Desert Shrub Species Research Articles

Nitrogen enrichment alters nutrient resorption and exacerbates phosphorus limitation in the desert shrub Artemisia ordosica.

Increasing nitrogen (N) deposition and precipitation are major drivers of global changes that are expected to influence plant nutrient resorption in desert ecosystems, where plant growth is often nutrient and water limited. However, knowledge on the effects of increased N and precipitation on them remain poorly understood. This study determined the effects of increased N (ambient, 60 kg N ha−1 year−1) and water supply (ambient, +20%, +40%), and their combination on the leaf nutrient resorption of Artemisia ordosica, a dominant shrub in the Mu Us Desert of northern China. After 2 years of treatments, only N addition significantly decreased the N resorption efficiency of A. ordosica. Both N and water addition had no effect on the phosphorus (P) resorption efficiency of this shrub, and there were no interactive effects of N and water availability on shrub nutrient resorption. The responses of shrub leaf N:P ratio tended to saturate as soil available N:P increased. The aboveground net primary productivity of A. ordosica was positively correlated with leaf P resorption efficiency, rather than N resorption efficiency. Our results suggest that N addition exacerbated the P limitation of the shrub growth and played a more fundamental role than water addition in controlling the nutrient resorption process of the desert shrub A. ordosica. This information contributes to understand the relationship between nutrient conservation strategy and plant growth of desert shrub species under global environmental changes.

Photosynthetic gas-exchange and PSII photochemical acclimation to drought in a native and non-native xerophytic species (Artemisia ordosica and Salix psammophila)

The mechanisms of plant photosynthetic acclimation to drought remains incomplete. We examined the photosynthetic performance through plant photosystem-fluorescence response to a long-lasting, anomalous summer drought (i.e., lasting for 61 days, with soil water content <0.1 m3·m−3) and subsequent post-drought recovery of a native and a non-native desert shrub species (Artemisia ordosica and Salix psammophila, respectively) growing in northwest China. Key indicators to the examination are derived estimates of plant stomatal conductance (gs), transpiration (Tr), water use efficiency (WUE), maximum and actual photochemical efficiency (Fv/Fm and ФPSII), non-photochemical quenching (NPQ), and leaf photosynthesis (PN) from simultaneous, continuous in situ measurements of gas exchange and chlorophyll fluorescence. Chlorophyll fluorescence-associated indicator “Fv/Fm” for both species was down regulated in response to deficits in soil water content (SWC), with differential SWC thresholds of 0.07 and 0.08 m3 m−3 for A. ordosica and S. psammophila, respectively. The results revealed that both species acclimate to summer drought by a stomatal-regulation mechanism of reducing gs and WUE and by a PSII NPQ mechanism of dissipating the excessive light energy, indicative of a water-conservation strategy in the acclimation to drought. In comparison with the non-native species, the native species generally had greater photosynthetic performance under water-deficit conditions demonstrating higher transpiration, net CO2 assimilation, and greater PSII photochemical efficiency. Lower WUE and greater drought resistance and resilience, combined with minor changes in PSII photochemical efficiency are consistent with a well-defined water-conservation response by A. ordosica. The results support the hypothesis that native desert-shrub species can potentially outperform non-native shrub species, due to stronger resistance and faster overall recovery from long-lasting drought. Therefore, with increased drought severity and duration anticipated with future climate change, the native shrub species is suggested to be used in desertification control and ecological restoration for sustainable ecosystem management.

