Boreal forests contain over 30% of Earth’s terrestrial carbon and are an important component of the land carbon sink. However, the future ability of the boreal forest to maintain a net carbon sink is uncertain and depends on the interactions of atmospheric CO2fertilization, warmer temperatures, hotter drought conditions, and diversity in plant hydraulic strategy. Numerical vegetation models are important tools when investigating the impacts of climate change. Yet, most vegetation models do not accurately represent boreal ecology and include a simplistic representation of vegetation water stress. These model deficiencies impact our ability to forecast (a) the vulnerability of boreal forests to changing water availability, (b) and the feedbacks on key environmental conditions. Here, we perform a regional-scale model evaluation using the boreal forest version of Ecosystem Demography model 2 that includes a dynamic soil organic layer, 5 boreal-specific plant functional types, and a fully mechanistic plant hydraulic scheme. We then use ED2 simulations and observations from the Alberta Forest Inventory to ask the following questions. (1) How do changes in forest water use strategy impact emergent soil moisture and canopy vapor pressure deficit? (2) How does diversity in plant hydraulic strategy impact forest resilience to changing environmental conditions?
Results/Conclusions
Our model evaluation using the Alberta Forest Inventory shows that simulations with a simplistic empirical water limitation factor substantially overestimate forest growth over the past half century. However, when a process-based water limitation scheme is implemented and leaf and stem water potential are tracked and used to solve for root zone water uptake, transport of water vertically through the sapwood, and transpiration, and variability in hydraulic traits determine species-specific responses, the positive bias is reduced by ~60%. Further, water limitation scheme substantially impacts emergent soil moisture and canopy vapor pressure deficit, such that trees in the mechanistic simulations experience substantially lower soil moisture and higher canopy vapor pressure deficit. We then use the ED2 model to examine how species diversity and diversity in tree hydraulic strategy affects the forest growth response to environmental conditions from the 1960s to present. Given that drought-limited growth is expected to become more prevalent with climate change, these results suggest that models may underestimate future water limitation on boreal forest productivity and neglect the importance that diversity in plant hydraulic strategy has on mediating these impacts.