Publications by authors named "David D Breshears"

35 Publications

The growing challenge of vegetation change.

Science 2021 05;372(6544):786-787

School of Natural Resources and the Environment, University of Arizona, Tucson, AZ, USA.

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http://dx.doi.org/10.1126/science.abi9902DOI Listing
May 2021

Radionuclide resuspension across ecosystems and environmental disturbances.

J Environ Radioact 2021 Jul 25;233:106586. Epub 2021 Mar 25.

Los Alamos National Laboratory, Mail Stop J978, Los Alamos, NM, 87545, USA.

Exposure assessment from radionuclides and other soil-bound contaminants often requires quantifying the amount of contaminant resuspended in the air. Rates and controlling factors of radionuclide resuspension and wind erosion of soil are clearly related but have largely been studied separately. Here, we review both and then integrate wind erosion measurements with the radiological resuspension paradigm to provide better estimates of resuspension factors across a broad range of ecosystems and environmental conditions. Radionuclide resuspension by wind was initially investigated during the era of aboveground nuclear weapons testing. Predictive dose models were developed from empirically-derived ratios of air and soil concentrations, otherwise called the resuspension factor. Resuspension factors were shown to generally predict radionuclide concentrations in air, but they were site-specific and largely derived from the arid and semi-arid environments surrounding nuclear weapons testing locations. In contrast, wind erosion studies from the agricultural and environmental sciences have produced more mechanistic models and a relatively robust data set of wind erosion rates and model parameters across a range of ecosystems. We sequentially show the mathematics linking measured sediment flux from wind erosion rate measurements to resuspension factors using the concept of transport capacity and its relationship to the deposition velocity. We also describe the conceptual framework describing how resuspension factors change through time and the mathematical models describing this decrease. We then show how vertical mass flux measurements across ecosystems were categorized and used to calculate ecosystem-based resuspension factors. These calculations allow generalized estimation of radionuclide resuspension factors across ecosystem types as a function of disturbance and as input for dose calculations.
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http://dx.doi.org/10.1016/j.jenvrad.2021.106586DOI Listing
July 2021

Underappreciated plant vulnerabilities to heat waves.

New Phytol 2021 07 23;231(1):32-39. Epub 2021 Apr 23.

Environmental and Conservation Sciences, Murdoch University, Murdoch, WA, 6150, Australia.

With climate change, heat waves are becoming increasingly frequent, intense and broader in spatial extent. However, while the lethal effects of heat waves on humans are well documented, the impacts on flora are less well understood, perhaps except for crops. We summarize recent findings related to heat wave impacts including: sublethal and lethal effects at leaf and plant scales, secondary ecosystem effects, and more complex impacts such as increased heat wave frequency across all seasons, and interactions with other disturbances. We propose generalizable practical trials to quantify the critical bounding conditions of vulnerability to heat waves. Collectively, plant vulnerabilities to heat waves appear to be underappreciated and understudied, particularly with respect to understanding heat wave driven plant die-off and ecosystem tipping points.
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http://dx.doi.org/10.1111/nph.17348DOI Listing
July 2021

Subcontinental heat wave triggers terrestrial and marine, multi-taxa responses.

Sci Rep 2018 08 30;8(1):13094. Epub 2018 Aug 30.

Centre of Excellence for Climate Change, Woodland and Forest Health, School of Veterinary and Life Sciences, Murdoch University, Perth, Western Australia, Australia.

Heat waves have profoundly impacted biota globally over the past decade, especially where their ecological impacts are rapid, diverse, and broad-scale. Although usually considered in isolation for either terrestrial or marine ecosystems, heat waves can straddle ecosystems of both types at subcontinental scales, potentially impacting larger areas and taxonomic breadth than previously envisioned. Using climatic and multi-species demographic data collected in Western Australia, we show that a massive heat wave event straddling terrestrial and maritime ecosystems triggered abrupt, synchronous, and multi-trophic ecological disruptions, including mortality, demographic shifts and altered species distributions. Tree die-off and coral bleaching occurred concurrently in response to the heat wave, and were accompanied by terrestrial plant mortality, seagrass and kelp loss, population crash of an endangered terrestrial bird species, plummeting breeding success in marine penguins, and outbreaks of terrestrial wood-boring insects. These multiple taxa and trophic-level impacts spanned >300,000 km-comparable to the size of California-encompassing one terrestrial Global Biodiversity Hotspot and two marine World Heritage Areas. The subcontinental multi-taxa context documented here reveals that terrestrial and marine biotic responses to heat waves do not occur in isolation, implying that the extent of ecological vulnerability to projected increases in heat waves is underestimated.
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http://dx.doi.org/10.1038/s41598-018-31236-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6117366PMC
August 2018

Research frontiers for improving our understanding of drought-induced tree and forest mortality.

New Phytol 2018 04;218(1):15-28

Estación Experimental de Zonas Áridas, Consejo Superior de Investigaciones Científicas, Carretera de Sacramento s/n, E-04120 La Cañada, Almería, Spain.

