Postglacial dynamics of Olympic Peninsula forests: Comparing predictions and observations

Background/Question/Methods

Climate is recognized to be a significant controller of plant distributions, but other endogenous factors -- plant traits related to establishment, competition, and response to disturbance -- are also required to explain a forest stand’s composition, age, and extent. Therefore, making accurate and precise predictions, on a local or regional scale, about a forest’s response to climate change, requires a model that is driven by environmental variables and also simulates stand-level forest dynamics. This study utilizes LPJ-GUESS, a process-based vegetation model, to simulate the growth of cohorts of up to 20 different arboreal species, at 5 sites on the Olympic Peninsula. The model is driven by transient climate data, temporally and spatially downscaled from a general circulation model. Pollen records exist at each site and serve as a critical validation dataset for the simulated vegetation. Transient simulations are performed for the same temporal coverage of the pollen records, generally beginning with the most recent deglaciation.

Results/Conclusions

Results of the climate downscaling process show interannual variability dominates millennial-scale variability since the Last Glacial Maximum. Results of LPJ-GUESS show that arboreal species of the coastal temperate rainforests, which are often treated as one plant functional type in global dynamic vegetation models, can be successfully differentiated by their temperature sensitivities and relative shade tolerances. This study supports the importance of a realistically variable climate in transient vegetation modeling applications, as well as the hypothesis that growth of Olympic Peninsula forests is energy-limited.