不同降水条件下两种荒漠植物的水分利用策略

自然降水是干旱、半干旱地区荒漠植物重要的水分来源。为了说明自然降水量的变化对干旱、半干旱地区荒漠植物水分利用策略的影响, 研究了两种常见荒漠植物油蒿(Artemisia ordosica)和白刺(Nitraria tangutorum)在3个不同自然降水地区(内蒙古的杭锦旗和磴口县及甘肃的民勤县)的水分来源、水分利用效率及植物的抗逆能力的变化。测定了不同地区的植物茎水、各潜在水源(降水、地下水和土壤水)的δD和δ¹⁸O值, 并利用IsoSource模型分析了这两种植物在不同地区对这些潜在水源的选择性利用情况; 同时测定了叶片的δ¹³C和游离脯氨酸浓度。结果表明: 在年降水量最高的杭锦旗, 这两种植物对浅层土壤水的利用比例最高, 其中油蒿主要利用0-50 cm土层中的水源; 在年降水量相对较低的磴口和民勤, 植物利用的主要水源为深层土壤水和地下水。随着年降水量的增加, 这两种植物的水分利用效率逐渐降低。白刺的脯氨酸浓度大于油蒿, 与水分利用效率无关, 但油蒿的水分利用效率和脯氨酸浓度成正比。研究表明, 荒漠植物能通过改变其水分利用策略和其他生理特性适应自然降水量的变化, 但不同植物种采用的策略可能有所不同。     

Ecophysiological response and morphological adjustment of two Central Asian desert shrubs towards variation in summer precipitation

As part of global climate change, variation in precipitation in arid ecosystems is leading to plant adaptation in water-use strategies; significant interspecific differences in response will change the plant composition of desert communities. This integrated study on the ecophysiological and individual morphological scale investigated the response, acclimation and adaptation of two desert shrubs, with different water-use strategies, to variations in water conditions. The experiments were carried out on two native dominant desert shrubs, Tamarix ramosissima and Haloxylon ammodendron, under three precipitation treatments (natural, double and no precipitation, respectively), in their original habitats on the southern periphery of Gurbantonggut Desert, Central Asia, during the growing season in 2005. Changes in photosynthesis, transpiration, leaf water potential, water-use efficiency, above-ground biomass accumulation and root distribution of the two species were examined and compared under the contrasting precipitation treatments. There were significant interspecific differences in water-use strategy and maintenance of photosynthesis under variation in precipitation. For the phreatophyte T. ramosissima, physiological activity and biomass accumulation rely on the stable groundwater, which shields it from fluctuation in the water status of the upper soil layers caused by precipitation. For the non-phreatophyte H. ammodendron, efficient morphological adjustment, combined with strong stomatal control, contributes to its acclimation to variation in precipitation. On account of its positive responses to increased precipitation, H. ammodendron is predicted to succeed in interspecific competition in a future, moister habitat.     

Interactive Effects of Warming and Increased Precipitation on Community Structure and Composition in an Annual Forb Dominated Desert Steppe

To better understand how warming, increased precipitation and their interactions influence community structure and composition, a field experiment simulating hydrothermal interactions was conducted at an annual forb dominated desert steppe in northern China over 2 years. Increased precipitation increased species richness while warming significantly decreased species richness, and their effects were additive rather than interactive. Although interannual variations in weather conditions may have a major affect on plant community composition on short term experiments, warming and precipitation treatments affected individual species and functional group composition. Warming caused C₄ grasses such as Cleistogenes squarrosa to increase while increased precipitation caused the proportions of non-perennial C₃ plants like Artemisia capillaris to decrease and perennial C₄ plants to increase.     

Selection on leaf ecophysiological traits in a desert hybrid Helianthus species and early-generation hybrids

