VARIABILITY AMONG DANTHONIA SERICEA (GRAMINEAE) POPULATIONS IN RESPONSES TO SUBSTRATE MOISTURE LEVELS

Populations of Danthonia sericea from the New Jersey Pine Barrens were investigated as to possible ecological differentiation in moisture tolerances at germination, seedling, and mature plant stages. Field studies had indicated that populations with pubescent lemmas and leaf sheaths are restricted to well-drained sandy upland sites and that relatively glabrous populations are found in open bogs or low wet areas bordering rivers or ponds. Greenhouse studies of responses of clonal and seed materials to saturated, moist, and dry moisture levels showed significant differences among populations in germination, growth, and survival. Reciprocal transplants at upland and bog sites provided a field evaluation (1969–1974) of the survival and performance of the morphological variants in each other's habitat under competitive conditions. Restriction to respective habitats was found to be primarily intolerance of wet site factors for the pubescent plants coupled with an inability of glabrous plants to successfully compete in upland sites.     

INFLUENCE OF ENVIRONMENT AND POPULATION ORIGIN ON SURVIVORSHIP AND REPRODUCTION IN RECIPROCAL TRANSPLANTS OF AMPHICARPIC PEANUTGRASS (AMPHICARPUM PURSHII)

Reciprocal transplants of both seeds and seedlings were utilized to determine whether populations of the annual grass Amphicarpum purshii have become locally adapted to specific habitats due to the consistent production of cleistogamous subterranean seeds from year to year. The hypothesis was that subterranean seeds placed in the same habitat as the parents will produce seedlings of greater vigor and adults of higher reproductive capacity than plants from seeds transplanted to a different habitat far removed from the parents. For both seed and seedling transplant experiments involving three sites in the Pine Barrens of New Jersey, the effects of site on shoot dry weight and production of aerial spikelets, subterranean spikelets, and seeds were generally much more significant than the effects of population origin. With one exception, there was no tendency for seedlings (or plants from seeds) replanted into their home sites to outperform alien seedlings (or plants from seeds) transplanted into these same sites. The overriding importance of environmental factors (relative to genetic differences among populations) in determining the phenotypic expression of life history characters, and selection occurring during succession at a site may retard the evolution of genetic adaptation to local habitat conditions in this species.     

Habitat and Population Effects on the Germination and Early Survival of the Invasive Weed, Lythrum salicaria L. (purple loosestrife)

In this study, we examined how environmental factors and differences among populations of Lythrum salicaria L. (Lythraceae; purple loosestrife) influenced the establishment of this species into both its preferred wetland habitat and secondary upland habitat. Seeds and seedlings from five parents from each of the three populations were planted in each wetland and upland of two sites in both disturbed and undisturbed plots. More seeds germinated in wetland than in upland sites (31% as compared to 11%) and, although a disturbance resulted in greater germination (23% as compared to 17% in undisturbed plots), there was no interaction with habitats. The results of a simultaneous greenhouse germination study in which seeds were planted under saturated and drained conditions suggest that the higher germination observed in wetland sites was due to the higher moisture present at such sites (79% vs. 45% in saturated and drained conditions, respectively). Disturbance enhanced both germination and growth of L. salicaria and this was especially so for seedlings. Dry weights of seedlings from disturbed plots were five times greater than those from undisturbed plots ( =9.2 vs. 1.8g in disturbed and undisturbed conditions, respectively). In addition to environmental factors, population differences are likely to play an important role in the spread of L. salicaria as we found differences between the populations in both life stages examined. One of the populations germinated equally well in both moisture treatments. In our seedling establishment experiment, one of the populations produced both the smallest plants and the most and earliest flowers, suggesting superior reproduction.     

