-52 a 2-32b 2-22b 1-48a 5 2-68b 1-88a 1.67 a 2-39b 250b L66a 6 2.59 b 2-00 a 1.97 a 2-25 b 2-85c 1-67a 7 2.34 a 2.09 a 2.17 a 2.27 a 2-76b 1.91 a 8 2-45a 2-24a 2.14 a 2.27 ab 2-63b 2-12 a 9 1-84a 1-92a 2.03 a 2.32 ab 2-72 b 1-97a

Overali average 2.34 1-77 1-65 2-25 2.28 1-58 l W~thineach year, mean values withùi a row foilowed by the same letter are not significantly diffierent (p < 0.05, Tukey's HSD). 1998, overd yield in the poplar intercrop was 70% of the control. However, in the maple intercrop the yield was comparable to the control plot. Yield in the maple intercrop surpasses that observed in the control at a distance of 5 m fkom the tree row. Altliough yield increases with distance in the poplar intercrop, it never exceeds that of the control.

Figure 5.3 depicts the relative yield of intercropped soybeans compared to the control.

5.3.2 Photosynthesis

Rates of photosynthesis of the maize crop varied with tirne ofday, tirne of year and relative location to the tree row. Maximum rates were typically observed at locations 6 m fkom the tree row (Figures 5-4 - 5.8; see appendi Tables Al. 1 - A1 -6). Naturally, rates of photosynthesis and the PPFD are correlated. The total area and distance shaded are considerably greater under the poplar in the morning and afternoon due to its greater height. As canopy width of the two tree species was simiIar, noon time PPFD (Figures 5.9

- 5.13 ; see appendix, Tables A1 -7 - Al. 12) was very similar under both species at each location. Maximum rates of photosynthesis occur in mid July. This reflects the combination of maximal rates of incident PPFD and the development stage of the plant.

Through the month of July in both years the rates of photosynthesis within the intercrop treatments in locations not influenced by shade during any one theperiod were generally similar to the sarne locations in the wntrol. Measurements taken on July 29,

1997 (Figure 5.4) and August of both years (Figures 5.5, 5.8) indicate that plants close to the tree (2 m east or west) did not respond to increased Light levels in the afiernoon.

Morning measurements in August of both years were impossible due to heavy dew on the plants. Wst Distance from tree row (m) East

b al .- 0- - )r I \ ,* -- .-t 75- \ YcP -Q> \ 0- - a 501 N - - fl %

25-

o,~.-.. [ . . s , . . . 1 . . .i, . . r. . .' -10 -8 -6 4 -2 O 2 4 6 8 10 Wst Distance from tree row (m) East - Maple - - Poplar

Figure 5.3 Relative yield of soybean in the Wo intemps as a percentage of the conbol plot 6 m East 2 m East Tree 2 m West 6 m West

Figure 5.4 Rate of photosynthethis measured at the top of the maize canopy, July29 1997. 6 rn East 2 m East Tree 2mwdt 6mWest

Figure 5.5 Rate of photosynthesis meas ured at the top of the maia canopy, August 29 1997. 6 m East 2 m East Tree 2 m West 6 rn West I i

Figure5.6 Rab of photospthesis measured at #a top of îhe mai= canopy, Julyl 1998. 6 m East 2 m East Tree 2 m West 6 m West

Figure 5.7 Rate of photosynthesis measurad at îhe top of the maize canopy, July 16 1998- 6 m East 2 m East Tree 2 m West 6 m West 3

Figure 5.8 Rateofphotosynthesis measuredatthebp of the main canopy, August 19 1998. 6 m East 2 m East Tree 2 m West 6 m West

Figure 5.9 Photosynthetic photon fluxdens ity (PPFû) received at the top of the m aize canopy, July29 1997. 6 m East 2 m East Tree 2 m West 6 m West

Figure 5.1 0 Photosynthetic photon luxdensity (PPFD) receiwd at the top of the mai= canopy, August 29 1997. 6 m East 2 m East Tree 2 m West 6 m West

Figure 5.1 1 Phobosynthetic photon fluxdensity(PPFD) receiwd atthe top of the mai= canopy, July 1 1998. 6 m East 2 m East Tree 2 m West 6 m West

Figure 5.1 2 Photosynthetic phobn fluxdensity (PPFD) reœivad at the top of the mai= canopy, July 16 1998. 6 m East 2 m East Tree 2 m West 6 m West

Figure 5.1 3 Phobsynthetic photon fluxdensity (PPFD) recekd at the top of îhe mai= canopy, August 19 1998. The trends in photosynthesk observed in the soybean plots are similar to that in the maize plots (Figures 5-14 - 5.18). The maximum rates of photosynthesis occur in mid-

July. In general, photosynthetic rates are a fimaion ofproràmity and orientation to the tree row. However, there ern'st some interesthg differences-

Aftemoon measurements in locations west ofthe tree row within the ùitercrop treatments generaliy show a lower rate of photosynthesis than comparable locations in the control, except for the Jdy 3, 1998 sample penod figure 5.16). For instance, the atlernoon of July 15, 1998 (Figure 5.19) exhibited a large variation in incident PPFD during the aftemoon sampling penod. PPFD received on the West (unshaded) side of the poplar intercrop was 2 - 3 times higher than that observed in the maple intercrop or the control. Despite these much higher PPFD values the rate of photosynthesis at 2 m West of the poplar row was comparable to that under the maple and significantly less than the control. Even at 6 m west ofthe tree row photosynthesis in the poplar intercrop equaled that of the control. A sunilar response was observed on August 29, 1997 and August 19,

1998.

5.3.3 Water Use Efficiency and Plant Water Deficit

Calculated estimates of water use efficiency (WU'E) in the maize intercrops do not exhibit any consistent trends (Tables 5.5 - 5.9). For July and August, 1997, lowea WUE values correspond to locations closest to the tree, however, there is no significant proximity effect (p < 0.05). 6 m East 2 m East Tree 2 m West 6 m West I 1

Figure 5.14 Rate of photosynthesis measured at the top of the soybecrn canopy. July 29 1997. 6 m East 2 m East Tree 2 m West 6 m West

Figure 5-15 Rab of photosynthesis measured atîhe top of the soybean canopy, August 29 1997. 6 m East 2 m East Tree 2 m West 6 m West

Figure 5.1 6 Rate of photosynthesis measured at îhe bpof the soybean canopy, July3 1998. 6 m East 2 m East Tree 2 m West 6 m West

Figure 5.17 Rats of photosynthesis measured atthe top ofthe soybean canopy. July 15 1998. 40 PM

30 -

20

10 -

O; 6 m East 2 m East Tree 2 m West 6 rn West

Figure 5.1 8 Rate of photosynthesis measured at îhe tup of the soybean canopy. August 19 1998. 6 m East 2 m East Tree 2 rn West 6 m West