Seasonal Changes in Photosynthetic Energy Utilization in a Desert Shrub (Artemisia ordosica Krasch.) during Its Different Phenophases

Our understanding of the mechanisms of plant response to environment fluctuations during plants’ phenological phases (phenophases) remains incomplete. Continuous chlorophyll fluorescence (ChlF) measurements were acquired from the field to quantify the responses in a desert shrub species (i.e., Artemesia ordosica Krasch. (A. ordosica)) to environmental factors by assessing variation in several ChlF-linked parameters and to understand plant acclimation to environmental stresses. Maximal quantum yield of PSII photochemistry (Fv/Fm) was shown to be reduced by environmental stressors and to be positively correlated to air temperature (Ta) during the early and late plant-growing stages, indicating a low temperature-induced inhibition during the leaf expansion and coloration phases. Effective quantum yield of PSII photochemistry (ΦPSII) was negatively correlated to incident photosynthetically active radiation (PAR) irrespective of phenophase, suggesting excessive radiation-induced inhibition at all phenophases. The main mechanism for acclimating to environmental stress was the regulatory thermal dissipation (ΦNPQ) and the long-term regulation of relative changes in Chl a to Chl b. The relative changes in photosynthetic energy utilization and dissipation in energy partitioning meant A. ordosica could acclimatize dynamically to environmental changes. This mechanism may enable plants in arid and semi-arid environments to acclimatize to increasingly extreme environmental conditions under future projected climate change.

Simulating canopy conductance of the Haloxylon ammodendron shrubland in an arid inland river basin of northwest China

Accurate estimates of canopy conductance (gc) are essential for quantifying the carbon, water and energy fluxes of ecosystem and understanding the patterns of water utilization of vegetation in arid regions. To this end, gc of Haloxylon ammodendron community, dominated by three desert shrub species, was calculated using the inversed Penman-Monteith equation from measurements of sap flow and concurrent microclimate over two main growing seasons (2014–2015). Then, it was used to examine the Jarvis-Stewart (JS) models which comprised different response functions and the simplified process-based model (BTA) to select the best one for our study species and climate. Photosynthetically active radiation and vapor pressure deficit typically covary throughout the day and are known have opposite effects on gc. When this effects was taken into account, both the JS model and the BTA model produced better gc fittings. Selection of proper vapor pressure deficit function and air temperature function significantly improved the performance of the JS model. The best JS model given a correlation coefficient of 0.89, RMSE of 1.99mms−1 and average percent error of 19% in comparison with the PM-calculated gc, while the best BTA model outperformed this model, reflected by higher correlation coefficient (0.90), and lower RMSE (1.93mms−1) and average percent error (9%). The average decoupling coefficient was 0.28, indicating canopies of H. ammodendron community were well coupled from the atmosphere. These findings addressed the importance of selection of stress function and consideration of air temperature for improving gc estimation in arid region, and gain new knowledge on the environmental control on canopy conductance.

Effect of salt stress on fatty acid and α-tocopherol metabolism in two desert shrub species.

Compared to Artemisia ordosiea Kraschen, a higher content of α-tocopherol in Artemisiasphaerocephala Kraschen under salt stress inhibits the conversion of linoleic acid (C18:2) into linolenic acid (C18:3), maintains cell membrane stability and contributes to higher salt resistance. Artemisia sphaerocephala Kraschen and Artemisia ordosiea Kraschen are widely distributed in the arid and semiarid desert regions of the northwest of China. Under salt stress, it has been known that α-tocopherol(α-T) improves membrane permeability and maintains Na+/K+ homeostasis; however, the function of α-T in regulating membrane components of fatty acids is unknown. In this study, 100-day-old plants of A. ordosiea and A. sphaerocephala are subjected to various NaCl treatments for 7, 14, and 21days. Compared to A. ordosiea, A. sphaerocephala has a higher Na+ concentration, higher chlorophyll content and dry weight in all NaCl treatments, but lower relative electric conductivity. The stable unsaturated levels of the lipids in A. sphaerocephala may be attributed to higher level of C18:2. Under 200mM NaCl treatment, α-T and C18:2 contents in A. sphaerocephala increase significantly, while the Na+, C18:1, C18:3 and jasmonic acid (JA) contents decrease. Moreover, α-T is positively correlated with C18:2, but negatively correlated with C18:3.