Leaf ecophysiological traits related to carbon gain and resource use are expected to be under strong selection in desert annuals. We used comparative and phenotypic selection approaches to investigate the importance of leaf ecophysiological traits for Helianthus anomalus, a diploid annual sunflower species of hybrid origin that is endemic to active desert dunes. Comparisons were made within and among five genotypic classes: H. anomalus, its ancestral parent species (H. annuus and H. petiolaris), and two backcrossed populations of the parental species (designated BC2ann and BC2pet) representing putative ancestors of H. anomalus. Seedlings were transplanted into H. anomalus habitat at Little Sahara Dunes, Utah, and followed through a summer growing season for leaf ecophysiological traits, phenology, and fitness estimated as vegetative biomass. Helianthus anomalus had a unique combination of traits when compared to its ancestral parent species, suggesting that lower leaf nitrogen and greater leaf succulence might be adaptive. However, selection on leaf traits in H. anomalus favored larger leaf area and greater nitrogen, which was not consistent with the extreme traits of H. anomalus relative to its ancestral parents. Also contrary to expectation, current selection on the leaf traits in the backcross populations was not consistently similar to, or resulting in evolution toward, the current H. anomalus phenotype. Only the selection for greater leaf succulence in BC2ann and greater water-use efficiency in BC2pet would result in evolution toward the current H. anomalus phenotype. It was surprising that the action of phenotypic selection depended greatly on the genotypic class for these closely related sunflower hybrids grown in a common environment. We speculate that this may be due to either phenotypic correlations between measured and unmeasured but functionally related traits or due to the three genotypic classes experiencing the environment differently as a result of their differing morphology.     

Patterns in leaf traits of leguminous and non-leguminous dominant trees along a rainfall gradient in Ghana

Aims Both dominance distribution of species and the composition of the dominant species determine the distribution of traits within community. Leaf carbon (C) and nitrogen (N) isotopic composition are important leaf traits, and such traits of dominant species are associated with ecosystem C, water and N cycling. Very little is known how dominant species with distinct traits (e.g. N-fixing leguminous and non-leguminous trees) mediate resource utilization of the ecosystems in stressful environment.Methods Leaves of 81 dominant leguminous and non-leguminous trees were collected in forest (moist semi-deciduous and dry semi-deciduous ecosystems) and savanna (costal savanna, Guinean savanna and west Sudanian savanna ecosystems) areas and the transitional zone (between the forest and the savanna) along the transect from the south to the north of Ghana. We measured leaf traits, i.e. leaf δ 13 C, leaf δ 15 N, leaf water content, leaf mass per area (LMA) and C and N concentration. Correlation analyses were used to examine trait–trait relationships, and relationships of leaf traits with temperature and precipitation. We used analysis of covariance to test the differences in slopes of the linear regressions between legumes and non-legumes.Important findings Leaf δ 13 C, δ 15 N, leaf water content and LMA did not differ between leguminous and non-leguminous trees. Leaf N concentration and C:N ratio differed between the two groups. Moreover, leaf traits varied significantly among the six ecosystems. δ 13 C values were negatively correlated with annual precipitation and positively correlated with mean annual temperature. In contrast, leaf δ 15 N of non-leguminous trees were positively correlated with annual precipitation and negatively correlated with mean annual temperature. For leguminous trees, such correlations were not significant. We also found significant coordination between leaf traits. However, the slopes of the linear relationships were significantly different between leguminous and non-leguminous trees. Our results indicate that shifts in dominant trees with distinct water-use efficiency were corresponded to the rainfall gradient. Moreover, leguminous trees, those characterized with relative high water-use efficiency in the low rainfall ecosystems, were also corresponded to the relative high N use efficiency. The high proportion of leguminous trees in the savannas is crucial to mitigate nutrient stress.     

Correlations between carbon isotope ratio and microhabitat in desert plants

Summary Water is usually considered to be the key limiting factor for growth of desert plants, yet there is little information available of the water-use efficiency of species within a desert community. Leaf carbon isotope ratios, an indicator of long-term intercellular carbon dioxide concentrations and thus of water-use efficiency in C₃ plants, were measured on species occurring within a Sonoran Desert community, consisting of wash, transition and slope microhabitats. Along a soil moisture gradient from the relatively wetter wash to the relatively drier slope, leaf carbon isotope ratios increased in all species, indicating that water-use efficiency increased as soil water availability decreased. Leaf carbon isotope ratios of long-lived perennials were substantially more positive than in short-lived perennials, even though plants were growing adjacent to each other. Leaf carbon isotope ratio and leaf duration (evergreen versus deciduous) were not correlated with each other. The results are discussed in terms of how the efficiency of water use may affect community structure and composition.     

