Impacts of long‐term precipitation manipulation on hydraulic architecture and xylem anatomy of piñon and juniper in Southwest USA

Hydraulic architecture imposes a fundamental control on water transport, underpinning plant productivity, and survival. The extent to which hydraulic architecture of mature trees acclimates to chronic drought is poorly understood, limiting accuracy in predictions of forest responses to future droughts. We measured seasonal shoot hydraulic performance for multiple years to assess xylem acclimation in mature piñon (Pinus edulis ) and juniper (Juniperus monosperma ) after 3+ years of precipitation manipulation. Our treatments consisted of water addition (+20% ambient precipitation), partial precipitation‐exclusion (?45% ambient precipitation), and exclusion‐structure control. Supplemental watering elevated leaf water potential, sapwood‐area specific hydraulic conductivity, and leaf‐area specific hydraulic conductivity relative to precipitation exclusion. Shifts in allocation of leaf area to sapwood area enhanced differences between irrigated and droughted K _L in piñon but not juniper. Piñon and juniper achieved similar K _L under ambient conditions, but juniper matched or outperformed piñon in all physiological measurements under both increased and decreased precipitation treatments. Embolism vulnerability and xylem anatomy were unaffected by treatments in either species. Absence of significant acclimation combined with inferior performance for both hydraulic transport and safety suggests piñon has greater risk of local extirpation if aridity increases as predicted in the southwestern USA.     

Regulation and acclimation of leaf gas exchange in a piñon–juniper woodland exposed to three different precipitation regimes

Leaf gas‐exchange regulation plays a central role in the ability of trees to survive drought, but forecasting the future response of gas exchange to prolonged drought is hampered by our lack of knowledge regarding potential acclimation. To investigate whether leaf gas‐exchange rates and sensitivity to drought acclimate to precipitation regimes, we measured the seasonal variations of leaf gas exchange in a mature piñon–juniper Pinus edulis–Juniperus monosperma woodland after 3 years of precipitation manipulation. We compared trees receiving ambient precipitation with those in an irrigated treatment (+30% of ambient precipitation) and a partial rainfall exclusion (?45%). Treatments significantly affected leaf water potential, stomatal conductance and photosynthesis for both isohydric piñon and anisohydric juniper. Leaf gas exchange acclimated to the precipitation regimes in both species. Maximum gas‐exchange rates under well‐watered conditions, leaf‐specific hydraulic conductance and leaf water potential at zero photosynthetic assimilation all decreased with decreasing precipitation. Despite their distinct drought resistance and stomatal regulation strategies, both species experienced hydraulic limitation on leaf gas exchange when precipitation decreased, leading to an intraspecific trade‐off between maximum photosynthetic assimilation and resistance of photosynthesis to drought. This response will be most detrimental to the carbon balance of piñon under predicted increases in aridity in the southwestern USA.     

Woodland recovery following drought‐induced tree mortality across an environmental stress gradient

Recent droughts and increasing temperatures have resulted in extensive tree mortality across the globe. Understanding the environmental controls on tree regeneration following these drought events will allow for better predictions of how these ecosystems may shift under a warmer, drier climate. Within the widely distributed piñon–juniper woodlands of the southwestern USA, a multiyear drought in 2002–2004 resulted in extensive adult piñon mortality and shifted adult woodland composition to a juniper‐dominated, more savannah‐type ecosystem. Here, we used pre‐ (1998–2001) and 10‐year post‐ (2014) drought stand structure data of individually mapped trees at 42 sites to assess the effects of this drought on tree regeneration across a gradient of environmental stress. We found declines in piñon juvenile densities since the multiyear drought due to limited new recruitment and high (>50%) juvenile mortality. This is in contrast to juniper juvenile densities, which increased over this time period. Across the landscape, piñon recruitment was positively associated with live adult piñon densities and soil available water capacity, likely due to their respective effects on seed and water availability. Juvenile piñon survival was strongly facilitated by certain types of nurse trees and shrubs. These nurse plants also moderated the effects of environmental stress on piñon survival: Survival of interspace piñon juveniles was positively associated with soil available water capacity, whereas survival of nursed piñon juveniles was negatively associated with perennial grass cover. Thus, nurse plants had a greater facilitative effect on survival at sites with higher soil available water capacity and perennial grass cover. Notably, mean annual climatic water deficit and elevation were not associated with piñon recruitment or survival across the landscape. Our findings reveal a clear shift in successional trajectories toward a more juniper‐dominated woodland and highlight the importance of incorporating biotic interactions and soil properties into species distribution modeling approaches.     

