Juniperus Monosperma Research Articles

Aspect reduces soil moisture and tree cover, but not nitrogen mineralization or grass cover, in semiarid pinyon-juniper woodlands of the Southwestern United States

Global climate change is expected to increase the aridity of semiarid landscapes by increasing heat stress and decreasing soil moisture, negatively impacting plant survival, growth, and reproduction and forcing major shifts in plant community composition. To better understand potential vegetation shifts, we investigated how aspect-mediated differences in soil moisture and nitrogen influence tree-grass and tree species coexistence in pinyon-juniper woodlands in the southwestern United States (at the Sevilleta Long Term Ecological Research Station in New Mexico). We measured soil moisture, total mineralized soil nitrogen, grass cover, and woody cover and demography of the two dominant trees—two-needle pinyon pine (Pinus edulis) and one-seeded juniper (Juniperus monosperma)—along 32 transects on north-facing and south-facing slopes. Tree cover was greater on north-facing than on south-facing slopes, reflecting significantly higher soil moisture on northern aspects. Aspect did not affect soil nitrogen or grass cover. Population structure for juniper was influenced by aspect while reproductive output was influenced by aspect in both species, suggesting that differences in drought tolerance between these codominant tree species may be responsible for differences in the utilization of resources in our study area. Our results suggest that soil moisture can affect tree cover and tree-grass coexistence more than does nitrogen. Increasing aridity due to global climate change is likely to decrease woody cover in semiarid landscapes, resulting in systems codominated by grasses and woody species.

Linking nonstructural carbohydrate dynamics to gas exchange and leaf hydraulic behavior in Pinus edulis and Juniperus monosperma.

Leaf hydraulics, gas exchange and carbon storage in Pinus edulis and Juniperus monosperma, two tree species on opposite ends of the isohydry-anisohydry spectrum, were analyzed to examine relationships between hydraulic function and carbohydrate dynamics. Leaf hydraulic vulnerability, leaf water potential (Ψl ), leaf hydraulic conductance (Kleaf ), photosynthesis (A), stomatal conductance (gs) and nonstructural carbohydrate (NSC) content were analyzed throughout the growing season. Leaf hydraulic vulnerability was significantly lower in the relatively anisohydric J. monosperma than in the more isohydric P. edulis. In P. edulis, Ψl dropped and stayed below 50% loss of leaf hydraulic conductance (P₅₀) early in the day during May, August and around midday in September, leading to sustained reductions in Kleaf . In J. monosperma, Ψl dropped below P₅₀ only during August, resulting in the maintenance of Kleaf during much of the growing season. Mean A and gs during September were significantly lower in P. edulis than in J. monosperma. Foliar total NSC was two to three times greater in J. monosperma than in P. edulis in June, August and September. Consistently lower levels of total NSC in P. edulis suggest that its isohydric strategy pushes it towards the exhaustion of carbon reserves during much of the growing season.

Carbohydrate dynamics and mortality in a piñon-juniper woodland under three future precipitation scenarios.

Drought-induced forest mortality is an increasing global problem with wide-ranging consequences, yet mortality mechanisms remain poorly understood. Depletion of non-structural carbohydrate (NSC) stores has been implicated as an important mechanism in drought-induced mortality, but experimental field tests are rare. We used an ecosystem-scale precipitation manipulation experiment to evaluate leaf and twig NSC dynamics of two co-occurring conifers that differ in patterns of stomatal regulation of water loss and recent mortality: the relatively desiccation-avoiding piñon pine (Pinus edulis) and the relatively desiccation-tolerant one-seed juniper (Juniperus monosperma). Piñon pine experienced 72% mortality after 13-25 months of experimental drought and juniper experienced 20% mortality after 32-47 months. Juniper maintained three times more NSC in the foliage than twigs, and converted NSC to glucose and fructose under drought, consistent with osmoregulation requirements to maintain higher stomatal conductance during drought than piñon. Despite these species differences, experimental drought caused decreased leaf starch content in dying trees of both species (P < 0.001). Average dry-season leaf starch content was also a good predictor of drought-survival time for both species (R(2) = 0.93). These results, along with observations of drought-induced reductions to photosynthesis and growth, support carbon limitation as an important process during mortality of these two conifer species.

