Awareness is rapidly increasing that predictions of tree mortality rates and behavior is a top research priority. We are experiencing increased frequency and/or intensity in droughts, extreme temperatures, wildfires, biological invasions, and land-use change, all which strongly impact forest survival. This is a major concern because forests account for 45% of the global terrestrial carbon stocks, and serve as large carbon sinks of anthropogenic CO2 emissions. We emphasize the urgency to quantify and accurately predict which regions of the world are transitioning faster from carbon sinks to sources (or vice versa), and which forests types will have the largest impact on carbon stocks, or are more likely to not recover from forest die-offs. To answer these questions, we use a cutting edge demographic vegetation model (DVM) with dynamic vegetation processes called FATES (Functionally Assembled Terrestrial Ecosystem Simulator) which is also coupled to state-of-the-science Earth System Models (ESMs), to predict tree mortality at regional and global scales.
Results/Conclusions
A advantage of FATES is that turnover rates are dynamic, partially a result of mortality rates which respond to changing climates and resource competition. Tree mortality is govern by many plant traits and can occur either by carbon starvation based on respiration and resource acquisition via allometry, hydraulic stress, freezing tolerance, fires, age-dependent senescence and background mortality, which is further separated by understory or canopy level trees.
We explored the mortality and biomass response to (hypothetical) extreme droughts, concluding that FATES produced nonlinear responses to extreme droughts. Due to differences in process representations, sensitivity of biomass loss diverged based on either duration or intensity of droughts. CO2 fertilization alone did not buffer ecosystems from mortality during extreme droughts in the majority of our simulations. Our findings highlight discrepancies in process formulations and model uncertainties, notably related to availability in plant carbohydrate storage and the diversity of plant hydraulic schemes. After including a trait-based plant hydraulic scheme that dynamically changes with continuous plant growth in FATES, we investigated shifts in mortality when permafrost melts for an Alaskan boreal site. Simulations showed that hydraulic-based mortality became less seasonally variable, and no significant change in hydraulic mortality with melting permafrost, but carbon starvation mortality increased. As a result there was a shift in plant type from evergreens to deciduous plants. To improve future modeling predictions, we further evaluate and define plant traits that buffer multiple mortality types, from a functional perspective that is capable in FATES.