Accumulating evidence highlights increased mortality risks for trees during severe drought, particularly under warmer temperatures and increasing vapour pressure deficit (VPD). Resulting forest die-off events have severe consequences for ecosystem services, biophysical and biogeochemical land-atmosphere processes. Despite advances in monitoring, modelling and experimental studies of the causes and consequences of tree death from individual tree to ecosystem and global scale, a general mechanistic understanding and realistic predictions of drought mortality under future climate conditions are still lacking. We update a global tree mortality map and present a roadmap to a more holistic understanding of forest mortality across scales. We highlight priority research frontiers that promote: (1) new avenues for research on key tree ecophysiological responses to drought; (2) scaling from the tree/plot level to the ecosystem and region; (3) improvements of mortality risk predictions based on both empirical and mechanistic insights; and (4) a global monitoring network of forest mortality. In light of recent and anticipated large forest die-off events such a research agenda is timely and needed to achieve scientific understanding for realistic predictions of drought-induced tree mortality. The implementation of a sustainable network will require support by stakeholders and political authorities at the international level.
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http://dx.doi.org/10.1111/nph.15048DOI Listing
April 2018

Beyond greenness: Detecting temporal changes in photosynthetic capacity with hyperspectral reflectance data.

PLoS One 2017 27;12(12):e0189539. Epub 2017 Dec 27.

School of Natural Resources and the Environment, University of Arizona, Tucson, Arizona, United States of America.

Earth's future carbon balance and regional carbon exchange dynamics are inextricably linked to plant photosynthesis. Spectral vegetation indices are widely used as proxies for vegetation greenness and to estimate state variables such as vegetation cover and leaf area index. However, the capacity of green leaves to take up carbon can change throughout the season. We quantify photosynthetic capacity as the maximum rate of RuBP carboxylation (Vcmax) and regeneration (Jmax). Vcmax and Jmax vary within-season due to interactions between ontogenetic processes and meteorological variables. Remote sensing-based estimation of Vcmax and Jmax using leaf reflectance spectra is promising, but temporal variation in relationships between these key determinants of photosynthetic capacity, leaf reflectance spectra, and the models that link these variables has not been evaluated. To address this issue, we studied hybrid poplar (Populus spp.) during a 7-week mid-summer period to quantify seasonally-dynamic relationships between Vcmax, Jmax, and leaf spectra. We compared in situ estimates of Vcmax and Jmax from gas exchange measurements to estimates of Vcmax and Jmax derived from partial least squares regression (PLSR) and fresh-leaf reflectance spectroscopy. PLSR models were robust despite dynamic temporal variation in Vcmax and Jmax throughout the study period. Within-population variation in plant stress modestly reduced PLSR model predictive capacity. Hyperspectral vegetation indices were well-correlated to Vcmax and Jmax, including the widely-used Normalized Difference Vegetation Index. Our results show that hyperspectral estimation of plant physiological traits using PLSR may be robust to temporal variation. Additionally, hyperspectral vegetation indices may be sufficient to detect temporal changes in photosynthetic capacity in contexts similar to those studied here. Overall, our results highlight the potential for hyperspectral remote sensing to estimate determinants of photosynthetic capacity during periods with dynamic temporal variations related to seasonality and plant stress, thereby improving estimates of plant productivity and characterization of the associated carbon budget.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0189539PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5744967PMC
January 2018

A multi-species synthesis of physiological mechanisms in drought-induced tree mortality.

Nat Ecol Evol 2017 Sep 7;1(9):1285-1291. Epub 2017 Aug 7.

Department of Plant Sciences, University of Oxford, Oxford, OX1 3RB, UK.

Widespread tree mortality associated with drought has been observed on all forested continents and global change is expected to exacerbate vegetation vulnerability. Forest mortality has implications for future biosphere-atmosphere interactions of carbon, water and energy balance, and is poorly represented in dynamic vegetation models. Reducing uncertainty requires improved mortality projections founded on robust physiological processes. However, the proposed mechanisms of drought-induced mortality, including hydraulic failure and carbon starvation, are unresolved. A growing number of empirical studies have investigated these mechanisms, but data have not been consistently analysed across species and biomes using a standardized physiological framework. Here, we show that xylem hydraulic failure was ubiquitous across multiple tree taxa at drought-induced mortality. All species assessed had 60% or higher loss of xylem hydraulic conductivity, consistent with proposed theoretical and modelled survival thresholds. We found diverse responses in non-structural carbohydrate reserves at mortality, indicating that evidence supporting carbon starvation was not universal. Reduced non-structural carbohydrates were more common for gymnosperms than angiosperms, associated with xylem hydraulic vulnerability, and may have a role in reducing hydraulic function. Our finding that hydraulic failure at drought-induced mortality was persistent across species indicates that substantial improvement in vegetation modelling can be achieved using thresholds in hydraulic function.
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http://dx.doi.org/10.1038/s41559-017-0248-xDOI Listing
September 2017

Synergistic Ecoclimate Teleconnections from Forest Loss in Different Regions Structure Global Ecological Responses.

PLoS One 2016 16;11(11):e0165042. Epub 2016 Nov 16.

Department of Forestry, Michigan State University, East Lansing, MI, 48824, United States of America.

