Tree Shelters Influence Growth and Survival of Carob (Ceratonia Siliqua L.) and Cork Oak

Tree shelters influence growth and survival of carob (Ceratonia siliqua L.) and cork oak

(Quercus suber L.) plants on degraded

Mediterranean sites

P.M. Marques, L. Ferreira, 0.Correia & M.A. Martins-Lou@o Centro de Ecologia e Biologia Vegetal, Faculdade de Ci6ncias da

Universidade de Lisboa, Portugal

Abstract

Tree shelters were used in an ecological restoration effort in 1999 and 2000 to test the decrease in transplant shock and increase in growth and survival of two selected Mediterranean species, carob tree (Ceratonia siliqua L.) and cork oak (Quercus suber L.), planted in a dry degraded region. At plantation, two treatments were established, one planted with 60 cm tall TUBEX

MinitubesTMand the other planted without tree shelters. Results have shown that tree shelters dramatically increased survival of sheltered plants and also stimulated height growth, probably caused by reduced light regimes inside shelters, inducing shade adaptation. Different biomass partition occurred between sheltered and unsheltered plants, although no effect in increased biomass production was observed. This work shows that individual tree shelters successfully increase establishment of newly planted plants in dry, degraded areas of the Mediterranean.

1 Introduction

Land degradation and drought are serious global problems, which threaten extensive marginal lands all around the world. In the Mediterranean region, the arid, semiarid and dry sub-humid areas are particularly susceptible to erosion and soil degradation. There has been an increased awareness towards

rehabilitation of these degraded ecosystems, which is causing socio-economic changes based on improved land-use management practices. One result of these changes has been the encouragement of ecological reforestation of these unproductive and marginal lands. Unfortunately, the establishment of trees on degraded ecosystems can be very difficult [l]. The use of individual tree shelters as an improved planting technique can greatly increase early growth of seedlings, by acting as a small greenhouse providing a different microclimate and by reducing wind and browsing damage to the young plants [2, 3, 41. Survival can also be positively affected [l, 51. However, some authors have expressed some concern in relation to this practice. Economic and practical considerations [6, 71; prevention of the hardening off of seedlings prior to the onset of cold weather, leading to decreased resistance to freezes; decreased light quality and quantity [6, 81 and root growth inhibition [g] have all been reported. Despite these drawbacks, tree shelters are being used in increasingly higher numbers in various regions around the world, with varying degrees of success. Although many studies with tree shelters have been made in temperate regions, so far few experiments have been reported in dry regions, where high temperatures combined with water stress create serious limitations to a reforestation effort [g]. This study has been designed to evaluate the effect of individual tree shelters in the successful establishment of two autochthonous Mediterranean species, cork oak (Quercus suber L.) and carob tree (Ceratonia siliqua L.), planted in a degraded dry sub-humid Mediterranean field site in the Algarve, Portugal.

2 Material and methods

2.1 Site description

Two field trials were established on public property (Terras da Ordem), in Odeleite, Algarve, SE Portugal, in mid February on two consecutive years, 1999 (referred to as Casa Branca) and 2000 (referred to as Portela Alta). Casa Branca (37'19' N, 7'31' W) and Portela Alta (37'19' N, 7'29' W) field sites were geographically near each other (3 km), located 130 m and l10 m above sea level, consisted of a sloping hillside, 11-14% and 4-1 l%, with a NE and a NW to NE aspect, respectively. Both sites were dominated by a degraded maquis consisting mainly of evergreen sclerophyllous and drought semi- deciduous shrubs such as Cistus ladanifer (average 100-150 cm height) * patched with C. monspeliensis, Genista hirsuta and Lavandula stoechas, vegetation typical of degraded Mediterranean dry areas. The shallow (average 20-30 cm deep), stony soil, developed over schist, is a sandy loam with poor organic matter content (average 1.65% in Casa Branca and 0.65% in Portela Alta) and average pH of 5.7 It is considered a Leptosol. Mean annual temperature and rainfall were 18' C and 513 mm, respectively. Annual

rainfall distribution (1998-2000) has been characterised (table 1). These sites have a Bagnouls-Gaussen aridity index (BGI) of 184, indicating very dry conditions and making them moderately eroded, fragile areas concerning desertification [10].

