Forest productivity under climate change - projections across species and forest types in a coastal British Columbia landscape

Background/Question/Methods

Forests provide a number of important ecosystem services, but a changing climate is expected to affect forest processes, such as productivity, in a variety of ways that will vary by species and local environment. We have limited understanding of how the growth of individual, co-occurring tree species will be altered by climate change, but making such projections is important both for managing forest resources and for advancing knowledge of forest behavior under global change. Here we apply an ecophysiological forest production model (PnET-II) to estimate potential productivity (ANPP) in 12 tree species under recent and projected climate conditions across a managed forest landscape in coastal British Columbia. To account for environmental heterogeneity, the landscape was stratified into subregions based on climate and soils and PnET-II was parameterized separately for each. The model was also parameterized with species-level and functional group trait data identified in a thorough literature survey. We then examined model-generated estimates of productivity across species and site types under different climate conditions to test the effect of recent versus late 21st Century normal climate, the effect of interannual climate dynamics over the course of the next century, and the potential combined effects of CO₂fertilization and climate change.

Results/Conclusions

Validation and sensitivity analysis of modeled productivity estimates indicate that PnET-II performs reasonably in the study area, with ANPP averaging ~500gC/m²/yr under current conditions. Our results show that climate change is likely to affect forest productivity in coastal British Columbia, but that changes are contingent on species, local environment, and the potential effects of increased CO₂. Hardwood tree species are predicted to see increases in ANPP of up to 50%, assuming no CO₂ effects, while conifers will change relatively little (<10%), with growth likely to decline in some warmer, drier areas and increase in cooler, wetter areas. The most productive areas of the landscape will shift from warmer, lower elevation areas that receive relatively less precipitation to cooler, higher elevation areas receiving more precipitation, due to increased summer moisture demand. We found that CO₂ fertilization effects could be quite substantial, leading to increases of 60-70% in most conifer species and 120% in hardwoods. This ecophysiological modeling work suggests that in terms of growth rates, hardwood species are likely to benefit the most from climate change in these conifer-dominated forests, while the overall magnitude of change is highly contingent on the extent of CO₂ fertilization effects.