Figure 5.1 9 Phobsynthetic photon flux dens ity reœived at the top of the soybean canopy. July 15 1998. Table 5.5. Rates of actual water use eficiency (AWUE) for maize on July 30, 1997. Distance Water Use Eficiency fiam tree (pmol CO2 m -2 s-1 /(mm01 H20 m-2 s" ) row AM Noon PM Control a Po~lar Control Made Po~lar Control Maple poplm 6mEast 5.63 a 5,29a 5.54 a 5.22 ab 6.63 b 4.94 a 4.94 a 4.60a 5.15 a 2mEast 5.14 a 4.56 a 5.34 a 549 b 3.15 a 2.90 a 4,33 a 3.37 a 3.75 a 2m West 5.10 ab 6,71 b 3.09 a 4.83 a 4.97 a 5.10 a 4.66 a 4.93 a 5.40 a 6mWest 6,04a 6,02a 6.89a 5,66a 5,IOa 5.85a 4.78a 4.91a 5,23a Within each time period, values in each row followed by the same letter are not significantly different ('hkey's HSD, p < 0.05).

Table 5.6. Rates of actual water use efficiency (AWUE) for maize on August 28, 1997. 0i O) Distance Wata Use Eniciency fiom tree (pol CO2 m -2 s-1 /(mm01 H20m-2 s-1 ) TOW AM Noon PM Control Maple Poplar Control Maple Po~lar Control Ma* POD~ 6mEast N/A N/A N/A 9.34 a 9,60 a 10.38 a 7,47 a 8.38 a 8,60 a 2mEast N/A N/A N/A 10,42a 6.65 a 10.40 a 7,01 a 8.44 a 6.29 a 2rnWest N/A N/A N/A 8.62a 8,86a 11.60a 6,27a 10,13a -0,24a 6mWest N/A NIA NIA 8,44a 10,80a 8.73a 8S8a 8,lla 7,35a Within each tirne period, values in each row followed by the same letter are not significantly different (Tukeyis HSD, p < 0.05).

EEEE V)Nel\O Analysis ofthe plant water defi& data was, perhaps, even more ambiguous than that of the AWUE data However, this information was coiiected only once in 1998.

Within the maize crop, PWD was generaily greater in the rnaple intercrop than both the poplar intercrop and the control (Tables 5.10 - 5.12).

There are no apparent trends that would suggest that the WUE of soybeans is af5ected by the presence of trees (See Appendix - AWUE: Tables A15 - Al?;IWUE:

Tables A18 - NO). Plant water dekit was reIatÏveIy uniforni across treatments except for noon readigs in the maple intercrop and the aftemoon readings in the poplar intercrop on

July 29, 1997. However, the greater deficit at these times does not appear to have infiuenced the rate of photosynthesis.

5.3.4 Soil Moisture

Soil moisture content was determined gravimetrically at two depths, 5 and 15 cm on July 17, 1998. W~thinboth make intercrops soil moisture content was generdy significantly @ < 0.05) lower than the control (Table 5-13). Poplar was consistently lower than the maple although not significantly Merent- Soil moisture for both intercrops at both depths was lowest 2 rn West ofthe tree row.

Within the soybean intercrops soi1 moisture under poplar was consistently lower than the maple intercrop and the control (Table 5.14). U&e the maize intercrop, soil moisture under the maple was often signifïcantIy greater @ < 0.05) than a comparable location in the control. =38 --xx3aaa3 ui c 5 ww3s = 2 0 EEEE BOflag *Nelm

Table 5.13. Soil moisture content (%) at a depth of 5 cm for maize on July 16, 1998. Distance fiom tree row 5 cm 15 cm Control Maple Poplar Control Made Poplar Control Made Po~lar 6m East 8.20 a 8.71 a 8,68 a 8.51a 8.40a 8.35a 2m East 9.63 b 7.32 a 7.16 a 10.00 b 7.32 a 7.16 a 2m West 10.47 b 6.83 a 6.48 a 11.09 b 6,98 a 6,26a 6m West 9,62 c 7.85 b 6,45 a 10,09b 7.62 a 6.55 a For each depth, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05).

Table 5.14. Soil moisture content (%) at a depth of 15 cm for soybeans on July 16 1998, t...... ,**...** t...... *,,....*".*."..*.."" i..... i...... i ..... i.i ...... i. ...ri...... ".S. " " .S...... c...... m... l...... r.... "...".."...... m.?*..l...ii....l.*...."..... ".." ...... " ....,. *...- !E Distance fiom tree fow 5 cm Control Ma~le Po~Iar Control Ma~lq Poglar 6m East 6.91 ab 8.22 b 5.70 a 6.76 ab 9.28 b 6,00 a 2mEast 6.45 b 730 c 4,61 a 5,51 a 7,80 b 4.61 a 2m West 6.77 a 6.42 a 5.64 a 7.30b 6.79ab 6.18a 6m West 7.63 b 7.61 b 6.03 a 8,42 a 7.97 a 5.65 a For each depth, values in each row followed by the saine letter are not significantly different (Tukey's HSD, p < 0.05). 5.3.5 Land Equivrilcnt Ratio

Land equivdent ratios (LER) were dculated. The rate oftree growth was assumed to be the same under intercropping as may be obsemed in a plantation. Final stocking density in the intercrop system was caiculated as 133 trees ha-' (5 x 15 m) and for a plantation at 400 trees hcL (5 x 5 m). Thus the LER ofthe trees, for both species, is

0.3325. LER for the crops required adjustuig the yidd for land area lost to the tree rows.

At the 5 x 15 spacing thÏs resulted in a reduction of the yieid by 13%. The calculated

LER values are presented in Table 5.15 for both years. LER values greater than one indicate the system is more productive than growuig either as a sole crop. Table 5.15 Land equivalent ratios (iJX) determined for make and soybean intercropped with hyôrid poplar and silver maple in two successive years at the Agroforestry Research Station, Guelph, Ontario-

Maize Soy bean Ma~le Poplar Maple Poplar 1997 1.13 1-03 0-99 0-94 1998 1.O7 0-86 1.21 0-94 5.4 Discussion

5.4.1 Maize intercrop

As expected, rates of photosynthesis were dependent on the location of the plant

relative to the tree and the time of day. In the maize plots, there was some evidence of impaired photosynthesis regardless of the quantity or quality oflight received at locations in close proamity to the tree Measured rates of photosynthesis near the tree were lowest from mid - July through August in both years. These plants were often relatively unresponsive to changes fiom a shaded condition to fiill sunlight, especiaily plants

Unmediately west of the tree row. Height growth and rate of canopy closure of the crop at these locations were observed to be less than that of the plants further from the tree row.