Soil moisture control of sap-flow response to biophysical factors in a desert-shrub species, &amp;lt;i&amp;gt;Artemisia ordosica&amp;lt;/i&amp;gt;

Abstract. The current understanding of acclimation processes in desert-shrub species to drought stress in dryland ecosystems is still incomplete. In this study, we measured sap flow in Artemisia ordosica and associated environmental variables throughout the growing seasons of 2013 and 2014 (May–September period of each year) to better understand the environmental controls on the temporal dynamics of sap flow. We found that the occurrence of drought in the dry year of 2013 during the leaf-expansion and leaf-expanded periods caused sap flow per leaf area (Js) to decline significantly, resulting in transpiration being 34 % lower in 2013 than in 2014. Sap flow per leaf area correlated positively with radiation (Rs), air temperature (T), and water vapor pressure deficit (VPD) when volumetric soil water content (VWC) was greater than 0.10 m3 m−3. Diurnal Js was generally ahead of Rs by as much as 6 hours. This time lag, however, decreased with increasing VWC. The relative response of Js to the environmental variables (i.e., Rs, T, and VPD) varied with VWC, Js being more strongly controlled by plant-physiological processes during periods of dryness indicated by a low decoupling coefficient and low sensitivity to the environmental variables. According to this study, soil moisture is shown to control sap-flow (and, therefore, plant-transpiration) response in Artemisia ordosica to diurnal variations in biophysical factors. This species escaped (acclimated to) water limitations by invoking a water-conservation strategy with the regulation of stomatal conductance and advancement of Js peaking time, manifesting in a hysteresis effect. The findings of this study add to the knowledge of acclimation processes in desert-shrub species under drought-associated stress. This knowledge is essential in modeling desert-shrub-ecosystem functioning under changing climatic conditions.

Landscape genetic approaches to guide native plant restoration in the Mojave Desert.

Restoring dryland ecosystems is a global challenge due to synergistic drivers of disturbance coupled with unpredictable environmental conditions. Dryland plant species have evolved complex life-history strategies to cope with fluctuating resources and climatic extremes. Although rarely quantified, local adaptation is likely widespread among these species and potentially influences restoration outcomes. The common practice of reintroducing propagules to restore dryland ecosystems, often across large spatial scales, compels evaluation of adaptive divergence within these species. Such evaluations are critical to understanding the consequences of large-scale manipulation of gene flow and to predicting success of restoration efforts. However, genetic information for species of interest can be difficult and expensive to obtain through traditional common garden experiments. Recent advances in landscape genetics offer marker-based approaches for identifying environmental drivers of adaptive genetic variability in non-model species, but tools are still needed to link these approaches with practical aspects of ecological restoration. Here, we combine spatially explicit landscape genetics models with flexible visualization tools to demonstrate how cost-effective evaluations of adaptive genetic divergence can facilitate implementation of different seed sourcing strategies in ecological restoration. We apply these methods to Amplified Fragment Length Polymorphism (AFLP) markers genotyped in two Mojave Desert shrub species of high restoration importance: the long-lived, wind-pollinated gymnosperm Ephedra nevadensis, and the short-lived, insect-pollinated angiosperm Sphaeralcea ambigua. Mean annual temperature was identified as an important driver of adaptive genetic divergence for both species. Ephedra showed stronger adaptive divergence with respect to precipitation variability, while temperature variability and precipitation averages explained a larger fraction of adaptive divergence in Sphaeralcea. We describe multivariate statistical approaches for interpolating spatial patterns of adaptive divergence while accounting for potential bias due to neutral genetic structure. Through a spatial bootstrapping procedure, we also visualize patterns in the magnitude of model uncertainty. Finally, we introduce an interactive, distance-based mapping approach that explicitly links marker-based models of adaptive divergence with local or admixture seed sourcing strategies, promoting effective native plant restoration.