Water-use strategy of three central Asian desert shrubs and their responses to rain pulse events

Plant water-use strategy is considered to be a function of the complex interactions between species of different functional types and the prevailing environmental conditions. The functional type of a plant’s root system is fundamental in determining the water-use strategy of desert shrubs and the physiological responses of the plant to an occasional rainfall event, or rain pulse. In this current study of Tamarix ramosissima Ledeb. Fl.Alt., Haloxylon ammodendron (C.A.Mey.) Bunge and Reaumuria soongorica (Pall.) Maxim., three dominant shrub species in the Gurbantonggut Desert (Central Asia), plant root systems were excavated in their native habitat to investigate their functional types and water-use strategies. We monitored leaf water potential, photosynthesis and transpiration rate during a 39-day interval between successive precipitation events during which time the upper soil water changed markedly. Plant apparent hydraulic conductance and water-use efficiency were calculated for the varying soil water conditions. Our results show that: 1) The three species of shrub belong to two functional groups: phreatophyte and non-phreatophyte; 2) The photosynthetic capacity and leaf-specific apparent hydraulic conductance of the three species was stable during the time that the water condition in the upper soil changed; 3) Transpiration, leaf water potential and water-use efficiency in Tamarix ramosissima Ledeb. Fl.Alt. were stable during the period of observation, but varied significantly for the other two species. Tamarix ramosissima Ledeb. Fl.Alt., as a phreatophyte, relies mostly on groundwater for survival; its physiological activity is not inhibited in any way by the deficiency in upper soil water. Non-phreatophyte Haloxylon ammodendron (C.A.Mey.) Bunge and Reaumuria soongorica (Pall.) Maxim. use precipitation-derived upper soil water for survival, and thus respond clearly to rain pulse events in terms of leaf water potential and transpiration. The observed similarity in leaf-specific photosynthesis capacity among all three species indicates that the two non-phreatophyte species are able to maintain normal photosynthesis within a wide range of plant water status. The observed stability in leaf-specific apparent hydraulic conductance indicates that the two non-phreatophyte species are able to maintain sufficient water supply to their foliage via, mostly likely, effective morphological adjustment at the scale of the individual plant.Section editor: H. Lambers     

Precipitation Pattern Determines the Inter-annual Variation of Herbaceous Layer and Carbon Fluxes in a Phreatophyte-Dominated Desert Ecosystem

Arid and semi-arid ecosystems dominated by shrubby species are an important component in the global carbon cycle but are largely under-represented in studies of the effect of climate change on carbon flux. This study synthesizes data from long-term eddy covariance measurements and experiments to assess how changes in ecosystem composition, driven by precipitation patterns, affect inter-annual variability of carbon flux and their components in a halophyte desert community dominated by deep-rooted shrubs (phreatophytes, which depend on groundwater as their primary water source). Our results demonstrated that the carbon balance of this community responded strongly to precipitation variations. Both pre-growing season precipitation and growing season precipitation frequency significantly affected inter-annual variations in ecosystem carbon flux. Heavy pre-growing season precipitation (November–April, mostly as snow) increased annual net ecosystem carbon exchange, by facilitating the growth and carbon assimilation of shallow-rooted annual plants, which used spring and summer precipitation to increase community productivity. Sufficient pre-growing season precipitation led to more germination and growth of shallow-rooted annual plants. When followed by high-frequency growing season precipitation, community productivity of this desert ecosystem was lifted to the level of grassland or forest ecosystems. The long-term observations and experimental results confirmed that precipitation patterns and the herbaceous component were dominant drivers of the carbon dynamics in this phreatophyte-dominated desert ecosystem. This study illustrates the importance of inter-annual variations in climate and ecosystem composition for the carbon flux in arid and semi-arid ecosystems. It also highlights the important effect of changing frequency and seasonal pattern of precipitation on the regional and global carbon cycle in the coming decades.     

Rainfall variability and fine‐scale life history tradeoffs help drive niche partitioning in a desert annual plant community

Tradeoffs have long been an essential part of the canon explaining the maintenance of species diversity. Despite the intuitive appeal of the idea that no species can be a master of all trades, there has been a scarcity of linked demographic and physiological evidence to support the role of resource use tradeoffs in natural systems. Using five species of Chihuahuan desert summer annual plants, I show that demographic tradeoffs driven by short‐term soil moisture variation act as a mechanism to allow multiple species to partition a limiting resource. Specifically, by achieving highest fitness in either rainfall pulse or interpulse periods, variability reduces fitness differences through time that could promote coexistence on a limiting resource. Differences in fitness are explained in part by the response of photosynthesis to changing soil moisture. My results suggest that increasing weather variability, as predicted under climate change, could increase the opportunity for coexistence in this community.     