Table 1: Annual rainfall distribution (mm) for 199811999 and 1999/2000 and mean annual rainfall (1964- 1980) for Faro weather station (37'0 1' N, 07'58'.. .- W). l998Il999 l999I2OOO Mean Annual Rainfall

October - March 98 264 42 1.O April - June 33 168 73.1 July - September 33 18 19.4

2.2 Experiment description

Two treatments were applied: treatment TS+, with tree shelters (brown

TUBEX Minitubem, 60 cm tall and 7-1 1 cm in diameter) and treatment TS-, without tree shelters (control). Site preparation consisted of 40 X 40 X 40 cm individual planting holes. Container-grown plants, aged 7.5 months old (Casa Branca) and 10 months old (Portela Alta), were outplanted at a spacing of 2 m

X 2 m each. The surrounding vegetation was left as intact as possible. Each plot was made up of three replicate blocks with plants distributed in a completely randomised block design. In Casa Branca, each block had 34 control and 50 sheltered plants for carob tree and 7 control and 12 sheltered plants for cork oak. In Portela Alta, each block had 40 control and 40 sheltered plants for carob tree and 40 control and 80 sheltered plants for cork oak. For each field trial 309 plants were used in 1999 and 600 plants in 2000.

2.3 Sampling and analysis

Survival assessment was made 2 months after plantation (April 1999 and 2000), before summer (June 1999 and 2000) and after summer (October 1999 and September 2000). After the first summer, plant survival continued to be monitored in Casa Branca until the end of the second summer. Main stem height, basal diameter, number of branches, leaf area and plant dry mass (roots, leaves and stem, oven dried at 70' C) were sampled before and after summer, but only after summer results are shown. Leaf area ratio (LAR, determined as total leaf area per total plant dry weight, specific leaf weight (SLW, mg.cm2) determined as leaf area per leaf dry weight and root:shoot ratio (g.g-') were firther determined for biomass partition analysis. Physiological parameters, such as midday maximal photochemical efficiency (using a PAM-2000 portable chlorophyll fluorescence analyser, Waltz, Germany) to measure optimal quantum yield of PSI1 (Fv:Fm), as an indicator of environmental stresses on photosystem 11,

and predawn leaf water potential (using a Scholander pressure chamber) were also measured. All data were analysed by analysis of variance followed by Tukey's test for significant differences (p<0.05) using STATISTICA software (StatSoft, Inc.). Survival percentages were first transformed into arcsine to better approach a Gauss distribution.

3 Results

Tree shelters successfully influenced plant survival rates in Casa Branca, with higher mortality observed in unsheltered plants (figure 1). After the first summer in the field, significant differences were found between treatments in

C. siliqua plants (62.1% in sheltered vs. 12.4% in unsheltered plants). For Q. suber, although non-significant, sheltered plants had also higher survival rates (39.7% vs. 28.9%). In the second year, the same trend was maintained in Casa Branca. At the end of the second summer, survival was 34.0% in sheltered vs.

2.0% in unsheltered plants for C. siliqua and 22.7% vs. 9.5% for Q. suber.

-&TS+ Ceratonia siliqua -0-- TS------C-- TS+ Quercus suber . . .D.. TS-

15/02/99 20/04/99 14/06/99 24/ lO/99 17/04/00 01/06/00 25/09/00 blantation ) Sampling Date

Figure 1: Field survival of C. siliqua and Q. suber plants in Casa Branca (1999) TS+, plants with tree shelter; TS-, control plants (without tree shelter). Results are means (n=3) + standard errors.

Portela Alta exhibited the same pattern in plant survival (figure 2), with significant differences for both species after the end of the dry season (83.5% in sheltered vs. 33.9% in unsheltered plants for C. siliqua and 82.6% vs.

Ecos,ssrems and S~f~rairlnbleDe~>elopmenr 639

53.0% for Q. suber). In 2000 both species had an overall higher survival rate than in 1999.

15/02/00 17/04/00 0 1/06/00 25/09/00 (plantation ) Sampling Date

Figure 2: Field survival of C. siliqua and Q. suber plants in Portela Alta (2000). TS+, plants with tree shelter; TS-, control plants (without tree shelter). Results are means (n=3) standard + errors.