Shading certainiy played a critical role in &on assimilation during a significant part of the day. In addition, repeated stress events (shading or possibly drought) has likely created permanent physiological changes within the plant. Indeed, chronic stress factors may induce a 'state of exhaustion' within the plant resulting in an increased susceptibility to other stressors (Le. disease, Uiseas) (Larcher, 1995). This effect was apparent in 1997 where disease was observed to be more prevalent in plants receiving the greatest amount of shade.

In the maize crop, yield increases with distance fkom the tree row (fdow strip).

This can readidy be explained by the greater duration of shade at locations closer to the tree row. However, the apparent yield increase which occurs some distance fiom the tree row partially offsets the yield loss near the tree row. This phenornena is generaiiy observed in agricultural systems employing windbreaks and shelterbelts and is attniutable to modiication of the boundary layer chte. As the trees increase in size and cast a

greater shadow the benefits denved fiom the changing microclimate dlbe offset by

reduced photosynthesis at ever increasing distances fkom the tree row.

Figure 5-1 (PPFD received at the top of the maize canopy on July 1, 199%)depicts

rates of irradiance that are typicaiiy expected through the course of a day at diaérent

locations relative to the tree row. Actual rates of imdiance will Vary day to day

depending on climatic conditions, but the pattern shod be repetaive over tirne. If light

is the major limiting factor in carbon assimilation the plants nearest the trees are obviously

moa Sected. Rates of photosynthesis are dûectly and iinearly correlated to PPFD up to

certain point, which is species specifïc. In general, given non-limiting conditions, C3 plants

reach a maximum efficiency around 12,000 polmw2 s-' and for C4 plants this value is

approximately 18,000 polrn -2 s-1 . Plants growing firrther away than 6m fiom the tree

row, in this study, are afEected for a relatively short penod of the. AU other factors behg

equal, (Le. water, nutrients, etc. not limiting), as the shadow moves the rates of

photosynthesis in plants that have been recently shaded should increase to levels

comparable to that found in the control. Reviewhg the PPFD and photosynthesis data,

this appears to be the case.

Calculated estimates of both intrinsic and actuai water use efficiency show very few differences within a treatment and sample penod. The lack of any trend suggests that ciifferencesare due to either smdtemporal and spatial difEerences in the sampling or may be attributed to human or mechanical errors. Soi1 moishire content determïned on Iuly 16,

1998 was higher in the control plots for depths of both 5 and 15 cm. Contrast this information with plant water deficits determined on the same day . The only significant difEerence detected occurred in the afternoon sample period at a distance 6 m east ofthe

tree row. Surprisingly, the control plants exbiiited the highest deficit. This information,

combined with photosynthesis rate estimates suggest that competition for soil moisture is

not limiting to the success of the maire crop.

5.4.2 Soyberin intercrop

The lack of a yield response dong the east - West axis is somewhat surpcising. It has been shown that net photosynthesis in soybeans declines through mid-day, especialiy when the plant is already water stressed (Rawson et al. 1978). At this theof day soybeans in fuii sunlight can affiect stomatal closure and a partial reduction in turgor causing leaves in the upper canopy to droop. This aüows the plant to avoid evaporative fosses, thermal shock and phot~toxicity~Under these conditions photosynthesis is greatly reduced and, in extreme conditions, the plants may undergo photo-respiration. It would be expected that plants shaded du~gthis thne wodd not respond in the sarne manner as plants receiving fiiU sunlight. Assuming other factors are not limiting, especialiy moisture, partiaily shaded soybeans may be able to take advantage of scattered light or iight flecks and maintain a higher rate of carbon assimiiation. However, this response was not observed.

In general, patterns of photosynthesis follow the measured rates of PPFD.

However, numerous inconsistencies are apparent. For instance, comparing PPFD and rates of photosynthesis for Jdy 3, 1998, especidy the noon and afternoon measurements, suggests that some factor other than iight is affecting rates of carbon assimilation. Estirnates of actual water use efficiency indicate few differences between treatments. This same pattern held for intrinsic water use e5ciency .akhough, an interesting trend is suggested in the data. Despite a lack of statistical signincance

(statistics not presented) it is apparent that the lowest IWWE occurs in plants closest to the tree row. In addition it appears that the plants receiwig morning Sun and afternoon shade (located 2 m east of the tree row) consistently exhibit the lowest JWüE.

Sun flecks may play an important role in net assimilation of carbon Within an open grown soybean crop the lower leaves on the plant are partialIy shaded. These leaves typicdly require an initial input of iight to activate photosynthetic enzymes (especially

RUBISCO) and cause stomata to open (Pearcy et al., 1990). The presence oftrees dl create this environment in the upper canopy ofthe crop as weU. Thus, sun flecks may not necessarily provide sufficient energy to initiate a significant photosynthetic response, depending on the degree and duration of shade. This may also be a result of chronic stress factors, such as shade, afEigthe photosynthetic apparatus of the plant.

Soybeans had a negative response to the presence of poplar across the entire research plot. The leaforientation of soybean is descnied as rnosaic. This is in contrast to the erect nature of many modem maize hybrids. The mosaic arrangement results in low

Light interception when the Sun is at an oblique angle. Thus, plants that are shaded during the period of maximum potential photosynthesis, arowid mid-day, are at a severe disadvantage. However, even plants that are shaded for oniy part of the penod of high potential photosynthesis are disadvantaged. Making a general cornparison of the soy bean response to make plants at a wrrespondiig position in the landscape, maize is better adapted to utiliie any available Light, primarily due to leaf orientation. This is despite the generally accepted fkct that Q aops have a higher photosynthetic efficiency at low light

levels.

In addition to above ground competition, it appears that poplar is providing strong

competition for soi1 moisture. Data collectecl on Iuly 16, 1998 shows that soi1 moisture

under the poplar intercrop is less than or similar to that of the control. However, in the

maple intercrop values are similar to, or higher than the control. In part, this explains the

much lower yield in the poplar intercrop.

The potential benefits of a windbreak effect are not realized by the soybean crop.

Although yield greatly increases even a short distance fion the tree row the negative

impact of the trees is evident throughout the sampled area. The yield reduction extends

weU beyond where tree roots were expected. The soi1 moisture data and estimates of

suggest that we may have grossly underestimated the extent to which tree roots,

especiaiiy poplar, have extended into the crop area.

As Cj crops becorne Iight saturated at approxhnately 50% of fùll suniight it is

apparent that the moming and dernoon shade across the crop stnp greatly reduces the

daily net assimilation of carbon. A C crop, such as maize, may welI be able to

compensate for these penods oflight stress due to a more efficient use of high levels of

PPFD. Additiondy, one of the characteristics of modem maize cultivars is erect leaves.

This adaptation improves the ability of the whole plant to intercept radiation arriving fiom

an oblique angle, including scattered Li@.