Integrating ecophysiology and forest landscape models to improve projections of drought effects under climate change

Fundamental drivers of ecosystem processes such as temperature and precipitation are rapidly changing and creating novel environmental conditions. Forest landscape models (FLM) are used by managers and policy-makers to make projections of future ecosystem dynamics under alternative management or policy options, but the links between the fundamental drivers and projected responses are weak and indirect, limiting their reliability for projecting the impacts of climate change. We developed and tested a relatively mechanistic method to simulate the effects of changing precipitation on species competition within the LANDIS-II FLM. Using data from a field precipitation manipulation experiment in a piñon pine (Pinus edulis) and juniper (Juniperus monosperma) ecosystem in New Mexico (USA), we calibrated our model to measurements from ambient control plots and tested predictions under the drought and irrigation treatments against empirical measurements. The model successfully predicted behavior of physiological variables under the treatments. Discrepancies between model output and empirical data occurred when the monthly time step of the model failed to capture the short-term dynamics of the ecosystem as recorded by instantaneous field measurements. We applied the model to heuristically assess the effect of alternative climate scenarios on the piñon-juniper ecosystem and found that warmer and drier climate reduced productivity and increased the risk of drought-induced mortality, especially for piñon. We concluded that the direct links between fundamental drivers and growth rates in our model hold great promise to improve our understanding of ecosystem processes under climate change and improve management decisions because of its greater reliance on first principles.

Dynamics of leaf water relations components in co‐occurring iso‐ and anisohydric conifer species

Because iso- and anisohydric species differ in stomatal regulation of the rate and magnitude of fluctuations in shoot water potential, they may be expected to show differences in the plasticity of their shoot water relations components, but explicit comparisons of this nature have rarely been made. We subjected excised shoots of co-occurring anisohydric Juniperus monosperma and isohydric Pinus edulis to pressure-volume analysis with and without prior artificial rehydration. In J. monosperma, the shoot water potential at turgor loss (Ψ(TLP)) ranged from -3.4 MPa in artificially rehydrated shoots to -6.6 MPa in shoots with an initial Ψ of -5.5 MPa, whereas in P. edulis mean Ψ(TLP) remained at ∼ -3.0 MPa over a range of initial Ψ from -0.1 to -2.3 MPa. The shoot osmotic potential at full turgor and the bulk modulus of elasticity also declined sharply with shoot Ψ in J. monosperma, but not in P. edulis. The contrasting behaviour of J. monosperma and P. edulis reflects differences in their capacity for homeostatic regulation of turgor that may be representative of aniso- and isohydric species in general, and may also be associated with the greater capacity of J. monosperma to withstand severe drought.

Recent advances in tree hydraulics highlight the ecological significance of the hydraulic safety margin

Drought-induced cavitation resistance varies considerably between tree species and forest ecosystems (Maherali et al., 2004; Delzon et al., 2010) and is closely linked to survival under severe drought in both conifers (Brodribb & Cochard, 2009; Brodribb et al., 2010) and angiosperms (Kursar et al., 2009; Anderegg et al., 2012; Barigah et al., 2013; Urli et al., 2013). Choat et al. (2012) recently reported that most trees operate very close to their threshold of cavitation, leaving them potentially vulnerable to drought-induced mortality in a warmer/drier world (Engelbrecht, 2012). Indeed, species growing in dry environments are more resistant to droughtinduced cavitation (more negative water potential at 50% cavitation, P50) but experience a more negative minimumwater potential (Pmin) than those growing in wet environments. The so-called hydraulic safety margin, the difference between the level of water stress experienced by a species in the field (Pmin) and the level of water stress leading to hydraulic failure, is, therefore, remarkably narrow, whatever the forest species and biome considered (Choat et al., 2012). This pattern provides clues to the global droughtinduced mortality currently observed, even in very wet environments, such as tropical forests (Allen et al., 2010). Klein et al. (2014) play down the functional significance of the hydraulic safety margin in the vulnerability of forests to drought, pointing out the important role played by additional mechanisms, such as the ability of trees to repair embolism. While it is obvious that drought-induced forest dieback is a complex process involving a number of biotic and abiotic factors, we would like to draw the attention of scientists to the state of evidence for embolism repair, thereby guiding research on tree drought resistance into the most relevant and fruitful directions

Is Differential Use of Juniperus monosperma by Small Ruminants Driven by Terpenoid Concentration?

