Juniperus Monosperma Research Articles

Environmental and endogenous drivers of tree mast production and synchrony in piñon–juniper–oak woodlands of New Mexico

AbstractTree masting (synchronous, episodic reproduction) is common worldwide and is hypothesized to be especially important in ecosystems with low species diversity and productivity as in semi‐arid woodlands of the American Southwest. We analyzed mast dynamics of three dominant tree species, one‐seed juniper (Cupressaceae: Juniperus monosperma), two‐needle piñon pine (Pinaceae: Pinus edulis), and Sonoran scrub oak (Fagaceae: Quercus turbinella), on six sites during 1997–2016 within the Sevilleta National Wildlife Refuge, New Mexico. We tested multiple hypotheses regarding environmental and endogenous variables as controlling mechanisms, analyzing time‐series mast production data in relation to weather variables (precipitation, temperature, relative humidity, vapor pressure deficit, wind) over different time frames, and assessed evidence for weather tracking, resource switching, resource depletion, resource cycling, and pollen limitation. We found that (1) in univariate regression analyses, mast production in all three species was inversely related to lagged late‐summer/autumn temperatures during fruit primordia formation (1‐yr lag for juniper and oak, 2‐yr lag for piñon pine). Juniper mast production (n = 412 trees) was positively related to current‐year late‐winter (February–April) precipitation, combined with a negative relationship with current‐year summer temperatures. Piñon pine mast production (n = 210 trees) was positively related to 1‐yr lagged total annual precipitation and negatively associated with current‐year spring and summer temperatures. Oak mast production (n = 194 trees) also was positively related to current‐year late‐winter precipitation but was not affected by subsequent summer temperatures. (2) Multivariate environmental logistic regression models produced reasonable fits to observed field mast values. (3) High‐mast years in juniper and oak were characterized by greater proportions of trees producing mast and increased mast production per tree. (4) Juniper, oak, and piñon pine mast years were highly synchronized. (5) We found indirect support for resource depletion in all three species at some sites and indirect support for resource cycling in oak populations. (6) We observed only marginal indirect evidence for resource switching in piñon pine and oak. (7) Analyses of atmospheric pollen abundance relative to mast production produced no significant relationships. Predictive models of mast production have direct applications to regional silviculture, wildlife management, and ecosystem services in Southwestern woodlands.

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.

Plant Community Responses to Mastication and Mulching of One-Seed Juniper (Juniperus monosperma)

Mechanical cutting and mastication of juniper trees aims to restore grassland habitat by reducing the density of encroaching woody species. However, the associated soil disturbance may also create conduits for invasive species, a risk that must be mitigated by land managers. We characterized herbaceous communities in treated and adjacent untreated areas in a piñon-juniper (Pinus edulis and Juniper monosperma) woodland in northern Arizona 2.5 years after treatment. Untreated plots had 4× the herbaceous cover (82%) than treated plots (21%). Within treated plots, native species cover (19%) was 10× higher than invasive species cover (2%). Furthermore, treated plots exhibited greater plant community variability and diversity than untreated plots, driven by an increase in the diversity of native grasses and non-native forbs. No new recruits were Arizona listed noxious weeds, indicating that, at least in the short term, mastication is not producing invasive species hot spots in this piñon-juniper woodland.

Unsaturation of vapour pressure inside leaves of two conifer species

Stomatal conductance (gs) impacts both photosynthesis and transpiration, and is therefore fundamental to the global carbon and water cycles, food production, and ecosystem services. Mathematical models provide the primary means of analysing this important leaf gas exchange parameter. A nearly universal assumption in such models is that the vapour pressure inside leaves (ei) remains saturated under all conditions. The validity of this assumption has not been well tested, because so far ei cannot be measured directly. Here, we test this assumption using a novel technique, based on coupled measurements of leaf gas exchange and the stable isotope compositions of CO2 and water vapour passing over the leaf. We applied this technique to mature individuals of two semiarid conifer species. In both species, ei routinely dropped below saturation when leaves were exposed to moderate to high air vapour pressure deficits. Typical values of relative humidity in the intercellular air spaces were as low 0.9 in Juniperus monosperma and 0.8 in Pinus edulis. These departures of ei from saturation caused significant biases in calculations of gs and the intercellular CO2 concentration. Our results refute the longstanding assumption of saturated vapour pressure in plant leaves under all conditions.

