Journal of the Torrey Botanical Society 134(2), 2007, pp. 281-288
Variability in needle morphology and water status of Pinus cemhroides across an elevational gradient in the Davis Mountains of west Texas, USA1
2 H. M. Poulos and G. P. Berlyn ,3 School of Forestry and Environmental Studies, Yale University, Greeley Memorial Laboratory, 370 Prospect St., New Haven, CT 06511, USA
POULOS, H. M. AND G. P. BERLYN (Yale School of Forestry and Environmental Studies, Yale University, Greeley Memorial Laboratory, 370 Prospect St., New Haven, CT 06511, USA). Variability in needle morphology and water status of Pinus cembroides across an elevational gradient in the Davis Mountains of west Texas, USA. J. Torrey Bot. Soc. 134: 281-288. 2007.-The pinyon pines (Pinaceae, Pinus subsections Cembroides and Rzedowskianae) are a widely distributed group of site generalist species that dominate many of the middle to upper elevation semi-arid regions of North America. We investigated the physiological and morphological response of Pinus cembroides var. bicolor Little across an elevational gradient in the Davis Mountains of west Texas to test the hypothesis that variability in needle morphology, relative water content, and transpiration allow this species to exist across a range of elevations and local site conditions. Results from our study showed significant increases in P. cembroides needle length, mass, and area and plant water status (relative water content and transpiration) with elevation. Our findings suggested that this species is able to adapt to changes in local environment over short distances, which is an important factor responsible for the wide distribution of pinyon pines in North America. Key words: elevation gradient, gradient analysis, needle morphology, Pinus cembroides, pinyon pine, transpiration, pinyon-juniper woodlands, water relations.
Pinyon pines (Pinaceae, Pinus subsections ex Schltdl.), which are generally more re Cembroides and Rzedowskianae) are some of stricted in their geographic and elevational the most widely distributed trees in the semi distributions (Hanawalt and Whittaker 1976, arid regions of the northern hemisphere Neiring and Lowe 1984, Bailey and Hawsk (Fig. 1). They dominate over 325,000 square worth 1987, Allen et al. 1991, Poulos et al. in kilometers in North America. Their latitudinal press). range spans from southern Idaho in the Water availability is the most limiting factor United States (42°N) to southern Puebla, to tree growth in the Southwest (Meko et al. Mexico (l8°N), and their longitudinal distri 1995, Hidalgo et al. 2001, Adams and Kolb bution covers a similarly large portion of 2005), and the drought tolerance of pinyon North America from Puebla (97°W) to Cali pines in this region is considered the primary fornia (l200 W) (Fig. 1) (Aldon and Springfield factor regulating their distributions across 1973, Barger and Ffolliot 1972, Little 1999). climatic and soil moisture gradients (Barton The biogeographic range of the pinyon pines is 1993, Barton and Teeri 1993, Lajtha and Getz large compared to many other pine species in 1993, Linton et al. 1998). Moisture and southwestern North America (i.e., Pinus stro temperature can vary more than twofold over biformis Englem., P. arizonica Englem., P. elevational gradients inhabited by Southwest flexilis James, P. leiophylla Schltdl. & Cham., ern pinyon pines (West 1988), with a generally P. engelmannii Carriere, P. ayacahuite Ehrenb. increasing trend in moisture and decreasing trend in temperature with elevation (Barry 1992). These changes in local environmental I Funding for this research was provided by the Joint Fire Sciences Program of the Department of conditions with elevation are presumed to the Interior (# 03-3-3-13). increase water availability to plants, allowing 2 We thank the Davis Mountains Preserve of The for increased carbons sequestration and Nature Conservancy of west Texas for the logistical support necessary to carry out this project. Dr. Ann growth (Barnes 1986, Padien and Lajtha Camp, Dr. Mark Ashton and two anonymous 1992, Lajtha and Getz 1993). reviewers provided helpful comments on earlier The wide distribution of pinyon pines across drafts of this manuscript. the elevational gradients of southwestern 3 Address for correspondence: E-mail: helen.mills@ yale.edu North America has been quantified in a range Received for publication September 12, 2006, and of studies (St. Andre et al. 1965, Whittaker in revised form February 12, 2007. and Niering 1965, 1968, 1975, Niering and 281 282 JOURNAL OF THE TORREY BOTANICAL SOCIETY [VOL 134
range in Texas, spanning from 1525 to 2555 m in elevation. The forests of DMTNC are dominated by pinyon-juniper-oak and mixed conifer woodlands. Chihuahuan desert grass lands bound the site at lower elevations, and relict montane conifer forests form the upper elevational boundary (Hinckley 1944). The Davis Mountains are 35-39 million years old, and originated in the same Eocene to Oligo cene orogeny that formed most of the Front Range of the Rocky Mountains (Turner 1977). Geologic substrates in DMTNC are derived ~ P. pinareena _ P. monophy!la from the erosional remnant of the once widespread Davis Mountains volcanic field, DP.nelSDnjj _ P. cembroides and consequently, the underlying rocks are