Differential plant use by herbivores has been observed for several woody plant species and has frequently been attributed to plant secondary metabolites. We examined the relationship between terpenoid concentration and Juniperus monosperma herbivory by small ruminants. Two groups of animals (10 goats or 5 goats plus 4 sheep) browsed 16 paddocks (20 × 30 m) containing one-seed juniper for six days during two seasons. Juniper leaves were sampled from 311 saplings immediately after browsing. Saplings were categorized by size (short [<0.5 m], medium [0.5-1.0 m], or tall [>1.0 m]), and by browsing intensity (light [<33 %], moderate [33-66 %], or heavy [>66 %]). Juniper bark was collected from 12 saplings during spring. Total estimated terpenoid concentrations in leaves and bark were 18.3 ± 0.3 and 8.9 ± 0.8 mg/g, respectively, and the dominant terpene in both tissues was α-pinene (11.1 ± 0.2 and 7.6 ± 0.7 mg/g, respectively). Total terpenoid concentration of juniper leaves was greater in spring than summer (20.6 ± 0.5 vs. 16.7 ± 0.3 mg/g, respectively) and was lower in short saplings than medium or tall saplings (16.5 ± 0.6 vs. 19.8 ± 0.4 and 19.5 ± 0.4 mg/g, respectively). Total terpenoid concentration of leaves also differed among the three defoliation categories (21.2 ± 0.6, 18.7 ± 0.5, and 16.1 ± 0.4 mg/g for light, moderate, and heavy, respectively). The smallest subset of terpenoids able to discriminate between light and heavy browsing intensity categories included eight compounds ([E]-β-farnesene, bornyl acetate, γ-eudesmol, endo-fenchyl acetate, γ-cadinene, α-pinene, cis-piperitol, and cis-p-menth-2-en-1-ol). Our results suggest terpenoid concentrations in one-seed juniper are related to season, sapling size, and browsing by small ruminants.

The effects of prescribed fire on soil nematodes in an arid juniper savanna

Prescribed fire produced a landscape with two types of severely burned patches: charred shrub patches and charred patches with tree trunks at the center. Soil nematodes were more abundant in burned and unburned juniper (Juniperus monosperma) tree patches than in yucca-shrub patches. There were no differences in nematode abundance between burned and unburned patches during the late spring and summer samples. Nematode abundance was significantly (p i.e., by loss of tree canopies, litter accumulation, and shrub foliage, which affects soil temperatures and water redistribution.

On the observed ecohydrologic dynamics of a semiarid basin with aspect-delimited ecosystems

[1] In semiarid complex terrain, the combination of elevation and aspect promotes variations in the water and energy balance, resulting in slopes with distinct ecologic and hydrologic properties. Quantifying the differential energy and water dynamics of opposing slopes can provide essential information on the potential effects of climate variability on landscapes. In this study, we use observations from a network of hydrologic sensors deployed on the slopes of a semiarid catchment in central New Mexico, USA, to quantify the ecohydrologic dynamics of two coexisting and contrasting ecosystems: a juniper (Juniperus monosperma) savanna on a north facing slope (NFS) and a creosote (Larrea tridentata) shrubland on a south facing slope (SFS). Our analyses show that: (1) energy loads exert a first-order control on the dynamics of evapotranspiration and soil moisture residence times in the catchment, with vegetation imposing a second-order control at the onset of the growing season; (2) soils exhibit a characteristic progression of moisture and temperature along the slope-aspect continuum that is preserved throughout the year, going from a wetter and cooler NFS to a drier and warmer SFS; (3) there are remarkable differences in the runoff dynamics among the catchment slopes, with a smaller precipitation threshold triggering larger SFS runoff amounts than at its NFS counterpart; and (4) seasonal water balances of the NFS and SFS follow opposite trajectories in the year and point to distinct soil water pools for evapotranspiration demands. The results of this study have important implications for understanding landscape changes in areas of complex topography under current and future climate variability.