In the maize crop, the presence of rnould near the tree row in 1997 can be attributed to two factors. These plants were already severely stressed as evidenced by stunted growth and chlorotic and necrotic spots on the leaves. Additionally, the cool, humid nights through most of August created very moist conditions, ideal for the growth of mould. With the addition of shaded conditions reducing potential evaporation Eom leaf surfaces dong with interruption of the wind flow (windbreak effect) these plants remained moist for extended peciods of time. In the most heavily shaded areas the dew remained on the plant as Iate as I 1:O0 AM.

Response curves were estimated for yield and nnal height for each treatment in both years- In ali cases the quadratic eqyation provided the best fit although not aiways a good fit. There are two problems with these estimates. The control plots exhibit a general lack of response to distance fiom the tree row beyond 1 - 2 m corn the tree row where yield was depressed due to edge effects. In the rnaple and poplar treatments the estimates were a better fit, however, they hold true for oniy a short distance fiom the tree row. Beyond this distance we can assume that the yield will rernain relatively constant until the next tree row is encountered. The design of the ARS precludes this type of modeling as there are very few paired rows containhg the same tree species or at least trees of the sarne height. Also, there is insufficient variation in crop row widths (12.5 or

15 m) to generate more accurate estimates. However, it does provide a working hypothesis for fiture consideration.

The distance at which the estimates begin to fail is a hction of some effect of the trees. However, it remains uncIear as to the exact mechanism. The height of the tree is not the dominant factor as the estirnates fail at approximately the same distance fiom the tree row for both species, despite the height differential. Crown width is kely a more important characteristic as the zone of greatest yïeld depression corresponds to the area shaded during the potentidy most productive part of the day. This is confounded by the potential for significant below ground competition not accounted for in this study.

Soil moisture merences observed for both the maize and soybean suggest that poplar may be exploiting shaliow soü layers, at least up to a depth of 15 cm, for moisture.

Despite the greater extent of shade provided by poplar, soi1 moishire was consistently lower throughout the sampled area. This may have serious implications in ternis of potential yield, especiaiiy in years where drought occurs. This appears to be the case in

1998. Soybean yields appear to have been most affecteci as a result oflow rainfd through the critical period of pod development. Aithough the poplar Euramericana phenotype is characterized by extensive lateral root development other research at the ARS indicates that the presence of roots is negiigibie at a distance of 6 m fiom the tree (Gray, pers. comm.).

An alternative to the trees using this moisture is that the crops may be rapidly trampiring thus depleting the soil of moisture. This does not seem possible given the low rates of photosynthesis in soybeans compared to the maize intercrop. In addition, estimates of plant water deficit do not support this. PWD in the poplar intercrop are generaiîy lower than under both the maple intercrop and the control.

Within the maize intercrop, the moisture data are more easily interpreted. Soil rnoisture content, determined on July 17, 1998 was typically lower in both intercrops compared to the wrresponding sample location in the control. It has been shown that

ABA produced in the roots ofmoisture stressed plants is responsible for stomatal dosure even before water becomes lirniting to photosynthesis (Tardieu et al., 1992; Davies and

Zhang, 199 1). explains the relative low rates of photosynthesis of the intercropped plants exposed to full sun However, 1do not know why the soi1 is depleted of moisture. Logidy, plants transpiring at a lower rate combined with shading durkg part of the day should lead to conservation of soil water. A likely explanation is that the trees are competing for this water. The density ofroots in the soil at a distance of 6 m from the tree row is quite Iow (Gray, pers. cornm.). However, the tree root surfêce area that exists in the shallow soif profüe may be suf16cient to dowfor signincant depletion of the avaiiable moisture,

The calculated LER for each intercrop does not proMde compelling evidence that intercropping provides a signincant advantage in overall site productivity. However,

Uicreasing the density of trees within the row and spacing the rows further apart will Wrely irnprove these ratios. As an example, increasing the crop row width to 25 m and doubhg the number oftrees in each row (2.5 m spacing) increases the LER by 0.07 and reduces the amount of land lost to trees fiom 13% to 8% given the sarne level of management. 6.0 Conclusions

The hypotheses tested by this research were 1: that Light levels in the intercrops were sufficient to aUow for rates of net assimilation that would result in no loss of econornic yield, and 2: that inter-specifk competition for soii moisture would not bt crop growth. A loss in economic yield was determined within the intercropped areas, especidy close to the tree rows.

The greater yield reduction ofboth soybeans and maize under the poplar intercrop rnay be due to two factors. Primarily, these trees are considerably tder and cast a greater shadow that persists across the cropped area longer than the maples used in this study.

However, the leaf arrangement of the poplar is such that a greater amount of scattered light rnay penetrate the crown. The erect leaves of the maize rnay weil be able to efficiently utilize scattered iight as weU as direct radiation arriving fiom an oblique angle.

Soybeans rnay not reaiize such benefits due to its leafarrangernent.

The second concem is the level of cornpetition that occurs below ground. Silver maple typicaily has an extensive, fibrous root system. However, there is no idormation on the degree to wtuch silver maple can compete with other species for below ground resources, especialiy water. The native habitat for silver maple are lowlands and floodplains where water is less likely to be limiting. Poplas is adapted to a wide range of soil conditions and rnay be inherently more successfùl in direct competition for soi1 resources. The type of hybrid used in North America (Eurumericma) typicaliy possess strong iateral roots (pNnary) with secondary roots plunging vcrtically, thus providing most of the mechanical support. Finer roots, responsible for the majority of nutrient and water uptake, rnay develop anywhere dong the supporthg root structure. It appears that cornpetition for tight rnay be a les signifïcant factor than beiow ground cornpetition for water. Given the inherent fertility of the soi1 in addition to the amount ofinorganic feailizers applied in the spring, water rnay be the most limiting factor to crop success. This hypothesis comes with a caveat. Reduction ofcrop yield near the tree row (within 6 m) rnay be quite significantly affecteci by shade. Shading within this zone occurs du~gthe time of greatest potential photosynthesis of the crop. This, in conjunction wïth below ground cornpetition results in an uneconomic yield being produced within this zone. However, CQcrops posses a higher photosynthetic efficiency at Iow light levels than C crops- This trait rnay be exploited to confer an advantage to intercropping systems through crop species andlor cultivar selection.

Future challenges are to establish a method of cultivation that would ensure an economic yield in the near tree zone. This rnay be achieved through a combination of cultivation, prunhg and crown thinning. The optimum distance between tree rows must be established. The current spacing ofeither 12.5 or 15 m is likely too close to econornically grow current high performance cultivars of maize and soybeans. The close spacing of the tree rows favours the use of shade tolerant (ginger, ginseng) or short season intercrops (spring grains).