Phenotypic selection on leaf water use efficiency and related ecophysiological traits for natural populations of desert sunflowers

Plant water-use efficiency (WUE) is expected to affect plant fitness and thus be under natural selection in arid habitats. Although many natural population studies have assessed plant WUE, only a few related WUE to fitness. The further determination of whether selection on WUE is direct or indirect through functionally related traits has yielded no consistent results. For natural populations of two desert annual sunflowers, Helianthus anomalus and H. deserticola, we used phenotypic selection analysis with vegetative biomass as the proxy for fitness to test (1) whether there was direct and indirect selection on WUE (carbon isotope ratio) and related traits (leaf N, area, succulence) and (2) whether direct selection was consistent with hypothesized drought/dehydration escape and avoidance strategies. There was direct selection for lower WUE in mesic and dry H. anomalus populations, consistent with dehydration escape, even though it is the longer lived of the two species. For mesic H. anomalus, direct selection favored lower WUE and higher N, suggesting that plants may be “wasting water” to increase N delivery via the transpiration stream. For the shorter lived H. deserticola in the direr habitat, there was indirect selection for lower WUE, inconsistent with drought escape. There was also direct selection for higher leaf N, succulence and leaf size. There was no direct selection for higher WUE consistent with dehydration avoidance in either species. Thus, in these natural populations of two desert dune species higher fitness was associated with some combination direct and indirect selection for lower WUE, higher leaf N and larger leaf size. Our understanding of the adaptive value of plant ecophysiological traits will benefit from further consideration of related traits such as leaf nitrogen and more tests in natural populations.     

DIFFERING SELECTION ON PLANT PHYSIOLOGICAL TRAITS IN RESPONSE TO ENVIRONMENTAL WATER AVAILABILITY: A TEST OF ADAPTIVE HYPOTHESES

I used phenotypic selection analysis to test the prediction from functional and comparative studies of plants that smaller leaves and more efficient water use are adaptive in drier environments. I measured selection gradients on leaf size and instantaneous water-use efficiency (a measure of carbon gain per unit water loss) in experimental populations of Cakile edentula var. lacustris placed into wet and dry environments in the field. Linear and nonlinear selection differed significantly between the two environments as predicted. Water-use efficiency was selected to be higher, and leaf area was selected toward a small intermediate optimum, in the dry environment. There was also significant positive correlational selection on water-use efficiency and leaf size, suggesting that the optimum leaf size in the dry environment is greater for plants with higher water-use efficiency. In contrast, neither leaf size nor water-use efficiency were selected in the wet environment, though larger leaves resulted in greater vegetative biomass. Path analysis of the linear selection gradients found that water-use efficiency affected plant fitness primarily because it increased vegetative biomass, as suggested by the hypotheses about the function of physiological traits. These results were not only consistent with the functional hypotheses but also with the observed genetic differentiation in water-use efficiency and leaf size between wet and dry site populations.     

Summer rain pulses may stimulate a CO2 release rather than absorption in desert halophyte communities

Background and aims

The response of plants and soil to rain pulses determines seasonal variations in the exchange of materials and energy at the ecosystem scale in arid and semi-arid regions. We assessed how the ecosystem carbon exchange (NEE) of desert halophyte communities of different plant functional-types responds to summer precipitation pulses in Tamarix and Haloxylon communities.

Methods

Plant water status, photosynthetic gas exchange, soil respiration and net ecosystem carbon exchange were measured to test the hypothesis that high physiological sensitivity may induce a greater changes in NEE resulting from the summer precipitation pulses in Haloxylon community.

Results

Plant water status and photosynthetic assimilation did not differ before and after summer precipitation pulses in either community. In contrast, soil respiration and NEE responded strongly to summer precipitation events in both communities. At the ecosystem level, precipitation pulses induced a pulse of CO₂ release, rather than absorption. The NEE response to summer precipitation was less in the deep-rooted Tamarix community, compared to the shallow-rooted Haloxylon community, which was even converted into a carbon source after summer precipitation inputs. As a result, the effect of summer precipitation inputs on soil respiration was more important than the plant (carbon assimilation) response in determining the ecosystem response to episodic precipitation pulses.     