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.

Substituting ground woody plants for cottonseed hulls in kid goat feedlot diets: growth performance and blood serum chemistry.

Boer × Spanish kid goats (n = 48) were used to evaluate effects of using ground woody products in feedlot diets on growth performance and blood serum chemistry. A completely randomized study design was used with 2 feeding periods (Period 1 = 70% concentrate, days 0 to 26; Period 2 = 86% concentrate, days 27 to 64). Goats were individually fed 1 of 6 diets that differed only by roughage source (n = 4 wether males and 4 females/treatment; initial BW = 22 ± 2 kg): cottonseed hulls (CSH; control) or ground wood consisting of redberry (RED), blueberry (BLUE), one-seed (ONE), or eastern red cedar (ERC) Juniperus spp., or Prosopis glandulosa (MESQ). Ground woody diets were individually compared with CSH. During Period 1, goats fed CSH had greater (P < 0.05) average daily DMI (DMI), ADG, and G:F than goats fed MESQ and tended to have greater (P < 0.10) ADG and G:F than goats fed BLUE. A Treatment × d interaction (P = 0.008) was observed for goat BW during Period 1 and goats fed CSH tended (P < 0.09) to have greater BW on day 27 than goats fed BLUE or MESQ. During Period 2, Treatment × d interactions were not observed (P > 0.29) for DMI, ADG, G:F, or BW and no differences were observed between goats fed CSH and goats fed any of the treatment diets. Various blood serum variables were different between CSH and goats fed diets containing woody plants (mainly during Period 1); however, blood serum profiles did not indicate hepatotoxicity or any other health issue. Collectively, results suggested that ground Juniperus pinchotii, Juniperus ashei, or Juniperus monosperma can completely replace CSH in goat feedlot diets without negatively affecting growth performance or animal health. During Period 1, feeding diets to goats that contain 30% Juniperus virginiana (ERC) or P. glandulosa (MESQ) may not be economically justifiable in most scenarios, even though goat health, assessed by blood serum profiles, was not negatively affected. However, using 14% J. virginiana (ERC) or P. glandulosa (MESQ) in finishing diets is warranted.

Historical and event-based bioclimatic suitability predicts regional forest vulnerability to compound effects of severe drought and bark beetle infestation.

Vulnerability to climate change, and particularly to climate extreme events, is expected to vary across species ranges. Thus, we need tools to standardize the variability in regional climatic legacy and extreme climate across populations and species. Extreme climate events (e.g., droughts) can erode populations close to the limits of species' climatic tolerance. Populations in climatic-core locations may also become vulnerable because they have developed a greater demand for resources (i.e., water) that cannot be enough satisfied during the periods of scarcity. These mechanisms can become exacerbated in tree populations when combined with antagonistic biotic interactions, such as insect infestation. We used climatic suitability indices derived from Species Distribution Models (SDMs) to standardize the climatic conditions experienced across Pinus edulis populations in southwestern North America, during a historical period (1972-2000) and during an extreme event (2001-2007), when the compound effect of hot drought and bark beetle infestation caused widespread die-off and mortality. Pinus edulis climatic suitability diminished dramatically during the die-off period, with remarkable variation between years. P.edulis die-off occurred mainly not just in sites that experienced lower climatic suitability during the drought but also where climatic suitability was higher during the historical period. The combined effect of historically high climatic suitability and a marked decrease in the climatic suitability during the drought best explained the range-wide mortality. Lagged effects of climatic suitability loss in previous years and co-occurrence of Juniperus monosperma also explained P.edulis die-off in particular years. Overall, the study shows that past climatic legacy, likely determining acclimation, together with competitive interactions plays a major role in responses to extreme drought. It also provides a new approach to standardize the magnitude of climatic variability across populations using SDMs, improving our capacity to predict population's or species' vulnerability to climatic change.