Dp,edulis predominantly extrusives, consisting of lavas EZ22 P. quadrifolia and pyroclastics (Savage and Morin 2002). Soils are generally shallow to moderately deep, FIG. 1. Distributions of the dominant pinyon and are volcanic in origin. pine species in North America. Derived from Pinus cembroides is distributed across the Little (1999). entire elevational gradient of DMTNC. It is the dominant pine species at low elevations, where it exists in association with Juniperus Lowe 1985, Barton 1993, Martens et al. 2001, deppeana-Steud., and several oak species Poulos et al. in press). However, virtually no (Quercus grisea Liebm. and Q. emoryi Torr.). information exists that identifies the anatom At middle and upper elevations, it mixes with ical, morphological, and physiological vari P. ponderosa Douglas ex C. Lawson var. ability in pinyon pines that underscores their arizonica (Engelmann), P. strobiformis En wide distribution across North America (but gelm., and other oak associates (Q. grisea, Q. see Jaindl et al. 1995). hypoleucoides A. Camus, and Q. rugosa Nee), This study examined the ecophysiological and it is the highest elevation tree species in the mechanisms responsible for the extensive Davis Mountains. We sampled a low range of Mexican pinyon pine (Pinus cem (1820 m), an intermediate (2113 m), and a high broides var. bicolor Little) by measuring a suite (2513 m) elevation stand of P. cembroides trees of morphological and physiological traits on to test the hypothesis that leaf morphology adult trees that spanned the elevational and water relations in P. cembroides vary gradient of The Nature Conservancy Preserve according to elevation, and that such variation of the Davis Mountains in west Texas allows this species to colonize the range of (DMTNC). To our knowledge, no previous available sites in DMTNC. studies have been conducted on the physio The distribution of P. cembroides is limited logical ecology of this species, though it is the to west Texas and southern New Mexico in the most common and economically important United States, though it spans a wide geo pinyon pine in Mexico (Suzim-Aspiri et al. graphical range in Mexico (Fig. I) (Little 2002). We hypothesized that Mexican pinyon 1999). The Davis Mountains form part of pine would respond to changes in environ the northern limit of the distribution of P. mental conditions along the elevational gradi cembroides, being replaced by Pinus edulis ent through variation in needle morphology, Engelm. in the Guadalupe Mountains, which plant water status, and transpiration. is the next mountain range 200 km to the north. Materials and Methods. SITE DESCRIPTION. The climate is arid, characterized by cool The Nature Conservancy Preserve of the winters and walID summers. Mean annual Davis Mountains is located in Jeff Davis precipitation is 400 mm in Ft. Davis, Texas, County in the Trans-Pecos region of western and is distributed bi-modally in late-summer Texas (Fig. I). The Davis MOW1tains (300 50' and winter with the majority of precipitation N; 103 0 50' W) comprise the largest mountain falling as rain in the summer as part of the 2007] POULOS AND BERLYN: NEEDLE MORPHOLOGY IN PINUS CEMBROIDES 283
North American Monsoon System (Bomar elevation (n = 30). Samples were recut under 1995). Mean monthly minimum temperatures degassed distilled water immediately after range from 0.0 DC in January to 17.7 DC in returning from the field. They were then July, while mean monthly maximum tempera placed upright in test tubes and left in diffuse tures range from 15.5 °C in January to 32.6 °C light for 48 hours prior to the experiment. At in June. the onset of the experiment, twigs were removed from the test tubes, blotted dry, FIELD SAMPLING. Sun needles of Pinus weighed, and placed horizontally on screen cembroides were sampled on the top branches racks. Twigs were reweighed on an electronic of 15 trees at 1820m, 2113m, and 2513m balance periodically for the next 15 h. The elevation (n = 45) at DMTNC on May 24, temperature ranged from 16.7-20.5 DC during 2004. The 1820 m site represented the lower the course of the experiment. The relative end of the range of P. cembroides in these humidity was not controlled, and ranged from mountains, and the 2513 m site represented 16-30 %. At the end of the experiment, twigs the highest elevation stand of trees in the were oven dried at 70°C until they reached Davis Mountain Range. Branches were chosen a constant mass, and weighed a final time to from vigorous canopy trees that were approx obtain their dry mass. imately 15 cm diameter at breast height and Transpiration was measured using transpi 8 m tall in an effort to sample trees that were ration decline curves as the rate of water loss similar in size and age. Tree height was over time (mg min-I g dry mass-I) (Jarvis and measured using a Laser Tech Impulse 200 Jarvis 1963). It was assumed that the stomata (Laser Technology Inc., Centennial, CO). were fully open at time 0, and that stomatal Branches were cut with a pole cutter from and cuticular transpiration could be differen the top of the canopy with a southern tiated by a slowing in the transpiration rate