Leaf traits of natural populations of <Emphasis Type="Italic">Adiantum reniforme</Emphasis> var. <Emphasis Type="Italic">sinensis</Emphasis>, endemic to the Three Gorges region in China

Leaf mass per unit area (LMA), carbon and nitrogen contents, leaf construction cost, and photosynthetic capacity (P _max) of Adiantum reniforme var. sinensis, an endangered fern endemic to the Three Gorges region in southwest China, were compared in five populations differing in habitat such as soil moisture and irradiance. The low soil moisture and high irradiance habitat population exhibited significantly higher LMA, area-based leaf construction (CC_A), and carbon content (C_A), but lower leaf nitrogen content per unit dry mass (N_M) than the other habitat populations. The high soil moisture and low irradiance habitat populations had the lowest CC_A, but their cost/benefic ratios of CCA/P _max were similar to the medium soil moisture and irradiance habitat population due to their lower leaf P _max. Hence A. reniforme var. sinensis prefers partially shaded, moist but well-drained, slope habitats. Due to human activities, however, its main habitats now are cliffs or steeply sloped bare rocks with poor and thin soil. The relatively high energy requirements and low photosynthetic capacity in these habitats could limit the capability of the species in extending population or interspecific competition and hence increase its endangerment.     

Habitat variability in the effects of predation and microclimate on mycophagous fly communities

Ant predation and soil moisture have direct and interactive effects on the abundance and community structure of mycophagous flies Here, we replicated an experiment across three climatically different habitat types to describe how these small-scale processes (microclimate and predation) are affected by macroclimatic variability at a larger spatial scale (among habitats) Each week for eight weeks during the summer of 1993, 18 Agaricus bisporus mushrooms were placed on dry, moist, or wet potting soil, within predator access or predator exclusion treatments cups, at six sites in the piedmont of South Carolina, USA Two sites were moist hemlock ravines and four sites were dry ridgetops Mushrooms and soil were collected after one week and fly metamorphs were counted and sorted by species We described the effects of ant predation, soil moisture, site and week on the frequency of host use, metamorph abundance, and Simpson's diversity All three measures were affected by macroclimatic differences among sites and across weeks At wet sites and during rainy weeks, more mushrooms were used, more metamorphs emerged, and the communities were more diverse than at dry sites or during dry periods The small-scale effect of soil moisture was strongly affected by large-scale macroclimatic conditions In dry ridgetop sites and in dry weeks, abundance and diversity increased with increasing soil moisture In moist sites or dunng rainy weeks, however, soil moisture was unimportant and had no effect on abundance or community structure Predation was unaffected by large-scale differences in climate, but was affected by small-scale differences in soil moisture, becoming more intense as soil moisture increased This research demonstrates that the effect of climate on predation is a scale-dependent phenomenon, and that microclimatic effects are mediated by climatic conditions at larger spatial scales We relate these findings to hierarchical theory and hypotheses concerning the relative effects of tolerances, competition, and predation under different levels of environmental stress     

Differential seed mortality among habitats limits the distribution of the invasive non-native shrub Ardisia elliptica

During the early phases of biological invasions, both limited dispersal and habitat preference may contribute to observed patterns of distribution for non-native species. In a disturbed area of Everglades National Park, the non-native Ardisia elliptica (Myrsinaceae) does not occur in all habitat types nor is it evenly distributed among habitats where it does occur. One goal of this study was to determine whether the patchy distribution of A. elliptica may be due to differential effects of habitat on seed fates and germination. Screen mesh bags containing seeds of A. elliptica were buried in the surface litter of five habitats and were later sampled periodically over a three month period to measure seed germination, survival and death. Seed mortality and mean germination speed was higher in open exposed habitats relative to closed canopied forests. During the study period, which coincided with the dry season, there was no germination of seeds that had been placed in pine forests or in Schinus thickets that were highly disturbed pine forests. Because hydroperiod is a critical determinant of habitat type in the Everglades ecosystem, a second goal of this study was to determine whether water availability was correlated with seed germination and seedling survival in five sites where A. elliptica has invaded. Seeds were sown under mesh boxes and monitored for germination and seedling survival for ten months. Estimates of water availability taken as predawn water potentials on A. elliptica were positively correlated with percent seed germination but not with percent seedling survival. Additionally, pre-dawn water potential of A. elliptica differed among sites. This study suggests that differential water availability among habitats may have influenced the patchy distribution of A. elliptica in Everglades National Park. Lack of sufficient moisture availability for seeds in exposed habitats such as pine forests and wetland prairies during the dry season has contributed to its distribution. Because some of the habitats where A. elliptica currently occurs were pine forests and wetland prairies prior to disturbance, this study suggests that substrate disturbance and canopy formation have altered the soil microenvironment, favoring the establishment of A. elliptica.     