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 KL in piñon but not juniper. Piñon and juniper achieved similar KL 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.

Woodland resilience to regional drought: Dominant controls on tree regeneration following overstorey mortality

Abstract Drought events occurring under warmer temperatures (i.e. “hotter droughts”) have resulted in widespread tree mortality across the globe, and may result in biome‐level 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. 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. 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. 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.

Substituting ground woody plants for cottonseed hulls in lamb feedlot diets: Growth performance, blood serum chemistry, and rumen fluid parameters1

Effects of using ground woody plants in Rambouillet wether lamb feedlot diets on growth performance, blood serum, and rumen parameters were evaluated. A randomized design study was used with 2 feeding periods (70% concentrate diet from d 0 to 27 [Period 1] and 86% concentrate diet from d 28 to 57 [Period 2]); the concentrate portion of the diets mainly consisted of dried distiller's grains with solubles and sorghum grain. Lambs were individually fed 6 diets that differed only by roughage source ( = 8 animals/treatment; 32.9 ± 3.2 kg initial BW): either cottonseed hulls (CSH; control) or ground wood consisting of redberry juniper (RED), blueberry juniper (BLUE), one-seeded juniper (ONE), or eastern red cedar (ERC) spp. or honey mesquite (MESQ; ). Using ground wood vs. CSH as the roughage source did not affect ( > 0.12) BW. There tended to be a treatment × day interaction ( = 0.07) for lamb DMI, attributed to Period 1 when DMI was greater ( 0.10) among all lambs. Dietary treatments did not affect ( > 0.15) ruminal pH, but treatment × day interactions ( 0.10) were observed within day. Treatment × day interactions ( < 0.05) were observed for blood serum glucose, γ-glutamyl transferase, the albumin:globulin ratio, total bilirubin, β-hydroxybutyrate, P, Cl, and Mg, with most results being less for lambs fed the wood-based diets than for lambs fed the CSH diets. Results suggested that even though lamb DMI was reduced during the growing period when diets contained 30% ground woody products (RED, ONE, ERC, and MESQ), animal health and rumen fluid parameters were not negatively affected by ground woody plants.

Tree water dynamics in a drying and warming world.

Disentangling the relative impacts of precipitation reduction and vapour pressure deficit (VPD) on plant water dynamics and determining whether acclimation may influence these patterns in the future is an important challenge. Here, we report sap flux density (FD ), stomatal conductance (Gs ), hydraulic conductivity (KL ) and xylem anatomy in piñon pine (Pinus edulis) and juniper (Juniperus monosperma) trees subjected to five years of precipitation reduction, atmospheric warming (elevated VPD) and their combined effects. No acclimation occurred under precipitation reduction: lower Gs and FD were found for both species compared to ambient conditions. Warming reduced the sensibility of stomata to VPD for both species but resulted in the maintenance of Gs and FD to ambient levels only for piñon. For juniper, reduced soil moisture under warming negated benefits of stomatal adjustments and resulted in reduced FD , Gs and KL . Although reduced stomatal sensitivity to VPD also occurred under combined stresses, reductions in Gs , FD and KL took place to similar levels as under single stresses for both species. Our results show that stomatal conductance adjustments to high VPD could minimize but not entirely prevent additive effects of warming and drying on water use and carbon acquisition of trees in semi-arid regions.

Intestinal Lymphatic Transport: an Overlooked Pathway for Understanding Absorption of Plant Secondary Compounds in Vertebrate Herbivores.