Evaluating theories of drought‐induced vegetation mortality using a multimodel–experiment framework

Model-data comparisons of plant physiological processes provide an understanding of mechanisms underlying vegetation responses to climate. We simulated the physiology of a piñon pine-juniper woodland (Pinus edulis-Juniperus monosperma) that experienced mortality during a 5yr precipitation-reduction experiment, allowing a framework with which to examine our knowledge of drought-induced tree mortality. We used six models designed for scales ranging from individual plants to a global level, all containing state-of-the-art representations of the internal hydraulic and carbohydrate dynamics of woody plants. Despite the large range of model structures, tuning, and parameterization employed, all simulations predicted hydraulic failure and carbon starvation processes co-occurring in dying trees of both species, with the time spent with severe hydraulic failure and carbon starvation, rather than absolute thresholds per se, being a better predictor of impending mortality. Model and empirical data suggest that limited carbon and water exchanges at stomatal, phloem, and below-ground interfaces were associated with mortality of both species. The model-data comparison suggests that the introduction of a mechanistic process into physiology-based models provides equal or improved predictive power over traditional process-model or empirical thresholds. Both biophysical and empirical modeling approaches are useful in understanding processes, particularly when the models fail, because they reveal mechanisms that are likely to underlie mortality. We suggest that for some ecosystems, integration of mechanistic pathogen models into current vegetation models, and evaluation against observations, could result in a breakthrough capability to simulate vegetation dynamics.

The significance of atmospheric nutrient inputs and canopy interception of precipitation during ecosystem development in piñon–juniper woodlands of the southwestern USA

In arid ecosystems, widely spaced vegetation and prolonged dry periods may enhance canopy capture of nutrients from dry deposition. Additionally, differences in precipitation type, plant canopy architecture, and soil nutrient limitation could affect canopy exchange of atmospherically derived nutrients. We collected bulk precipitation and throughfall underneath piñon pine (Pinus edulis) and one-seed juniper (Juniperus monosperma) along a substrate age gradient to determine if canopy interception or throughfall chemistry differed among tree species, season, or substrate age. The Substrate Age Gradient of Arizona consists of four sites with substrate ages ranging from 1 ky to 3000 ky-old, which exhibit classic variations in soil nitrogen (N) and phosphorus (P) availability with substrate age. Greater nutrient inputs below canopies than in intercanopy areas suggest throughfall contributes to the “islands of fertility” effect. Canopy interception of precipitation did not differ between tree species, but was greater in the summer/fall than winter/spring. We found that net canopy retention of atmospherically derived N was generally greater when N availability in the soil was low, but retention also occurred when N availability was relatively high. Taken together, our results were inconclusive in determining whether the degree of soil nutrient limitation alters canopy exchange of plant growth-limiting nutrients.

Hydrologic control of the oxygen isotope ratio of ecosystem respiration in a semi-arid woodland

Abstract. We conducted high frequency measurements of the δ18O value of atmospheric CO2 from a juniper (Juniperus monosperma) woodland in New Mexico, USA, over a four-year period to investigate climatic and physiological regulation of the δ18O value of ecosystem respiration (δR). Rain pulses reset δR with the dominant water source isotope composition, followed by progressive enrichment of δR. Transpiration (ET) was significantly related to post-pulse δR enrichment because the leaf water δ18O value showed strong enrichment with increasing vapor pressure deficit that occurs following rain. Post-pulse δR enrichment was correlated with both ET and the ratio of ET to soil evaporation (ET/ES). In contrast, the soil water δ18O value was relatively stable and δR enrichment was not correlated with ES. Model simulations captured the large post-pulse δR enrichments only when the offset between xylem and leaf water δ18O value was modeled explicitly and when a gross flux model for CO2 retro-diffusion was included. Drought impacts δR through the balance between evaporative demand, which enriches δR, and low soil moisture availability, which attenuates δR enrichment through reduced ET. The net result, observed throughout all four years of our study, was a negative correlation of post-precipitation δR enrichment with increasing drought.

