Simulation modeling of the effects of disturbance on forest structure and nutrient cycling in the Lake Tahoe Basin
AuthorKaram, Sarah Lynne
AdvisorWeisberg, Peter J
Environmental and Natural Resource Sciences
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It is widely acknowledged that fire and biomass harvesting can have substantial effects on forest structure, but their interactions with nutrient cycling dynamics are less understood. In water-limited forests with frequent fire, it is hypothesized that fire-related ecosystem fluxes (e.g., volatilization) may exceed water-related fluxes (e.g., leaching). Because conventional sawlog harvesting primarily removes nutrient-poor wood from the ecosystem, it may not have the same importance as fire to nutrient cycling, even though it can have similar effects on forest composition and age structure. We developed NuCycling-Succession, a novel nutrient cycling extension for the LANDIS-II landscape model of forest dynamics, to examine the influence of fire and biomass harvesting regimes on nutrient cycling at multiple spatial scales in the Lake Tahoe Basin. NuCycling-Succession models carbon (C), nitrogen (N), and phosphorus (P) nutrient fluxes and masses associated with the living biomass, dead biomass, soil organic matter, soil mineral N and P, charcoal, and bedrock nutrient pools. It includes direct effects of disturbance events on nutrient cycling as well as the indirect effects of disturbance as mediated through changes in forest composition and structure. NuCycling-Succession represents the continuum of decomposition and associated changes in chemistry using annual cohorts of leaf and fine root litter. This formulation may provide a more realistic representation of decomposition dynamics and their interactions with disturbances that affect the forest floor, like fire, than that in most widely used nutrient cycling models. Modeled results for soil C and N are not significantly different from field measurements and the majority of nutrient pools and fluxes fall within the range of measured values from empirical studies in similar forests. We used NuCycling-Succession to examine the influences of fire and biomass harvesting regimes on N cycling at multiple spatial scales in the Lake Tahoe Basin. The modeling scenario included the historical trajectory of changes in fire and biomass harvesting regimes, including the pre-settlement fire regime, subsequent fire exclusion and coincident clearcut logging, and then biomass harvesting for forest management. At the watershed scale, these changes in the disturbance regimes result in increasing masses of nutrient pools and magnitudes of nutrient fluxes over simulation time that are primarily related to an increasing fire rotation period associated with fire exclusion. At this scale, fire and biomass harvesting do not drive the majority of ecosystem nutrient fluxes. In regions where fire rotation periods are 10 or 15 years and annual evapotranspiration (AET) is less than 240 mm yr-1, fire-related volatilization flux exceeds water-related leaching flux of N overall. At the site scale, fire-related ecosystem N fluxes are dominant in 98.6% of fire years and fire-related N transfer fluxes between nutrient pools exceed other transfer fluxes in the majority of fire years for up to three years after fire. Although biomass harvesting can have substantial effects on forest structure at the site scale, its effects on N cycling are marginal, indicating biomass harvesting is not an adequate surrogate for fire. The relative importance of fire to N cycling varies with time since the previous fire event and in relation to fire rotation and AET, providing boundary conditions under which fire is more important than water. Considering the dominance of fire-related nutrient cycling dynamics under particular environmental conditions, forest management that incorporates fire may more effectively reduce the masses of nutrient pools and restore nutrient cycling dynamics to their pre-settlement conditions than management based on biomass harvesting.