New Phytologist Special Feature: Drought-induced Forest Mortality

Special Feature Articles: Editorial Improving our knowledge of drought-induced forest mortality through

experiments, observations, and modelling Nate G. McDowell, Michael G. Ryan, Melanie J. B. Zeppel, David T. Tissue

Commentary

Thirsty roots and hungry leaves: unravelling the roles of carbon and water dynamics in tree mortality

Anthony P. O’Grady, Patrick J. M. Mitchell, Elizabeth A. Pinkard, David T. Tissue

Our limited ability to predict vegetation dynamics under water stress

Chonggang Xu, Nate G. McDowell, Sanna Sevanto and Rosie A. Fisher

Climate-driven tree mortality: insights from the piñon pine die-off in the United States

Jeffrey A. Hicke, Melanie J. B. Zeppel

Tansley Reviews Evaluating theories of drought-induced vegetation mortality using a

multimodel–experiment framework Nate G. McDowell, Rosie A. Fisher, Chonggang Xu, J. C. Domec, Teemu

Hölttä, D. Scott Mackay, John S. Sperry, Amanda Boutz, Lee Dickman, Nathan Gehres, Jean Marc Limousin, Alison Macalady, Jordi Martínez-

Vilalta, Maurizio Mencuccini, Jennifer A. Plaut, Jérôme Ogée, Robert E. Pangle, Daniel P. Rasse, Michael G. Ryan, Sanna Sevanto, Richard H.

Waring, A. Park Williams, Enrico A. Yepez, William T. Pockman

Rapid Reports Shoot desiccation and hydraulic failure in temperate woody

angiosperms during an extreme summer drought Andrea Nardini, Marta Battistuzzo and Tadeja Savi

Full Papers

High temperature causes negative whole-plant carbon balance under mild drought

Junbin Zhao, Henrik Hartmann, Susan Trumbore, Waldemar Ziegler, Yiping Zhang

Thirst beats hunger – declining hydration during drought prevents

carbon starvation in Norway spruce saplings Henrik Hartmann, Waldemar Ziegler, Olaf Kolle, Susan Trumbore

Confronting model predictions of carbon fluxes with measurements of

Amazon forests subjected to experimental drought Thomas L. Powell, David R. Galbraith, Bradley O. Christoffersen, Anna Harper,

Hewlley M. A. Imbuzeiro, Lucy Rowland, Samuel Almeida, Paulo M. Brando, Antonio Carlos Lola da Costa, Marcos Heil Costa, Naomi M. Levine, Yadvinder

Malhi, Scott R. Saleska, Eleneide Sotta, Mathew Williams, Patrick Meir, Paul R. Moorcroft

Increased vapor pressure deficit due to higher temperature leads to greater

transpiration and faster mortality during drought for tree seedlings common to the forest–grassland ecotone

Rodney E. Will, Stuart M. Wilson, Chris B. Zou, Thomas C. Hennessey

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

Jennifer A. Plaut, W. Duncan Wadsworth, Robert Pangle, Enrico A. Yepez, Nate G. McDowell, William T. Pockman

Drought-induced defoliation and long periods of near-zero gas exchange

play a key role in accentuating metabolic decline of Scots pine Rafael Poyatos, David Aguadé, Lucía Galiano, Maurizio Mencuccini, Jordi

Martínez-Vilalta

Tree regeneration following drought- and insect-induced mortality in piñon–juniper woodlands

Miranda D. Redmond, Nichole N. Barger

Precipitation thresholds and drought-induced tree die-off: insights from patterns of Pinus edulis mortality along an environmental stress gradient

Michael J. Clifford, Patrick D. Royer, Neil S. Cobb, David D. Breshears, Paulette L. Ford

Mortality and community changes drive sudden oak death impacts on

litterfall and soil nitrogen cycling Richard C. Cobb, Valerie T. Eviner, David M. Rizzo

Cover image: Dying Pinus edulis and Juniperus monosperma trees at a seven year, 47% precipitation reduction experiment in central New Mexico, USA. Courtesy of Michael G. Ryan.