3.2 Growth development

3.2.1 Height and diameter The effect of tree shelters on the height development of both species is summarised in figure 3. At the time of plantation, no differences were observed between plants of each species. However, after the end of the first summer, and for both field sites, height increment was notorious in sheltered plants. Carob sheltered plants experienced an increase in height, although non-significant, of 29.9% and 39.3% in relation to unsheltered plants, in 1999 and 2000, respectively. The response of Q. suber was even more pronounced being significantly increased in both years. Sheltered plants grew 70.4% and 62.7% more than unsheltered ones, in 1999 and 2000 respectively.

Unsheltered plants of both species had an almost nil growth rate in the field, for both field sites. No differences were found in stem diameter between treatments for both species and for both field sites (table 2). However, in 1999, cork oak control plants tended to display thicker stems than sheltered plants.

In Q. suber, the branching rate appeared to be higher in both years in the case of unsheltered plants (data not shown).

9 - Ceratonia siliqua a

h E 6- W S M a .m

3- . . -0. . TS-TS+ Casa Branca Portela Alta

M) - Quercus suber

q ....-----E M - - _.Q..---...ar - 20 g CasaBranca Portela Alta

10/ 12/98 14/06/99 241 lO/99 20/0 1/00 02/06/00 26/09/00 Sampling Date

Figure 3: Height growth of C. siliqua and Q. suber plants in Casa Branca (1999) and Portela Alta (2000). TS+, plants with tree shelter; TS-, control plants (without tree shelter). Results are means

(n=30) + standard errors. Values with different letters indicate differences between treatments within the same date (pS0.05).

Table 2: Stem diameter of C. siliqua and Q. suber plants in Casa Branca (1999) and Portela ~lta(2000).TS+, plants with tree shelter; TS-, control plants (without tree shelter). Results are means (n=30) + standard deviations. For each site and species, values with different letters indicate differences between treatments (~10.05).

Casa Branca Portela Alta TS+ TS- TS+ TS-

C. siliqua 2.26k0.38a 2.47M.87a 3.48f0.76a 3.51M.72a Q. suber 4.45f0.23a 5.2850.20a 3.75k0.24a 3.58+0.37a

3.2.2 Biomass Total biomass production was not affected by the use of tree shelters. No differences were found for both species and field sites (table 3). However, sheltered plants exhibited lower root:shoot ratios than non-sheltered plants (figure 4). This trend, presented in both species, represented different allocation strategies dependent on species. While in cork oak this lower root:shoot ratio was due to an higher biomass allocation towards leaves, in

carob was mainly due to a reduction in root biomass (data not shown).

Table 3: Total biomass, SLW and LAR of C, siliqua and Q. suber plants in Casa Branca (1999) and Portela Alta (2000). TS+, plants with tree

shelter; TS-, control plants (without tree shelter). Results are means (n=5) rt standard deviations. For each site and species, values with different letters indicate differences between treatments ($50.05).

Casa Branca Portela Alta C, siliqua TS+ TS- TS+ TS- Total Biomass (g) 1.2OkO.85a 1.97fO.93a 2.09rt0.30a 3.00rt0.99a

SLW (mg.cm-*) 11 .93+0.6Ob 16.1 1+0.94a 16.9f4.37b 2 1.4rt2.85a LAR (~rn~.~.')23.07f7.71a 19.25f8.92a 17.39f5.68a 8.43f4.74b Q. suber

Total Biomass (g) 7.99f3.91a 7.42f l.7la 9.24f3.24a 8.46f3.63a SLW (mg.cm-') 10.36f2.02a 9.40+1.17a 13.58f3.72b 17.01f5.78a LAR (cm2.n-l) 18.46f7.88a 12.91f4.82a 11.81k1.78a 4.49f4.13b

5 ClTS+ 0 TS-

0 Casa Branca Portela Alta Casa Branca Portela Alta Ceratonia siliqua Quercus suber

Figure 4: Root:shoot ratio of C. siliqua and Q. suber plants in Casa Branca (1999) and Portela Alta (2000). TS+, plants with tree shelter; TS-, control plants (without tree shelter). Results are means (n=5) -+ standard errors. For each site and species, values with different letters indicate differences between treatments (p10.05).

3.2.3 Specific leaf weight and leaf area ratio

Carob sheltered plants presented lower values of specific leaf weight (SLW) in both years. In Q. suber such differences were observed only in 2000 (table 3). Leaf area ratio (LAR) was higher in sheltered plants for both species and field sites (table 3).

