Juan Camilo Villegas

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

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 (≈4 °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.

Partitioning evapotranspiration across gradients of woody plant cover: Assessment of a stable isotope technique

in the stable isotopic composition of water vapor (d 2 H). Our technique employs a newly‐developed laser‐based isotope analyzer and the Keeling plot approach for surface flux partitioning. The applicability of the technique was verified by comparison to separate, simultaneous lysimeter and sap flow estimates of ET partitioning. The results showed an expected increase in fractional contribution of transpiration to evapotranspiration as woody cover increased—from T/ET =0 .61 at 25% woody cover toT/ET = 0.83 at 100% cover. Further development of this technique may enable field characterization of evapotranspiration partitioning across diverse woody cover gradients, a central issue in addressing dryland ecohydrological responses to land use and climate change. Citation: Wang, L., K. K. Caylor, J. C. Villegas, G. A. Barron‐ Gafford, D. D. Breshears, and T. E. Huxman (2010), Partitioning evapotranspiration across gradients of woody plant cover: Assessment of a stable isotope technique, Geophys. Res. Lett., 37, L09401, doi:10.1029/2010GL043228.

Toward accounting for ecoclimate teleconnections: intra- and inter-continental consequences of altered energy balance after vegetation change

ContextVegetation is projected to continue to undergo major structural changes in coming decades due to land conversion and climate change, including widespread forest die-offs. These vegetation changes are important not only for their local or regional climatic effects, but also because they can affect climate and subsequently vegetation in other regions or continents through “ecoclimate teleconnections”.ObjectivesWe propose that ecoclimate teleconnections are a fundamental link among regions within and across continents, and are central to advancing large-scale macrosystems ecology.Methods and resultsWe illustrate potential ecoclimate teleconnections in a bounding simulation that assumes complete tree cover loss in western North America due to tree die-off, and which predicts subsequent drying and reduced net primary productivity in other areas of North America, the Amazon and elsewhere. Central to accurately modeling such ecoclimate teleconnections is characterizing how vegetation change alters albedo and other components of the land-surface energy balance and then scales up to impact the climate system. We introduce a framework for rapid field-based characterization of vegetation structure and energy balance to help address this challenge.ConclusionsEcoclimate teleconnections are likely a fundamental aspect of macrosystems ecology needed to account for alterations to large-scale atmospheric-ecological couplings in response to vegetation change, including deforestation, afforestation and die-off.

Density-Dependent Ecohydrological Effects of Piñon–Juniper Woody Canopy Cover on Soil Microclimate and Potential Soil Evaporation

Abstract Many rangeland processes are driven by microclimate and associated ecohydrological dynamics. Most rangelands occur in drylands where evapotranspiration normally dominates the water budget. In these water-limited environments plants can influence abiotic and biotic processes by modifying microclimate factors such as soil temperature and potential soil evaporation. Previous studies have assessed spatial variation in microclimate and associated ecohydrological attributes within an ecosystem (e.g., under vs. between woody canopies) or across ecosystems (e.g., with differing amounts of woody canopy cover), but generally lacking are assessments accounting systematically for both, particularly for evergreen woody plants. Building on recently quantified trends in near-ground solar radiation associated with a piñon–juniper gradient spanning 5% to 65% woody canopy cover, we evaluated trends in soil temperature and associated estimates of potential soil evaporation as a function of amount of woody canopy cover for sites overall and for associated canopy vs. intercanopy locations. Quantified soil temperature trends decreased linearly with increasing woody canopy cover for intercanopy as well as canopy patches, indicating the coalescing influence of individual canopies on their neighboring areas. Notably, intercanopy locations within high-density (65%) woody canopy cover could be as much as ∼10°C cooler than intercanopy locations within low-density (5%) cover. Corresponding potential soil evaporation rates in intercanopies within high-density woody canopy cover was less than half that for intercanopies within low density. Our results highlight ecohydrological consequences of density-dependent shading by evergreen woody plants on soil temperature and potential soil evaporation and enable managers to rapidly estimate and compare approximate site microclimates after assessing amounts of woody canopy cover. Such predictions of microclimate have general utility for improving management of rangelands because they are a fundamental driver of many key processes, whether related to understory forage and herbaceous species or to wildlife habitat quality for game or nongame species.

Factoring in canopy cover heterogeneity on evapotranspiration partitioning: Beyond big-leaf surface homogeneity assumptions

The vast majority of water on Earths terrestrial surface is lost through evapotranspiration (ET; vaporization processes that include evaporation [E] of intercepted water, E from free-water surfaces, and transpiration [T] from vegetation [Savenije 2004]) (Jasechko et al. 2013). Management and conservation of water resources require explicit understanding of ET, particularly due to the potential for global change to alter water fluxes. Although mostly considered by its hydrological nature, ET is the result of a suite of both physical and biological processes interacting at multiple spatial and temporal scales (Jarvis 1995) and constitutes a key driver of ecosystem function via the effects of T on ecosystem water and energy balance, impacting productivity (Jackson et al. 2001). During the twentieth century, important empirical and theoretical models that described ET based on its physical drivers—particularly relevant to agriculture and water resource management—as well as sophisticated measurement techniques relevant to local scales were developed (Shuttleworth 2007). Although vegetation is acknowledged to strongly influence ET, theories that explicitly considered vegetation applied generally to two extreme cases: bare or fully vegetated soil (Shuttleworth 2007; Caylor et al. 2005). Widely used empirical models for ET, mostly derived from the Penman-Montheith equation (Montheith 1965), use a simplifying assumption…

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

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.