Reduced transpiration response to precipitation pulses precedes mortality in a piñon–juniper woodland subject to prolonged drought

Global climate change is predicted to alter the intensity and duration of droughts, but the effects of changing precipitation patterns on vegetation mortality are difficult to predict. Our objective was to determine whether prolonged drought or above-average precipitation altered the capacity to respond to the individual precipitation pulses that drive productivity and survival. We analyzed 5yr of data from a rainfall manipulation experiment in piñon-juniper (Pinus edulis-Juniperus monosperma) woodland using mixed effects models of transpiration response to event size, antecedent soil moisture, and post-event vapor pressure deficit. Replicated treatments included irrigation, drought, ambient control and infrastructure control. Mortality was highest under drought, and the reduced post-pulse transpiration in the droughted trees that died was attributable to treatment effects beyond drier antecedent conditions and reduced event size. In particular, trees that died were nearly unresponsive to antecedent shallow soil moisture, suggesting reduced shallow absorbing root area. Irrigated trees showed an enhanced response to precipitation pulses. Prolonged drought initiates a downward spiral whereby trees are increasingly unable to utilize pulsed soil moisture. Thus, the additive effects of future, more frequent droughts may increase drought-related mortality.

Drought predisposes piñon–juniper woodlands to insect attacks and mortality

To test the hypothesis that drought predisposes trees to insect attacks, we quantified the effects of water availability on insect attacks, tree resistance mechanisms, and mortality of mature piñon pine (Pinus edulis) and one-seed juniper (Juniperus monosperma) using an experimental drought study in New Mexico, USA. The study had four replicated treatments (40 × 40 m plot/replicate): removal of 45% of ambient annual precipitation (H2 O-); irrigation to produce 125% of ambient annual precipitation (H2 O+); a drought control (C) to quantify the impact of the drought infrastructure; and ambient precipitation (A). Piñon began dying 1 yr after drought initiation, with higher mortality in the H2 O- treatment relative to other treatments. Beetles (bark/twig) were present in 92% of dead trees. Resin duct density and area were more strongly affected by treatments and more strongly associated with piñon mortality than direct measurements of resin flow. For juniper, treatments had no effect on insect resistance or attacks, but needle browning was highest in the H2 O- treatment. Our results provide strong evidence that ≥ 1 yr of severe drought predisposes piñon to insect attacks and increases mortality, whereas 3 yr of the same drought causes partial canopy loss in juniper.

Ringtail (Bassariscus astutus) Ecology And Behavior In Central New Mexico, USA

ABSTRACT. I studied the ecology and behavior of ringtails (Bassariscus astutus) in juniper (Juniperus monosperma) woodland on the Sevilleta National Wildlife Refuge in central New Mexico. Average home-range sizes of ringtails (males 462 ha, females 94 ha, 95% fixed kernel) were among the largest reported to date. Females moved least in summer and most in autumn, but males showed no seasonal trend in movements. The average stay in a single den was 1.3 days. I found no effect of the closeness of approach of the den observer on the distance moved between successive dens. Ringtails used a variety of den settings: rocks, trees, shrubs, rocks and shrubs, and holes in the ground. Males used trees and rocks more and shrubs less than females. In summer and autumn, ringtails used rocks more and shrubs less than expected, and in winter ringtails used rocks less and shrubs and holes in the ground more than expected. There was no tendency to reuse certain types of dens, and 31.6% of dens were reoccupied at least once....

