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. In the Lake Tahoe Basin, historical reconstructions of vegetation from photographs, dendrochronology, and pollen analysis indicate that lower elevation forests were pine-dominated and open in structure with a shrub understory. Complementary reconstructions of historical fire regimes suggest that low severity fires frequently burned through these forests, selectively removing smaller trees and forest floor mass to create openings for plant establishment.

Although cycling of nitrogen (N) and phosphorus (P) is exceptionally important to forest health and water quality, these ecosystem functions are seldom considered within a historical context. Because historical data are unavailable, we have developed a new nutrient cycling extension for LANDIS-II, a spatially explicit forest landscape simulation model, to investigate spatial and temporal variation in N and P cycling in the Lake Tahoe Basin .

Empirical studies have suggested that fire is the primary mode of N mineralization in semi-arid forests, such as those in the Sierra Nevada. In the Lake Tahoe Basin, 120 years of fire suppression have likely increased forest floor N and P contents and decreased available N and P because of slow decomposition rates. Forest floor nutrient pools possibly decrease water quality through nutrients leaching into surface runoff, and the increase in these potential pools due to fire suppression may have increased the potential magnitude of this source. Under a simulated historical fire regime, modeled forest floor and available N and P contents were significantly less than those under fire suppression due to the larger nutrient pools. Variance partitioning of the source of these differences suggests that in addition to direct fire effects on nutrient pools, changes in species composition (especially N-fixers) and total biomass as a result of fire suppression also affected nutrient cycling.