GERMINATION ECOLOGY OF DANTHONIA SERICEA POPULATIONS

Seed were collected from nine populations of Danthonia sericea on either wet or well-drained, sandy sites ranging from Georgia to New Jersey. Germination tests were conducted in the laboratory and in field plots established on contrasting habitat types. Factors considered were temperature, light, prechilling, scarification, and after-ripening. The majority of freshly harvested seed from all nine populations exhibited innate dormancy the intensity of which decreased with age. Dormancy was removed by scarification and prechilling treatments of 60 days. Seed from wet habitats showed significantly greater germination in light (alternating night-day) than in dark, while seed from well-drained habitats had a higher germination percentage in dark than in light. Both responses are considered to be physiological adaptations of the populations to their respective habitats. Alternating night-day temperatures of 15–25 C produced the greatest overall germination for the nine populations; 15–20 and 10–25 C produced the next highest responses. Field observations confirmed that peak germination occurred in the spring, at the time soil temperatures were in the same range found to be optimum in the laboratory. A comparison of the germination responses obtained from six New Jersey populations in field plots on contrasting habitat types further revealed distinctive physiological adaptations for populations from differing habitats.     

Irregular rain cues and the germination and seedling survival of a Panamanian shrub (Hybanthus prunifolius)

Summary Seeds of the Panamanian shrub, Hybanthus prunifolius (Schult.) Schulze (Violaceae) are dispersed at different times in different years ((March to June) and are exposed to the irregular rainfall of the dry season in some years. Fluctuations in soil moisture in the dry season represent suboptimal conditions for germination and seedling survival. There are no mechanisms to prevent germination prior to the arrival of consistent rains in the wet season.Among three natural cohorts of seeds followed in two years, the cohort experiencing the longest time from sowing to consistent rains had the highest germination, but it also had the longest time lag from sowing to, beginning of germination, longest germination period, and lowest survival of seedlings 3 months after sowing.Seeds were also induced experimentally to germinate under 14 different moisture patterns. The patterns encompassed 1) varying lengths of moisture before a dry period, 2) inconsistent moisture, and 3) varying lengths of dryness prior to any moisture.Mortality of seeds by fungal infection occurred if the wet period was delayed. But germination was less affected by fluctuations than was seedling survival. Length of the first wet period and frequency of occurrence of the wet period both affected germination levels. Survival and development of seedlings was influenced by the number of days exposed to dry conditions and by the stage of development at the beginning of the dry period. Young seedlings suffered attrition due to drought stress, and older seedlings died from fungal attack.Results from field and experimental sowing of seeds both indicate that this perennial species has minimal defense against germination when conditions are suboptimal for seedling survival. Undoubtedly there is more recruitment in some years than in others.     

Improving success of rare plant seed reintroductions: a case study of Dalea carthagenensis var. floridana,a rare legume with dormant seeds

Recent reviews of rare plant reintroduction success indicate that far fewer studies have been conducted with seeds than whole plants, and of these, less than 10% have established or had long‐term population persistence reported. Because seed reintroductions are relatively less expensive than plant reintroductions, determining ways to increase efficacy of using seeds to establish rare populations has conservation benefits. In laboratory trials, we examined seed germination of an endangered legume, Dalea carthagenensis var. floridana, endemic in South Florida, U.S.A. Laboratory treatments confirmed that seeds are hard seeded, remaining viable for 1,452 days even when moist; nicking, heat, and freezing triggered higher and more rapid germination than controls. Field trials begun in 2009, using pretreated (frozen) and untreated seeds within two habitats (natural and novel) revealed that freezing pretreatment increased germination in both habitats. However, plants matured, reproduced, and established seedlings only in natural habitat, not in novel habitat. By 2012, seed treatment plots in natural pine rockland had significantly greater numbers of reproductive plants and seedlings than controls. In a restoration context, using seed pretreatments to stimulate germination can improve establishment success in suitable habitats. When paired with essential vegetation management and a controlled burn, seed augmentation helped rescue the population from the brink of extinction.     

