Northern Sierra Historical Range of Variability and Current Landscape Departure

Kevin McGarigal, Becky Estes, Scott Conway, Danielle Perrot, Marilyn Tierney, Terri Walsh, Christiana Liang, Laurie Perrot, Edward Smith

This report describes modeling historical range of variability (HRV) and current departure for landscapes within the Northern Sierra ecoregion in California and Nevada. We discuss the need for this study with respect to the historical and contemporary context of the ecoregion, including background on the natural range of variability concept and the use of simulation modeling to quantify it. For several focal landscapes within the Northern Sierra ecoregion, we simulated wildfire disturbance and succession processes under historical reference period conditions (circa 1550-1850) at the 5 m spatial resolution. Based on the output, we quantified the mean Fire Return Interval (FRI) under historical dynamic equilibrium conditions and estimated departure of the contemporary fire interval from the historical FRI for each cell. We also quantified the FRI for high-mortality wildfire (i.e., complete overstory mortality) as a proxy for vegetation persistence under historical reference period conditions, in which short return intervals represented locations where early-seral vegetation would be expected to predominate over time and, conversely, in which long return intervals represented locations where late-seral vegetation would be expected to predominate over time. Lastly, we quantified the HRV in forest vegetation patterns and compared the results to the patterns present in the current landscape to quantify departure at three scales or levels: Sites, Geophysical Units (GPUs) and Subbasins. We defined Sites as 40-m radius plots (1.25 acre) centered on each focal cell, which corresponds to the predominant ecological neighborhood of a single mature tree. We further classified each Site to one of four Biophysical Classes (BPCs) that discriminated Sites on the basis of simulated tree productivity (from low to high) under historical dynamic equilibrium conditions. We quantified HRV as the range of variation across Sites and over time under historical dynamic equilibrium conditions for three different vegetation metrics. We quantified the current range of variability (CRV) as the range of variation across Sites in the current landscape and compared it to HRV to estimate current departure for Sites in each BPC. We illustrate the use of the Site-level metrics to inform unit- and project-level planning and management, as well as their use for monitoring vegetation changes over time across the landscape. We defined Geophysical Units (GPUs) as contiguous spatial units discriminated on the basis of relative homogeneity in 10 geophysical variables and averaging 50 acres in extent. We quantified HRV for each GPU independently for a suite of 11 vegetation composition and configuration metrics, and compared the current condition to HRV for each metric separately and in combination to estimate current departure for each GPU. We illustrate the use of the GPU-level metrics to identify and prioritize local areas for treatments designed to restore historical reference conditions, as well as their use in monitoring treatment implementation. We defined Subbasins based on Hydrologic Unit Code (HUC) level 12 watersheds, which averaged approximately 15,000 acres in size. We further partitioned each Subbasin into BPCs. We quantified HRV for each Subbasin-by-BPC partition independently for a suite of 6 vegetation composition and configuration metrics, and compared the current condition to HRV for each metric separately and in combination to estimate current departure for each landscape partition. We illustrate the use of the Subbasin-level metrics to identify and prioritize broad areas for projects designed to restore historical reference conditions.