Diurnal response of effective quantum yield of PSII photochemistry to irradiance as an indicator of photosynthetic acclimation to stressed environments revealed in a xerophytic species

Monitoring responses of plants to stresses in situ using the chlorophyll-fluorescence (ChlF) technique has been of growing interest, but challenged by lack of stable daytime indicators, limiting the method’s application. Seasonal changes in diurnal responses of effective quantum yield of photosystem II (PSII) photochemistry (ΦPSII) to photosynthetically active radiation (PAR) have been assumed to reflect changes in physiological status of plants in changing environments. We examined the responses of diurnal ΦPSII to PAR in relation to environmental factors through continuous season-long monitoring of ChlF of PSII in a desert shrub species, Artemisia ordosica. Response data were analysed with a modification of the diurnal regression method by Durako (2012). Diurnal variation in ΦPSII for A. ordosica was largely controlled by PAR, decreasing with increasing PAR. The rates of the response of diurnal ФPSII to PAR (slopes) were down-regulated by seasonal changes in environmental factors, in particular, high values for daily mean air temperature (Ta) and vapor pressure deficit (VPD), and up-regulated with increases in soil water content (SWC). The y-intercepts of diurnal ФPSII-PAR regressions were linearly related to maximal quantum yield of PSII photochemistry (Fv/Fm) during the growing season, increasing with increasing SWC. Our results confirm the suggestion by Durako (2012) that the y-intercept of diurnal ФPSII-PAR relationship serves as a good proxy for Fv/Fm in non-invasive monitoring of the physiological status of plants with the ChlF technique.

Individual‐based integral projection models: the role of size‐structure on extinction risk and establishment success

Summary Matrix models or integral projection models (IPMs) are commonly used to study the dynamics of structured populations, where discrete or continuous traits influence survival, growth or reproduction. When a population's size is small, as is often the case for threatened species or potentially invasive species arriving in novel habitats, extinction risk may be substantial due to demographic stochasticity. Branching processes, which are individual‐based counterparts to matrix models and IPMs, allow one to quantify these risks of extinction. For discretely structured populations, the theory of multitype branching processes provides analytic methods to compute how extinction risk changes over time and how it depends on the size and composition of the population. Building on prior work on continuous‐state branching processes, we extend these analytic methods to individual‐based models accounting for any mixture of discrete and continuous population structure. The individual‐based IPMs are defined by probabilistic update rules at the level of the individual which determine how each individual with a given trait value dies, changes trait values (e.g. grows in size) or produces individuals with the same or other trait values. We show that probabilities of extinction can be analytically determined by probability‐generating functionals associated with the individual‐based IPMs. In particular, we present analytical expressions for how extinction probabilities change over time and depend on the initial abundance and trait distribution of the population. We illustrate how to numerically implement these methods using data from the short‐lived desert shrub species Cryptantha flava and provide a more general discussion of how to implement these methods to other data sets including those involving fluctuating environmental conditions. As most IPM studies have the necessary data to parameterize individual‐based IPMs, these methods provide a computationally efficient means to explore how continuously structured populations differing in their evolutionary history and environmental context may differ in their vulnerability to extinction or ability to colonize new habitats.

Water relations and photosynthesis along an elevation gradient for Artemisia tridentata during an historic drought

Quantifying the variation in plant-water relations and photosynthesis over environmental gradients and during unique events can provide a better understanding of vegetation patterns in a future climate. We evaluated the hypotheses that photosynthesis and plant water potential would correspond to gradients in precipitation and soil moisture during a lengthy drought, and that experimental water additions would increase photosynthesis for the widespread evergreen shrub Artemisia tridentata ssp. vaseyana. We quantified abiotic conditions and physiological characteristics for control and watered plants at 2135, 2315, and 2835 m near Mammoth Lakes, CA, USA, at the ecotone of the Sierra Nevada and Great Basin ecoregions. Snowfall, total precipitation, and soil moisture increased with elevation, but air temperature and soil N content did not. Plant water potential (Ψ), stomatal conductance (g s), maximum photosynthetic rate (A max), carboxylation rate (V cmax), and electron transport rate (J max) all significantly increased with elevations. Addition of water increased Ψ, g s, J max, and A max only at the lowest elevation; g s contributed about 30 % of the constraints on photosynthesis at the lowest elevation and 23 % at the other two elevations. The physiology of this foundational shrub species was quite resilient to this 1-in-1200 year drought. However, plant water potential and photosynthesis corresponded to differences in soil moisture across the gradient. Soil re-wetting in early summer increased water potential and photosynthesis at the lowest elevation. Effects on water relations and photosynthesis of this widespread, cold desert shrub species may be disproportionate at lower elevations as drought length increases in a future climate.