Herbivores employ numerous strategies to reduce their exposure to toxic plant secondary chemicals (PSCs). However, the physiological mechanisms of PSC absorption have not been extensively explored. In particular, the absorption of PSCs via intestinal lymphatic absorption has been largely overlooked in herbivores, even though this pathway is well recognized for pharmaceutical uptake. Here, we investigated for the first time whether PSCs might be absorbed by lymphatic transport. We fed woodrats (Neotoma albigula) diets with increasing concentrations of terpene-rich juniper (Juniperus monosperma) either with or without a compound that blocks intestinal lymphatic absorption (Pluronic L-81). Woodrats consuming diets that contained the intestinal lymphatic absorption blocker exhibited increased food intakes and maintained higher body masses on juniper diets. Our study represents the first demonstration that PSCs may be absorbed by intestinal lymphatic absorption. This absorption pathway has numerous implications for the metabolism and distribution of PSCs in the systemic circulation, given that compounds absorbed via lymphatic transport bypass first-pass hepatic metabolism. The area of lymphatic transport of PSCs represents an understudied physiological pathway in plant-herbivore interactions.

Seasonal Shifts in Soil Moisture throughout a Semiarid Hillslope Ecotone during Drought: A Geoelectrical View

Core Ideas Resistivity imaging revealed dynamic subsurface response to drought across an ecotone. Resistivity responses restricted to rooting zones of vegetation type on each slope. Drought led to a deepening of interpreted soil moisture losses. Rewetting after drought did not immediately return to pre‐drought conditions. This study investigated the spatial distribution and seasonal variation of soil moisture during a drought year throughout a first‐order drainage basin whose hillslopes have different vegetation and soil. The north‐facing slope has juniper trees [Juniperus monosperma (Engelm.) Sarg.] and finer soil textures, while the south‐facing slope has creosote bushes [Larrea tridentata (DC.) Coville]. Time‐lapse, two‐dimensional electrical resistivity measurements, which can detect changes due to soil moisture, were conducted to capture subsurface moisture changes across the ecotone, from one hillslope to the other and along the ecotone down the valley axis. After the onset of the drought, the upper 3 m of the entire hillslope was mostly drying (resistivity was increasing). As the drought progressed and when there was less available shallow soil moisture, the resistivity profiles suggested that both juniper and creosote bushes sequestered moisture stored from increasingly greater depths, eventually reaching their respective average maximum rooting depths of ∼6 m for juniper and ∼3 m for creosote. However, this was also accompanied by decreases in resistivity (increases in soil moisture) at shallower depths, suggesting hydraulic redistribution. With shallow rewetting of the hillslope following storm events near the end of the study period, shallow areas could once again support vegetation on both hillslopes. Electrical resistivity imaging is effective for studying the spatiotemporal dynamics of soil moisture throughout the critical zone in response to water stress and can be used to inform ecohydrologic connections.

Substituting ground woody plants for cottonseed hulls in lamb feedlot diets: Growth performance, blood serum chemistry, and rumen fluid parameters.

Effects of using ground woody plants in Rambouillet wether lamb feedlot diets on growth performance, blood serum, and rumen parameters were evaluated. A randomized design study was used with 2 feeding periods (70% concentrate diet from d 0 to 27 [Period 1] and 86% concentrate diet from d 28 to 57 [Period 2]); the concentrate portion of the diets mainly consisted of dried distiller's grains with solubles and sorghum grain. Lambs were individually fed 6 diets that differed only by roughage source ( = 8 animals/treatment; 32.9 ± 3.2 kg initial BW): either cottonseed hulls (CSH; control) or ground wood consisting of redberry juniper (RED), blueberry juniper (BLUE), one-seeded juniper (ONE), or eastern red cedar (ERC) spp. or honey mesquite (MESQ; ). Using ground wood vs. CSH as the roughage source did not affect ( > 0.12) BW. There tended to be a treatment × day interaction ( = 0.07) for lamb DMI, attributed to Period 1 when DMI was greater ( < 0.05) for lambs fed CSH vs. RED, ONE, ERC, or MESQ during the first 14 d and greater ( < 0.05) during d 14 to 28 vs. lambs fed ERC. Overall, ADG was less for lambs fed ERC ( < 0.10) or MESQ ( < 0.05) vs. lambs fed CSH, but G:F was similar ( > 0.10) among all lambs. Dietary treatments did not affect ( > 0.15) ruminal pH, but treatment × day interactions ( < 0.05) were observed for rumen fluid ammonia N or molar proportions of propionate and butyrate; few differences were observed within day. However, overall, lambs fed RED or MESQ had greater ( < 0.05) total rumen VFA than lambs fed CSH. A treatment × day interaction ( = 0.04) was observed for the acetate:propionate ratio, but no differences ( > 0.10) were observed within day. Treatment × day interactions ( < 0.05) were observed for blood serum glucose, γ-glutamyl transferase, the albumin:globulin ratio, total bilirubin, β-hydroxybutyrate, P, Cl, and Mg, with most results being less for lambs fed the wood-based diets than for lambs fed the CSH diets. Results suggested that even though lamb DMI was reduced during the growing period when diets contained 30% ground woody products (RED, ONE, ERC, and MESQ), animal health and rumen fluid parameters were not negatively affected by ground woody plants.