Forest loss in hotspots around the world impacts not only local climate where loss occurs, but also influences climate and vegetation in remote parts of the globe through ecoclimate teleconnections. The magnitude and mechanism of remote impacts likely depends on the location and distribution of forest loss hotspots, but the nature of these dependencies has not been investigated. We use global climate model simulations to estimate the distribution of ecologically-relevant climate changes resulting from forest loss in two hotspot regions: western North America (wNA), which is experiencing accelerated dieoff, and the Amazon basin, which is subject to high rates of deforestation. The remote climatic and ecological net effects of simultaneous forest loss in both regions differed from the combined effects of loss from the two regions simulated separately, as evident in three impacted areas. Eastern South American Gross Primary Productivity (GPP) increased due to changes in seasonal rainfall associated with Amazon forest loss and changes in temperature related to wNA forest loss. Eurasia's GPP declined with wNA forest loss due to cooling temperatures increasing soil ice volume. Southeastern North American productivity increased with simultaneous forest loss, but declined with only wNA forest loss due to changes in VPD. Our results illustrate the need for a new generation of local-to-global scale analyses to identify potential ecoclimate teleconnections, their underlying mechanisms, and most importantly, their synergistic interactions, to predict the responses to increasing forest loss under future land use change and climate change.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0165042PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5112850PMC
June 2017

Rainfall intensity switches ecohydrological runoff/runon redistribution patterns in dryland vegetation patches.

Ecol Appl 2015 Dec;25(8):2094-100

Effectively managing net primary productivity in drylands for grazing and other uses depends on understanding how limited rainfall input is redistributed by runoff and runon among vegetation patches, particularly for patches that contrast between lesser and greater amounts of vegetation cover. Due in part to data limitations, ecohydrologists generally have focused on rainfall event size to characterize water redistribution processes. Here we use soil moisture data from a semiarid woodland to highlight how, when event size is controlled and runoff and interception are negligible at the stand scale, rainfall intensity drives the relationship between water redistribution and canopy and soil patch attributes. Horizontal water redistribution variability increased with rainfall intensity and differed between patches with contrasting vegetation cover. Sparsely vegetated patches gained relatively more water during lower intensity events, whereas densely vegetated ones gained relatively more water during higher intensity events. Consequently, range managers need to account for the distribution of rainfall event intensity, as well as event size, to assess the consequences of climate variability and change on net primary productivity. More generally, our results suggest that rainfall intensity needs to be considered in addition to event size to understand vegetation patch dynamics in drylands.
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http://dx.doi.org/10.1890/15-0550.1DOI Listing
December 2015

Global change-type drought-induced tree mortality: vapor pressure deficit is more important than temperature per se in causing decline in tree health.

Ecol Evol 2013 Aug 10;3(8):2711-29. Epub 2013 Jul 10.

School of Natural Resources and the Environment, University of Arizona Tucson, Arizona, 85721 ; Department of Ecology and Evolutionary Biology, University of Arizona Tucson, Arizona, 85721.

Drought-induced tree mortality is occurring across all forested continents and is expected to increase worldwide during the coming century. Regional-scale forest die-off influences terrestrial albedo, carbon and water budgets, and land-surface energy partitioning. Although increased temperatures during drought are widely identified as a critical contributor to exacerbated tree mortality associated with "global-change-type drought", corresponding changes in vapor pressure deficit (D) have rarely been considered explicitly and have not been disaggregated from that of temperature per se. Here, we apply a detailed mechanistic soil-plant-atmosphere model to examine the impacts of drought, increased air temperature (+2°C or +5°C), and increased vapor pressure deficit (D; +1 kPa or +2.5 kPa), singly and in combination, on net primary productivity (NPP) and transpiration and forest responses, especially soil moisture content, leaf water potential, and stomatal conductance. We show that increased D exerts a larger detrimental effect on transpiration and NPP, than increased temperature alone, with or without the imposition of a 3-month drought. Combined with drought, the effect of increased D on NPP was substantially larger than that of drought plus increased temperature. Thus, the number of days when NPP was zero across the 2-year simulation was 13 or 14 days in the control and increased temperature scenarios, but increased to approximately 200 days when D was increased. Drought alone increased the number of days of zero NPP to 88, but drought plus increased temperature did not increase the number of days. In contrast, drought and increased D resulted in the number of days when NPP = 0 increasing to 235 (+1 kPa) or 304 days (+2.5 kPa). We conclude that correct identification of the causes of global change-type mortality events requires explicit consideration of the influence of D as well as its interaction with drought and temperature. This study disaggregates the influence of temperature and vapour pressure deficit on net primary productivity of an Australian woodland and their interactions with drought as potential causal agents in recent widespread forest mortality.
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http://dx.doi.org/10.1002/ece3.664DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3930053PMC
August 2013

The critical amplifying role of increasing atmospheric moisture demand on tree mortality and associated regional die-off.

Front Plant Sci 2013 2;4:266. Epub 2013 Aug 2.

The School of Natural Resources and the Environment, The University of Arizona Tucson, AZ, USA ; Department of Ecology and Evolutionary Biology, The University of Arizona Tucson, AZ, USA.