In the case of mahe, increasing the size ofthe crop rows rnay take advantage of the windbreak effect. Increasing the height to live crown through pruning wili hprove early and late day light regimes, especially near the tree row. This will also result in improved airflow through the crop close to the tree row. This should reduce the humidity in this zone, thus îimiting conditions favourable to rnould growth.

There is much work to be done. The extent to which tree roots are competing directly with the crops needs to be estabtished. This entails an extensive survey ofthe rooting habits of potential agroforestry tree species. Additionaliy, a detailed mode1 of shade patterns under varying intercropping configurations would be usefui to develop more specinc hypotheses. Such a mode1 should incorporate variables descnbhg crown structure and Ieafarrangement for a varïety of tree morphologies.

The findings of this project suggest that crops adapted to fiiii sun rnay not penorm to a desired standard under the current management system. There are several options available that rnay make the system more acceptable in the current economic environment.

One of the primary goals of the research conducted at the Guelph Agroforestry Research

Station is to grow trees with a high ttture value. This is expected to offset the hunediate econornic loss of crop yield reduction due to shading and the smailer land base for crop production. However, this is a long term investment. Increasing the size of the crop strips dlreduce the area of crop that is direçtly cornpethg with the trees for available resources. It is also possible to increase the spacing within the tree rows, thus allowing more light to reach individuai crop plants over the course of a day. Both ofthese options reduce the number of trees that rnay be harvested at a fiiture date as weii as reducing the potential environmentai benefits. The economics of such scenarios rnay be investigated, but at first glance, do not look promising.

Aitemative to these scenarios is something more radical, and potentially more profitable. Crops adapted to conditions of full Sun may not be appropnate to intercropping with trees. Alternative crops rnay provide a unique source of income whiie dowing the successfiil establishment of valuable trees. As has been mentioned, ginger is one such potential crop. Given the current screage of ginseng grown in Ontario, it rnay also be quite promising- Ginseng cultivation rnay even promote the potential for even higher tree densities.

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Table Al. 1. Rates of photosynthesis for maize on July 30, 1997. Distance Rate of photosynthesis fiom tree (pmol rn -2 s-1 ) row AM Nwn PM Control Ma~le Podar Controi Ma~le Poplar Control Ma~le Popkir 6mEast 38.5 a 42.3 a 28.2 a 41.3 a 45.2 a 34.3 a 39,Ob 24,4 ab 21,6 a 2mEast 36.3 b 20.5 a 26,3a 39.7 b 13.0 a 13.5 a 368 b 18,6a 13,3 a 2mWest 29,6 b 30,O b $3 a 38.5 a 22,l a 21.3 a 4O,7 b 25.4 a 22,7 a 6mWest 31.5ab 38.6b 27,la 40.8a 32.9a 31.5a 39.8b 41.7b 25.7a Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05). m W Table A1.2. Rates of photosynthesis for maize on August 28, 1997. Distance Rate of photosynthesis îiom t ree (pmol m-2 s-1 ) fow AM Noon PM Control Maple Poplar Control Maple Po~lar Control Matde Po~lar 6mEast N/A N/A N/A 20,8a 21.5a 18Sa 14,Oa 16,Sa 18,6a 2mEast N/A NIA N/A 16.1a 10.3s 12,Ba 13,3a 12,la I1Ja 2mWest N/A NIA N/A 17Sa 22.6a 7,sa 18.3 a 14,O a 62a 6m West NIA NIA N/A 20.3 a 27.3 a 14,O a 20,2 a 22,3 a 14.0 a Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05). Table A1.3 Rates of photosynthesis for maize on July 1, 1998. .".C"..C...... "...WI...... "...... "*."...... W.... "..."...... ""U.)...... "...... "...... Distance Rate of photosynthesis fiom tree (pmol m -2 s-1 ) row AM Noon PM Control Maple Po~lar Controi Maple Podar Control Made Po~lar 6mEast 36.1 a 40.7a 28.5a 27.1 a 37.3 a 27.4a 36,4b 44,2 b 2L4a 2mEast 31.6a 27.3a 22.9a 22,6a 17,6a 17,la 26.1a 12,3a 14.5 a 2mWest 27,Ob 15,2a 9.5a 23.7a 18.4a 17Sa 30.2a 33.4a 27,la 6mWest 32.0a 28.9a 28.38 30.2a -19,3a 293a 29.4a 31.4a 31,5a Within each time period, values in each row followed by the same Ietter are not significantly different (Tukey's HSD, p < 0.05).

Table A1.4. Rates of photosynthesis for mai* on July 16, 1998. Distance Rate of photosynthesis fiom tree mol m-2 s-1 ) row AM Noon PM control Ma~le Po~lar control Mapie Po~jar controj Podar 6mEast 18,6a 34.9b 25,2a 18.5a 31.1b 27.1ab 16.6a 23,la 18.6 a 2mEast 16,4a 18,4 ab 25.0 b 183 a 23,2 a 21,l a 21.9 b 9,8 a 11,9a 2m West 15,7a 16.5 a 11.6a 17.5 a 16,8a 18.1 a 13.7a 12.1 a 13.2 a 6mWest 21.h 22.3a 30,Ob 19.6a 34.5b 23,Oa 17.4b 11.4a 20.1a Within each tirne period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05). I Table Al.5. Rates of photosynthesis for maize on July 18 1998. ..." ...... i""...~..W...... ~rr.....rn...... *W.." ..... ri.. "i.."i... " S..... *...... ".....W.. .W...... ? a...... "...... M...... "."...,...... Distance Rate of photosynthesis fioni tree @mol m -2 s-1 ) row AM Noon PM Control Maple Poplar Control Ma Poplar Control Ma Poolar 6mEast 28.6 b 18.7 a 28.5 b 17.9 a 32.3 b 27.9 b 27.4 ab 39,7b 15.6 a 2m East 24.9 a 20.2 a 17.7 a 23.3 b 19,4ab 11.6 a 13.8 a 10.4 a 13,s a 2m West 21.0 b 9,4 a 8,3 a 25.4 b 9,4 a 14,6 a 14.7 a 16.5 a 8,O a 6m west 31.4b 17.1 a 11.2a 24.8a 22.8a 24.6a 23.6ab 29.4 b 21.8a Within each time period, values in each row followeâ by the same letter are not significantly different (Tukey's HSD, p < 0.05).