Making up for lost time: Biophysical constraints on the temporal abundance of two fiddler crabs in wet–dry tropical mangroves

Biophysical models are used to predict the spatial distributions of organisms. Nevertheless, understanding factors influencing the temporal distributions of animals may often be additionally required. It is expected that intertidal macrofauna of the wet–dry tropics face a multitude of temporal challenges because there is not only seasonal drying but also variation in surface moisture over the circatidal cycle. Activities of fiddler crabs (Uca spp.) depend on adequate surface moisture being available for feeding and respiration. A recent study monitored crab abundance during spring tides and found that one Uca species in the mangroves of Darwin Harbour, Australia, U. flammula, is most abundant in the wet season, while another, U. elegans, is most abundant in the dry season. We hypothesized here that these seemingly contradictory abundance patterns are driven by temporal variation in the availability of soil moisture within each species habitat. We thus monitored crab abundance and measured soil moisture content across four types of habitat (low gap centres, low gap edges, mid‐height gap centres and high gap centres) seasonally and across the circatidal cycle. We found that crab abundance and soil moisture both varied over time among habitat types. We used a log‐linear model to show that habitat type influenced soil moisture and this in turn influenced crab abundance. Sampling across the circatidal cycle showed that U. flammula was more abundant in the wet season, as reported previously, while the abundance of U. elegans did not vary between seasons. Our model suggested that U. elegans ‘makes up for lost time’ in the dry season by undertaking all activities during spring tide low water as only at this time is the substratum moist enough for feeding and respiration. We highlight the importance of measuring multiple variables across habitats over small and large scales when assessing temporal abundance patterns of intertidal tropical organisms.     

Germination of seven exotic weeds and seven native speciesin south-western Australia under steady and fluctuating water supply

The germination of seven common weeds and seven species native to south-western Australia was studied at three sites where they co-occur and in the laboratory. Under field conditions, final germination of the exotic species was much higher than that of the native species, and the number of days to reach 50 % final germination was lower. For some of the germination period, soil moisture was lower than expected after 48 h without rain, with one dry period lasting 10 d. Overall, germination rose with increasing wetness of the three sites, especially among the weeds. Most exotic species germinated under laboratory conditions needed less than 10 d to reach 50 % of final germination which was further delayed by 2 d when seeds were removed from their moist substrate and allowed to dry out (air-drying) for 48 h. Most native species took 10–35 d, with a mean delay of 7 d after 48 h of air-drying. Total germination of the weeds was close to 100 % and was little affected by periods of air-drying. Germination of the native species was more variable and greatly reduced by air-drying in two species. We conclude that water availability as it varies through time and space is significant in controlling germination even during the wet season. The quicker and greater responses of weed seeds to moisture may contribute toward the superior ability of weeds to colonise disturbed habitats. Differences in life form, seed size, flatness, presence of appendages, and ability to absorb or retain water only partially explain these results.     

TRACHEID LENGTH AS AN ECOTYPIC CHARACTER IN ACER NEGUNDO L.

Laboratory germinated seedlings of Acer negundo grown under controlled environmental conditions showed patterns of decreasing cell length (tracheids) with decrease in length of growing season of parental tree's habitat. Populations tested represented a range in latitude of 15 degrees (Texas to Canada) and the results indicate populational response to shorter growing seasons being reflected in xylem development.     

Spatial and Temporal Distribution of Northern Waterthrushes Among Nonbreeding Habitats in Southwestern Puerto Rico1

We surveyed nonbreeding Northern Waterthrushes (Seiurus noveboracensis) in several coastal habitats in southwestern Puerto Rico, West Indies. Waterthrush abundance was highest (mean = 2.2/point) in well‐developed mangrove habitats characterized by large trees and standing water, and lowest (mean = 0.1/point) in dry coastal scrub and savannah. Within the latter habitats, waterthrushes were more common in moist microhabitats such as temporary ponds and dry washes. Abundance in most habitats declined between October and January, probably a result of the presence of transients during the October surveys, Between January and March, abundance in mature red mangrove (Rhizophora mangle) forests increased dramatically, suggesting that this habitat provides a refuge for birds at the end of the dry season and prior to northward migration. In conjunction with other data demonstrating the use of coastal red mangroves as nocturnal roosting sites, these results suggest that mangroves are the most important nonbreeding habitat for this species. Conservation of this globally threatened habitat may thus be important for the long‐term stability of waterthrush populations.     