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Introduction

Research on drought-induced forest mortality has become more prominent in recent years. The ever-growing desire to unravel the causes and consequences of drought mortality is driven in part by predicted increases in the frequency and intensity of droughts associated with climate change, and consistent observations suggesting we are already witnessing global mortality at a scale unprecedented in recorded history. Drought-induced forest mortality is an issue that has piqued the attention of a wide range of researchers, including ecologists, ecohydrologists, plant ecophysiologists and landscape ecologists, all of whom endeavour to examine drought-induced forest mortality across a broad range of ecosystems and through a diverse range of approaches. Some of the most recent advances in this research are presented in this Feature Issue of New Phytologist. The breadth of this topic is evident in the studies included in this collection, which features a wide range of geographies– Europe, North America, Amazonia – and covers various ecosystem types. This Special Feature Issue has its origins in a series of presentations held at the Ecological Society of America conference in August 2012, which included contributions from theorists, modelers, and field scientists, and again, this diversity is reflected in the collection. The goal of this collection is to identify the general features that lead to drought-induced mortality in a wide variety of plant species and ecosystems across the world, leading to a rapid advancement of our understanding of widespread plant mortality.

Jarrah (Eucalyptus marginata) with drought top kill and adjacent unaffected marri

(Corymbia calophylla). Photograph courtesy of Michael G. Ryan.

http://onlinelibrary.wiley.com/doi/10.1111/nph.12502/full

Anthony P. O’Grady, Patrick J. M. Mitchell, Elizabeth A. Pinkard, David T. Tissue

New Phytologist (2013) doi: 10.1111/nph.12451

Summary

Research into the mechanisms driving drought-related plant mortality has seen a focussed effort in recent years. Drought and water availability are pervasive factors influencing the distribution of forests and woodlands globally, particularly in water-limited environments, where evaporation exceeds rainfall and is therefore a major constraint on productivity. However, the lack of a predictive framework for forest mortality remains an important knowledge gap in ecosystem and biogeochemical models (Roxburgh et al., 2004). The paper by Hartmann et al. (pp. 340–349) in the Feature on drought-related mortality in this issue of New Phytologist provides some novel insights into the relative contributions of hydraulic failure and carbon (C) starvation in trees. In conjunction with recent activity focussed on unravelling the relative importance of the putative mechanisms of plant mortality, our goal should be to improve our understanding of C, water and nutrient dynamics in ecosystems and improve our capacity to predict conditions under which mortality is likely to occur.

Key words: carbon starvation, ecosystem resilience, hydraulic

failure, low CO2, mortality

Commentary Thirsty roots and hungry leaves: unravelling the roles of carbon and water dynamics in tree mortality

Author for correspondence: Anthony P. O’Grady

Tel: +61 3 62375658 Email: [email protected]

http://onlinelibrary.wiley.com/doi/10.1111/nph.12451/abstract

Chonggang Xu, Nate G. McDowell, Sanna Sevanto and Rosie A. Fisher

Author for correspondence: Chonggang Xu

Tel: +1 505 665 9773 Email: [email protected]


Summary

Recently, drought-induced changes in vegetation have received increasing attention (Allen et al., 2010) and models have been revised to specifically simulate vegetation responses to drought (e.g. Fisher et al., 2010; Domec et al., 2012). However, few rigorous tests have been conducted to evaluate how well vegetation models simulate drought-caused vegetation responses (Galbraith et al., 2010). In this issue of New Phytologist, Powell et al. (pp. 350–365) conducted a critical data-model comparison study to assess model simulations of vegetation and ecosystem responses to drought manipulation experiments in the Amazon. This unique study revealed key limitations of five state-of-the-art biosphere models and one hydrodynamic terrestrial ecosystem model. They found that the biosphere models accurately captured the carbon (C) fluxes in control plots, but poorly simulated C and water fluxes, seasonal leaf-area indices, and vegetation mortality under imposed drought. This result is significant because it clearly points the way towards specific model developments and field experiments needed to better predict terrestrial ecosystem responses to drought.