8 Table A1.6. Rates of photosynthesis for maize on August 19, 1998. Distance Rate of photosynthesis fiom tree (pmol m-2 s-1 ) row AM Nwn PM Control hda~le Poplar Control Maple Poph Controî Ma~e Po~lar 6mEast N/A N/A N/A lL4a 1L8a 7,8a 13.9a 11,6a 6,4 a 2mEast N/A N/A NIA 13.8b 6,4a 3,7a 12.4a 6,4 a 6,3 a 2m West N/A N/A NIA 10,4 a 7,4 a 2.7 a 11,O b 5,6 ab 3.6 a 6m West N/A N/A NIA 8.8 a 8.8 a 8,3 a 9,4 a 10,3 a 6,O a Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05). Table A1 -7.Incident PPFD measured at the top of the maize canopy on Juiy 30 1997. .-... W....." ...... "...... "...--S...-.-. "... S...... S...... ".."...".. ...-.,.".r""..+-.-.----.-- .-...... ri...... "L"."".-."...--.,".-..-...... " ,...... , Distance Incident PPFD fiom tree row AM Control Maple Po~lar Controi Ma~le Pootar Control Made Po~lar

Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05).

Distance Incident PPFD fiom t ree (pmoi mo2S.') row AM Noon Control Made Po~lar Control Ma~le Po~lar Control Ma~le Po~lar 6mEast N/A N/A NIA 607a 1143 a 554a 366a 480a 837 a 2m East N/A N/A NIA 765b 252a 361 a 522a 310a 336 a 2m West NIA N/A NIA 785 a 946 a 479 a 608 a 706 a 484 a 6m West N/A N/A NIA 581a 826a 482a 792a 933a 489 a Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05). Table A1.9. Incident PPFD measured at the top of the maize canoey on Ju?y 1, 1998. U.."..~.~....".~"."""~.."..,..""","...,...."..,...... ""...... ".....".."....."...... -.W.""...... ". W...... rr"" ".."..m.....r."...".rt...."*."...... mmr.mnm,*"*.""* Distance Incident PPFD fiom tree (pmol m-2 s-1 ) row AM Noon PM Controi Maple Po~Iar controi Ma~le PopIar controi Ma~le Poplar 6mEast 1808a 1777a 1656a 1247a 1864a 1596a 1804a 1635a 1194a 2mEast 1798b 1109a 1626ab 1144a 722a 914a 1717b 256s 711ab 2m West 1778 b 602 a 230 a 1329 a 788 a 991 a 1753 a 1668 a 1668 a 6mWest 1771a 1664a 1635a 1492a 1444a 1711a 1691ab 1662a 1753b Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05).

00 -J ....cc".c-...... ~-.-..~...... ~...... ~~...... w~-.-."~...... Table Al. 10. Incident PPFD measured at the top...... 1+.r..."11ttNNN"n..+..+.. of the maize canopy...... iir.... on July..... 16,..i...... +...... i1....NN.N...... ~~...... ~...~..~~~...... 1998. Distance Incident PPFD fiom t ree (pmol rn-2 s-1 ) row AM Nwn PM Control Ma@e Poplar controi Ma~le Po~lar controî Ma~le Po~lar 6mEast 559a 1106b 1277c S90a 1157b 1518c 51Sa 677b 584 a 2mEast 504a 572a 1247b 605a 661ab 801b 815c 259a 456 b 2m West 580 c 420 b 345 a 633 a 554 a 1091 b 511 a 704 ab 773 b 6m West 655 a 655 a 1252 b 643 a 1548 b 889a 627 a 362 a 1018 b Within each time period, values in each row followed by the same letter are not significantly diflerent (Tukey's HSD, p < 0.05). Table Al. 11. Incident PPFD measured at the top of the maize canopy on July 18 1998...... ""--"-""-..-"-...." -...... m... ""--"-""-..-"-...." ...... ri ...... ".l..-..".,. Distance Incident PPFD fiom tree (pmol m -2 s-1 ) row AM Noon PM Control Maple Po~lar Control Ma~le Poplar controi Ma~le Po~lar 6mEast 1410a 772a 1569a 642a 1626b 1961b 1524a 1982a 604a 2mEast 1772b 904a 1343ab 1606b 471a 339a 449a 270a 790 a 2mWest 1815b 175a 194a 1843b 186a 1446b 848a 1667s 2035a 6m West 1843 c 620 b 289 a 1796 a 933 a 1690 a 1856 a 1867 a 1898 a Within each time period, values in each row followed by the same letter are not significantly different (TukeytsHSD, p < 0.05).

aE ....Table"t.1...... -..~...~-~-....-..~.-..~w...... -....~...... w...~...... "...~."~ Al. 12. Incident PPFD measured at the top...... of the maize canopy on Aupst.m.+rn-...n..n...tttttn~..~~~.nn~~~~...*.~.+....w.-..~~..~ 19, 1998. Distance Incident PPFD fiom tree (pmol m-2 s4 ) row AM Noon PM ......

Control Maple Poplar Control Maple Po~ontroÏ ' Maple Po~lat: 6mEast N/A NIA N/A 1540b 1241a 1669c 1730c 1158b 269a 2m East NIA NIA NIA 1650 b 289 a 446 a 1549 c 132 b 870 c 2mWest N/A N/A NIA 1814b 686a 380a 1592a 1641a 1559a 6m West NIA NIA N/A 1615a 1617 b 1535a 1660b 1384a 1551 ab Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05). Table Al.13. Rates of actual water use eficiency (AWUE) for maize on July 30, 1997. "".tm-"m...."."*....."nn*.,..-,m...u...... ".. m.. ..."...""...... ""."...... +."..."...... r. ""...... M.""...mr...."..*.""*...... "...."..".. Distance Water Use Eficiency fiom tree (pmol CO2 rnm2s" /(mm01 H20mm2 s" ) row AM Nwn PM Control Made Po~lar Control Ma~le Po~lar Controî Maoie Poplar 6mEast 5.63a 5,29a 534a 5.22ab 6,63b 4.94a 4.94a 4.60a 5.15a 2mEast 5.14 a 4.56 a 5.34 a 5,49 b 3.15 a 2.90 a 4.33 a 3,37 a 3,75 a 2m West 5.10 ab 6.71 b 3.09 a 4,83 a 4.97 a 510 a 4,66 a 4.93 a 5,40 a 6mWest 6.04a 6,02a 6,89a 5.66a 5,IOa 5Ma 4.78a 4,91a 5,23a Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05).