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 (R² = 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.     

Stem radial growth and water storage responses to heat and drought vary between conifers with differing hydraulic strategies

We investigated stem radial growth and water storage dynamics of 2 conifer species differing in hydraulic carbon strategies, Juniperus monosperma and Pinus edulis, under conditions of ambient, drought (～45% reduction in precipitation), heat (～4.8 °C temperature increase), and the combination of drought + heat, in 2013 and 2014. Juniper maintained low growth across all treatments. Overall, the relatively isohydric piñon pine showed significantly greater growth and water storage recharge than the relatively anisohydric juniper across all treatments in the average climate year (2014) but no differences in the regionally dry year (2013). Piñon pine ceased growth at a constant predawn water potential across all treatments and at a less negative water potential threshold than juniper. Heat has a greater negative impact on piñon pines' growth and water storage than drought, whereas juniper was, in contrast, unaffected by heat but strongly impacted by drought. The whole‐plant hydraulic carbon strategies, in this case captured using the isohydric/anisohydric concept, translate into alternative growth and water storage strategies under drought and heat conditions.     

Hydraulic limits preceding mortality in a piñon-juniper woodland under experimental drought

Drought‐related tree mortality occurs globally and may increase in the future, but we lack sufficient mechanistic understanding to accurately predict it. Here we present the first field assessment of the physiological mechanisms leading to mortality in an ecosystem‐scale rainfall manipulation of a piñon–juniper (Pinus edulis–Juniperus monosperma) woodland. We measured transpiration (E) and modelled the transpiration rate initiating hydraulic failure (E_crit). We predicted that isohydric piñon would experience mortality after prolonged periods of severely limited gas exchange as required to avoid hydraulic failure; anisohydric juniper would also avoid hydraulic failure, but sustain gas exchange due to its greater cavitation resistance. After 1 year of treatment, 67% of droughted mature piñon died with concomitant infestation by bark beetles (Ips confusus) and bluestain fungus (Ophiostoma spp.); no mortality occurred in juniper or in control piñon. As predicted, both species avoided hydraulic failure, but safety margins from E_crit were much smaller in piñon, especially droughted piñon, which also experienced chronically low hydraulic conductance. The defining characteristic of trees that died was a 7 month period of near‐zero gas exchange, versus 2 months for surviving piñon. Hydraulic limits to gas exchange, not hydraulic failure per se, promoted drought‐related mortality in piñon pine.     

Experimental drought and heat can delay phenological development and reduce foliar and shoot growth in semiarid trees

Higher temperatures associated with climate change are anticipated to trigger an earlier start to the growing season, which could increase the terrestrial C sink strength. Greater variability in the amount and timing of precipitation is also expected with higher temperatures, bringing increased drought stress to many ecosystems. We experimentally assessed the effects of higher temperature and drought on the foliar phenology and shoot growth of mature trees of two semiarid conifer species. We exposed field‐grown trees to a ~45% reduction in precipitation with a rain‐out structure (‘drought’), a ~4.8 °C temperature increase with open‐top chambers (‘heat’), and a combination of both simultaneously (‘drought + heat’). Over the 2013 growing season, drought, heat, and drought + heat treatments reduced shoot and needle growth in piñon pine (Pinus edulis) by ≥39%, while juniper (Juniperus monosperma) had low growth and little response to these treatments. Needle emergence on primary axis branches of piñon pine was delayed in heat, drought, and drought + heat treatments by 19–57 days, while secondary axis branches were less likely to produce needles in the heat treatment, and produced no needles at all in the drought + heat treatment. Growth of shoots and needles, and the timing of needle emergence correlated inversely with xylem water tension and positively with nonstructural carbohydrate concentrations. Our findings demonstrate the potential for delayed phenological development and reduced growth with higher temperatures and drought in tree species that are vulnerable to drought and reveal potential mechanistic links to physiological stress responses. Climate change projections of an earlier and longer growing season with higher temperatures, and consequent increases in terrestrial C sink strength, may be incorrect for regions where plants will face increased drought stress with climate change.     

