Response of biomass, hydrology and biogeochemistry to alternative approaches of cutting a northern forest: model comparisons

The biogeochemical model, PnET-BGC, was modified and parameterized using field data from an experimental whole-tree harvest of watershed (W5) in 1983–1984 at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA. The model simulated the hydrology, biomass accumulation, and soil solution and stream water chemistry responses to forest cutting. The parameterized model was then 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 evaluate the ability of the model to depict ecosystem responses to a range of cutting regimes. Revisions of algorithms of PnET-BGC improved model performance in predicting short- and long-term dynamics of major elements following various approaches to forest cutting. Despite some initial differences in species composition and biomass accumulation rates among the cut watersheds, simulations of total forest biomass for all three treated watersheds (W2, W4 and W5) were consistent with expectations based on the growth trajectory of a second-growth, reference watershed (W6) at the HBEF. The modified two-soil-layer PnET-BGC captured the immediate increase in stream concentrations of NO₃⁻, Ca²⁺, Mg²⁺ and Na⁺ as well as enhanced adsorption of SO₄²⁻ following cuttings and indicated a greater response for the devegetation and the whole-tree harvest treatments than the sequential strip-cut of W4. Simulations indicated intense NO₃⁻ leaching with the devegetation and herbicide treatment and consequent accelerated decline in soil base saturation and a slower recovery pattern during forest regrowth by the end of the simulation period (2100) compared to the other treatments.