aD CD Table A 1.14. Rates of acaial water use eficiency (AWUE) for maize on August 28, 1997. Distance Water Use Eniciency fiom tree (pmol CO2 m-2 s-' /(mm01 H20mm* s" ) row AM Noon PM ------Control Maple PO~IW Control Maple Poplar Control Ma~le PopIar 6mEast NIA N/A NIA 9,34 a 9.60 a 10.38 a 7,47 a 8,38 a 8,60 a 2m East NIA NIA NIA 10.42 a 6.65 a 10,40 a 7.01 a 8,44 a 629 a 2mWest N/A NIA NIA 8.62a 8.86a 11.60a 6,27a 10.13a -0.24a 6m West NIA NIA NIA 8.44 a 10.80 a 8.73 a 858 a 8,ll a 7,35 a Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05). Table Al. 15. Rates of actual water use eficiency (AWUEJ for maize on July 16, 1998...... -ri...... r-.".. -m...... "...... m...... "*."...... -"-...."." Distance Water Use Enlciency fiom tree @mol CO2 rn'? s" /(mm01 H20m -2 s-1 ) row AM Noon PM Control Maq- Po~lar Controi Maple POD~Control Ma~le Po~lar 6mEast 7,39a 5,97a 6.91a 4.80a 5.25a 5.49a 5.84a 4.64a 5.99 a 2mEast 7.68a 8.36a 6.73a 6.37a 5.75a 6.05a 6.51 a 6.05a 6.59a 2m West 6.06 a 8.28 a 6.72 a 6.94 a 6.42 a 5.39 a 4.96 a 5.49 a 6.66 a 6m West 6.73 a 7.82 a 6.28 a 7,10 b 5.72 ab 5,44 a 7.02 a 6.17 a 4.71 a Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05).

8 A 1.16. on 1 8, Table Rates of actual water use efliciency (AWUE) for maize July 1998. ..J Distance Water Use Eniciency fkom tree (polCOz mw2S.' /(mm01 H20mW2 8' ) row AM Noon PM Control Ma~lg Po~lar Control Maple Poplar Control Maple Po~lar 6mEast 5.29a 5.03a 6.98a 7.79a SSOa 5.12a 3.91a 5,104 3.96a 2mEast 4,72 a 7,30 b 9.30~ 5.62 a 6.33 a 4.46 a 4,09 a 3,48 a 4.13 a 2m West 5.03 a 5.39 a 6.62 a 5,49 a 5,10 a 5.32 a 4.98 a 4,06 a 3.00 a 6mWest 6.76a 6.46a 6.32a 6.13 a 4,89a 5.41 a 4.08a 5.09a 4,26a Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05).

Table Al.18. Rates of intrinsic water use efficiency QWUQfor maize on July 30 1997...... "..."..".--.--.-.,,"..."..... ".....,S.-....,.*..-...... m,.... S.,, ... ..,W. "i."i...... W.. i...... -, >"." ,,-,,...... "...... "..,+...... Distance Water Use EtTiciency fiom tree (pmol CO2 m -2 s-1 /(mm01 Hz0 m -2 s-1 ) row AM Noon PM Control Ma~le Po~lar Control Ma~le Poplar Control Maple Poplar 6mEast 116a 113a 125a 121a 149a 130a 127a 132a 130 a 2rnEast 107a 118a 124a 128b 85ab 73a 115a 100a 94 a 2mWest 113a 179b 72a 103 a 142a 137a 120a 161 a 144 a 6m West 141 a 133 a 161 a 124a 136a 159a 119a 127a 133 a Within each time period, values in each row followed by the same letter are not signifiwtly different (Tukey's HSD, p < 0.05).

(O c\) Table Al. 19. Rates of intrinsic water use eficiency (IWUE) for maize on August 28, 1997. Distance Water Use Efkiency fiom tree (pmol COz m -2 s-1 /(mm01 H2Om -2 s-1 ) fow AM Noon PM Control Ma~le Po~lar Contrai Ma@ Podar Control Made Poplar 6m East N/A NIA NIA 98 a 120s 104 a 96a Ill a 144 a 2m East N/A NIA NIA 109a 75a 89 a 98 a 114 a 99 a 2m West NIA NIA NIA 91a 102a 59a 86a 1268 -3 a 6m West N/A NIA NIA 83a 130a 72a 118a 116a 112a Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05). 8 9! Y*S;S;33Q" Q " wmS3 =b EEEE eiee V,mN~ Table A1.22. Rates of intnnsic water use efficiency QWUE) for maize on Au@ 19, 1998...... - W...... S...... -...... Distance Water Use Efficiency fiom tree (pmol COz m-2 s-1 /(mm01 H20m -2 s-1 ) row AM Noon PM Controi Ma~le Popkir Control Maple Po~lar Control Maple Poplar 6mEast NIA NIA NIA 125 a 145 a 170a 155 a 138 a 187 a 2mEast N/A N/A NIA 188 b 141 a 109a 159s 227a 144 a 2mWest N/A N/A N/A 157 b 129ab 103 a 152 a 129a 143 a dm West N/A N/A NIA 154a 139a 156a 177a 224a 184 a Within each tirne period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05). EEEE wmm\O Table Al.25. Plant water deficit for maize on July 16, 1998, Distance Plant water deficit fiom tree îow AM Noon PM Control Made Poplar Control Maple Po~lar Controi Made Poplar 6m East -8.53 a -6.67a -5.60a -10.93a -9.87a -9.87a -16.07b -13.00ab -6.80a 2m East -8.27a -8.13 a -5.80a -1 1.80 a -10.07a -1 1.07 a -7,93a -4.07a -4.33 a 2m West -7.73a -7.93a -10.00a -1 1.13 a -12,53a -12.27a -8.33a -10,20a -10.47a dm West -9,27 a -7.53a -8.80a -12.33a -12.87a -8,40a -8,93a -4.60 a -1 1.53 a Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05). Table B 1.1. Rates ofphotosynthesis for soybean on July 29 1997. ....m..- .....a...... W...... m..-...... m...... m...... m...... m.."...... ""-.Ai...... ,m.,...."...".."rrm ...,"...... imiir...n.,,rr,*...... ~, Distance Rate of photosynthesis fiom tree (pmol m-2 sœ1) row AM Noon PM Control Maple Po~iar Controi Maple Poolar Control Maple Popiar 6mEast 19,l a 16.6 a 22.7 a 16.1 a 15.1 a 16.2 a 17.2 a 17.4 a 16,4a 2mEast 19,3a 20.9a 19.7a 20,Ob 8,Oa 13,5ab 17,4b 7.18 7.5 a 2mWest 23,Ob 6.6a 7,3a 16.4a 14Sa 11.0a 21,Sb 11,3a 13.8 a 6m West 19.5 a 18,5 a 8,4 a 14.2 a 21.8 a 21.7 a 18.9 a 18,2 a 19,6a Within each time period, values in each row followeû by the same letter are not significantly different (Tukey's HSD, p < 0.05).