Prolonged experimental drought reduces plant hydraulic conductance and transpiration and increases mortality in a piñon–juniper woodland

Plant hydraulic conductance (k_s) is a critical control on whole‐plant water use and carbon uptake and, during drought, influences whether plants survive or die. To assess long‐term physiological and hydraulic responses of mature trees to water availability, we manipulated ecosystem‐scale water availability from 2007 to 2013 in a piñon pine (Pinus edulis) and juniper (Juniperus monosperma) woodland. We examined the relationship between k_s and subsequent mortality using more than 5 years of physiological observations, and the subsequent impact of reduced hydraulic function and mortality on total woody canopy transpiration (E_C) and conductance (G_C). For both species, we observed significant reductions in plant transpiration (E) and k_s under experimentally imposed drought. Conversely, supplemental water additions increased E and k_s in both species. Interestingly, both species exhibited similar declines in k_s under the imposed drought conditions, despite their differing stomatal responses and mortality patterns during drought. Reduced whole‐plant k_s also reduced carbon assimilation in both species, as leaf‐level stomatal conductance (g_s) and net photosynthesis (A_n) declined strongly with decreasing k_s. Finally, we observed that chronically low whole‐plant k_s was associated with greater canopy dieback and mortality for both piñon and juniper and that subsequent reductions in woody canopy biomass due to mortality had a significant impact on both daily and annual canopy E_C and G_C. Our data indicate that significant reductions in k_s precede drought‐related tree mortality events in this system, and the consequence is a significant reduction in canopy gas exchange and carbon fixation. Our results suggest that reductions in productivity and woody plant cover in piñon–juniper woodlands can be expected due to reduced plant hydraulic conductance and increased mortality of both piñon pine and juniper under anticipated future conditions of more frequent and persistent regional drought in the southwestern United States.     

Postfire soil water repellency in piñon–juniper woodlands: Extent,severity, and thickness relative to ecological site characteristics and climate

Wildfires can create or intensify water repellency in soil, limiting the soil's capacity to wet and retain water. The objective of this research was to quantify soil water repellency characteristics within burned piñon–juniper woodlands and relate this information to ecological site characteristics. We sampled soil water repellency across forty‐one 1,000 m² study plots within three major wildfires that burned in piñon–juniper woodlands. Water repellency was found to be extensive—present at 37% of the total points sampled—and strongly related to piñon–juniper canopy cover. Models developed for predicting SWR extent and severity had R²_adj values of 0.67 and 0.61, respectively; both models included piñon–juniper canopy cover and relative humidity the month before the fire as coefficient terms. These results are important as they suggest that postfire water repellency will increase in the coming years as infilling processes enhance piñon–juniper canopy cover. Furthermore, reductions in relative humidity brought about by a changing climate have the potential to link additively with infilling processes to increase the frequency and intensity of wildfires and produce stronger water repellency over a greater spatial extent. In working through these challenges, land managers can apply the predictive models developed in this study to prioritize fuel control and postfire restoration treatments.     

Is desiccation tolerance and avoidance reflected in xylem and phloem anatomy of two coexisting arid‐zone coniferous trees?

Plants close their stomata during drought to avoid excessive water loss, but species differ in respect to the drought severity at which stomata close. The stomatal closure point is related to xylem anatomy and vulnerability to embolism, but it also has implications for phloem transport and possibly phloem anatomy to allow sugar transport at low water potentials. Desiccation‐tolerant plants that close their stomata at severe drought should have smaller xylem conduits and/or fewer and smaller interconduit pits to reduce vulnerability to embolism but more phloem tissue and larger phloem conduits compared with plants that avoid desiccation. These anatomical differences could be expected to increase in response to long‐term reduction in precipitation. To test these hypotheses, we used tridimensional synchroton X‐ray microtomograph and light microscope imaging of combined xylem and phloem tissues of 2 coniferous species: one‐seed juniper (Juniperus monosperma) and piñon pine (Pinus edulis) subjected to precipitation manipulation treatments. These species show different xylem vulnerability to embolism, contrasting desiccation tolerance, and stomatal closure points. Our results support the hypothesis that desiccation tolerant plants require higher phloem transport capacity than desiccation avoiding plants, but this can be gained through various anatomical adaptations in addition to changing conduit or tissue size.     