exposure following DeLucia and Berlyn upon closure of the stomata based on previous (1984). work by DeLucia and Berlyn (1984), who found this to be true for Abies balsamea (L.) RELATIVE WATER CONTENT AND TRANSPIRA P. Mill. TION DECLINE CURVES. Leaf relative water content (RWC) was determined for 5 needles MORPHOLOGICAL MEASUREMENTS. Morpho on 15 trees at each elevation (n = 75). R WC logical measurements were made on 75 needles for needles on each tree were averaged, pro 2 from each elevation. Needle area (cm ) and viding a sample size of n = 45. While, two and needle length (mm) were measured using three needle fascicles were present on Pinus a CID-202 Leaf Area Meter (CID Inc., cembroides, only three needle fascicles were Camas, WA). Leaves were then oven dried at chosen for use in this experiment to preserve 70 DC to a constant mass to determine their uniformity in the sampling design. Needles 2 dry mass. LMA (mg cm- ) was calculated as were detached and immediately weighed on the ratio of leaf dry weight to leaf area. a portable digital balance (Acculab model PP 2060, Newtown, PA) to obtain their fresh STATISTICAL ANALYSES. Differences in RWC, weight (FW). Needles were then saturated in leaf area, needle length, needle mass and LMA water to full turgor by immersing them in among the three sites were analyzed using water and leaving them in the dark for 24 h. a general linear model procedure (GLM). Needles were reweighed to obtain their turgid Elevational differences in stomatal and cutic weight (TW) and put into an oven at 70°C to ular transpiration were analyzed by a mixed dry the needles while preventing case harden model repeated measures procedure with ing. After 3 days, the needles were weighed. elevation as a fixed effect. All statistical Then, needles were placed in a 100°C oven for analyses were performed using the R Statisti 2 days and reweighed. Needles were placed cal Language (R Development Core Team back in the 100 DC oven until they reached 2006). a constant mass to obtain their dry weight (DW). R WC was calculated as: R WC = (FW Results. All measures of needle morphology - DW)/(TW - DW) X 100. showed significant linear, increasing trends The rate of water loss was measured with elevation (P < 0.05) except LMA gravimetrically on 2 twigs of 15 trees at each (Table 1, Fig. 2). Low elevation (1820 m) 284 JOURNAL OF THE TORREY BOTANICAL SOCIETY [VOL. 134
Table I. Needle morphology and relative water content (RWC) of Pinus cembraides along the e1evational gradient in DMTNC. Mean values for the traits are reported for each elevation. Standard errors are reported in parentheses.
Site RW (%) Needle area (cm') eedle length (mm) eedle mass (mg) LMA (mg cm") 1820 m 4446 (2.07) 1.25 (0.02) 3069 (0.05) 1069(7.13) 829(19.21) 2113 m 66.05 (1.83) 2.07 (006) 3385 (0.03) 1321 (881) 74.6 (14.03) 2513 m 70.58 (125) 2.29 (0.05) 3688 (0.03) 2233 (1489) 1226 (3892) P 0.041 0.0001 0.0001 0.0400 n.s.
2 pinyons had the smallest (1.25 cm ) and elevation (P = 0.05) (Fig. 3) Middle elevation shortest (20.69 mm) needles with the least branches had the lowest transpiration through mass (J 32.1 mg). Pinyon needle morphology out the experiment. Transpiration was high was intermediate at middle elevations (2113 m) est for low elevation branches during the compared to the other two sites, with mean stomatal phase of transpiration, followed by values of 2.07 cm2 for needle area, 33.85 mm high-, and then mid-elevation branches. High for needle length, and 132. I mg for mass. elevation branch transpiration surpassed mid High elevation (2513 m) pinyon needles were dle and low elevation water loss during the 2 the largest (2.29 cm ), longest (22.85 mm), and cuticular phase of transpiration after stomatal heaviest (223.3 mg) of all needles, suggesting closure. greater carbon assimilation in needles of high elevation pinyons relative to other elevations. Discussion. The variability in Pinus cem Needle RWC also showed a significant bra/des morphology and water relations was increasing trend with elevation (P = 0.04), manifested by a positive relationship between indicating a rise in plant water status along the elevation and RWC, needle length, needle elevational gradient of DMTNC. Mean values mass, and needle area. The changes in P. were 44.46% at 1820 m, 66.05% at 2113 ill, cembra/des needle morphology and plant and 70-58% at 2513 m elevation. water status with elevation potentially reflect Stomatal and cuticular transpiration did not the adaptation ofeach pinyon pine population exhibit linear relationships with elevation. The to local environmental conditions, explaining stomatal and cuticular phases of transpiration why this species is able to exist across a wide were differentiated by the slowing in water loss range of site conditions. that took place 60 minutes after excision The low stomatal and cuticular transpira (Fig. 3). Stomatal transpiration did not differ tion of mid-elevation pinyon pine branches by elevation (P = 0.12). Cuticular transpira relative to other elevations suggests that these tion differed between the sites (P = 0.05), and individuals may have been the most adapted the rate of branch water loss increased with to varia ble environmental site conditions.
