Effects of changing disturbance regimes on watershed-scale nutrient cycling: An integrated modeling approach

Historical ranges of variability (HRV) for fire regimes and forest structure are increasingly used to guide forest management and provide reference conditions for landscape-level restoration. However, although important for water quality and forest health, variations in ecosystem function such as nitrogen (N) and phosphorus (P) cycling are seldom considered within this context. In the Lake Tahoe Basin, 120 years of fire suppression have likely increased forest floor nutrient contents and decreased available nutrient contents through slower nutrient cycling. Resulting high nutrient concentrations in surface runoff may contribute to lake eutrophication. A non-spatial nutrient cycling model was integrated into LANDIS-II, a spatially-explicit model of forest landscape dynamics, to examine spatial and temporal dynamics of N and P cycling under three sets of scenarios: (a) current conditions, (b) HRV for fire regimes and forest structure, and (c) various fuel management treatments. Under pre-settlement conditions simulated forest floor accumulations following fire were significantly lower (averaging 100 versus 500 kg·N ha^-1 and 4.5 versus 17  kg·P ha^-1) and available soil N was 40% greater than currently for lower elevation forests. Model results suggest that with continued fire suppression, available N may further decrease due to increased aboveground biomass and loss of shade-intolerant N-fixing species. Thinning to emulate historic forest structure will not emulate biogeochemical effects of fire, and therefore may lead to declining forest productivity. Use of HRV to inform management should extend to ecosystem processes, which respond differently than forest structure to altered disturbance regimes.