Changes in Forest Structure and Potential Fire Behavior in Old-Growth Forests from 1873-2010 in the Lake Tahoe Basin, USA Anna Vandervlugt The Pennsylvania State University, University Park, PA
Introduction The suppression of frequent, low- to moderate-severity fires in lower elevation pine dominated forests in the American Southwest and California Sierra has allowed fire-intolerant species such as white fir to increase in density, creating fuel buildups that increase the risk of historically unusual high severity crown fires. Uncharacteristic stand-replacing fires in mixed conifer forest put wildlife habitat and critical watersheds at risk, affecting endangered species and humans who benefit from ecosystem services these watersheds provide. My study area, the Lake Tahoe Basin, is currently experiencing the impacts from landscape-altering fire suppression policies, although no one has yet quantified changes in potential fire behavior. Within the basin, there is a consensus that knowledge of pre-Euro-American settlement conditions should inform the management plans and practices and that restoration should aim to create conditions similar to pre-settlement conditions. Although most forests in the basin were logged in the late 19th century, eliminating most of the historical evidence needed for constructing reference conditions, existing old-growth forests can be used to help us identify and understand how a functioning fire regime shaped vegetation structure. This study identified forest reference conditions and how forests have changed in terms of structure and fuels over an extended period of fire suppression. These data were used to estimate how forest changes affected potential fire behavior. The reconstructed forest data can be used to provide a range of target structure s for restoration goals .
(1) Identify the composition and structure of contemporary forests including size, age, and species structure and quantify current canopy and surface fuel load of forest stands (2) Identify the composition and structure of pre-suppression reference forests, including canopy and surface fuel load, using dendroecological, historical methods, and model estimates (3) Quantify changes in forest structure and potential fire behavior caused by fire suppression using the reconstructed vegetation and fuels data and fire behavior models.
Methods Change in the Tahoe Basin: extensive logging in the mid-1800s gave way to fire suppression policies and a young, dense contemporary forest.
Study Area The Tahoe Basin is situated on the eastern side of the Sierra Nevada. The lake surface sits at an elevation of about 1920 m, and the surrounding watershed rises to 3000 m. The watershed was glaciated and the resulting topography is diverse, sloping up to 30º or more from the flat valley floor. The climate is Mediterranean with cool wet winters and warm dry summers. Annual precipitation is 500-1500 mm, with 80% falling as snow in the winter. Mean daily temperatures at lake elevation range from -6ºC in winter to 24 ºC in summer, with a 75 day frost-free growing season (Barbour et al. 2002). My study area is located in and around the General Creek Watershed (GCW) on the west side of the basin, where the most extensive stands of unlogged old growth forest are located. Species composition varies across sites, with pine dominated stands on south facing slopes, fir dominated stands on north slopes, and valley bottoms dominated by Jeffrey pine and lodgepole pine. The GCW was not logged and the study sites I used have not been burned by prescribed fire. The last natural fires occurred in the late 19th century.
Lake Tahoe Basin, West Shore
Plot densities significantly increased from 1873 to 2010 (p <0.001). Between individual species, shade tolerant white fir increased most dramatically over time, from a mean density of 82 stems/ha in 1873 to 347 stems/ha in 2010 (p <0.01).
Relative importance values of fire-intolerant fir have increased on average for all plots because of the pulse of regeneration since the time of the last fire, although pine species still dominate the larger size classes. In the contemporary forest, white fir is disproportionately more abundant than other species. In 1873 species composition was more evenly distributed.
Contemporary forest structure fuel load of old-growth mixed conifer forests were measured in each of thirteen 0.5 ha plots on the west shore of Lake Tahoe. These plots were originally established in 1999 and used in a previous study of changes in fire disturbance. Diameter at breast height (DBH) and heights of stems >5 cm DBH were measured, and dead trees and logs were visually assigned classes based on decomposition. Surface fuels were measured along transects at four points in each plot. Canopy fuels were measured by analyzing hemispherical photographs. Pre-suppression forest conditions were estimated for the 1873, the year of the last widespread fire in the study area. Trees that established after 1873 eliminated from the stands, and the diameters of trees alive in 1873 were estimated by subtracting growth from live trees, and estimating diameter of trees dead in 2010 using decomposition rate equations. The spatial distributions of stems by size and age class for the 2010 and 1873 forests were analyzed using Ripley’s K(t) and Moran’s I statistical tests. Potential fire behavior was simulated for the 2010 forest and the 1873 forest using the Fire and Fuels Extension to the Forest Vegetation Simulator (FVSFFE). Changes in stand structure and fire behavior variables from 1873 to 2010 were identified using Kruskall-Wallace H tests.
Hemispherical Canopy Photo Measurement of down and dead woody material
Fire models show that potential fire severity has significantly increased from 1873 to 2010 (p<0.001). In 1873, low intensity surface fires were seen at all fuel moisture percentiles. In 2010, all plots experienced a crown fire at all weather conditions. At the 98th percentile weather conditions, representing extreme fire weather, more than 60% of plots experienced an active crown fire.
The more open stand structures of the 1873 forests resulted in surface fires with low flame heights that killed only the smallest trees (above left). The dense structures of the contemporary forest provided ladder fuel to propel a fire into the crowns of trees, resulting in complete mortality of the stand (right).
Conclusions Over a century of fire suppression policies have resulted in significant changes to forest structure and potential fire behavior in the Tahoe Basin. Forest changes are similar to those quantified in other fire suppressed mixed conifer forests throughout California. The forest has shifted from an open pine-dominated structure to a dense fir-dominated structure. Without periodic surface fires to remove fir saplings and create small gaps in the canopy, fire dependent pine species risk being outcompeted and replaced by the shade tolerant firs. The dense understory of fir creates a heavy fuel load that is able to support extremely severe crown fire, putting the Basin at risk for widespread habitat and property loss. This study provides an understanding of how human activity alters disturbance regimes over time. My results have direct significance to the restoration of the Tahoe Basin, since my study area forests have been recognized by the Tahoe Watershed Assessment as the most critical to restore and to reduce fire hazard. Low and mid-elevation forests are closest to the urban interface, where high values are at risk and relative fire susceptibility index is highest. Managers have also decided to use reference conditions as a guide for a structural restoration of the Basin, and are using prescribed fire to remove fuels, reduce fire hazard, and begin to restore natural conditions to the highly altered forests. Future research could be done on regeneration patterns and mortality rates of different species in the old-growth stands to quantify patterns of succession over the past century of fire suppression. The short and long-term effects of prescribed burning could also be monitored to measure the success of fuel load reduction efforts.
Acknowledgements This project could not have been completed without the support, guidance, and summer research funding provided by my advisor, Dr. Alan Taylor. I am also grateful for the dedicated work of my field interns, Derek Furry, Melissa Harkavy, Warren Reed, and Ben Wharton, and the advice shared by my colleagues Stockton Maxwell and Andrew Pierce.
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