A framework for partitioning plant rooting profiles from neighbours using multiple data types

Aim Vertical root distributions (‘profiles’) influence plant water use and productivity, and the differentiation of root profiles between neighbouring species can indicate the degree of plant interactions and niche partitioning. However, quantifying multiple species' root distributions in the field can be labour intensive and highly destructive to the soil and plants. We describe a method for partitioning multiple species roots using minimally destructive methods to determine if neighbour interactions alter the root profile of a common desert shrub, Larrea tridentata (creosote bush). Location Sonoran Desert, central Arizona, USA. Methods We obtained root and soil samples from soil cores collected around Larrea growing alone and next to three different neighbouring species. Bulk root mass was measured for each soil sample, and Larrea and neighbouring species root presence was determined with molecular identification methods. Water extracted from the soil and paired stem samples was analysed for its stable isotope composition (D and 18O). Species-specific (i.e. Larrea and neighbouring species) root biomass and fractional active root area were estimated through a hierarchical statistical modelling approach that combined all three data sets and accounted for detection errors. Results The combined data model successfully partitioned Larrea root biomass from neighbouring plants and provided biologically relevant estimates of rooting profiles with greater certainty than individual analyses of each data source. The data model results indicate that plant neighbours alter Larrea's root profile; Larrea growing under tree species had significantly higher root biomass in shallow soil layers than Larrea growing alone. Conclusions Our framework requires minimally destructive sampling methods, and accounts for sampling errors associated with different methods. We demonstrate the utility of our approach with a common desert shrub species, which illustrated that plant neighbours can alter the Larrea vertical root profile. Our approach is useful in problematic study systems fraught with sample collection issues or supporting species with inhibitory compounds that prohibit the use of more sophisticated molecular methods to identify the presence of other species' roots.

Functionally relevant climate variables for arid lands: a climatic water deficit approach for modelling desert shrub distributions

AbstractAimWe have three goals. (1) To develop a suite of functionally relevant climate variables for modelling vegetation distribution on arid and semi‐arid landscapes of the Great Basin, USA. (2) To compare the predictive power of vegetation distribution models based on mechanistically proximate factors (water deficit variables) and factors that are more mechanistically removed from a plant's use of water (precipitation). (3) To quantify the climate gradients that control shrub distributions in a cold desert environment.LocationThe central basin and range ecoregion of the western USA (36–42°N).MethodsWe used a modified Thornthwaite method to derive monthly water balance variables and to depict them using a water balance climograph. Eighteen variables were calculated from the climograph, representing different components of the seasonal water balance. These were used in boosted regression tree models to derive distribution models for 18 desert shrub species. The water balance approach was compared with an approach that used bioclimatic variables derived from the PRISM (Parameter‐elevation Relationships on Independent Slopes Model) climate data.ResultsThe water balance and bioclimatic approaches yielded models with similar performance in predicting the geographical distribution of most shrub species. Cumulative annual climatic water deficit was consistently the most important water balance variable for predicting shrub type distributions, although predictions were improved by the inclusion of variables that describe the seasonal distribution of water balance such as water supply in the spring, fall actual evapotranspiration, monsoonality and summer decline in actual evapotranspiration.Main conclusionsThe water balance and bioclimatic approaches to species distribution modelling both yielded similar prediction accuracies. However, the water balance approach offers advantages over the bioclimatic approach because it is mechanistically derived to approximate physical processes important for plant growth.