Phenotypic plasticity and precipitation response in Sonoran Desert winter annuals

Temporal environmental variation has profound influences on population dynamics and community structure. Examination of functional traits that influence resource uptake and allocation can illuminate how co-occurring species translate environmental variation into different demographic outcomes, yet few studies have considered interspecific differences in trait plasticity. We experimentally manipulated soil moisture to test the hypothesis that differences in morphological plasticity contribute to species differences in demographic response to unpredictable precipitation in Sonoran Desert winter annual plants. We compared plasticity of leaf traits and biomass allocation between Pectocarya recurvata (Boraginaceae) and Stylocline micropoides (Asteraceae), co-occurring species that differ in long-term demographic patterns. The species with highly variable population dynamics, Stylocline, had striking increases in leaf area and root biomass in response to an experimental increase in soil moisture. In contrast, the species with buffered long-term population dynamics, Pectocarya, did not differ in leaf morphology or biomass allocation between soil moisture treatments. Regardless of water treatment, Pectocarya had earlier reproductive phenology and greater fecundity than Stylocline, suggesting that differences in the timing of the phenological transitions from vegetative to reproductive growth may affect species' responses to precipitation pulses. Combining long-term observations with experimental manipulations provides a window into the functional underpinnings and demographic consequences of trait plasticity.     

Monosoonal precipitation responses of shrubs in a cold desert community on the Colorado Plateau

South-eastern Utah forms a northern border for the region currently influenced by the Arizona monosoonal system, which feeds moisture and summer precipitation into western North America. One major consequence predicted by global climate change scenarios is an intensification of monosoonal (summer) precipitation in the aridland areas of the western United States. We examined the capacity of dominant perennial shrubs in a Colorado Plateau cold desert ecosystem of southern Utah, United States, to use summer moisture inputs. We simulated increases of 25 and 50 mm summer rain events on Atriplex canescens, Artemisia filifolia, Chrysothamnus nauseosus, Coleogyne ramosissima, and Vanclevea stylosa, in July and September with an isotopically enriched water (enriched in deuterium but not ¹⁸O). The uptake of this artificial water source was estimated by analyzing hydrogen and oxygen isotope ratios of stem water. The predawn and midday xylem water potentials and foliar carbon isotope discrimination were measured to estimate changes in water status and water-use efficiency. At. canescens and Ch. nauseosus showed little if any uptake of summer rains in either July or September. The predawn and midday xylem water potentials for control and treatment plants of these two species were not significantly different from each other. For A. filifolia and V. stylosa, up to 50% of xylem water was from the simulated summer rain, but the predawn and midday xylem water potentials were not significantly affected by the additional summer moisture input. In contrast, C. ramosissima showed significant uptake of the simulated summer rain (>50% of xylem water was from the artificial summer rain) and an increase in both predawn and midday water potentials. The percent uptake of simulated summer rain was greater when those rains were applied in September than in July, implying that high soil temperature in midsummer may in some way inhibit water uptake. Foliar carbon isotope discrimination increased significantly in the three shrubs taking up simulated summer rain, but pre-treatment differences in the absolute discrimination values were maintained among species. The ecological implications of our results are discussed in terms of the dynamics of this desert community in response to changes in the frequency and dependability of summer rains that might be associated with a northward shift in the Arizona monsoon boundary.     

Using multiple trait associations to define hydraulic functional types in plant communities of south-western Australia