Importance of juniper to birds nesting in piñon–juniper woodlands in northwest New Mexico

Piñon–juniper (Pinus spp.–Juniperus spp.) woodlands are common throughout western North America, yet relatively little is known about the habitat use and requirements for many members of its avian community. During summer 2005–2007, we assessed avian nesting substrates within piñon (Pinus edulis)–juniper (Juniperus osteosperma) woodlands in northwestern New Mexico. Of all nests in live trees, 86% were in junipers. The selection of juniper as a nest tree was significantly higher than expected from the region's piñon–juniper ratio (1:1.06) for the community as a whole, for both open cup and cavity nesting species, and for 8 species (of which 6 are piñon–juniper obligate or semi-obligate species). Nest survival, however, was not higher in juniper than in piñon for the nesting community as a whole or for chipping sparrows (Spizella passerina), the single species that was well represented nesting in piñon. The high use of juniper as a nesting substrate differs from previous studies, which have suggested that a presence of piñon is among the most important habitat features for many piñon–juniper species. Because of their importance to nesting birds, managers should avoid preferential thinning of junipers within piñon–juniper woodlands. © 2011 The Wildlife Society.     

Initial Response of Butterflies to an Overstory Reduction and Slash Mulching Treatment of a Degraded Piñon-Juniper Woodland

Overstory reduction and slash mulching (ORSM) has been shown to be an effective means for increasing herbaceous cover and diversity in degraded piñon (Pinus edulis) and juniper (Juniperus monosperma) woodlands of north‐central New Mexico. Local fire history, tree age‐class structure, and grazing records suggest that many areas now occupied by dense piñon‐juniper woodlands were formerly more open, with grassy understories that supported well‐developed soils and a fire regime. At Bandelier National Monument, studies are evaluating the use of ORSM treatments as a restoration management tool. In 1999 and 2001, we evaluated the effects of an ORSM treatment implemented in 1997 upon butterfly abundance and species richness between a pair of treated and control watersheds. Butterfly abundance and species richness were significantly greater on the treated watershed in both years, and these measures were correlated with significant increases in forb and grass cover in the treated watershed. Five of the 10 most common nectar and larval host plants had significantly greater cover in the treated watershed, including the legume Lotus wrightii. Our results suggest that the increased herbaceous cover resulting from an ORSM treatment of a single watershed induced a positive, initial response by butterflies. Using butterflies as indicators of site productivity and species richness, our results suggest ORSM is a promising technique for restoring biodiversity in degraded piñon‐juniper woodlands.     

Spatial patterns and ecological drivers of historic piñon–juniper woodland expansion in the American southwest

Apparent changes in the local occurrence and regional extent of southwestern U.S. piñon–juniper woodlands since Euro‐American settlement (i.e. historic expansion) are widely reported. These changes are commonly attributed to rapid onset of intensive and unregulated livestock grazing in western rangelands beginning ca 1850. However, other potential drivers of historic expansion including climatic warming, biological inertia, elevated CO₂, and post‐disturbance successional recovery have also been noted. Landscape patterns of woodland expansion have not been well characterized, thus the magnitude, extent, and timing of historic change remains uncertain and the relative importance of ecological drivers difficult to assess. Previous work within a monsoonal north‐central New Mexico study area demonstrated the utility of logistic modeling to address these questions. Here I expand these efforts by sampling and modeling piñon‐juniper woodlands across a climatically variable four‐state area (i.e. New Mexico, Arizona, Colorado, and Utah). Using MARS, a piece‐wise linear procedure, I model occurrence of pre‐ versus post‐settlement aged stands in relation to environmental factors, predictively map woodland expansion patterns, and evaluate grazing versus other potential drivers of historic change. Pre‐settlement woodlands were most commonly found in upland settings consistent with previous work, although this relationship was less pronounced in winter moisture areas on the Colorado Plateau. Post‐settlement stands in contrast were typically associated with more productive valley‐terrace and toe‐slope settings. However, predicted extent of expansion regionally was largely restricted to summer moisture areas within the range of one‐seed juniper. This dichotomy, where historic expansion was directional into depositional settings atypical of pre‐settlement woodland occurrence supports the view of intensive grazing as a disruptive disturbance. However, the more limited expansion predicted for winter moisture areas suggests grazing effects were likely contingent on bio‐climatic context including ecological potential of different juniper species and affected sites.     