Leaf nitrogen is closely coupled to phenophases in a desert shrub ecosystem in China

The understanding of the seasonal dynamics of leaf nitrogen (N) relative to leaf traits for desert shrub species is limited. We investigated the seasonal changes in leaf N, soil N, specific leaf area (SLA) and photosynthetic capacity (Pmax) in relation to the phenology of Artemisia ordosica in a desert shrubland area in northern China during the growing season of year 2012. We found that the amount of leaf N per unit dry mass varied in different phenological phases, having higher values during the early leaf expansion and leaf coloration and early defoliation periods and lower values during the expanded leaf and flowering periods. Pmax was positively related to leaf N from the late leaf expansion period to the flowering and bearing period. Photosynthetic nitrogen use efficiency (NUE) correlated with SLA during the growing period. However, similar relationships were not observed for the leaf expansion and defoliation periods. During the flowering and bearing period, leaf N content per unit dry mass remained relatively stable, although fluctuations occurred due to precipitation. Our findings could help with modelling the dynamics of plant-soil N coupling of desert shrub-land ecosystems under changing environment.

Adaptive, water-conserving strategies in Hedysarum mongolicum endemic to a desert shrubland ecosystem

Adaptive mechanisms in Hedysarum mongolicum to dry and hot conditions in desert areas remain unclear. In this work, the diurnal and seasonal dynamics of sap flow in H. mongolicum, endemic to a semi-arid desert shrubland ecosystem, were studied. Stem sap flow was continuously monitored with heat balance sensors from June to October, 2012. Diurnal pattern in sap flow per stem area (J s) revealed a clear seasonal trend, with J s peaking earlier in summer (~10:00 h, 3 h before solar noon, R s), than in autumn (at roughly the same time as R s). Seasonally, J s was positively correlated to leaf area index. Excluding effects of phenology, volumetric soil water content (VWC) affected J s the greatest. In general, VWC was revealed to regulate the degree of response of J s to meteorological factors [R s, temperature T, and water vapor pressure deficit (VPD)], with sap flow being more responsive to these factors when VWC was high (>0.09 m3 m−3). Under low VWC (<0.09 m3 m−3), positive response of J s to the meteorological factors became saturated (leveled off) or declined when dry-hot conditions exceeded an hourly mean T, R s, and VPD of 24 °C, 700 W m−2, and 2.1 kPa, respectively. The results showed that H. mongolicum shrubs adapt to hot-dry or water-limited conditions in summer by closing their stomata in mid-morning and reducing the sensitivity in J s to T, R s, and VPD. The understanding of adaptive mechanisms in desert shrub species could potentially provide important baseline information towards the ecological restoration and sustainable development of arid- and semi-arid ecosystems.

Impact of intra- versus inter-annual snow depth variation on water relations and photosynthesis for two Great Basin Desert shrubs.

Snowfall provides the majority of soil water in certain ecosystems of North America. We tested the hypothesis that snow depth variation affects soil water content, which in turn drives water potential (Ψ) and photosynthesis, over 10 years for two widespread shrubs of the western USA. Stem Ψ (Ψ stem) and photosynthetic gas exchange [stomatal conductance to water vapor (g s), and CO2 assimilation (A)] were measured in mid-June each year from 2004 to 2013 for Artemisia tridentata var. vaseyana (Asteraceae) and Purshia tridentata (Rosaceae). Snow fences were used to create increased or decreased snow depth plots. Snow depth on +snow plots was about twice that of ambient plots in most years, and 20 % lower on -snow plots, consistent with several down-scaled climate model projections. Maximal soil water content at 40- and 100-cm depths was correlated with February snow depth. For both species, multivariate ANOVA (MANOVA) showed that Ψ stem, g s, and A were significantly affected by intra-annual variation in snow depth. Within years, MANOVA showed that only A was significantly affected by spatial snow depth treatments for A. tridentata, and Ψ stem was significantly affected by snow depth for P. tridentata. Results show that stem water relations and photosynthetic gas exchange for these two cold desert shrub species in mid-June were more affected by inter-annual variation in snow depth by comparison to within-year spatial variation in snow depth. The results highlight the potential importance of changes in inter-annual variation in snowfall for future shrub photosynthesis in the western Great Basin Desert.