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http://dx.doi.org/10.3389/fpls.2013.00266DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3731633PMC
August 2013

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

New Phytol 2013 Oct 17;200(2):413-421. Epub 2013 Jun 17.

USDA Forest Service Rocky Mountain Research Station, Albuquerque, NM, 87102, USA.

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 600 mm 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.
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http://dx.doi.org/10.1111/nph.12362DOI Listing
October 2013

Modeling aeolian transport of soil-bound plutonium: considering infrequent but normal environmental disturbances is critical in estimating future dose.

J Environ Radioact 2013 Jun 1;120:73-80. Epub 2013 Mar 1.

Los Alamos National Laboratory, Environmental Stewardship Group, Mail Stop J978, Los Alamos, NM 87544, USA.

Dose assessments typically consider environmental systems as static through time, but environmental disturbances such as drought and fire are normal, albeit infrequent, events that can impact dose-influential attributes of many environmental systems. These phenomena occur over time frames of decades or longer, and are likely to be exacerbated under projected warmer, drier climate. As with other types of dose assessment, the impacts of environmental disturbances are often overlooked when evaluating dose from aeolian transport of radionuclides and other contaminants. Especially lacking are predictions that account for potential changing vegetation cover effects on radionuclide transport over the long time frames required by regulations. A recently developed dynamic wind-transport model that included vegetation succession and environmental disturbance provides more realistic long-term predictability. This study utilized the model to estimate emission rates for aeolian transport, and compare atmospheric dispersion and deposition rates of airborne plutonium-contaminated soil into neighboring areas with and without environmental disturbances. Specifically, the objective of this study was to utilize the model results as input for a widely used dose assessment model (CAP-88). Our case study focused on low levels of residual plutonium found in soils from past operations at Los Alamos National Laboratory (LANL), in Los Alamos, NM, located in the semiarid southwestern USA. Calculations were conducted for different disturbance scenarios based on conditions associated with current climate, and a potential future drier and warmer climate. Known soil and sediment concentrations of plutonium were used to model dispersal and deposition of windblown residual plutonium, as a function of distance and direction. Environmental disturbances that affected vegetation cover included ground fire, crown fire, and drought, with reoccurrence rates for current climate based on site historical patterns. Using site-specific meteorology, accumulation rates of plutonium in soil were modeled in a variety of directions and distances from LANL sources. Model results suggest that without disturbances, areas downwind to the contaminated watershed would accumulate LANL-derived plutonium at a relatively slow rate (<0.01 Bq m(-2) yr(-1)). However, model results under more realistic assumptions that include environmental disturbances show accumulation rates more than an order-of-magnitude faster. More generally, this assessment highlights the broader need in radioecology and environmental health physics to consider infrequent but normal environmental disturbances in longer-term dose assessments.
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http://dx.doi.org/10.1016/j.jenvrad.2013.01.011DOI Listing
June 2013

Nonstructural leaf carbohydrate dynamics of Pinus edulis during drought-induced tree mortality reveal role for carbon metabolism in mortality mechanism.

New Phytol 2013 Mar 11;197(4):1142-1151. Epub 2013 Jan 11.

Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, 85721, USA.

Vegetation change is expected with global climate change, potentially altering ecosystem function and climate feedbacks. However, causes of plant mortality, which are central to vegetation change, are understudied, and physiological mechanisms remain unclear, particularly the roles of carbon metabolism and xylem function. We report analysis of foliar nonstructural carbohydrates (NSCs) and associated physiology from a previous experiment where earlier drought-induced mortality of Pinus edulis at elevated temperatures was associated with greater cumulative respiration. Here, we predicted faster NSC decline for warmed trees than for ambient-temperature trees. Foliar NSC in droughted trees declined by 30% through mortality and was lower than in watered controls. NSC decline resulted primarily from decreased sugar concentrations. Starch initially declined, and then increased above pre-drought concentrations before mortality. Although temperature did not affect NSC and sugar, starch concentrations ceased declining and increased earlier with higher temperatures. Reduced foliar NSC during lethal drought indicates a carbon metabolism role in mortality mechanism. Although carbohydrates were not completely exhausted at mortality, temperature differences in starch accumulation timing suggest that carbon metabolism changes are associated with time to death. Drought mortality appears to be related to temperature-dependent carbon dynamics concurrent with increasing hydraulic stress in P. edulis and potentially other similar species.
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http://dx.doi.org/10.1111/nph.12102DOI Listing
March 2013

Drought-induced forest decline: causes, scope and implications.

Biol Lett 2012 Oct 14;8(5):689-91. Epub 2011 Dec 14.

CREAF/Unitat d'Ecologia, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain.