Effects of adding protein, condensed tannins, and polyethylene glycol to diets of sheep and goats fed one-seed juniper and low quality roughage

The biochemical mechanisms that limit voluntary intake of one-seed juniper by browsing ruminants are not well understood. Twelve Rambouillet ewes (78±2.3kgBW) and 12 Boer-Spanish does (54±1.4kgBW) were used in a split-plot sequence design to investigate the effects of adding protein, quebracho tannins (QTs), and polyethylene glycol (PEG) on one-seed juniper intake and preference and on ruminal VFA and plasma AA concentrations. Animals received sudangrass hay and isoenergetic basal diets (4 does and 4 ewes/diet) with either rumen degradable (RDP) or rumen undegradable (RUP) protein (12.5% CP) or no additional protein (control, 5% CP) during four 15-d periods. The control, RDP, and RUP diets were considered the main plot, whereas the four supplement treatments were analyzed as the split-plot effects. Period 1 allowed for adaptation to basal diets and served as a baseline phase. In periods 2–4, animals were offered juniper leaves and twigs (period 2), juniper plus QT (10% of basal diet; period 3), and juniper plus QT plus PEG (50g/animal; period 4). Juniper intake by sheep and goats was not affected (P>0.88) by RDP or RUP when animals were first exposed to juniper, but marginally increased (P<0.10) in period 4 (QT+PEG) regardless of the basal diet. Prior exposure to juniper did not affect (P=0.61) the preference ratio for juniper, but goats had higher preference ratio for juniper (P<0.01) when receiving PEG (period 4). Concentrations of total VFA tended to increase in sheep (P=0.10) and goats (P=0.14) fed protein supplements and molar proportions shifted toward acetate for goats fed RDP and RUP (P=0.07) and to butyrate for sheep fed RDP (P=0.01). Initial juniper exposure (period 2) elevated concentrations of acetate, propionate, and butyrate (P<0.01), but the effect was extinguished in periods 3 and 4 with addition of QT (P<0.05). Supplementation with PEG transiently mitigated the depressor effect of QT on acetate, propionate, and butyrate concentration at 12h post juniper feeding (P<0.01). RDP in goats and to a greater extent RUP in sheep increased plasma concentrations of various AA, especially the branched chained Val, Ile, and Leu (P<0.05). Plasma concentrations of several AA, including Met, Cyst, Glu, Gly, Gln, Asn, Thr, Ser, and Phe, decreased with ingestion of juniper (period 2) and juniper plus QT (period 3). Concentrations of some AA that were depressed by ingestion of juniper and QT were partially restored with supplemental PEG (P<0.05). Protein, terpenes, tannins, and PEG interacted to influence rumen VFA and plasma AA, which were related to intake of juniper and basal diets.

Soil C and N patterns in a semiarid piñon–juniper woodland: Topography of slope and ephemeral channels add to canopy–intercanopy heterogeneity

Carbon and nitrogen are crucial to semiarid woodlands, determining decomposition, production and redistribution of water and nutrients. Carbon and nitrogen are often greater beneath canopies than intercanopies. Upslope vs. downslope position and ephemeral channels might also cause variation in C and N. Yet, few studies have simultaneously evaluated spatial variation associated with canopy–intercanopy patches and topography. We estimated C and N upslope and downslope in an eroding piñon–juniper woodland for canopies beneath piñons ( Pinus edulis) and junipers, ( Juniperus monosperma), intercanopies, and ephemeral channels. Soil C and N in the surface and profile beneath canopies exceeded that of intercanopies and channels. Relative to intercanopies, channels had more profile C upslope but less downslope (profile N was not significant). Relative to upslope, profile C downslope for intercanopies was greater and for channels was less (profile N was not significant). Relative to profile, surface soil C and N exhibited less heterogeneity. Although some topographic heterogeneity was detected, results did not collectively support our redistribution hypotheses, and we are unable to distinguish if this heterogeneity is due to in situ or redistribution effects. Nonetheless, results highlight finer topographical spatial variation in addition to predominant canopy and intercanopy variation that is applicable for semiarid woodland management.