Drought-induced piñon mortality alters the seasonal dynamics of microbial activity in piñon–juniper woodland

Piñon (Pinus edulis) – Juniper (Juniperus monosperma) (PJ) woodlands cover 17+ million hectares in the western USA. These woodlands have been particularly sensitive to recent changes in climate, with drought induced die-offs of piñon occurring across 1.5 million ha, since 2000, triggering large changes in the structure and function of these biomes. To assess the effects of this large scale mortality on soil processes, we analyzed rhizosphere extracellular enzyme activity (EEA) and fungal biomass in soil samples collected beneath tree canopies at two piñon–juniper woodland sites: one site where piñons were experimentally killed by girdling, and a paired intact site with no piñon mortality that served as a reference. We also quantified soil water content (SWC) from soil samples, and sap flow density fluxes as an indicator of tree physiological status. Soil EEA patterns varied as functions of the identity of the closest competitor trees, host-tree physiological status, and SWC. At the girdled site, we observed higher plant cell wall related decomposition activities under live juniper canopies, compared to dead piñon trees. In contrast, at the control site, we observed higher plant cell wall decomposition rates under live piñon rather than juniper canopies, particularly with higher soil moisture availability. At the site level, ordination plots for intact PJ woodlands showed a decreasing trend in microbial cell wall decomposition activity as soil water availability, and fungal biomass increased. We observed the opposite trend at the girdled site. Overall, these results suggest that widespread piñon mortality significantly affects the functional behavior of rhizosphere microorganisms at multiple scales, in part by shifting the focal substrates of microbial community decomposition.

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 19-57days, 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.

Root-associated fungal community response to drought-associated changes in vegetation community

Recent droughts in southwestern USA have led to large-scale mortality of piñon (Pinus edulis) in piñon-juniper woodlands. Piñon mortality alters soil moisture, nutrient and carbon availability, which could affect the root-associated fungal (RAF) communities and therefore the fitness of the remaining plants. We collected fine root samples at a piñon-juniper woodland and a juniper savannah site in central New Mexico. Roots were collected from piñon and juniper (Juniperus monosperma) trees whose nearest neighbors were live piñon, live juniper or dead piñon. RAF communities were analyzed by 454 pyrosequencing of the universal fungal ITS region. The most common taxa were Hypocreales and Chaetothyriales. More than 10% of ITS sequences could not be assigned taxonomy at the phylum level. Two of the unclassified OTUs significantly differed between savanna and woodland, had few like sequences in GenBank and formed new fungal clades with other unclassified RAF from arid plants, highlighting how little study has been done on the RAF of arid ecosystems. Plant host or neighbor did not affect RAF community composition. However, there was a significant difference between RAF communities from woodland vs. savanna, indicating that abiotic factors such as temperature and aridity might be more important in structuring these RAF communities than biotic factors such as plant host or neighbor identity. Ectomycorrhizal fungi (EM) were present in juniper as well as piñon in the woodland site, in contrast with previous research, but did not occur in juniper savanna, suggesting a potential shared EM network with juniper. RAF richness was lower in hosts that were neighbors of the opposite host. This may indicate competitive exclusion between fungi from different hosts. Characterizing these communities and their responses to environment and plant neighborhood is a step toward understanding the effects of drought on a biome that spans 19 000 000 ha of southwestern USA.