Explaining Rarity of the Dry Grassland Perennial Astragalus exscapus

In Central Europe several plant species of dry grasslands are particularly rare. Here I investigate whether habitat requirements, reproduction, and dispersal potential can contribute to the rarity of Astragalus exscapus (Fabaceae) growing in dry grassland habitats in dry regions of Europe. In addition, I question whether historic events might have contributed to the present-day rarity of A. exscapus. To assess habitat requirements of A. exscapus, vegetation composition and soil characteristics were studied in 37 populations in central Germany. Production and dispersal potential of seeds were investigated in 10 populations, and germination and recruitment were assessed in experimental plots in three populations. Vegetation of the habitats included most dry grassland community types occurring in the central German dry region indicating a broad ecological niche of the species within dry subcontinental grasslands. Soil characteristics of the habitats also spanned a wide range. Seed production was moderate. 98% of the seeds sown in the laboratory germinated whereas under natural conditions 20% of the seeds developed seedlings. Half of these seedlings survived for one year but only 4.5% for two years. 90% of the seeds were dispersed less than 50 cm distance indicating a low dispersal potential. I conclude that A. exscapus is mainly limited in dispersal but recruitment limitation might also be important in explaining its rarity. Furthermore, former climate change and postglacial reforestation of the area very likely contribute to the rarity of A. exscapus.     

Seedling emergence patterns and dormancy/germination physiology of <Emphasis Type="Italic">Primula modesta</Emphasis> in a subalpine region

On the basis of the germination/dormancy responses of seeds to temperature and light, and local seed rain, we attempted to interpret the seedling emergence patterns of Primula modesta Bisset et Moore (Primulaceae) in two different types of habitats in a subalpine zone of Mt. Asama: an oligotrophic flat moor and a grassland with relatively dense herbaceous vegetation cover. The seasonal pattern of seedling emergence was well explained by the dormancy/germination physiology revealed in laboratory germination tests. The seeds were demonstrated to have a strict light requirement even after experiencing moist chilling, which might facilitate the incorporation of the seeds into the soil seed bank. Despite sufficient seed production, the seedlings emerging were far less at the grassy site than the moor site, but the number of seedlings was significantly dependent on the seed rain within previous season and on the litter cover of the microsite in both sites. Therefore, the spatiotemporal patterns of seedling emergence in the habitats could be well explained by the spatiotemporal patterns of seed rain and safe-sites for germination.     

盐旱互作对不同生境盐地碱蓬种子萌发和幼苗生长的影响

研究了盐旱互作对潮间带和盐碱地生境盐地碱蓬棕色种子萌发、地上部生长和离子积累的影响。不同盐浓度预处理后,未萌发的种子风干后复水,其萌发率与对照相比没有降低,说明两种生境盐地碱蓬种子萌发期间都耐干湿交替。400mmol/LNaCl溶液浇灌的潮间带生境盐地碱蓬幼苗在第3次干旱处理后萎蔫幼苗的百分比高于盐碱地生境的,而复水后正常幼苗的百分比却相反。400mmol/LNaCl溶液处理下,第3次干旱处理复水后,盐碱地生境盐地碱蓬幼苗地上部离子含量(主要是Na+和Cl-)高于潮间带生境的。表明在干旱情况下,盐碱地生境盐地碱蓬幼苗能积累更多的无机离子,降低渗透势,提高根系吸收水分的能力。上述结果说明,盐碱地生境盐地碱蓬幼苗较潮间带生境盐地碱蓬幼苗更耐盐与干旱的交互作用。     

The demography of a clonal herbaceous perennial plant, the longleaved sundew Drosera intermedia, in different heathland habitats

Eight populations of longleaved sundew Drosera intermedia Hayne ramets were studied in three hydrologically and phytosociologically differing heathland habitats over a 4 yr period a pool edge, an old path through the wet heath, a seepage area At the extreme sites, with high levels of water table fluctuations (pool edge and dry path), both survival and recruitment were low, resulting in small populations The pool edge populations went extinct At the less extreme sites (wet paths and seepage area), populations were stable, but ramet and genet turnover was higher at the path sites than in the seepage area Demographic variation between populations was explained by differences in adult survival and sexual recruitment (asexual recruitment did not differ clearly), which, in turn, were related to specific biotic and abiotic factors Likely adult mortality causes were summer drought (all habitats), winter frost (path habitat), cover with a crust of iron bacteria after heavy summer rainfall (seepage area) and shading by Sphagnum and algae (pool edge) Low seedhng recruitment was caused by a low germination succes (pool, seepage area) and seedling survival (seepage area, dry path)     

Plant coexistence in coastal heaths: Habitat segregation in the post-fire environment