Key words: carbon fluxes, data-model comparison, drought,

ecosystem response to water stress, net primary production (NPP),

photosynthesis, respiration, vegetation models

Commentary Our limited ability to predict vegetation dynamics under water stress


Jeffrey A. Hicke, Melanie J. B. Zeppel

Author for correspondence: Jeffrey A. Hicke



Summary

The global climate is changing, and a range of negative effects on plants has already been observed and will likely continue into the future. One of the most apparent consequences of climate change is widespread tree mortality (Fig. 1). Extensive tree die-offs resulting from recent climate change have been documented across a range of forest types on all forested continents (Allen et al., 2010). The exact physiological mechanisms causing this mortality are not yet well understood (e.g. McDowell, 2011), but they are likely caused by reductions in precipitation and increases in temperatures and vapor pressure deficit (VPD) that lead to enhanced soil moisture deficits and/or increased atmospheric demand of water from plants. When plant stomata close because of a lack of available soil water or high atmospheric demand, the plant cannot photosynthesize (leading to carbon (C) starvation) and/or cannot move water from roots to leaves (hydraulic limitation); either mechanism reduces growth, potentially leading directly to mortality and/or to reduced capacity to defend against insect or pathogen attack. Regardless of the mechanisms, few studies have documented relationships between climate and large-scale tree die-offs. In this issue of New Phytologist (pp. 413–421) Clifford et al. address this gap by reporting on a study of climate conditions during widespread piñon pine mortality that occurred in the early 2000s. This die-off occurred across 1.2 Mha of the southwestern United States (Breshears et al.,2005) and killed up to 350 million piñon pines (Meddens et al., 2012; Fig. 2). A combination of low precipitation, high temperatures and VPD, and bark beetles was reported to cause the mortality (Breshears et al., 2005).

Key words: climate change, modelling, plant functional type,

Precipitation, tree mortality, vapor pressure deficit (VPD)

Commentary Climate-driven tree mortality: insights from the piñon pine die-off in the United States


Nate G. McDowell, Rosie A. Fisher, Chonggang Xu, J. C. Domec, Teemu Hölttä, D. Scott Mackay, John S. Sperry, Amanda Boutz, Lee Dickman, Nathan Gehres, Jean Marc Limousin, Alison Macalady, Jordi Martínez-Vilalta, Maurizio Mencuccini, Jennifer A. Plaut, Jérôme Ogée, Robert E. Pangle, Daniel P. Rasse, Michael G. Ryan, Sanna Sevanto, Richard H. Waring, A. Park Williams, Enrico A. Yepez, William T. Pockman

Author for correspondence: Nate G. McDowell



Summary

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 modelling 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.

Key words: carbon starvation, cavitation, dieoff, dynamic global

vegetation models (DGVMs), hydraulic failure, photosynthesis,

process-based models.

Tansley Review Evaluating theories of drought-induced vegetation mortality using a multimodel–experiment framework


Andrea Nardini, Marta Battistuzzo and Tadeja Savi

Author for correspondence: Andrea Nardini



Summary

Plant water status and hydraulics were measured in six woody angiosperms growing in a karstic woodland, during an extreme summer drought. Our aim was to take advantage of an unusual climatic event to identify key traits related to species-specific drought damage. The damage suffered by different species was assessed in terms of percentage of individuals showing extensive crown desiccation. Stem water potential (Ψstem) and percent loss of hydraulic conductivity (PLC) were measured in healthy and desiccated individuals. Vulnerability to cavitation was assessed in terms of stem water potential inducing 50% PLC (Ψ50). Stem density (ρstem) was also measured. Species-specific percentage of desiccated individuals was correlated to Ψ50 and ρstem. Crown desiccation was more widespread in species with less negative Ψ50 and lower ρstem. Desiccated individuals had lower Ψstem and higher PLC than healthy ones, suggesting that hydraulic failure was an important mechanism driving shoot dieback. Drought-vulnerable species showed lower safety margins (Ψstem - Ψ50) than resistant ones. The Ψ50, safety margins and ρstem values emerge as convenient traits to be used for tentative predictions of differential species-specific impact of extreme drought events on a local scale. The possibility that carbohydrate depletion was also involved in induction of desiccation symptoms is discussed.

Key words: cavitation, crown desiccation, drought, hydraulic

failure, safety margin, stem density, stem water potential, tree mortality.