The effects of α‐cellulose extraction and blue‐stain fungus on retrospective studies of carbon and oxygen isotope variation in live and dead trees

Tree-ring carbon and oxygen isotope ratios from live and recently dead trees may reveal important mechanisms of tree mortality. However, wood decay in dead trees may alter the δ(13)C and δ(18)O values of whole wood obscuring the isotopic signal associated with factors leading up to and including physiological death. We examined whole sapwood and α-cellulose from live and dead specimens of ponderosa pine (Pinus ponderosa), one-seed juniper (Juniperous monosperma), piñon pine (Pinus edulis) and white fir (Abies concolor), including those with fungal growth and beetle frass in the wood, to determine if α-cellulose extraction is necessary for the accurate interpretation of isotopic compositions in the dead trees. We found that the offset between the δ(13)C or δ(18)O values of α-cellulose and whole wood was the same for both live and dead trees across a large range of inter-annual and regional climate differences. The method of α-cellulose extraction, whether Leavitt-Danzer or Standard Brendel modified for small samples, imparts significant differences in the δ(13)C (up to 0.4‰) and δ(18) O (up to 1.2‰) of α-cellulose, as reported by other studies. There was no effect of beetle frass or blue-stain fungus (Ophiostoma) on the δ(13)C and δ(18)O of whole wood or α-cellulose. The relationships between whole wood and α-cellulose δ(13)C for ponderosa, piñon and juniper yielded slopes of ~1, while the relationship between δ(18)O of whole wood and α-cellulose was less clear. We conclude that there are few analytical or sampling obstacles to retrospective studies of isotopic patterns of tree mortality in forests of the western United States.

Rare drought-induced mortality of juniper is enhanced by edaphic stressors and influenced by stand density

During 2001–2003, a severe sub-continental drought resulted in a mortality event involving multiple woody plant species. Mass-mortality (up to 65% mortality in the locality studied) of the most drought resistant tree in the region, Juniperus monosperma (Engelm)Sarq., resulted in a conspicuously heterogeneous dieback pattern. Mortality was over three times greater in grassland landscapes compared to adjacent woodlands. We investigated the relative importance of biotic stressors (abundance of grass, tree density), edaphic stressors (soil texture), and climatic stressors (heat load) in determination of mortality. Using a multivariate modeling approach we separated the correlated edaphic and climatic influences, and determined that soil texture was the primary driver. We hypothesize that hydraulic failure in juniper was influenced by small-scale variation in matrix water potential ( ψ m). Density of tree stands also exerted an apparent competitive effect in grasslands and an apparent facilitative effect in woodlands. This study offers a rare glimpse of the extreme drought response of an anisohydric tree. The characteristics of sites with high juniper mortality may allow insight into the consequences of climate change impacts, enabling prediction of the types of sites which may be affected in the future.

Climatic constraints on trait‐based forest assembly

Summary1. Climatic constraints on plant distributions are well known, but predicting community composition through knowledge of trait‐based environmental filtering remains an important empirical challenge. Here, we evaluate the maximum entropy (MaxEnt) model of trait‐based community assembly using forest communities occurring along a 12 °C gradient of mean annual temperature (MAT).We use independent cross‐validation to evaluate model predictions from sites where trait constraints are predicted from environmental conditions. We also test whether orthogonal axes of trait variation can be used as predictors to improve model parsimony and explore MaxEnt forecasts of species distributions in a warmer climate.2. Environmental factors explained between 31% and 74% of the community‐weighted mean trait values, indicating moderate‐to‐strong selection of traits along the environmental gradients. A model with 10 traits explained 54% of the variation in observed relative abundances, which approached the upper limit of 57% given the available environmental information. Three orthogonal axes accounted for 81% of the trait variation among species, and environmental factors explained between 47% and 67% of the variation in these axes. However, the axes only explained 18% of the variation in relative abundances, suggesting that minor axes of functional variation may be important or that models with many traits may achieve good predictive capacity through over‐fitting.3. Trait–environment relationships formed the basis for predicting vegetation change in a future scenario where MAT was increased by 2.5 °C. The results suggested that up to 78% of Pinus ponderosa forest in Arizona may transition to dominance by Juniperus monosperma, but this forecast likely overestimates the rates of species migration.4. Synthesis. MaxEnt is a mathematical translation of trait‐based environmental filtering of the species pool and performs moderately well in predicting forest community structure using empirical trait–environment relationships. MaxEnt required many traits to achieve good fits, and three orthogonal axes of trait variation performed poorly as predictors of community structure. To be useful predictors, traits must vary strongly among species and community‐weighted mean traits must vary predictably along environmental gradients.