Hydrologic stress suppresses tree regeneration and destabilizes the lower treeline in the U.S. Rocky Mountains

More frequent hydrologic stress events associated with increasing air temperatures and declining precipitation in the western U.S are resulting in more frequent and larger forest fires and tree die offs. It is also producing drier and hotter soils that are gradually becoming inadequate for seedlings, reducing the probability of recruitment and forest recovery and increasing the probability of permanent forest loss.

We use a spatially-distributed ecohydrologic model (Ech2o-SPAC) to simulate the spatial distribution of soil moisture and the conditions that generate water stress in plants at high resolution and regional extents. The model represents water stress in seedlings from a mechanistic point of view by simulating the water potential within the vascular system of seedlings. When the water potential within seedlings is very low, cavitation events that reduce water transport in the hydraulic column occur, which generate hydraulic stress. Time series of cavitation-induced low hydraulic conductivity events are combined into an index that integrates their intensity, duration and frequency to generate a dynamic stress index. The spatially distributed nature of the model permits to obtain maps of the dynamic stress index that can be directly related to the probability of seedling mortality and its influence on the regeneration potential of the lower treeline.

The model was calibrated for Pinus ponderosa seedlings using a glasshouse drought experiment and was tested using in situ monitoring data on seedling mortality from reforestation efforts. The calibrated model was used to simulate water-induced stress and mortality in seedlings in western Montana. Results show that low elevation, south facing, non-convergent topographic locations with high atmospheric demand and limited upslope water subsidies experienced the highest rates of modeled mortality. Furthermore, modeled drought mortality in seedlings from 2001-2015 correlated with the current distribution of forest cover near the lower treeline suggest that drought limits recruitment and ultimately constrains the low elevation extent of conifer forests within the region. Extrapolation of the results show that many low elevation forest regions in the western US may have crossed climatic thresholds that prevent recruitment and will probably not recover after disturbance.