Shoot desiccation and hydraulic failure in temperate woody angiosperms during an extreme summer drought


Junbin Zhao, Henrik Hartmann, Susan Trumbore, Waldemar Ziegler, Yiping Zhang

Author for correspondence: Junbin Zhao

Tel: +86 87165160904 Email: [email protected]


Summary

Theoretically, progressive drought can force trees into negative carbon (C) balance by reducing stomatal conductance to prevent water loss, which also decreases C assimilation. At higher temperatures, negative C balance should be initiated at higher soil moisture because of increased respiratory demand and earlier stomatal closure. Few data are available on how these theoretical relationships integrate over the whole plant. We exposed Thuja occidentalis to progressive drought under three temperature conditions (15, 25, and 35°C), and measured C and water fluxes using a whole-tree chamber design. High transpiration rates at higher temperatures led to a rapid decline in soil moisture. During the progressive drought, soil moisture-driven changes in photosynthesis had a greater impact on the whole-plant C balance than respiration. The soil moisture content at which wholeplant C balance became negative increased with temperature, mainly as a result of higher respiration rates and an earlier onset of stomatal closure under a warmer condition. Our results suggest that the effect of drought on whole-plant C balance is highly temperature- dependent. High temperature causes a negative C balance even under mild drought and may increase the risk of C starvation.

Key words: carbon compensation point, carbon limitation, drought,

high temperature, whole-plant chamber.

High temperature causes negative whole-plant carbon balance under mild drought


Henrik Hartmann, Waldemar Ziegler, Olaf Kolle and Susan Trumbore

Author for correspondence: Henrik Hartmann



Summary

Drought-induced tree mortality results from an interaction of several mechanisms. Plant water and carbon relations are interdependent and assessments of their individual contributions are difficult. Because drought always affects both plant hydration and carbon assimilation, it is challenging to disentangle their concomitant effects on carbon balance and carbon translocation. Here, we report results of a manipulation experiment specifically designed to separate drought effects on carbon and water relations from those on carbon translocation. In a glasshouse experiment, we manipulated the carbon balance of Norway spruce saplings exposed to either drought or carbon starvation (CO2 withdrawal), or both treatments, and compared the dynamics of carbon exchange, allocation and storage in different tissues. Drought killed trees much faster than did carbon starvation. Storage C pools were not depleted at death for droughted trees as they were for starved, well-watered trees. Hence drought has a significant detrimental effect on a plant’s ability to utilize stored carbon. Unless they can be transported to where they are needed, sufficient carbon reserves alone will not assure survival of a drought except under specific conditions, such as moderate drought, or in species that maintain plant water relations required for carbon re-mobilization.

Key words: carbon remobilization, carbon starvation, carbon storage

use, drought induced tree mortality, plant hydration.

Thirst beats hunger – declining hydration during drought prevents carbon starvation in Norway spruce saplings


Thomas L. Powell, David R. Galbraith, Bradley O. Christoffersen, Anna Harper, Hewlley M. A. Imbuzeiro,

Lucy Rowland, Samuel Almeida, Paulo M. Brando, Antonio Carlos Lola da Costa, Marcos Heil Costa, Naomi M. Levine, Yadvinder Malhi, Scott R. Saleska, Eleneide Sotta, Mathew Williams, Patrick Meir, Paul R. Moorcroft

Author for correspondence: Paul R. Moorcroft



Summary

Considerable uncertainty surrounds the fate of Amazon rainforests in response to climate change. Here, carbon (C) flux predictions of five terrestrial biosphere models (Community Land Model version 3.5 (CLM3.5), Ecosystem Demography model version 2.1 (ED2), Integrated Biosphere Simulator version 2.6.4 (IBIS), Joint UK Land Environment Simulator version 2.1 (JULES) and Simple Biosphere model version 3 (SiB3)) and a hydrodynamic terrestrial ecosystem model (the Soil–Plant–Atmosphere (SPA) model) were evaluated against measurements from two large-scale Amazon drought experiments. Model predictions agreed with the observed C fluxes in the control plots of both experiments, but poorly replicated the responses to the drought treatments. Most notably, with the exception of ED2, the models predicted negligible reductions in aboveground biomass in response to the drought treatments, which was in contrast to an observed c. 20% reduction at both sites. For ED2, the timing of the decline in aboveground biomass was accurate, but the magnitude was too high for one site and too low for the other. Three key findings indicate critical areas for future research and model development. First, the models predicted declines in autotrophic respiration under prolonged drought in contrast to measured increases at one of the sites. Secondly, models lacking a phenological response to drought introduced bias in the sensitivity of canopy productivity and respiration to drought. Thirdly, the phenomenological water-stress functions used by the terrestrial biosphere models to represent the effects of soil moisture on stomatal conductance yielded unrealistic diurnal and seasonal responses to drought.