Measuring canopy loss and climatic thresholds from an extreme drought along a fivefold precipitation gradient across Texas

Globally, trees are increasingly dying from extreme drought, a trend that is expected to increase with climate change. Loss of trees has significant ecological, biophysical, and biogeochemical consequences. In 2011, a record drought caused widespread tree mortality in Texas. Using remotely sensed imagery, we quantified canopy loss during and after the drought across the state at 30‐m spatial resolution, from the eastern pine/hardwood forests to the western shrublands, a region that includes the boundaries of many species ranges. Canopy loss observations in ~200 multitemporal fine‐scale orthophotos (1‐m) were used to train coarser Landsat imagery (30‐m) to create 30‐m binary statewide canopy loss maps. We found that canopy loss occurred across all major ecoregions of Texas, with an average loss of 9.5%. The drought had the highest impact in post oak woodlands, pinyon‐juniper shrublands and Ashe juniper woodlands. Focusing on a 100‐km by ~1,000‐km transect spanning the State's fivefold east–west precipitation gradient (~1,500 to ~300 mm), we compared spatially explicit 2011 climatic anomalies to our canopy loss maps. Much of the canopy loss occurred in areas that passed specific climatic thresholds: warm season anomalies in mean temperature (+1.6°C) and vapor pressure deficit (VPD, +0.66 kPa), annual percent deviation in precipitation (?38%), and 2011 difference between precipitation and potential evapotranspiration (?1,206 mm). Although similarly low precipitation occurred during the landmark 1950s drought, the VPD and temperature anomalies observed in 2011 were even greater. Furthermore, future climate data under the representative concentration pathway 8.5 trajectory project that average values will surpass the 2011 VPD anomaly during the 2070–2099 period and the temperature anomaly during the 2040–2099 period. Identifying vulnerable ecological systems to drought stress and climate thresholds associated with canopy loss will aid in predicting how forests will respond to a changing climate and how ecological landscapes will change in the near term.     

Multi‐factor climate change effects on insect herbivore performance

The impact of climate change on herbivorous insects can have far‐reaching consequences for ecosystem processes. However, experiments investigating the combined effects of multiple climate change drivers on herbivorous insects are scarce. We independently manipulated three climate change drivers (CO₂, warming, drought) in a Danish heathland ecosystem. The experiment was established in 2005 as a full factorial split‐plot with 6 blocks × 2 levels of CO₂ × 2 levels of warming × 2 levels of drought = 48 plots. In 2008, we exposed 432 larvae (n = 9 per plot) of the heather beetle (Lochmaea suturalis Thomson ), an important herbivore on heather, to ambient versus elevated drought, temperature, and CO₂ (plus all combinations) for 5 weeks. Larval weight and survival were highest under ambient conditions and decreased significantly with the number of climate change drivers. Weight was lowest under the drought treatment, and there was a three‐way interaction between time, CO₂, and drought. Survival was lowest when drought, warming, and elevated CO₂ were combined. Effects of climate change drivers depended on other co‐acting factors and were mediated by changes in plant secondary compounds, nitrogen, and water content. Overall, drought was the most important factor for this insect herbivore. Our study shows that weight and survival of insect herbivores may decline under future climate. The complexity of insect herbivore responses increases with the number of combined climate change drivers.     

