Explore the vast range of titles available.

The persistence of vegetation under climate change will depend on a plant’s capacity to exploit water resources. We analyzed water source dynamics in piñon pine and juniper trees subjected to precipitation reduction, atmospheric warming, and to both simultaneously. Piñon and juniper exhibited different and opposite shifts in water uptake depth in response to experimental stress and background climate over 3 yr. During a dry summer, juniper responded to warming with a shift to shallow water sources, whereas piñon pine responded to precipitation reduction with a shift to deeper sources in autumn. In normal and wet summers, both species responded to precipitation reduction, but juniper increased deep water uptake and piñon increased shallow water uptake. Shifts in the utilization of water sources were associated with reduced stomatal conductance and photosynthesis, suggesting that belowground compensation in response to warming and water reduction did not alleviate stress impacts for gas exchange. We have demonstrated that predicted climate change could modify water sources of trees. Warming impairs juniper uptake of deep sources during extended dry periods. Precipitation reduction alters the uptake of shallow sources following extended droughts for piñon. Shifts in water sources may not compensate for climate change impacts on tree physiology.

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 5 yr 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.

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 (H2O−); irrigation to produce 125% of ambient annual precipitation (H2O+); 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 H2O− 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 H2O− 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.

Stomatal Closure Point (SCP) has commonly been used to describe drought response strategies in plants, with isohydric species maintaining relatively high, constant SCP compared to anisohydric species that can lower SCP with increasing drought severity. However, there is evidence that, within these groups, SCP may respond dynamically to environmental conditions. Here, we explored how increasing water availability affects SCP in classically isohydric piñon pine and anisohydric one-seed or Utah juniper at various spatial- (i.e., from branch, to tree, to ecosystem) and temporal- (i.e., hours to decades) scales. Our results show that short-term increases in water availability decreased SCP in isohydric piñon pine, making it more anisohydric, while short-term rehydration had no effect on SCP in anisohydric juniper. Increasing mean annual precipitation, on the other hand, increased SCP in both species. Our findings are consistent with documented differences in the use of ABA to control stomata in iso- and aniso-hydric species on short timescales, and with structural acclimation in both species at long timescales. These results illustrate that the local environment plays a large role in determining SCP.

Global climate change is projected to produce warmer, longer, and more frequent droughts, referred to here as \"global change-type droughts\", which have the potential to trigger widespread tree die-off. However, drought-induced tree mortality cannot be predicted with confidence, because long-term field observations of plant water stress prior to, and culminating in, mortality are rare, precluding the development and testing of mechanisms. Here, we document plant water stress in two widely distributed, co-occurring species, piñon pine (Pinus edulis) and juniper (Juniperus monosperma), over more than a decade, leading up to regional-scale die-off of piñon pine trees in response to global change-related drought. Piñon leaf water potentials remained substantially below their zero carbon assimilation point for at least 10 months prior to dying, in contrast to those of juniper, which rarely dropped below their zero-assimilation point. These data suggest that piñon mortality was driven by protracted water stress, leading to carbon starvation and associated increases in susceptibility to other disturbances (eg bark beetles), a finding that should help to improve predictions of mortality during drought.

1. Drought events occurring under warmer temperatures (i.e. \"hotter droughts\") have resulted in widespread tree mortality across the globe, and may result in biomelevel vegetation shifts to alternate vegetation types if there is a failure of trees to regenerate.We investigated how overstorey trees, understorey vegetation, and local climatic and edaphic conditions interact to influence tree regeneration, a key prerequisite for resilience, in a region that has experienced severe overstorey tree mortality due to hotter droughts and beetle infestations. 2. We used detailed field observations from 142 sites that spanned a broad range of environmental conditions to evaluate the effects of climate and recent tree mortality on tree regeneration dynamics in the spatially extensive piñon (Pinus edulis)- juniper (Juniperus osteosperma, Juniperus monosperma) woodland vegetation type of the southwestern USA. We used a structural equation modelling framework to identify how tree mortality and local climatic and edaphic conditions affect piñon and juniper regeneration and electivity analyses to quantify the species-specific associations of tree juveniles with overstorey trees and understorey shrubs. 3. Piñon regeneration appears to be strongly dependent upon advanced regeneration, (i.e. the survival of juvenile trees that established prior to the mortality event), the survival of adult seed-bearing trees (inferred from basal area of surviving trees) and the facilitative effects of overstorey trees for providing favourable microsites for seedling establishment. Model results suggest that local edaphoclimatic conditions directly affected piñon and juniper regeneration, such that stands with hotter, drier local climatic conditions and lower soil available water capacity had limited tree regeneration following large-scale dieback. 4. Synthesis. We identify four indicators of resilience to hotter drought conditions: (1) abundant advance regeneration of tree seedlings; (2) sufficient canopy cover for survival of emergent seedlings and existing regeneration; (3) sufficient seed source from surviving trees with high reproductive output; (4) areas with cooler and wetter local climates and greater soil available water capacity. In the absence of these conditions, there is greater likelihood of woodlands transitioning to more xeric vegetation types following dieback.