A large number of episodes of forest mortality associated with drought and heat stress have been detected worldwide in recent decades, suggesting that some of the world's forested ecosystems may be already responding to climate change. Here, we summarize a special session titled 'Drought-induced forest decline: causes, scope and implications' within the 12th European Ecological Federation Congress, held in Ávila (Spain) from 25 to 29 September 2011. The session focused on the interacting causes and impacts of die-off episodes at the community and ecosystem levels, and highlighted recent events of drought- and heat-related tree decline, advances in understanding mechanisms and in predicting mortality events, and diverse consequences of forest decline. Talks and subsequent discussion noted a potentially important role of carbon that may be interrelated with plant hydraulics in the multi-faceted process leading to drought-induced mortality; a substantial and yet understudied capacity of many forests to cope with extreme climatic events; and the difficulty of separating climate effects from other anthropogenic changes currently shaping forest dynamics in many regions of the Earth. The need for standard protocols and multi-level monitoring programmes to track the spatio-temporal scope of forest decline globally was emphasized as critical for addressing this emerging environmental issue.
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http://dx.doi.org/10.1098/rsbl.2011.1059DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3440957PMC
October 2012

The interdependence of mechanisms underlying climate-driven vegetation mortality.

Trends Ecol Evol 2011 Oct 29;26(10):523-32. Epub 2011 Jul 29.

Los Alamos National Laboratory, Los Alamos, NM 87545, USA.

Climate-driven vegetation mortality is occurring globally and is predicted to increase in the near future. The expected climate feedbacks of regional-scale mortality events have intensified the need to improve the simple mortality algorithms used for future predictions, but uncertainty regarding mortality processes precludes mechanistic modeling. By integrating new evidence from a wide range of fields, we conclude that hydraulic function and carbohydrate and defense metabolism have numerous potential failure points, and that these processes are strongly interdependent, both with each other and with destructive pathogen and insect populations. Crucially, most of these mechanisms and their interdependencies are likely to become amplified under a warmer, drier climate. Here, we outline the observations and experiments needed to test this interdependence and to improve simulations of this emergent global phenomenon.
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http://dx.doi.org/10.1016/j.tree.2011.06.003DOI Listing
October 2011

When ecosystem services crash: preparing for big, fast, patchy climate change.

Ambio 2011 May;40(3):256-63

School of Natural Resources and the Environment, Department of Evolutionary Biology, University of Arizona, Tucson, AZ 85721-0043, USA.

Assessments of adaptation options generally focus on incremental, homogeneous ecosystem responses to climate even though climate change impacts can be big, fast, and patchy across a region. Regional drought-induced tree die-off in semiarid woodlands highlights how an ecosystem crash fundamentally alters most ecosystem services and poses management challenges. Building on previous research showing how choice of location is linked to adaptive capacity and vulnerability, we developed a framework showing how the options for retaining desired ecosystem services in the face of sudden crashes depend on how portable the service is and whether the stakeholder is flexible with regard to the location where they receive their services. Stakeholders using portable services, or stakeholders who can move to other locations to obtain services, may be more resilient to ecosystem crashes. Our framework suggests that entering into cooperative networks with regionally distributed stakeholders is key to building resilience to big, fast, patchy crashes.
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3357807PMC
http://dx.doi.org/10.1007/s13280-010-0106-4DOI Listing
May 2011

Interactive effects of grazing and burning on wind- and water-driven sediment fluxes: rangeland management implications.

Ecol Appl 2011 Jan;21(1):22-32

School of Natural Resources and the Environment, University of Arizona, Tucson, Arizona 85721, USA.

Rangelands are globally extensive, provide fundamental ecosystem services, and are tightly coupled human-ecological systems. Rangeland sustainability depends largely on the implementation and utilization of various grazing and burning practices optimized to protect against soil erosion and transport. In many cases, however, land management practices lead to increased soil erosion and sediment fluxes for reasons that are poorly understood. Because few studies have directly measured both wind and water erosion and transport, an assessment of how they may differentially respond to grazing and burning practices is lacking. Here, we report simultaneous, co-located estimates of wind- and water-driven sediment transport in a semiarid grassland in Arizona, USA, over three years for four land management treatments: control, grazed, burned, and burned + grazed. For all treatments and most years, annual rates of wind-driven sediment transport exceeded that of water due to a combination of ongoing small but nontrivial wind events and larger, less frequent, wind events that generally preceded the monsoon season. Sediment fluxes by both wind and water differed consistently by treatment: burned + grazed > burned > grazed > or = control, with effects immediately apparent after burning but delayed after grazing until the following growing season. Notably, the wind:water sediment transport ratio decreased following burning but increased following grazing. Our results show how rangeland practices disproportionally alter sediment fluxes driven by wind and water, differences that could potentially help explain divergence between rangeland sustainability and degradation.
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http://dx.doi.org/10.1890/09-2369.1DOI Listing
January 2011

Temperature sensitivity of drought-induced tree mortality portends increased regional die-off under global-change-type drought.

Proc Natl Acad Sci U S A 2009 Apr 13;106(17):7063-6. Epub 2009 Apr 13.

B2 EarthScience/Biosphere 2, P.O. Box 210158-B, University of Arizona, Tucson, AZ 85721, USA.