Growth and early seedling survival of four Atacama Desert shrub species under experimental light and water availability regimes

In arid environments, where light and water supply are patchy in space and time, differences in how seedlings tolerate drought and shade conditions will determine the ability of a species to establish. Understanding seedling responses to the interactive effects of water and shade is critical to predict the future impact of increasing drought frequencies on plant communities across many arid ecosystems of the world. In this study, we used an experimental field approach to assess how different light (15%, 75%, and 95%) and water (natural rainfall (NR), NR +50 mm, NR +100 mm) levels affect seedling survival and growth of four Atacama Desert shrub species (Calliandra chilensis, Encelia canescens, Proustia cuneifolia, and Senna cumingii). We predicted that under drought conditions, maximum seedling survival and growth would occur at intermediate levels of shade. Shade and water had independent effects on seedling survival of all species. In contrast, the interaction between shade and water only affected seedling survival of E. canescens. Seedling survival of the four species tended to be higher in environments with intermediate shade and 100 mm of added water than in high or low shade environments with either 50 mm or no additional water. Relative growth rates (RGR) of all species, except E. canescens, were only positive at intermediate or high shade levels with 50 or 100 mm of added water. Our results not only highlight the importance of water for plant recruitment in arid ecosystems but also stress the positive role of shade as a factor influencing seedling establishment.

Diversity of phenolic compounds and plant traits in coexisting Patagonian desert shrub species of Argentina

Desert shrubs often accumulate different types of phenolic compounds but what determines the amount and diversity of these compounds is an issue scarcely explored. The aim of this study was to assess differences in the amount and diversity of phenolic compounds in leaves among coexisting shrub species differing in rooting depth and leaf turnover. We hypothesized that the diversity and amount of phenolic compounds in leaves of desert shrubs are related to access to soil water through rooting depth, and to leaf turnover. The study was carried out in the Patagonian Monte of Argentina. We collected green leaves of six species representing the dominant shrub morphotypes (tall evergreen, tall deciduous, and medium evergreen shrubs) and assessed lignin concentration and groups of soluble phenols obtained by sequential extraction with ethyl ether, ethyl acetate, and amyl alcohol. We also assessed nitrogen concentration in leaves and leaf mass per unit area (LMA) as traits related to leaf lifespan. The diversity of phenolic compounds was higher in green leaves of tall shrubs with deep rooting depth than in those of medium evergreen shrubs with shallow rooting depth. Diversity of phenolic compounds in green leaves was negatively related to lignin concentration. Evergreen shrubs had higher amount of phenolic compounds in green leaves than deciduous ones and the total amount of phenolic compounds in green leaves was positively related to LMA. We concluded that access to soil water sources and leaf turnover were related to the amount and diversity of phenolic compounds in green leaves of desert shrub species and these results are consistent with those predicted by the resource availability theory for plants from resource-rich and resource-poor habitats.

Intraspecific variation of a desert shrub species in phenotypic plasticity in response to sand burial

Shoot elongation is one of the main plastic responses of plants to burial, a ubiquitous stress factor in dry ecosystems. Yet, intraspecific variation in this response to burial and the extent to which this variation is functionally coordinated with variation in other trait responses are largely unknown. We subjected seedlings of the shrub Caragana intermedia from 18 maternal parents (i.e. different half-sib families) to repeated partial burial to investigate how burial affects shoot growth, stem mechanical traits and associated plasticity. Burial increased both stem elongation and diameter growth of plants, but decreased biomass production. Half-sib families had different rates of shoot elongation, and differed in their response to burial with respect to biomechanical stem properties. Across half-sib families, the magnitude of these responses in mechanical traits was positively correlated with the magnitude of the stem elongation response. These results indicate that plasticity in different stem traits in response to sand burial and intraspecific variation therein are functionally coordinated with respect to mechanical stability. The results emphasize the importance of considering functionally coordinated traits when analyzing phenotypic plasticity in plants.