3.2.4 Physiological parameters Midday maximal photochemical efficiency was higher in sheltered plants of C. siliqua in 1999 and Q. suber in 2000. In predawn leaf water potential no differences were observed (table 4).

Table 4: Midday maximal photochemical efficiency (Fv:FmMD)and predawn leaf water potential (YPD)of C. siliqua and Q. suber plants in Casa Branca (1999) and Portela Alta (2000). TS+, plants with tree shelter;

TS-, control plants (without tree shelter). Results are means (n=5) k standard deviations. For each site and species, values with different letters indicate differences between treatments (p10.05).

Casa Branca Portela Alta TS+ TS- TS+ TS- C. siliqua Fv:F~MD 0.76k0.02a 0.67+0.10b 0.61f0.15a 0.54+0.11a

YpD(MPa) -4.02k0.91a -4.14k1. 17a Q. suber F~:F~MD0.80f0.03a 0.8&0.03a 0.74+0.05a 0.55f0. 19b

YpD(MPa) -2.98k1.05a -4.02e.04a

4 Discussion

Tree shelters dramatically improve plants establishment on dry sites by improving survival rates. They stimulated height growth (figure 3). This could

be due to the favourable microclimatic conditions found inside tree shelters, like increased temperature, relative humidity, C02 concentration and wind protection, already reported by several authors [2, 4, 8, 91, or it could be a consequence of etiolation caused by light reduction [g]. This last parameter also contributes to the reduction of stem diameter in sheltered plants [l l].

Wind protection has this same effect [12]. This can explain the tendency observed for Q. suber in 1999 (table 2). Branching pattern in this species gives support to the idea of shade adaptation, due to the maintenance of apical dominance in sheltered plants [8]. Wind damage, browsing and desiccation of vegetative buds followed by resprouting could also have caused higher

branching in unsheltered plants [l]. The lack of differences in total biomass between sheltered and unsheltered plants accompanied by height increase seem to indicate that sheltered plants are thinner, which is in accordance with lower sclerophylly values (table 3).

The height increase and lower root:shoot ratios of sheltered plants (figure 4) also indicate that height increase was achieved trough different biomass partitioning than by a general biomass increase. Burger et al. [9] attributed the differences in root:shoot ratio to a stimulation of stem growth at the expense of inhibition of root growth. This was mainly the case for C, siliqua, but not for Q. suber. Cork oak sheltered plants invested more in stems (figure 3) and in leaves without inhibition of root growth. In spite of these differences in allocation between species, a larger investment in leaf area of sheltered plants was noticed. Higher leaf area ratios and lower specific leaf weights of sheltered plants (table 3) are probably a result of reduced light regimes inside shelters, causing shade adaptation. This is in accordance with Dias [g] and Dias et al. [13]. Higher results of midday maximal photochemical efficiency found in sheltered plants (table 4) support the above statement about light reduction inside tree shelters, reducing light saturation stress.

The conditions inside tree shelters reduce transpiration rates and consequently affect water use efficiency [14], and could lead to reduced water stress [15]. In spite of these evidences, tree shelters did not seem to affect soil water availability, as seen by the lack of differences in predawn leaf water potential between sheltered and unsheltered plants (table 4). Significantly higher post-summer survival in sheltered plants was observed (figures 3 & 4), similarly to what has been described by Bainbridge in arid regions [l]. This was not due to reduced water stress, but most probably to reduced light saturation stress and to other favourable conditions inside shelters, as evidenced by our results. The differences in survival between 1999 and 2000 could have been caused by distinct rainfall distribution (table l), which explains the mortality before summer but not the higher mortality during summer. This mortality may probably be attributable to different seedling quality. Mediterranean climatic conditions are more extreme than the ones found in more temperate regions, where so far most of the work with shelters has been done. Higher temperatures, as well as water and light saturation stress all contribute to dramatically increase mortality of young plants, reducing the success of reforestation efforts.

From the economic point of view, the cost of shelter use must be taken into account. In these experiments, the use of TUBEX MinitubesTM increased plantation cost in 50%, which, although expensive, can be worth the cost in degraded regions due to much higher survival rates obtained. Landscape impact considerations must also be taken into account. Small brown shelters like the ones used in this work have lower visual impact than others, more conspicuous, kind of shelters. In an ecological restoration, shelters can greatly increase the success of these projects by enhancing survival of key species, which will help to restore degraded ecosystems.

References

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