Large-scale biogeographical shifts in vegetation are predicted in response to the altered precipitation and temperature regimes associated with global climate change. Vegetation shifts have profound ecological impacts and are an important climate-ecosystem feedback through their alteration of carbon, water, and energy exchanges of the land surface. Of particular concern is the potential for warmer temperatures to compound the effects of increasingly severe droughts by triggering widespread vegetation shifts via woody plant mortality. The sensitivity of tree mortality to temperature is dependent on which of 2 non-mutually-exclusive mechanisms predominates--temperature-sensitive carbon starvation in response to a period of protracted water stress or temperature-insensitive sudden hydraulic failure under extreme water stress (cavitation). Here we show that experimentally induced warmer temperatures (approximately 4 degrees C) shortened the time to drought-induced mortality in Pinus edulis (piñon shortened pine) trees by nearly a third, with temperature-dependent differences in cumulative respiration costs implicating carbon starvation as the primary mechanism of mortality. Extrapolating this temperature effect to the historic frequency of water deficit in the southwestern United States predicts a 5-fold increase in the frequency of regional-scale tree die-off events for this species due to temperature alone. Projected increases in drought frequency due to changes in precipitation and increases in stress from biotic agents (e.g., bark beetles) would further exacerbate mortality. Our results demonstrate the mechanism by which warmer temperatures have exacerbated recent regional die-off events and background mortality rates. Because of pervasive projected increases in temperature, our results portend widespread increases in the extent and frequency of vegetation die-off.
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http://dx.doi.org/10.1073/pnas.0901438106DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2678423PMC
April 2009

Vegetation responses to extreme hydrological events: sequence matters.

Am Nat 2009 Jan;173(1):113-8

South Florida Water Management District, West Palm Beach, Florida 33406, USA.

Extreme hydrological events such as flood and drought drive vegetation dynamics and are projected to increase in frequency in association with climate change, which could result in sequences of extreme events. However, experimental studies of vegetation responses to climate have largely focused on responses to a trend in climate or to a single extreme event but have largely overlooked the potential for complex responses to specific sequences of extreme events. Here we document, on the basis of an experiment with seedlings of three types of subtropical wetland tree species, that mortality can be amplified and growth can even be stimulated, depending on event sequence. Our findings indicate that the impacts of multiple extreme events cannot be modeled by simply summing the projected effects of individual extreme events but, rather, that models should take into account event sequences.
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http://dx.doi.org/10.1086/593307DOI Listing
January 2009

Vegetation synchronously leans upslope as climate warms.

Proc Natl Acad Sci U S A 2008 Aug 12;105(33):11591-2. Epub 2008 Aug 12.

School of Natural Resources, Institute for the Study of Planet Earth, Department of Ecology and Evolutionary Biology, and Biosphere 2/B2 Earthscience, University of Arizona, Tucson, AZ 85721, USA.

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http://dx.doi.org/10.1073/pnas.0806579105DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2575300PMC
August 2008

Mechanisms of plant survival and mortality during drought: why do some plants survive while others succumb to drought?

New Phytol 2008 14;178(4):719-739. Epub 2008 Apr 14.

Department of Biology, University of New Mexico, Albuquerque, NM 87131, USA.

Severe droughts have been associated with regional-scale forest mortality worldwide. Climate change is expected to exacerbate regional mortality events; however, prediction remains difficult because the physiological mechanisms underlying drought survival and mortality are poorly understood. We developed a hydraulically based theory considering carbon balance and insect resistance that allowed development and examination of hypotheses regarding survival and mortality. Multiple mechanisms may cause mortality during drought. A common mechanism for plants with isohydric regulation of water status results from avoidance of drought-induced hydraulic failure via stomatal closure, resulting in carbon starvation and a cascade of downstream effects such as reduced resistance to biotic agents. Mortality by hydraulic failure per se may occur for isohydric seedlings or trees near their maximum height. Although anisohydric plants are relatively drought-tolerant, they are predisposed to hydraulic failure because they operate with narrower hydraulic safety margins during drought. Elevated temperatures should exacerbate carbon starvation and hydraulic failure. Biotic agents may amplify and be amplified by drought-induced plant stress. Wet multidecadal climate oscillations may increase plant susceptibility to drought-induced mortality by stimulating shifts in hydraulic architecture, effectively predisposing plants to water stress. Climate warming and increased frequency of extreme events will probably cause increased regional mortality episodes. Isohydric and anisohydric water potential regulation may partition species between survival and mortality, and, as such, incorporating this hydraulic framework may be effective for modeling plant survival and mortality under future climate conditions.
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http://dx.doi.org/10.1111/j.1469-8137.2008.02436.xDOI Listing
July 2008

Phenology of mixed woody-herbaceous ecosystems following extreme events: net and differential responses.

Ecology 2008 Feb;89(2):342-52

Creekside Center for Earth Observation, Menlo Park, California 94025, USA.

Ecosystem responses to key climate drivers are reflected in phenological dynamics such as the timing and degree of "green-up" that integrate responses over spatial scales from individual plants to ecosystems. This integration is clearest in ecosystems dominated by a single species or life form, such as seasonally dynamic grasslands or more temporally constant evergreen forests. Yet many ecosystems have substantial contribution of cover from both herbaceous and woody evergreen plants. Responses of mixed woody-herbaceous ecosystems to climate are of increasing concern due to their extensive nature, the potential for such systems to yield more complex responses than those dominated by a single life form, and projections that extreme climate and weather events will increase in frequency and intensity with global warming. We present responses of a mixed woody-herbaceous ecosystem type to an extreme event: regional-scale piñon pine mortality following an extended drought and the subsequent herbaceous green-up following the first wet period after the drought. This example highlights how reductions in greenness of the slower, more stable evergreen woody component can rapidly be offset by increases associated with resources made available to the relatively more responsive herbaceous component. We hypothesize that such two-phase phenological responses to extreme events are characteristic of many mixed woody-herbaceous ecosystems.
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http://dx.doi.org/10.1890/06-2137.1DOI Listing
February 2008

Foliar absorption of intercepted rainfall improves woody plant water status most during drought.