Key words: Amazon, carbon cycle, drought, terrestrial biosphere

model, throughfall exclusion, tropical rainforest.

Confronting model predictions of carbon fluxes with measurements of Amazon forests subjected to experimental drought


Rodney E. Will, Stuart M. Wilson, Chris B. Zou, Thomas C. Hennessey

Author for correspondence: Rodney E. Will



Summary

Tree species growing along the forest–grassland ecotone are near the moisture limit of their range. Small increases in temperature can increase vapor pressure deficit (VPD) which may increase tree water use and potentially hasten mortality during severe drought. We tested a 40% increase in VPD due to an increase in growing temperature from 30 to 33°C (constant dewpoint 21°C) on seedlings of 10 tree species common to the forest–grassland ecotone in the southern Great Plains, USA. Measurement at 33 vs 30°C during reciprocal leaf gas exchange measurements, that is, measurement of all seedlings at both growing temperatures, increased transpiration for seedlings grown at 30°C by 40% and 20% for seedlings grown at 33°C. Higher initial transpiration of seedlings in the 33°C growing temperature treatment resulted in more negative xylem water potentials and fewer days until transpiration decreased after watering was withheld. The seedlings grown at 33°C died 13% (average 2 d) sooner than seedlings grown at 30°C during terminal drought. If temperature and severity of droughts increase in the future, the forest–grassland ecotone could shift because low seedling survival rate may not sufficiently support forest regeneration and migration.

Key words: drought, mortality, seedlings, transpiration, vapor

pressure deficit (VPD),water potential.

Increased vapor pressure deficit due to higher temperature leads to greater transpiration and faster mortality during drought for tree seedlings common to the forest–grassland ecotone


Jennifer A. Plaut, W. Duncan Wadsworth, Robert Pangle, Enrico A. Yepez, Nate G. McDowell, William T. Pockman

Author for correspondence: Jennifer A. Plaut



Summary

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 5 yr 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.

Key words: carbon starvation, die-off, hydraulic conductance, hydraulic

failure, mixed effects model, semi-arid.

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


Rafael Poyatos, David Aguadé, Lucía Galiano, Maurizio Mencuccini, Jordi Martínez-Vilalta

Author for correspondence: Rafael Poyatos

Tel: +34 935814677 Email: [email protected]


Summary

Drought-induced defoliation has recently been associated with the depletion of carbon reserves and increased mortality risk in Scots pine (Pinus sylvestris). We hypothesize that defoliated individuals are more sensitive to drought, implying that potentially higher gas exchange (per unit of leaf area) during wet periods may not compensate for their reduced photosynthetic area. We measured sap flow, needle water potentials and whole-tree hydraulic conductance to analyse the drought responses of co-occurring defoliated and nondefoliated Scots pines in northeast Spain during typical (2010) and extreme (2011) drought conditions. Defoliated Scots pines showed higher sap flow per unit leaf area during spring, but were more sensitive to summer drought, relative to nondefoliated pines. This pattern was associated with a steeper decline in soil-to-leaf hydraulic conductance with drought and an enhanced sensitivity of canopy conductance to soil water availability. Near-homeostasis in midday water potentials was observed across years and defoliation classes, with minimum values of -2.5 MPa. Enhanced sensitivity to drought and prolonged periods of near-zero gas exchange were consistent with low levels of carbohydrate reserves in defoliated trees. Our results support the critical links between defoliation, water and carbon availability, and their key roles in determining tree survival and recovery under drought.

Key words: canopy defoliation, hydraulic limits, nonstructural

carbohydrates, Pinus sylvestris, sap flow, tomatal conductance, tree

mortality, water potential.

Drought-induced defoliation and long periods of near-zero gas exchange play a key role in accentuating metabolic decline of Scots pine