Reply to Sala: Temperature sensitivity in drought-induced tree mortality hastens the need to further resolve a physiological model of death

Our recent study (1) of pinon pine (Pinus edulis) response to change in climate, on which Sala (2) comments, documented that drought-induced mortality was temperature-sensitive. In addition, we showed that time to tree mortality was predicted by leaf-level cumulative respiration for ambient and warmer treatments. Notably, our study experimentally assessed temperature sensitivity of drought mortality by tracking individual physiological responses throughout the death process. Ambient and warmer treatments did not differ in water balance in such a manner as to drive differences in mortality, yet higher respiration rates under warmer temperatures were associated with earlier death of individual trees. Two related studies provide additional support implicating carbon starvation via respiration during protracted water stress. First, modeling of physiological responses indicated that even short droughts drove leaf water potential of pinon pine—a drought-avoiding, isohydric species—quickly below its zero-carbon assimilation point (3). Second, long-term observational measurements of predawn water potential of pinon pine documented that trees could survive shorter but not longer periods of water stress below their zero-carbon assimilation point (4).

Reply to Leuzinger et al.: Drought-induced tree mortality temperature sensitivity requires pressing forward with best available science

Forest and woodland vulnerability to tree mortality in response to future drought and warmer temperatures is emerging as a potentially critical impact of global change (1). We directly addressed this issue experimentally in our recent study (2), on which Leuzinger et al. comment (3). Notably, we showed drought-induced tree mortality was highly temperature-sensitive, raising concern about future die-off. Other experimental studies isolating the effect of warmer temperature on drought-induced tree mortality are lacking—a major knowledge gap given how directly such a relationship underpins the potential impacts of climate change. The shorter survival period under warmer temperatures quantified in our study correspondeds to a difference in leaf-level cumulative respiration—a response consistent with the temperature sensitivity of carbon dynamics driving differences in mortality. A simple projection of this sensitivity using a 103-year historical record of drought indicated that warmer temperatures (+4.3 °C) could increase die-off frequency 5-fold. Leuzinger et al. (3) note methodological concerns regarding the study, some of which are helpful in prioritizing future research to refine insights, but nonetheless do not negate the main findings. Furthermore, these concerns should not cloud the urgency with which the research community pursues additional research to develop an improved model of plant mortality.

Vegetation cover and rainfall seasonality impact nutrient loss via runoff and erosion in the Colombian Andes

Mountain ecosystems provide key services to a large portion of the population in the tropics. However, they are particularly vulnerable to regional environmental changes such as soil degradation, via soil erosion and associated nutrient loss, both dissolved in runoff and suspended in sediment. Current trends in land use conversion combined with projections of intensified hydrological extremes potentially amplify these threats. We analyze the interactive effects of rainfall characteristics (at three time scales) and vegetation cover on the runoff–erosion–nutrient loss progression for a group of vegetation cover types that represent different land use conversion stages. After a year of observations we found, as expected, that natural forests have the highest potential for regulating precipitation–runoff–erosion–nutrient loss. The highest amounts of runoff occurred in pasturelands, and croplands had the highest erosion losses. Croplands showed the highest concentrations of soluble nutrients in runoff and in sediment. However, due to higher runoff amounts, pasturelands had the greatest loss of dissolved nutrients. Precipitation seasonality significantly influenced both erosion and nutrient loss. This is particularly critical in managed agricultural and pasture systems where increased runoff and erosion rates, combined with unsustainable management practices, may lead to alterations in soil and water quality. Our results indicate how agricultural practices need to adapt fertilization scheduling to rainfall seasonality to minimize potential environmental impacts. Collectively, our results highlight a fundamental management need in tropical mountains where the combination of rapid land use change and altered climate threatens ecosystem integrity and ecosystem services.

Human Computation as an Educational Opportunity

“Citizen science” refers to the emerging practice in which individuals in the community, often en masse, partner with researchers to assist with data collection, analysis or interpretation. Such partnerships benefit researchers through access to data at a scale not possible for individuals or small teams. To date, the benefits to the citizen scientists have been less apparent, although some have argued that participation increases critical thinking and appreciation for science methodologies. The present chapter reports a case study in which 12-year-old citizen scientists contributed to a major research investigation of evapotranspiration and, in turn, deepened their own understanding of the water cycle.