Responses of two semiarid conifer tree species to reduced precipitation and warming reveal new perspectives for stomatal regulation.

Relatively anisohydric species are predicted to be more predisposed to hydraulic failure than relatively isohydric species, as they operate with narrower hydraulic safety margins. We subjected co-occurring anisohydric Juniperus monosperma and isohydric Pinus edulis trees to warming, reduced precipitation, or both, and measured their gas exchange and hydraulic responses. We found that reductions in stomatal conductance and assimilation by heat and drought were more frequent during relatively moist periods, but these effects were not exacerbated in the combined heat and drought treatment. Counter to expectations, both species exhibited similar gs temporal dynamics in response to drought. Further, whereas P. edulis exhibited chronic embolism, J. monosperma showed very little embolism due to its conservative stomatal regulation and maintenance of xylem water potential above the embolism entry point. This tight stomatal control and low levels of embolism experienced by juniper refuted the notion that very low water potentials during drought are associated with loose stomatal control and with the hypothesis that anisohydric species are more prone to hydraulic failure than isohydric species. Because direct association of stomatal behaviour with embolism resistance can be misleading, we advocate consideration of stomatal behaviour relative to embolism resistance for classifying species drought response strategies.

An allometry‐based model of the survival strategies of hydraulic failure and carbon starvation

AbstractA simplified soil–plant–atmosphere–continuum model of carbon starvation and hydraulic failure is developed and tested against observations from a drought‐manipulation experiment in a woodland dominated by piñon pine (Pinus edulis) and juniper (Juniperus monosperma) in New Mexico. The number of model parameters is reduced using allometric relationships. The model can represent more isohydric (piñon) and more anisohydric (juniper) responses. Analysis of the parameter space suggests four main controls on hydraulic failure and carbon starvation: xylem vulnerability curve, root:shoot area ratio, rooting depth and water use efficiency. For piñon, an intermediate optimal (1.5–2 m2 m−2) tree leaf area index reduces the risk of hydraulic failure. For both piñons and junipers, hydraulic failure was relatively insensitive to root:shoot ratio across a range of tree LAI. Higher root:shoot ratios however strongly decreased the time to carbon starvation. The hydraulic safety margin of piñons is strongly diminished by large diurnal variations in xylem/leaf water potential. Diurnal drops of water potential are mitigated by high maximum hydraulic conductivity, high root:shoot ratio and stomatal regulation (more isohydric). The safety margin of junipers is not very sensitive to diurnal drops in water potential so that there is little benefit in stomatal regulation (more anisohydric). Narrower tracheid diameter and a narrower distribution of tracheid diameters reduce the risk of hydraulic failure and carbon starvation by reducing diurnal xylem water potential drop. Simulated tree diameter‐dependent mortality varies between these two species, with piñon mortality decreasing with increasing tree size, whereas juniper mortality increases with tree size. Juvenile piñons might thus be overimpacted by water stress. Copyright © 2015 John Wiley &amp; Sons, Ltd.

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

Plant hydraulic conductance (ks) 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 ks 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 (EC) and conductance (GC). For both species, we observed significant reductions in plant transpiration (E) and ks under experimentally imposed drought. Conversely, supplemental water additions increased E and ks in both species. Interestingly, both species exhibited similar declines in ks under the imposed drought conditions, despite their differing stomatal responses and mortality patterns during drought. Reduced whole-plant ks also reduced carbon assimilation in both species, as leaf-level stomatal conductance (gs) and net photosynthesis (An) declined strongly with decreasing ks. Finally, we observed that chronically low whole-plant ks 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 EC and GC. Our data indicate that significant reductions in ks 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.