Miranda D. Redmond, Nichole N. Barger

Author for correspondence: Miranda D. Redmond



Summary

Widespread piñon (Pinus edulis) mortality occurred across the southwestern USA during 2002–2003 in response to drought and bark beetle infestations. Given the recent mortality and changes in regional climate over the past several decades, there is a keen interest in postmortality regeneration dynamics in piñon–juniper woodlands. Here, we examined piñon and Utah juniper (Juniperus osteosperma) recruitment at 30 sites across southwestern Colorado, USA that spanned a gradient of adult piñon mortality levels (10–100%) to understand current regeneration dynamics. Piñon and juniper recruitment was greater at sites with more tree and shrub cover. Piñon recruitment was more strongly facilitated than juniper recruitment by trees and shrubs. New (post-mortality) piñon recruitment was negatively affected by recent mortality. However, mortality had no effect on piñon advanced regeneration (juveniles established pre-mortality) and did not shift juvenile piñon dominance. Our results highlight the importance of shrubs and juniper trees for the facilitation of piñon establishment and survival. Regardless of adult piñon mortality levels, areas with low tree and shrub cover may become increasingly juniper dominated as a result of the few suitable microsites for piñon establishment and survival. In areas with high piñon mortality and high tree and shrub cover, our results suggest that piñon is regenerating via advanced regeneration.

Key words: climate change, disturbance, Ips confusus, Juniperus

osteosperma, Pinus edulis, recruitment, species interactions,

soil properties.

Tree regeneration following drought- and insect-induced mortality in piñon–juniper woodlands


Michael J. Clifford, Patrick D. Royer, Neil S. Cobb, David D. Breshears, Paulette L. Ford

Author for correspondence: Michael J. Clifford



Summary

Recent regional tree die-off events appear to have been triggered by a combination of drought and heat – referred to as ‘global-change-type drought’. To complement experiments focused on resolving mechanisms of drought-induced tree mortality, an evaluation of how patterns of tree die-off relate to highly spatially variable precipitation is needed. Here, we explore precipitation relationships with a die-off event of pinyon pine (Pinus edulis Engelm.) in southwestern North America during the 2002–2003 global-change-type drought. Pinyon die-off and its relationship with precipitation was quantified spatially along a precipitation gradient in north-central New Mexico with standard field plot measurements of die-off combined with canopy cover derived from normalized burn ratio (NBR) from Landsat imagery. Pinyon die-off patterns revealed threshold responses to precipitation (cumulative 2002–2003) and vapor pressure deficit (VPD), with little to no mortality (< 10%) above 600mm and below warm season VPD of c. 1.7 kPa. [Correction added after online publication 17 June 2013; in the preceding sentence, the word ’below’ has been inserted.] Our results refine how precipitation patterns within a region influence pinyon die-off, revealing a precipitation and VPD threshold for tree mortality and its uncertainty band where other factors probably come into play – a response type that influences stand demography and landscape heterogeneity and is of general interest, yet has not been documented.

Key words: climate change, die-off, drought, mortality, Pinus edulis,

pinyon pine, pinyon–juniper woodlands, threshold.

Precipitation thresholds and drought-induced tree die-off: insights from patterns of Pinus edulis mortality along an environmental stress gradient


Richard C. Cobb, Valerie T. Eviner, David M. Rizzo

Author for correspondence: Richard C. Cobb



Summary

Few studies have quantified pathogen impacts to ecosystem processes, despite the fact that pathogens cause or contribute to regional-scale tree mortality. We measured litterfall mass, litterfall chemistry, and soil nitrogen (N) cycling associated with multiple hosts along a gradient of mortality caused by Phytophthora ramorum, the cause of sudden oak death. In redwood forests, the epidemiological and ecological characteristics of the major overstory species determine disease patterns and the magnitude and nature of ecosystem change. Bay laurel (Umbellularia californica) has high litterfall N (0.992%), greater soil extractable NO3– N, and transmits infection without suffering mortality. Tanoak (Notholithocarpus densiflorus) has moderate litterfall N (0.723%) and transmits infection while suffering extensive mortality that leads to higher extractable soil NO3–N. Redwood (Sequoia sempervirens) has relatively low litterfall N (0.519%), does not suffer mortality or transmit the pathogen, but dominates forest biomass. The strongest impact of pathogen-caused mortality was the potential shift in species composition, which will alter litterfall chemistry, patterns and dynamics of litterfall mass, and increase soil NO3–N availability. Patterns of P. ramorum spread and consequent mortality are closely associated with bay laurel abundances, suggesting this species will drive both disease emergence and subsequent ecosystem function.

Key words: community–pathogen feedback, ecosystem ecology,

emerging infectious disease, nitrification, nitrogen mineralization,

Phytophthora ramorum, redwood forests.

Mortality and community changes drive sudden oak death impacts on litterfall and soil nitrogen cycling


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New Phytologist Special Feature: Drought-induced Forest Mortality