In an era of rapid climate and land-use change, quantifying the patterns and processes of past ecosystems vitally informs the stewardship of contemporary ecosystems. In California, the Public Land Survey System (PLSS) systematically collected tree inventory data between 1872-1884 at 93.2 km2 (36mi2) resolution. Although these data provide an extensive record of settlement-era forest conditions, reconstructions using PLSS data have been limited and controversial in western landscapes. Recent improvements in the application of plotless density estimators (PDE) have made reconstructions more accurate and robust. The purpose of this study was to use PDE to reconstruct the settlement-era forest conditions in Six Rivers National Forest – a floristically diverse temperate forest in the Klamath Mountains of northwestern California – in order to quantify differences with modern conditions. Records of fires and harvests were used in conjunction with the PLSS data to understand the influence of forest management during the previous century. Our questions were 1) Has tree density and basal area of the contemporary forest increased in comparison to settlement-era forest conditions? 2) Were oak and pine dominated forests more common during the settlement era? 3) Do documented changes in the disturbance regime since the settlement era explain the differences in forest structure and composition?
Results/Conclusions
The contemporary forest in Six Rivers contains three times more trees than in the settlement era with a comparable increase in tree basal area. Forest composition during the settlement era was predominantly Douglas-fir (34.4%), pine (24.2%), and oak (21.9%) by basal area. Contemporary forests support more Douglas-fir (45.2%) while oaks have decreased by more than half (9.3%). These increases in tree abundance occurred despite extensive, mid-century timber harvesting in Six Rivers. Although large fires have burned in Six Rivers between 2000 and 2019, far fewer fires occurred during the twentieth century. Our results suggest that effective fire suppression contributed to the densification of the contemporary forests in Six Rivers. The magnitude of difference between settlement and modern forests may aid California land managers tasked with setting and evaluating restoration practices.