CD Table B 1.2. Rates of photosynthesis for soybean on August 29, 1997, -J Distance Rate of photosynthesis from tree (pmol m -2 s-1 ) row AM Noon PM Control Ma~le Poplu Control MapIe Poplar Control Maole Po~lar 6mEast NIA NIA N/A 16.2a 13.1a 13.1a 9,7a 8,0 a 11.4s 2mEast NIA N/A N/A 16,Sa 9,6a 10.6 a 12.0 a 4.3a 6.2 a 2mWest N/A N/A N/A 19.7a 8,6a 14,6a 11.9a 6,Ua 8,7 a 6mWest NIA NIA N/A 16,6a 12,4a 11.6a 13,7a 12,l a 9,4 a Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05). EEEE \Orneau3 EEEE V)elC1\O Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05).

a

8 ~...a..C...... w...... 1....tt.1...... n...... n.....Table B1.9.Incident PPFD measured at the top...... a...... *.. of the soybeana-t....i...w.tt..*nr canopy. r...r.rr..wiintt on July i.l*.~.n 3 1998...... ii.r...**ettttet*t~***.....*..t.sa*...*..~.~.~v.~ Distance Incident PPFD fiom tree (pmol m-2 s*I ) row AM Noon

Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05). Table B 1.10. Incident PPFD measured at the top of the sqbean canopy on July 15, 1998...... W...... - ...... W...... - ~-"...... ".-...... ~...... -...~""-*.".."---."...... ~....~...... W..... S...... "...m...... m...... m...... Distance Incident PPFD fiom tree (pmoi m -2 s-1 ) row AM Nwn PM Control Ma Po~lar Control Ma~k Po~lar Control Maoie Poplar 6mEast 1716a 1598a 1604a 1822a 1824a 889a 897a 398a 607 a 2mEast 1826b 392a 1335b 1530b 226a 349a 878a 193s 660 a 2mWest 1711b 989ab 255a 1889a 1733b 1419a 741a 473a 1526 b 6mWest 1639a 1657a 1521a 1138a 1398a 1790a 443a 372a 1218 b Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05).

a O Table B 1.11. Incident PPFD measured at the top of the soybean canopy on Aupst 19 1998, CC...... "...... "*tt,* .,,.ri +...S... 4 ....,*...*.,*,.. .,.."...... " ...... *...,*...... ".**.." ...... *...?,...... ,,.,,... "... .,.,,.*...,.,...... ,,...,., Distance Incident PPFD fiom tree (pmol m-2 s-1 ) row AM Nwn PM Control Maple Poplar Control hda~le Po~lar Control Maple Po~lar 6m East NIA NIA NIA 1863c 1725b 1587a 1738b 910a 491 a 2m East N/A NIA N/A 1821 b 139a 149a 1589 b 113 a 524 a 2m West NIA NIA N/A 1848b 1674ab 1367a 1779b 1574a 1670a 6m West N/A NIA NIA 1678 a 1744 a 1678a 1662 a 1693 a 1581 a Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD,p < 0.05). Table B 1.12. Rates of actual water use efficieng (AWUE) for soybean on July 29 1997...... "...... ""."...... ".. "..."...... r r..... m...... "m...... ".. .".."."?...."...,.."* ,.,...... Distance Water Use Eniciency -2 -1 fkom tree (pmol CO2 rn'l S.' /(mm01 H20m s ) row AM Noon PM Control Ma~le PopIar Controi Ma~le Popkir Control Maple Podar 6mEast 3,56a 3,96a 4,92a 3.25a 2S4a 3,48a 2.85a 2,94a 2,95a 2mEast 3.64 a 5,12 a 4.37 a 3.51 b 1.96a 2.86 ab 3.20 a 2,37 a 2,30a 2m West 3.68 a 2.34 a 4,OOa 2.43 a 3,19a 3.03 a 3.78 a 3,06 a 3,28 a 6mWest 3,70a 4.57a 5.39a 2,17a 3,19ab 3S3b 2,84a 2,74a 3,OSa Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05).

A O Table B 1.13. Rates of actuat water use efficiency (Am)for soybean on August 28.1 997. h) Distance Water Use Eniciency tiom tree (pmol COI rn-' s-' /(mm01 H20mw2 S.') row AM Noon PM Contml Made Poplar Control Maple Pogar Control Maple Po~Iar 6m East NIA N/A N/A 4.82 a 4.13 a 5,39 a 4.62 a 3,29 a 3,71 a 2m East NIA NIA NIA 4,26a 3.10a 5.11a 4.1Sa 2.70a 2,17a 2m West NIA N/A NIA 4,75a 3,07a 6,21a 3,60a 406s 3,97a 6m West NIA N/A N/A 4,95 a 4,60 a 5.08 a 3.59 a 3,71 a 3,65 a Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05). Table B 1.14. Rates of actual water use efficiency (AWUE) for soybean on August 19, 1998, .W.. ~~.."".*.....Ir...."...".,.,~w"....."."...... ,.,...... ,.,,,.,*...... ".",.,,,...,., ...... m.... .W.. ri..."... .."...... ".."*.."...... "...... m. "...... ""...-...... *, Distance Water Use Eficiency from tree (pmol CO2 m-2 s-1 /(mm01 Hz0 rn -2 s-1 ) row AM Nwn PM Control Maple Po~lar Control Maple Po~lar Control Maple Poplar 6m East NIA N/A NIA 2,50a 3.33a 2,55a 1.84a 3.60a 3.16a 2mEast N/A N/A NIA 2.40 a 1.62 a 2,32 a 1.48 a 1.34 a 2,04 a 2m West N/A N/A N/A 2.74a 2.38a 2,61a 1.92a 2J7a 1,41a 6m West N/A N/A NIA 2.51a 3.46a 2.67a 2J0a 2,88b 2,10a Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05). - - Control Ma~ie Poplar Control Ma Po~lar controi Maple Poplar 6m East 65 a 71 a 80 a 69 a 45 a 74 a 64 a 66 a 58 a 2m East 68 a 81 a 67 a 77 a 43 a 52 a 78 a 54 a 56 a 2m West 52 ab 33 a 63 b 45 a 78 a 79 a 79 a 85 a 92 a 6m West 63 a 72 a 71 a 46 a 58 a 72 a 64 a 61 a 72 a Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD, p < 0.05).

Table B 1.16. Rates of intrinsic water use efficiency (IWUE) for soybean on August 28, 1997. Distance Water Use Efficiency fiom tree (wol COz m-2 s-1 /(mm01 H20mœ2 s-') row AM Noon PM Control Maple Poplar Controi Made Poplar Control Maple Po~lar dm East NIA NIA NIA 40 a 38 a 43 a 64 a 42 a 51 a 2mEast N/A NIA NIA 38 a 28 a 42 a 52 a 38 a 30 a 2m West NIA NIA NIA 31 a 35 a 50 a 41 a 61 a 70 a dm West NIA NIA NIA 38 a 51 a 43 a 36 a 44 a 55 a Within each time period, values in each row followed by the same letter are not significantly different (Tukey's HSD,p < 0.05). - aS9s Lameg-grne uP-;.*4' ziy 8 O O rn! c b 3plBaa qiS E w w 33 si.%0 * EEEE @:ainct;.g rDc.lc.lrD