Extreme drought conditions accompanied by rising temperatures have characterized the American Southwest during the past decade, causing widespread tree mortality in piñon-juniper woodlands. Piñon pine (Pinus edulis Engelm.) mortality is linked primarily to outbreaks of the pinyon ips (Ips confusus (Leconte)) precipitated by drought conditions. Although we searched extensively, no biotic agent was identified as responsible for death in Juniperus L. spp. in this study; hence this mortality was due to direct drought stress. Here we examine the relationship between tree abundance and patterns of mortality in three size classes (seedling/sapling, pre-reproductive, reproductive) during the recent extended drought in three regions: southwest Colorado, northern New Mexico, and northern Arizona. Piñon mortality varied from 32% to 65%, and juniper mortality from 3% to 10% across the three sites. In all sites, the greatest piñon mortality was in the larger, presumably older, trees. Using logistic regression models, we examined the influence of tree density and basal area on bark beetle infestations (piñon) and direct drought impacts (juniper). In contrast to research carried out early in the drought cycle by other researchers in Arizona, we did not find evidence for greater mortality of piñon and juniper trees in increasingly high density or basal area conditions. We conclude that the severity of this regional drought has masked densitydependent patterns visible in less severe drought conditions. With climate projections for the American Southwest suggesting increases in aridity and rising temperatures, it is critical that we expand our understanding of stress responses expected in widespread piñon-juniper woodlands.

Global changes in temperature and aridity are increasing the frequency of extreme drought events. Such changes can have pronounced impacts on dryland ecosystems, which exist at the margins of plant physiological tolerances. Pinyon–juniper (PJ) woodlands—a dryland vegetation type spanning 40 million ha in western North America—are a model system for the impacts of drought, where recurrent short‐interval drought events may trigger feedback mechanisms that influence future drought resistance. Leveraging a long‐term monitoring network in PJ woodlands of the United States (US) Southwest, we sought to understand how interactions between recurrent drought events influence tree mortality risk. We developed generalized linear mixed models to predict patterns of recent (i.e., 2014–2023) tree mortality based on biophysical variables, tree size, and prior drought‐driven changes (ca. 1998–2014) in forest conditions. We then used these models to quantify how mortality risk has shifted over time. Tree density and stand basal area declined substantially throughout our 1998–2023 monitoring period. Since 2014, tree mortality was more common and spatially extensive than new tree recruitment, and nearly half of the surviving trees experienced crown dieback. Tree size influenced biotic interactions and responses to environmental conditions, and soil organic matter and mycorrhizal fungi communities buffered individuals against drought. Shifts in woodland demographics (e.g., reduced stand densities, crown dieback) led to a 28.2% increase in mortality risk between 2014 and 2023 for trees that survived this period, a pattern that was consistent across species. Recent drought events have triggered widespread tree mortality and dieback in PJ woodlands of the US Southwest. These events also increase future tree mortality risk, overcoming system inertia created by local edaphic conditions and compensatory responses. Global changes in temperature and aridity can have pronounced impacts on dryland ecosystems, which exist at the margins of plant physiological tolerances. We investigated the drivers of recent tree mortality in pinyon–juniper (PJ) woodlands—a dryland vegetation type spanning 40 million ha in western North America. Shifts in woodland demographics (e.g., reduced stand densities, crown dieback) led to a 28.2% relative increase in mortality risk between 2014 and 2023, overcoming system inertia created by local edaphic conditions and compensatory responses.

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, Ψₗdropped 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, Ψₗdropped below P₅₀only during August, resulting in the maintenance of Kₗₑₐfduring much of the growing season. Mean A and gₛduring 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.

Aims (1) To develop a 3D root distribution model for piñon-juniper woodland using only tree species, sizes and locations as input. (2) To interpret a two-year time series of soil moisture relative to root distributions. Methods The study was conducted in a piñon ( Pinus edulis (Englem.)) -juniper ( Juniperus monosperma (Englem.) Sarg.) woodland in New Mexico. We extracted roots from 720 soil blocks (30 cm × 10 cm × 10 cm) cut from the walls of three 10-m long and 1.5-m deep trenches. Roots were sorted by species and diameter class. Distribution models were developed for the dry weight of roots ≤5 mm in diameter. Soil water content and water potentials were measured in soil profiles under tree cluster and canopy gaps for 2 years, including a protracted dry-down period. Results Piñon had twice the root dry mass of juniper, similar to the ratio of canopy projection areas. Root densities were ca. 50% lower in soils under canopy gaps compared to tree clusters and the species ratio did not significantly differ between clusters and gaps. Piñon root density declined faster with soil depth and distance from the stem compared to juniper. A hard caliche layer at 60–80 cm soil depth had no apparent effect on the already low root density at that depth. Overall, the models explained 66% (piñon) and 54% (juniper) of the spatial variation in root density at the scale of sampled soil blocks. During an 8-month dry period, soil moisture declined faster in regions of higher root density: in shallow soil and under tree clusters. This left a reserve of plant-available soil water in the deep soil under canopy gaps. Conclusions Horizontal variation in root density in these open woodlands is predictable and an important component of the dynamic interactions between plant and soil. Under wet and dry conditions, soil water content is substantially different under tree clusters and canopy gaps.