Evaluating theories of drought‐induced vegetation mortality using a multimodel–experiment framework

Abstract

305I.305II.306III.310IV.311V.314VI.316VII.317VIII.318318References318 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 modeling 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.

Tree mortality predicted from drought-induced vascular damage

“…A detailed plant hydraulics model 21 was parameterized using published vulnerability-to-cavitation curves and extensive branch and whole-tree hydraulic data from aspen trees of the study region and then evaluated against published estimates of transpiration and whole-tree hydraulic conductance in three aspen stands. This model was then used to examine levels of CWD above which mortality occurred by calculating the time spent at high percent loss of conductivity from cavitation and soil drying, indicated as being a key mortality predictor variable in a previous multi-model comparison study that included this hydraulic model 26 . Using the field-derived threshold in CWD (5,470 ± 153 mm), we then compared predictions of 'mortality likely' areas to three mortality data sets: published high-resolution remotely sensed maps of mortality severity generated using Landsat imagery 30 ; polygons of high mortality determined via aerial surveys; and an independent set of 57 field sites distributed throughout the region.…”

“…Simulations using modelled monthly soil moisture from 2000-2013 at the field sites where soil characteristics had been measured revealed a strong relationship between CWD and time spent at high percent loss of hydraulic conductivity ( Supplementary Fig. 4), indicated previously as a useful mortality predictor variable 26 . Furthermore, high canopy mortality occurred at a threshold CWD value similar to that observed in branch conductivities ( Supplementary Fig.…”

Predicting the response of the Amazon rainforest to persistent drought conditions under current and future climates: a major challenge for global land surface models

“…The use of different WSF formulations in different land surface models (Egea et al, 2011;Zhou et al, 2013) reflects our inability to define the general behavior(s) for multi-species biomes in which the physiological processes are not yet fully understood. The use of hydrodynamic models that do not include empirical soil moisture response functions, but instead predict drought-induced stomatal closure from the simulation of hydraulic potential, in the continuum soil-plant-atmosphere, has demonstrated some promising results (Williams et al, 2001;Fisher et al, 2006Fisher et al, , 2007Zeppel et al, 2008;McDowell et al, 2013).…”

“…Mortality is a complex process, highly nonlinear in both time and space (Allen et al, 2010;Fisher et al, 2010;McDowell et al, 2011), and is represented by a wide array of algorithms in commonly used LSMs (McDowell et al, 2013). The inability to simulate drought-induced tree mortality is expected from a compartment carbon model such as ISBA CC that has no deterministic climate-mortality relationship.…”

Improving the representation of hydrologic processes in Earth System Models

“…An alternative is to model the soil-plant hydraulic continuum using both the hydraulic properties of soil and cavitation in plant xylem represented with s-shaped curves representing the distinct changes in root, stem, and leaf hydraulic conductances with declining xylem pressure [Sperry et al, 1998]. This approach has been used to extend the TREES model [Mackay et al, 2003] to accurately predict root water uptake in response to drought [McDowell et al, 2013].…”