2513 m
FIG. 2. Morphological characteristics of Pinus cembroides needles at 1820 m, 2113 In, and 2513 In eleva tion. 2007] POULOS AND BERLYN: NEEDLE MORPHOLOGY IN PINUS CEMBRO/DES 285
1.B resulting in ameliorated growing conditions at 1.6 high elevations in southwestern North Amer .,- ~2513m ica. Low elevations in the Davis Mountains (Jl 1.4 ~2113m (Jl --0--- 1B20 m are hot and dry most of the year. In contrast, '"E 1.2 (Jl ~ (Jl high elevations of the Davis Mountains have .2 "tJ '" a more varied climate, with cooler and wetter '" . summer weather, and intermittent below ~ ".!: O.B E freezing conditions in winter. 0.6 '" Temperature and water availability have -S 0.4 major effects on plant growth and carbon
0.2 assimilation (Taiz and Zeiger 2006). Leaves that develop under conditions of low water 0 0 200 400 600 BOO 1000 supply are usually correspondingly smaUer and Time (min) have a smaller surface area (Billings and Mooney 1968, LarcheI' 2003, Fitter and Hay FIG. 3. The rate of water loss per gram dry 2002). The ameliorated climatic conditions weight of needles excised from mature twigs of Pinus at middle and upper elevations during the cembroides. Each point represents a mean rate for growing season in DMTNC were probably that time interval and was plotted at the center of that interval. Means were calculated from 15 responsible for the greater R WC, needle length, individuals from 1820 m, 2113 ill, and 2513 m needle mass and needle area of Mexican pinyon elevation. pine at high elevations. Likewise, the hot, dry environments of low elevations in DMTNC Their location in between the hot, dry condi probably similarly limited carbon assimilation. tions of low elevations and the cooler, wetter The relationship between needle morpholo environments of upper elevations in DMTNC gy and elevation that we observed in Pinus may mean that these individuals were exposed cembroides was consistent with other work on to a wider range of environmental conditions conifers in semi-arid regions, although the than Mexican pinyon pines at other elevations, opposite trend has been observed in montane and their low transpiration rates during the conifers at higher latitudes. Callaway et al. experiment may have reflected this adaptation. (1994) found a similar trend of increasing Middle elevations in DMTNC also proba biomass allocation in high versus low eleva bly provided more optimum conditions for tion P. ponderosa var. scopulorul17 Engelm. in photosynthesis. The cuticles and total cuticu another semi-arid locale in Nevada. Leaf lar layer are laid down after the upper cell wall mass, sapwood mass, and total tree height is developed (see Berlyn et al. 1993). This is increased with elevation and precipitation, and a secondary allocation and requiring addition decreased with temperature, suggesting a gen al carbon. Optimal allocation to the total eral trend of increased plant growth with cuticular layer requires adequate carbon re elevation in semi-arid regions of western serves that stressed foliage at low elevations North America. Yet, the opposite trend has may not provide, which potentially explains been documented in other alpine regions of the high cuticular transpiration by low-eleva North America, where needle area, length, tion relative to mid-elevation needles. mass, LMA, and epicuticular wax content showed decreasing trends with elevation (De RELATIVE WATER CONTENT AND NEEDLE Lucia and Berlyn 1984, Berlyn et a1. 1993, MORPHOLOGY. For pines, differences in needle Richardson et a1. 200 I) in response to harsher morphology have been suggested as adapta winter climatic conditions than those in the tions to water stress (Haller 1965, Neilson Davis Mountains, suggesting that these pat 1987, Tausch and West 1987, Jaindl et al. terns vary with latitude and the length of the 1995). The trend of increasing needle length, elevational gradient area, and mass with elevation changed in accord with local environmental variability TRANSPtRATIO . The low transpiration of across the elevational gradient. The environ middle elevation branches relative to low mental lapse rate causes lower temperatures elevation branches is surprising in light of and higher relative humidity at higher eleva the smaller needle size of low elevation versus tions in montane ecosystems (Barry 1992), middle elevation Mexican pinyon pines. How- 286 JOURNAL OF THE TORREY BOTANICAL SOCIETY [VOL. 134 ever, the trend of greater water loss of Pinus lauke, NH, though it had needles that were cembroides at higher elevations during cuticu almost twice as long (DeLucia and Berlyn 1984). lar transpiration was potentially the product Similarly, P. albicaulus Engelm. had higher of needles having thinner needle cuticles at transpiration than P. cernbroides (Adams and high elevations. While we did not measure Kolb 2005), suggesting that this species is also needle cuticle thickness, it can vary dramati more drought hardy than other species that are cally with elevation and is known to have also adapted to harsh, dry conditions near pronounced effects on transpiration (Gale treeline. 