Coupled ecohydrology and plant hydraulics modeling predicts ponderosa pine seedling mortality and lower treeline in the US Northern Rocky Mountains

Abstract

Summary We modeled hydraulic stress in ponderosa pine seedlings at multiple scales to examine its influence on mortality and forest extent at the lower treeline in the northern Rockies. We combined a mechanistic ecohydrologic model with a vegetation dynamic stress index incorporating intensity, duration and frequency of hydraulic stress events, to examine mortality from loss of hydraulic conductivity. We calibrated our model using a glasshouse dry‐down experiment and tested it using in situ monitoring data on seedling mortality from reforestation efforts. We then simulated hydraulic stress and mortality in seedlings within the Bitterroot River watershed of Montana. We show that cumulative hydraulic stress, its legacy and its consequences for mortality are predictable and can be modeled at local to landscape scales. We demonstrate that topographic controls on the distribution and availability of water and energy drive spatial patterns of hydraulic stress. Low‐elevation, south‐facing, nonconvergent locations with limited upslope water subsidies experienced the highest rates of modeled mortality. Simulated mortality in seedlings from 2001 to 2015 correlated with the current distribution of forest cover near the lower treeline, suggesting that hydraulic stress limits recruitment and ultimately constrains the low‐elevation extent of conifer forests within the region.

The Topographic Signature of Ecosystem Climate Sensitivity in the Western United States

“…Hydrological processes and their persistence are highly dependent upon the topography, soil, geology, vegetation, anthropogenic modification, and the amount of water and energy available in a given landscape (Wagener et al, ). In the future, it will be important for ecological studies to explicitly identify and incorporate the spatial distribution of active hillslope hydrology processes, and their potential to change with climate shifts, if we are to fully predict ecosystem trajectories in water‐limited landscapes (Fan et al, ; Simeone et al, ; Tai et al, ). One potential way forward is to perform site‐specific analysis across the ecosystem sensitivity gradient (Figures d and a) using field‐based hydrology approaches (e.g., Martin et al, ; Hawthorne & Miniat, ; Hoylman et al, ) or distributed ecohydrology modeling frameworks that couple the water and carbon cycles (e.g., Maneta & Silverman, ; Simeone et al, ; Tague & Band, )…”

“…In the future, it will be important for ecological studies to explicitly identify and incorporate the spatial distribution of active hillslope hydrology processes, and their potential to change with climate shifts, if we are to fully predict ecosystem trajectories in water-limited landscapes (Fan et al, 2019;Simeone et al, 2018;Tai et al, 2017). One potential way forward is to perform site-specific analysis across the ecosystem sensitivity gradient (Figures 2d and 3a) using field-based hydrology approaches (e.g., Martin et al, 2017;Hawthorne & Miniat, 2018;Hoylman et al, 2019) or distributed ecohydrology modeling frameworks that couple the water and carbon cycles (e.g., Maneta & Silverman, 2013;Simeone et al, 2018;Tague & Band, 2004) We present a simple and mappable empirical method to identify ecosystems that are buffered from annual climate fluctuations (e.g., Figure S2a), an important consideration when describing local ecosystem-climate relationships and for identifying conservation and management priorities across scales (Morelli et al, 2016). It is well known that factors such as climate and climate change, combined with topography, soil types, nutrient availability, and disturbance regimes, are central drivers of the state and potential trajectory of ecosystems (Box, 1996;Delcourt et al, 1982;McCarty, 2001).…”

Ecohydrological modelling with EcH2O‐iso to quantify forest and grassland effects on water partitioning and flux ages

“…Historically, most hydrological models conceptualize vegetation as a static element with prescribed constants that parameterize the physical processes of evapotranspiration, disregarding the strong coupling between evapotranspiration and the physiological processes that drive plant phenology and water use (Fatichi et al, ; Speich, Lischke, Scherstjanoi, & Zappa, ; Wegehenkel, ). Over the past 15 years, various ecohydrological models have explicitly included dynamic vegetation parameterization to overcome such limitations (e.g., RheSYSS [Tague & Band, ], EcH 2 O [Maneta & Silverman, ; Kuppel, Tetzlaff, Maneta, & Soulsby, ; Simeone et al, ], tRIBS‐VEGGIE [Ivanov, Bras, & Vivoni, ], Cathy [Niu et al, ], Tethys‐Chloris [Fatichi, Ivanov, & Caporali, ], and FLETCH2 [Mirfenderesgi et al, ]). However, the verification of these models is often focused on short‐term to midterm hydrologic (e.g., streamflows and soil moisture) and ecological dynamics (e.g., seasonal phenology), and rarely are these models are compared with long‐term direct metrics of vegetation dynamics (e.g., biomass production and transpiration) that affect the water balance.…”

Plant Hydraulic Stress Explained Tree Mortality and Tree Size Explained Beetle Attack in a Mixed Conifer Forest

“…Topographic gradients create variations of local water supply and energy demand (Fan, ; Tai et al, ). Coupling plant hydraulics with hydrological models allows mechanistic characterizations of plant hydraulic stress across the landscape (Mencuccini et al, ) and has been useful to predict the spatial pattern of tree morality mediated by variations in physical environment (e.g., water availability) induced by topography (W. R. Anderegg et al, ; Schwantes et al, ; Simeone et al, ; Tai et al, ).…”