Ecology 2008 Jan;89(1):41-7

School of Natural Resources, Institute for the Study of Planet Earth, and Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona 85721-0043, USA.

A large proportion of rainfall in dryland ecosystems is intercepted by plant foliage and is generally assumed to evaporate to the atmosphere or drip onto the soil surface without being absorbed. We demonstrate foliar absorption of intercepted rainfall in a widely distributed, continental dryland, woody-plant genus: Juniperus. We observed substantial improvement in plant water status, exceeding 1.0 MPa water potential for drought-stressed plants, following precipitation on an experimental plot that excluded soil water infiltration. Experiments that wetted shoots with unlabeled and with isotopically labeled water confirmed that water potential responded substantially to foliar wetting, that these responses were not attributable to re-equilibration with other portions of the xylem, and that magnitude of response increased with water stress. Foliar absorption is not included in most ecological, hydrological, and atmospheric models; has implications for interpreting plant isotopic signatures; and not only supplements water acquisition associated with increases in soil moisture that follow large or repeated precipitation events, but also enables plants to bypass soil water uptake and benefit from the majority of precipitation events, which wet foliage but do not increase soil moisture substantially. Foliar absorption of intercepted water could be more important than previously appreciated, especially during drought when water stress is greatest.
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http://dx.doi.org/10.1890/07-0437.1DOI Listing
January 2008

Uranium partition coefficients (Kd) in forest surface soil reveal long equilibrium times and vary by site and soil size fraction.

Health Phys 2007 Jul;93(1):36-46

Los Alamos National Laboratory, Radiation Protection Division, Mail Stop G761, Los Alamos, NM 87544, USA.

The environmental mobility of newly deposited radionuclides in surface soil is driven by complex biogeochemical relationships, which have significant impacts on transport pathways. The partition coefficient (Kd) is useful for characterizing the soil-solution exchange kinetics and is an important factor for predicting relative amounts of a radionuclide transported to groundwater compared to that remaining on soil surfaces and thus available for transport through erosion processes. Measurements of Kd for 238U are particularly useful because of the extensive use of 238U in military applications and associated testing, such as done at Los Alamos National Laboratory (LANL). Site-specific measurements of Kd for 238U are needed because Kd is highly dependent on local soil conditions and also on the fine soil fraction because 238U concentrates onto smaller soil particles, such as clays and soil organic material, which are most susceptible to wind erosion and contribute to inhalation exposure in off-site populations. We measured Kd for uranium in soils from two neighboring semiarid forest sites at LANL using a U.S. Environmental Protection Agency (EPA)-based protocol for both whole soil and the fine soil fraction (diameters<45 microm). The 7-d Kd values, which are those specified in the EPA protocol, ranged from 276-508 mL g-1 for whole soil and from 615-2249 mL g-1 for the fine soil fraction. Unexpectedly, the 30-d Kd values, measured to test for soil-solution exchange equilibrium, were more than two times the 7-d values. Rates of adsorption of 238U to soil from solution were derived using a 2-component (FAST and SLOW) exponential model. We found significant differences in Kd values among LANL sampling sites, between whole and fine soils, and between 7-d and 30-d Kd measurements. The significant variation in soil-solution exchange kinetics among the soils and soil sizes promotes the use of site-specific data for estimates of environmental transport rates and suggests possible differences in desorption rates from soil to solution (e.g., into groundwater or lung fluid). We also explore potential relationships between wind erosion, soil characteristics, and Kd values. Combined, our results highlight the need for a better mechanistic understanding of soil-solution partitioning kinetics for accurate risk assessment.
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http://dx.doi.org/10.1097/01.HP.0000258924.55225.cdDOI Listing
July 2007

From dust to dose: Effects of forest disturbance on increased inhalation exposure.

Sci Total Environ 2006 Sep 17;368(2-3):519-30. Epub 2006 Apr 17.

Los Alamos National Laboratory, Health Physics Measurements Group, Mail Stop J573, Los Alamos NM 87545, USA.