Assessing the hydrological imbalance and associated land degradation issues facing much of southern Australia and other parts of the world requires a better understanding of the defining features of ecosystem water use and the design of sustainable agroecosystems. Thus, by grouping species with similar water-use strategies into 'hydraulic functional types' (HFTs), we investigated the characteristics of water use in species-rich plant communities of south-western Australia. HFTs were determined using multiple-trait associations between morphological and physiological traits relating to water transport, water-use efficiency and response to water deficit. Sixteen traits were assessed from a subset of 21 species from three plant communities located along a topographically determined soil- and water-availability gradient. Multivariate analyses showed that trait variation was least at sites with shallower soils and putatively lower water availability, suggesting a convergence of water-use strategies at sites where plants are exposed to large seasonal water deficits. Stem hydraulic parameters, including stem-specific hydraulic conductivity, conduit diameter and maximum percentage embolism, were positively correlated, indicating the generality that larger conduit diameter permits greater hydraulic efficiency and is associated with greater seasonal reductions in hydraulic conductivity in this ecosystem. Wood density was not correlated with these traits, but closely associated with species' ability to withstand more negative water potentials during summer. Long-term integrated water-use efficiency was lower in shallow-rooted species that exhibited more negative summer water potentials. Specific leaf area and minimum leaf water potential were correlated with a number of separate traits, and appear to represent key axes of trait variation that describe the water-use strategies of different HFTs along the topographic gradient. Five HFTs were classified using a resemblance analysis according to combinations of traits that pertain to different water-use strategies among species; year-round active tree, year-round active shrub, hemiparasite, drought-suppressed broad-leaved shrub and drought-suppressed narrow-leaved shrub.     

Photosynthetic pathway and ecological adaptation explain stomatal trait diversity amongst grasses

? The evolution of C(4) photosynthesis in plants has allowed the maintenance of high CO(2) assimilation rates despite lower stomatal conductances. This underpins the greater water-use efficiency in C(4) species and their tendency to occupy drier, more seasonal environments than their C(3) relatives. ? The basis of interspecific variation in maximum stomatal conductance to water (g(max) ), as defined by stomatal density and size, was investigated in a common-environment screening experiment. Stomatal traits were measured in 28 species from seven grass lineages, and comparative methods were used to test for predicted effects of C(3) and C(4) photosynthesis, annual precipitation and habitat wetness on g(max) . ? Novel results were as follows: significant phylogenetic patterns exist in g(max) and its determinants, stomatal size and stomatal density; C(4) species consistently have lower g(max) than their C(3) relatives, associated with a shift towards smaller stomata at a given density. A direct relationship between g(max) and precipitation was not supported. However, we confirmed associations between C(4) photosynthesis and lower precipitation, and showed steeper stomatal size-density relationships and higher g(max) in wetter habitats. ? The observed relationships between stomatal patterning, photosynthetic pathway and habitat provide a clear example of the interplay between anatomical traits, physiological innovation and ecological adaptation in plants.     

Species‐specific adaptations explain resilience of herbaceous understorey to increased precipitation variability in a Mediterranean oak woodland

To date, the implications of the predicted greater intra‐annual variability and extremes in precipitation on ecosystem functioning have received little attention. This study presents results on leaf‐level physiological responses of five species covering the functional groups grasses, forbs, and legumes in the understorey of a Mediterranean oak woodland, with increasing precipitation variability, without altering total annual precipitation inputs. Although extending the dry period between precipitation events from 3 to 6 weeks led to increased soil moisture deficit, overall treatment effects on photosynthetic performance were not observed in the studied species. This resilience to prolonged water stress was explained by different physiological and morphological strategies to withstand periods below the wilting point, that is, isohydric behavior in Agrostis, Rumex, and Tuberaria, leaf succulence in Rumex, and taproots in Tolpis. In addition, quick recovery upon irrigation events and species‐specific adaptations of water‐use efficiency with longer dry periods and larger precipitation events contributed to the observed resilience in productivity of the annual plant community. Although none of the species exhibited a change in cover with increasing precipitation variability, leaf physiology of the legume Ornithopus exhibited signs of sensitivity to moisture deficit, which may have implications for the agricultural practice of seeding legume‐rich mixtures in Mediterranean grassland‐type systems. This highlights the need for long‐term precipitation manipulation experiments to capture possible directional changes in species composition and seed bank development, which can subsequently affect ecosystem state and functioning.     