Environmental drivers interactively affect individual tree growth across temperate European forests

Forecasting the growth of tree species to future environmental changes requires a better understanding of its determinants. Tree growth is known to respond to global‐change drivers such as climate change or atmospheric deposition, as well as to local land‐use drivers such as forest management. Yet, large geographical scale studies examining interactive growth responses to multiple global‐change drivers are relatively scarce and rarely consider management effects. Here, we assessed the interactive effects of three global‐change drivers (temperature, precipitation and nitrogen deposition) on individual tree growth of three study species (Quercus robur/petraea, Fagus sylvatica and Fraxinus excelsior). We sampled trees along spatial environmental gradients across Europe and accounted for the effects of management for Quercus. We collected increment cores from 267 trees distributed over 151 plots in 19 forest regions and characterized their neighbouring environment to take into account potentially confounding factors such as tree size, competition, soil conditions and elevation. We demonstrate that growth responds interactively to global‐change drivers, with species‐specific sensitivities to the combined factors. Simultaneously high levels of precipitation and deposition benefited Fraxinus, but negatively affected Quercus’ growth, highlighting species‐specific interactive tree growth responses to combined drivers. For Fagus, a stronger growth response to higher temperatures was found when precipitation was also higher, illustrating the potential negative effects of drought stress under warming for this species. Furthermore, we show that past forest management can modulate the effects of changing temperatures on Quercus’ growth; individuals in plots with a coppicing history showed stronger growth responses to higher temperatures. Overall, our findings highlight how tree growth can be interactively determined by global‐change drivers, and how these growth responses might be modulated by past forest management. By showing future growth changes for scenarios of environmental change, we stress the importance of considering multiple drivers, including past management and their interactions, when predicting tree growth.     

Seed and seedling ecology of piñon and juniper species in the pygmy woodlands of western North America

Knowledge of the seed and seedling ecology of the piñon and juniper woodlands of western North America is essential for understanding both the northward migration and expansion of the woodlands during the Holocene (< 11,500 B.P.), and the accelerated expansion of the woodlands since settlement of the West by Anglo-Americans around 200 years ago. We follow the fates of seeds and seedlings of the different piñon and juniper species within the woodlands from seed development to seedling establishment, and discuss the implications of this information for the past and present expansion of the woodlands. While seed development requires about two and one-half years in pinons, it is species-dependent in junipers and can take one, two, or even three years. Substantial seed losses can occur during seed development due to developmental constraints, and before or after seed maturation as a result of insects, pathogens, or predatory animals. In piñon pines, the primary seed dispersers are scatterhoarding birds (corvids) and rodents that harvest seeds from the trees or after seed fall and cache them in the soil. In contrast, most junipers appear to be dispersed primarily by frugivorous birds and mammals that ingest the seeds and defecate them onto the soil surface. We have recently documented that scatter-hoarding rodents also disperse juniper seeds. Disperser effectiveness, or the contribution a disperser makes to the future reproduction of a plant population, may vary among species of piñons and especially junipers. Piñon seeds are short-lived and exhibit little dormancy, and they probably only germinate the spring following dispersal. Juniper seeds are long-lived and seed dispersal can occur over one or more years. Seed germination can be delayed for several years due to impermeable seed coats, embryo dormancy, or the presence of inhibitors. Seedling establishment of piñon pines is facilitated by nurse plants but, while junipers often establish beneath nurse plants, they are capable of establishing in open environments. In the southwestern United States, higher establishment of juniper occurs in open environments due to more favorable precipitation, and competition may be more important than facilitation in determining establishment. When considering the mechanisms involved in the past and present expansion of the woodlands, short-distance dispersal, local population growth, and long-distance dispersal are all important. Different classes of dispersers, some of which appear to have coevolved with the tree species, appear to be responsible for local (short-distance) vs. long-distance dispersal in pinons and junipers. Because ecotones form the interface between the woodlands and adjacent communities, they can provide valuable information on both the seed dispersal and seedling establishment processes responsible for tree expansion. Disturbance regimes and, recently, the effects of humans on those regimes have major effects on the expansion and contraction of the woodlands. Before Anglo-American settlement, fires occurred as frequently as every 50–100 years throughout much of the woodlands. During this century, fire frequencies have been reduced due to the indirect effects of livestock grazing and the direct effects of removing Native Americans from the ecosystem and implementing active fire-prevention programs. The result has been an increase in tree-dominated successional stages at the expense of grass-dominated stages. Various management techniques, including controlled burning and chaining, have been implemented to reduce tree dominance, but their effects depend largely on the life histories of the tree species and the disturbance characteristics. Several areas relating to the seed and seedling ecology of the piñon and juniper require additional research if we are to truly understand the dynamics of the woodlands.     