Abstract Previous studies of heaths on Pleistocene coastal sands showed consistent variation in patterns of floristic composition between areas on ridges and slopes but not within them. A large wild-fire that swept this system provided an opportunity to observe temporal processes in habitat segregation of species. Ridges and slopes were found to differ in species richness of seedlings in 0.0625 m2 quadrats and there was no evidence that the species richness in the two habitats was converging over 3 years. This suggests that initially these processes differ in space between the two habitats with more species in the wet heath being packed into a smaller area than in the dry heath. Seed-banks of species did not saturate available space for recruitment in either habitat, but seedling densities differed asymmetrically between habitats across two pairs of species studied. Experimental manipulation of seeds among habitats also showed distinct differences in establishment and survival among representative species from different habitats. The presence of seed-banks of wet-heath species in dry heath suggested that recruitment of their seedlings may occur there, but experiments showed that seedlings that arise from them do not survive there. Conversely, although no seed-bank of either dry-heath species studied was found in wet heath, our manipulations showed that their seedlings could grow there. Thus, superficially, physiological tolerance appeared to limit regeneration of wet-heath species in dry heath. However, there appeared to be no physiological limit for dry-heath plants to survive in wet heath. This paper shows that segregation of species may operate at either of two stages of the life cycle: at dispersal when safe sites for establishment are required, and at recruitment after seedlings have established. These findings highlight the importance of the regeneration niche in structuring community composition in coastal heaths, and contrast with traditional explanations of species segregation invoking physiological tolerance and competition at later life-history stages.     

Differential effect of drought regimes on the seedling performance of six floodplain grassland species

• The performance of seedlings is crucial for the survival and persistence of plant populations. Although drought frequently occurs in floodplains and can cause seedling mortality, studies on the effects of drought on seedlings of floodplain grasslands are scarce. We tested the hypotheses that drought reduces aboveground biomass, total biomass, plant height, number of leaves, leaf area and specific leaf area (SLA), and increases root biomass and root‐mass fraction (RMF) and that seedlings from species of wet floodplain grasslands are more affected by drought than species of dry grasslands.
• In a greenhouse study, we exposed seedlings of three confamilial pairs of species (Pimpinella saxifraga, Selinum carvifolia, Veronica teucrium, Veronica maritima, Sanguisorba minor, Sanguisorba officinalis) to increasing drought treatments. Within each plant family, one species is characteristic of wet and one of dry floodplain grasslands, confamilial in order to avoid phylogenetic bias of the results.
• In accordance with our hypotheses, drought conditions reduced aboveground biomass, total biomass, plant height, number of leaves and leaf area. Contrary to our hypotheses, drought conditions increased SLA and decreased root biomass and RMF of seedlings. Beyond the effects of the families, the results were species‐specific (V. maritima being the most sensitive species) and habitat‐specific. Species indicative of wet floodplain grasslands appear to be more sensitive to drought than species indicative of dry grasslands.
• Because of species‐ and habitat‐specific responses to reduced water availability, future drought periods due to climate change may severely affect some species from dry and wet habitats, while others may be unaffected.

     

在旱化土壤条件下来自不同生境的20种植物幼苗根系的形态反应的比较(英文)

在浇水和未浇水的塑料管中栽培了 2 0个植物种 ,测量了其幼苗的根深、根重和茎重。这些种的原始生境含水状况差异较大 ,是从沼泽到沙漠的系列。植物种原始生境的水分状况用Ellenberg水分序数定量。幼苗首先在湿沙中生长 2 1d ,然后进入为期也是 2 1d的处理阶段 (浇水和不浇水 )。浇水植株的根深与Ellenberg水分序数无关。在旱化的沙层中 ,源于干旱生境的植物的根深趋向于增加 ,来自湿润生境的则减少。根深塑性 (即未浇水的根深 /浇水的根深 )与Ellenberg水分序数显著相关 (R2 =0 .5 6 ) ,茎 /根比值塑性也与Ellenberg水分序数相关 ,但不如根深塑性的关系明显。根深塑性表现最为明显的植物种具有在未浇水沙层中维持茎生长的最大能力。有迹象表明 ,在浇水处理时 ,来自很干旱生境的植物生长减弱。研究结果表明 :幼苗利用深层水分的能力是植物对干旱生境的主要适应。