Comparisons of carbon isotope discrimination in populations of aridland plant species differing in lifespan

Summary Carbon isotope discrimination () was compared between populations of dominant perennial plant species, differing in life expectancy, in two deserts with contrasting vegetation types. In both deserts, plants of the shorter-lived species showed significantly higher and greater intrapopulation variance in this character compared to the long-lived species. These results indicate underlying differences in gas-exchange physiology, and suggest a positive correlation between water-use efficiency and lifespan in desert plants. Differences in variance for this character may reflect greater microenvironmental variation experienced by shorter-lived plants and/or different forms of selection acting on water-use traits. Spatial distributions were significantly clustered for the shorter-lived species and significantly uniform for the long-lived species, indicating that competition has been important in the development of the long-lived populations. The long-lived Larrea tridentata showed a significant, negative correlation between and Thiessen polygon area, suggesting a positive relationship between water-use efficiency and longevity within this species. This relationship was weakly supported in the other warm desert species, Encelia farinosa, but was not observed within populations of the cold desert species, Gutierrezia microcephala and Coleogyne ramosissima. These results suggest that reflects key aspects of plant metabolism related to lifespan; these differences may ultimately influence interactions among desert plants and the structure of desert plant communities.     

Water-use efficiency and whole-plant performance of nine tropical tree species at two sites with contrasting water availability in Panama

Across their natural distributions, tropical tree species are regularly exposed to seasonal droughts of varying intensities. Their ability to tolerate drought stress plays a vital role in determining growth and mortality rates, as well as shaping the functional composition of tropical forests. In order to assess the ability of species to acclimate to contrasting levels of drought stress, physiological and structural traits involved in drought adaptation—wood C isotope discrimination (δ¹³C), wood specific gravity, and wood C content—of 2-year-old saplings of nine tropical tree species were evaluated in common garden experiments at two study sites in Panama with contrasting seasonality. We assessed co-variation in wood traits with relative growth rates (RGR_BD), aboveground biomass, and basal diameter and the plasticity of wood traits across study sites. Overall, species responded to lower water availability by increasing intrinsic water-use efficiency, i.e., less negative wood δ¹³C, but did not exhibit a uniform, directional response for wood specific gravity or wood C content. Trait plasticity for all wood traits was independent of RGR_BD and tree size. We found that the adaptive value of intrinsic water-use efficiency varied with water availability. Intrinsic water-use efficiency increased with decreasing RGR_BD at the more seasonal site, facilitating higher survival of slower growing species. Conversely, intrinsic water-use efficiency increased with tree size at the less seasonal site, which conferred a competitive advantage to larger individuals at the cost of greater susceptibility to drought-induced mortality. Our results illustrate that acclimation to water availability has negligible impacts on tree growth over short periods, but eventually could favor slow-growing species with conservative water-use strategies in tropical regions experiencing increasingly frequent and severe droughts.     

Plant functional traits improve diversity‐based predictions of temporal stability of grassland productivity

Synthesis The temporal stability of plant production is greater in communities with high than low species richness, but stability also may depend on species abundances and growth‐related traits. Annual precipitation varied by greater than a factor of three over 11 years in central Texas, USA leading to large variation in production. Stability was greatest in communities that were not dominated by few species and in which dominant species rooted shallowly, had dense leaves, or responded to the wettest year with a minimal increase in production. Stability may depend as much on species abundances and functional traits as on species richness alone. Aboveground net primary productivity (ANPP) varies in response to temporal fluctuations in weather. Temporal stability of community ANPP may be increased by increasing plant species richness, but stability often varies at a given richness level implying a dependence on abundances and functional properties of member species. We measured stability in ANPP during 11 years in field plots (Texas, USA) in which we varied the richness and relative abundances of perennial grassland species at planting. We sought to identify species abundance patterns and functional traits linked to the acquisition and processing of essential resources that could be used to improve richness‐based predictions of community stability. We postulated that community stability would correlate with abundance‐weighted indices of traits that influence plant responses to environmental variation. Annual precipitation varied by a factor of three leading to large inter‐annual variation in ANPP. Regression functions with planted and realized richness (species with > 1% of community ANPP during the final four years) explained 32% and 25% of the variance in stability, respectively. Regression models that included richness plus the fraction of community ANPP produced by the two most abundant species in combination with abundance‐weighted values of either the fraction of sampled root biomass at 20–45 cm depth, leaf dry matter content (LDMC), or response to greater‐than‐average precipitation of plants grown in monocultures explained 58–69% (planted richness) and 58–64% (realized richness) of the variance in stability. Stability was greatest in communities that were not strongly dominated by only two species and in which plants rooted shallowly, had high values of LDMC, or responded to the wettest year with a minimal increase in ANPP. Our results indicate that the temporal stability of grassland ANPP may depend as much on species abundances and functional traits linked to plant responses to precipitation variability as on species richness alone.