Abstract
Fire is an important ecological disturbance that can dramatically alter ecological communities. In northeastern California, the frequency and extent of fires are dependent on a variety of factors including climate, elevation, topography, vegetation, edaphic conditions, and human cultural practices. In 1905, with the inception of the United States Forest Service (USFS), a strict policy of fire suppression was initiated. This practice of fire suppression virtually eliminated naturally occurring low to moderate severity fires from forested ecosystems, including those of northeastern California. Shifts in fire frequency have been shown to alter the structure of forest communities. To examine the effect of changing fire frequency on forests in northeastern California I compared species composition, size class distributions, and forest cover between pre-1905 and post-1905 periods within the Lassen National Forest (LNF). The hypotheses of my study are: 1) post-settlement decrease in fire frequency will cause a shift towards fire intolerant tree species in the LNF; 2) post-settlement decrease in fire frequency will cause a shift towards a greater number of smaller diameter trees in LNF; and 3) forest cover within LNF has increased as a consequence of post-settlement decreases in fire frequency. This study was conducted at the Eagle Lake Ranger District (ELRD) of the LNF, located in Susanville, California. Three sampling areas (Harvey, Antelope, and Pegleg mountains) were selected within LNF. To determine whether a greater number of fireintolerant smaller-diameter trees now dominate LNF, General Land Office (GLO) survey protocols were followed to collect tree species composition and tree diameter size class data at 154 section and quarter section corners located across the three sampling areas. At each section or quarter section corner, the tree nearest the corner in each inter-cardinal direction was sampled for identity, diameter at breast height (dbh), and distance to the corner. Pre-settlement (pre-1905) linear distances of forest cover were compared to postsettlement (post-1905) linear distances of forest cover across Harvey, Antelope, and Pegleg mountains to determine whether forest cover within Lassen National Forest has increased as a consequence of post-settlement reduction in fire frequency. Orthorectified aerial photos taken in 1998 were overlaid with a Geographic Information System (GIS) data layer that depicted locations of pre-settlement forest community types. Total linear distance of forest cover within the study area at each point in time (pre-and postsettlement) was digitized and quantified using ArcGIS 9.2. Species composition differed between pre- and post-settlement witness trees. Specifically, a significant decrease was observed in the number of fire tolerant ponderosa pine (Pinus ponderosa) and a marginally significant increase was observed in the number of fire intolerant white fir (Abies concolor). Across the study area, pre- and post-settlement tree size class distributions differed. Individual confirmed pre-settlement witness trees were distributed across 13 tree diameter size classes. The majority (64%) of post-settlement individual confirmed witness trees were concentrated in two of the three smallest size classes, identifying a greater number of smaller diameter witness trees post-settlement. Mean corner to tree distance across all three mountains was significantly lower in the post-settlement period compared to the pre-settlement period (t (0.05,179) = 7.040; P < 0.001). Mean (± SE) corner to tree distance in the pre-settlement period was 13.08 ± 1.35 meters versus 8.19 + 1.08 meters in the post-settlement period. Across the entire study area forest cover increased significantly from the pre- to the post-settlement period. Mean (± SE) forest cover was 0.05 ± 0.132 km2 in the presettlement period compared to 0.067 ± 0.155 km2 in the post-settlement period. The overall increase in forest cover observed across the study area was due to a 50% increase in cover at Harvey Mountain. No significant difference in forest cover between pre- and post-settlement periods was observed at either Antelope or Pegleg Mountains. The results of this study suggest that post-settlement reduction in fire frequency at Lassen National Forest (LNF) is responsible for a shift toward fire- intolerant white fir, a greater number of smaller diameter trees, a reduced mean distance from corner to witness tree (suggesting an increase in tree density), and an increase in forest cover. It is important to remember that post-settlement differences observed in species composition, size class distribution, and forest cover were identified at the local rather than regional scale. As a strict policy of fire suppression continues to be practiced by the U.S. Forest Service, it is likely based on the results of this study that the number of fire-intolerant seedlings at Lassen National Forest will increase and tree density will increase. As tree density increases, total forest cover will most likely also continue to increase, and the risk for larger, higher severity fires also will heighten. This study also demonstrated that local, rather than regional forest attributes may be important to consider when managing ponderosa pine forests. The degree to which past patterns of forest structure differ from current conditions at Lassen National Forest identified in this study can help to guide future management strategies such as controlled burning and selection of forest management alternatives which can serve to reverse undesirable trends resulting from post-European settlement influences. These management practices can be used as tools to help to restore the ecosystem integrity that has been degraded over time.