Ecosystem disturbances that remove vegetation and disturb surface soils are major causes of excessive soil erosion and can result in accelerated transport of soils contaminated with hazardous materials. Accelerated wind erosion in disturbed lands that are contaminated is of particular concern because of potential increased inhalation exposure, yet measurements regarding these relationships are lacking. The importance of this was highlighted when, in May of 2000, the Cerro Grande fire burned over roughly 30% of Los Alamos National Laboratory (LANL), mostly in ponderosa pine (Pinus ponderosa) forest, and through areas with soils containing contaminants, particularly excess depleted and natural uranium. Additionally, post-fire thinning was performed in burned and unburned forests on about 25% of LANL land. The first goal of this study was to assess the potential for increased inhalation dose from uranium contaminated soils via wind-driven resuspension of soil following the Cerro Grande Fire and subsequent forest thinning. This was done through analysis of post-disturbance measurements of uranium air concentrations and their relationships with wind velocity and seasonal vegetation cover. We found a 14% average increase in uranium air concentrations at LANL perimeter locations after the fire, and the greatest air concentrations occurred during the months of April-June when wind velocities are highest, no snow cover, and low vegetation cover. The second goal was to develop a methodology to assess the relative contribution of each disturbance type towards increasing public and worker exposure to these resuspended soils. Measurements of wind-driven dust flux in severely burned, moderately burned, thinned, and unburned/unthinned forest areas were used to assess horizontal dust flux (HDF) in these areas. Using empirically derived relationships between measurements of HDF and respirible dust, coupled with onsite uranium soil concentrations, we estimate relative increases in inhalation doses for workers ranging from 15% to 38%. Despite the potential for increased doses resulting from these forest disturbances, the estimated annual dose rate for the public was <1 microSv yr(-1), which is far below the dose limits for public exposures, and the upper-bound dose rate for a LANL worker was estimated to be 140 microSv yr(-1), far below the 5 x 10(4) microSv yr(-1) occupational dose limit. These results show the importance of ecosystem disturbance in increasing mobility of soil-bound contaminants, which can ultimately increase exposure. However, it is important to investigate the magnitude of the increases when deciding appropriate strategies for management and long-term stewardship of contaminated lands.
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http://dx.doi.org/10.1016/j.scitotenv.2006.03.003DOI Listing
September 2006

Increased wind erosion from forest wildfire: implications for contaminant-related risks.

J Environ Qual 2006 Mar-Apr;35(2):468-78. Epub 2006 Feb 2.

Los Alamos National Laboratory, Health Physics Measurements Group, Mail Stop J573, Los Alamos, NM 87545, USA.

Assessments of contaminant-related human and ecological risk require estimation of transport rates, but few data exist on wind-driven transport rates in nonagricultural systems, particularly in response to ecosystem disturbances such as forest wildfire and also relative to water-driven transport. The Cerro Grande wildfire in May of 2000 burned across ponderosa pine (Pinus ponderosa Douglas ex P.&C. Lawson var. scopulorum Englem.) forest within Los Alamos National Laboratory in northern New Mexico, where contaminant transport and associated post-fire inhalation risks are of concern. In response, the objectives of this study were to measure and compare wind-driven horizontal and vertical dust fluxes, metrics of transport related to wind erosion, for 3 yr for sites differentially affected by the Cerro Grande wildfire: unburned, moderately burned (fire mostly confined to ground vegetation), and severely burned (crown fire). Wind-driven dust flux was significantly greater in both types of burned areas relative to unburned areas, by more than one order of magnitude initially and by two to three times 1 yr after the fire. Unexpectedly, the elevated dust fluxes did not decrease during the second and third years in burned areas, apparently because ongoing drought delayed post-fire recovery. Our estimates enable assessment of amplification in contaminant-related risks following a major type of disturbance-wildfire, which is expected to increase in intensity and frequency due to climate change. More generally, our results highlight the importance of considering wind- as well as water-driven transport and erosion, particularly following disturbance, for ecosystem biogeochemistry in general and human and ecological risk assessment in particular.
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http://dx.doi.org/10.2134/jeq2005.0112DOI Listing
September 2006

Regional vegetation die-off in response to global-change-type drought.

Proc Natl Acad Sci U S A 2005 Oct 10;102(42):15144-8. Epub 2005 Oct 10.

School of Natural Resources, Institute for the Study of Planet Earth, and Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721-0043, USA.

Future drought is projected to occur under warmer temperature conditions as climate change progresses, referred to here as global-change-type drought, yet quantitative assessments of the triggers and potential extent of drought-induced vegetation die-off remain pivotal uncertainties in assessing climate-change impacts. Of particular concern is regional-scale mortality of overstory trees, which rapidly alters ecosystem type, associated ecosystem properties, and land surface conditions for decades. Here, we quantify regional-scale vegetation die-off across southwestern North American woodlands in 2002-2003 in response to drought and associated bark beetle infestations. At an intensively studied site within the region, we quantified that after 15 months of depleted soil water content, >90% of the dominant, overstory tree species (Pinus edulis, a piñon) died. The die-off was reflected in changes in a remotely sensed index of vegetation greenness (Normalized Difference Vegetation Index), not only at the intensively studied site but also across the region, extending over 12,000 km2 or more; aerial and field surveys confirmed the general extent of the die-off. Notably, the recent drought was warmer than the previous subcontinental drought of the 1950s. The limited, available observations suggest that die-off from the recent drought was more extensive than that from the previous drought, extending into wetter sites within the tree species' distribution. Our results quantify a trigger leading to rapid, drought-induced die-off of overstory woody plants at subcontinental scale and highlight the potential for such die-off to be more severe and extensive for future global-change-type drought under warmer conditions.
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http://dx.doi.org/10.1073/pnas.0505734102DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1250231PMC
October 2005