How do trees die? A test of the hydraulic failure and carbon starvation hypotheses

Despite decades of research on plant drought tolerance, the physiological mechanisms by which trees succumb to drought are still under debate. We report results from an experiment designed to separate and test the current leading hypotheses of tree mortality. We show that piñon pine (Pinus edulis) trees can die of both hydraulic failure and carbon starvation, and that during drought, the loss of conductivity and carbohydrate reserves can also co‐occur. Hydraulic constraints on plant carbohydrate use determined survival time: turgor loss in the phloem limited access to carbohydrate reserves, but hydraulic control of respiration prolonged survival. Our data also demonstrate that hydraulic failure may be associated with loss of adequate tissue carbohydrate content required for osmoregulation, which then promotes failure to maintain hydraulic integrity.     

Do local moisture stress responses across tree species reflect dry limits of their geographic ranges?

Under future climate drought‐induced tree mortality may result in the contraction of species ranges and the reorganization of community composition where abundant and peripheral species exchange their patterns of dominance. Predicting these changes will be challenging because the future suitable habitat may be a mismatch for the current bioclimatic envelope because of discrepancies between the realized and fundamental niche. Here we evaluate the extent of the discrepancy, as applied to tree species in relation to their relative field‐recorded drought sensitivities and their observed range‐wide environmental moisture envelopes. The hypothesis tested was that different species levels of drought‐induced damage at sites where they co‐occur will be positively associated with the minimum moisture availability in the most drought‐prone part of each species current geographic range. We tested the hypothesis using drought damage measurements for 13 Australian Myrtaceae (including Eucalyptus) tree species at a site where all co‐occur, together with 120 years of climate data across their geographical ranges. With limited statistical power the results generated only modest support for the hypothesis suggesting limited capacity to predict future distributions under climate change scenarios. In spite of the poor dispersal capacities of Eucalyptus and allied genera, but consistent with knowledge of breeding systems and genetic variability within Eucalyptus, the findings also suggest that many species have a capacity for rapid adaptive response to climate change, including the vicissitudes of the late Quaternary.     

Bivalve aquaculture‐environment interactions in the context of climate change

Coastal embayments are at risk of impacts by climate change drivers such as ocean warming, sea level rise and alteration in precipitation regimes. The response of the ecosystem to these drivers is highly dependent on their magnitude of change, but also on physical characteristics such as bay morphology and river discharge, which play key roles in water residence time and hence estuarine functioning. These considerations are especially relevant for bivalve aquaculture sites, where the cultured biomass can alter ecosystem dynamics. The combination of climate change, physical and aquaculture drivers can result in synergistic/antagonistic and nonlinear processes. A spatially explicit model was constructed to explore effects of the physical environment (bay geomorphic type, freshwater inputs), climate change drivers (sea level, temperature, precipitation) and aquaculture (bivalve species, stock) on ecosystem functioning. A factorial design led to 336 scenarios (48 hydrodynamic × 7 management). Model outcomes suggest that the physical environment controls estuarine functioning given its influence on primary productivity (bottom‐up control dominated by riverine nutrients) and horizontal advection with the open ocean (dominated by bay geomorphic type). The intensity of bivalve aquaculture ultimately determines the bivalve–phytoplankton trophic interaction, which can range from a bottom‐up control triggered by ammonia excretion to a top‐down control via feeding. Results also suggest that temperature is the strongest climate change driver due to its influence on the metabolism of poikilothermic organisms (e.g. zooplankton and bivalves), which ultimately causes a concomitant increase of top‐down pressure on phytoplankton. Given the different thermal tolerance of cultured species, temperature is also critical to sort winners from losers, benefiting Crassostrea virginica over Mytilus edulis under the specific conditions tested in this numerical exercise. In general, it is predicted that bays with large rivers and high exchange with the open ocean will be more resilient under climate change when bivalve aquaculture is present.