Estimation of root cohesion for desert shrub species in the Lower Colorado riparian ecosystem and its potential for streambank stabilization

Quantifying mechanical properties of native vegetation for streambank stability is a critical need of the Lower Colorado riparian revegetation effort. In the present study we estimated root cohesion for four representative native desert shrub species: Artiplex lentiformis (Torr.) S. Watson, Lycium andersonii A. Gray, Larrea tridentata (DC.) Coville, and Allenrolfea occidentalis (S. Watson) Kuntze to understand their suitability in streambank stabilization in the framework of a revegetation campaign. Field experiments were conducted to measure root length, root length density, root area ratio, and root tensile strength. Finally, the root cohesion values were assessed using a simple perpendicular model.Root area ratio estimates showed that on average plant roots occupy 0.46% of the area under the crown of the selected species. The root tensile strength (Tr) was greatest for L. tridentata (62.23MPa) followed by L. andersonii (53.53MPa), A. lentiformis (49.17MPa), and A. occidentalis (35.03MPa). The root cohesion values could be used to rank the species according to their potential for shallow bank slope stabilization in riparian ecosystems of a desert environment. The maximum root cohesion in the present study was estimated for A. lentiformis (97.6kPa) followed by L. andersonii (89.3kPa), L. tridentata (35.6kPa), and A. occidentalis (34.8kPa). Root cohesion values were also estimated using Fiber bundle model (FBM) and compared to the perpendicular root model of Wu et al. (1979). The comparative root cohesion values for root diameter (>0.5mm) suggest that Wu's model estimates are greater than those of the FBM by a reduction factor ranges between 0.35 and 0.56 for our studied species.

No evidence of a trade‐off between drought and shade tolerance in seedlings of six coastal desert shrub species in north‐central Chile

AbstractQuestionIs there evidence of a trade‐off between drought and shade tolerance in coastal desert shrubs?LocationArid scrubland, Atacama desert, north‐central Chile.MethodsOne‐week‐old seedlings of six coastal desert shrub species (Centaurea chilensis,Encelia canescens,Flourensia thurifera,Pleocarphus revolutus,Senna cumingiiandHaplopappus parvifolius) were planted in the field under six treatment combinations: three light environments (low, intermediate and high radiation) and two water levels (natural rainfall (55.4 mm) and natural rainfall plus 100 mm irrigation). We quantified seedling survival for each species during 22 weeks, estimated relative growth rate, and examined how light and water affect whole‐plant responses, particularly specific leaf area, leaf mass ratio and root–shoot ratio.ResultsWe found species‐specific differences in the temporal pattern of mortality. Water and/or light levels affected seedling survival of all species, excludingC. chilensis. Relative growth rate (RGR) increased in low‐light conditions inC. chilensisandP. revolutus, but otherwise did not vary in response to differences in either light or water, independently or to their interaction. Across species, the effect of water on specific leaf area (SLA) was inconsistent, increasing both in drought conditions (C. chilensis) and in treatments with supplemental water (S. cumingii). Additionally,SLAtended to increase with decreasing light levels for most species (F. thurifera,H. parvifolius,C. chilensis). In our study, onlyF. thruriferaandC. chilensisshowed changes in leaf mass ratio (LMR) and only with respect to light levels; specifically,LMRtended to increase with decreasing light level. Biomass allocation was independent of light and water for all species exceptF. thurifera, which showed an increase in root biomass in drought conditions.ConclusionsOverall, we did not find evidence to support a trade‐off between drought and shade tolerance during early plant establishment. Our results suggest that water is the most important resource that limits recruitment in the coastal desert. Therefore, increased aridity in this system is likely to severely hinder seedling recruitment of the native coastal desert shrubs.