Modeling the interactions of forest cutting and climate change on the hydrology, biomass and biogeochemistry of a northeastern forest

Understanding both short- and long-term impacts of harvesting practices (e.g., cutting rotation length, intensity) on forest dynamics in the context of climate change is a key factor in developing criteria and guidelines for sustainable forest management practices. The biogeochemical model, PnET-BGC has been used to simulate forest biomass, and soil and stream chemistry at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA. PnET-BGC was modified and tested using field observations from an experimentally whole-tree harvested northern hardwood watershed (W5) at HBEF. The parametrized/modified model was applied to other experimentally cut watersheds at the HBEF; including a devegetation experiment (W2; devegetation and herbicide treatment) and a commercial strip-cut (W4) to confirm the ability of the model to depict ecosystem response to a range of harvesting regimes. The confirmed model was used as a heuristic tool to investigate long-term changes in aboveground biomass accumulation and nutrient dynamics under three different harvesting intensities (40%, 60%, 80% watershed cutting) for three rotation lengths (30, 60, 90 years) under both constant (current climate) and changing (MIROC5-RCP4.5) future climate.

The modified multi-element soil-layer model PnET-BGC was able to depict differences in stream water, soil chemistry and element budgets resulting from different tree cutting experiments. The model also captured the ability of all cut watersheds to limit stream nutrient losses by rapid regrowth of new vegetation. Biomass accumulation in the cut watersheds was found to approach similar levels by the end of the simulations period. Simulations suggest that tree harvesting under constant current climate should affect living tree biomass and woody debris more than soil carbon, while under climate change, loss of soil organic matter pools may adversely affect site fertility. Depletion of soil base cations is accelerated under climate change due to increases in soil mineralization, coupled with increased plant uptake and enhanced biomass accumulation. Simulations show that all management options under climate change enhance both timber production and carbon storage in comparison to stationary climate, but with greater potential for a reduction in long-term soil fertility.