1972, Smith and Geller 1979, Berlyn and DeLucia 1984, Berlyn et aL ] 993, Apple et PINYON PINE AS A SITE GENERALIST. Results aL 2000). Berlyn and DeLucia (1984) and from this study confirm the hypothesis that Berlyn et al. (1993) obtained results similar to leaf morphology and water relations in Pinus ours on the relationship between transpiration cembroides vary according to elevation, and and needle epicuticular wax content at treeline that such variation allows this species to (- 1450 m) in Abies balsamea on Mt. Moo colonize the range of available sites in silauke, NH and in Picea rubens Sarg. on DMTNC. This suggests that the wide distri Whiteface Mountain, NY, respectively. Like bution of pinyon pines across North America wise, Tranquillini (1979) reported that Picea is related to its high drought tolerance relative abies (L) H. Karst. twigs growing at 1940 m to other pine species, and its ability to adapt to had three times the transpiration rate ofshoots changes in local growing conditions over short growing at 1000 m. This suggests that high distances. The morphological variability in elevation trees may be more stressed than trees needle characteristics and differences in tran at intermediate elevations, making them un spiration of P. cembroicles across the eleva able to allocate carbon to wax production. tional gradient of DMTNC suggest that this However, our results appear to be unique for species has the potential to survive under pinyon pine. Previous work on P. monophylla a wide range of climatic conditions. Further Torr. & Frem. in the Great Basin showed no more, studies of historical (Lanner and Van significant seasonal or elevational variation in Devender 1998, Gray et at. 2006) and contem transpiration, suggesting that this pattern may porary (Blackburn and Teuller 1970, Lanner not be a general trend for pinyon pines as 198 I, Bahre 1991) range expansions of pinyon a whole (Jaindl et aL J995). pine support the idea of its ability to thrive Middle elevations likely represented the under a variety of climate scenarios. most favorable growing conditions 1D This research represents the only study that DMTNC. This environment probably facili has investigated the morphological response of tated greater carbon fixation (i.e., higher pinyon pine to changes in local environmental photosynthesis) leading to thicker cuticles, conditions across an elevational gradient in greater total cuticular thickness, and reduced North America, and it represents only the transpiration by middle elevation needles. Low second study to examine the influence of elevations represented the hottest and driest elevation on pinyon pine physiology (i.e., growing conditions in DMTNC. The lighter Jaindl et al. 1995). Further studies that color of low elevation pinyon needles (Poulos, investigate the morphological, anatomical, pers. obs.) and the harsher growing conditions and physiological mechanisms that underscore probably resulted in lower needle chlorophyll pinyon pine distribution patterns across larger content and photosynthesis, yielding less avail biogeographic areas, in addition to studies of able carbon for growth. This in turn, could niche differentiation by the range of pinyon have led to thinner cuticles at low versus pine species, are warranted for furthering our intermediate elevations, and higher transpira understanding of the mechanisms that un tion by low elevation needles. derscore the distribution patterns of the Pinus cembroides transpiration was also pinyon pines of across North America. much lower than other transpiration studies on other short-needle conifers in the United Literature Cited States, highlighting its high drought tolerance ADAMS, H. D. Al'.'D T E. KOLB. 2005. Tree growth relative to other, more-temperate conifers. response to drought and temperature in a moun Stomatal transpiration of P. cembroides was tain landscape in northern Arizona, USA. J. of half that of Abies balsarnea on Mt. Moosi- Biogeog. 32: 1629-1640. 2007] POULOS AND BERLYN: NEEDLE MORPHOLOGY IN PINUS CEMBROJDES 287
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