Impacts of global change and fire suppression on diverse forest communities in the Klamath mountains (CA) from 1969 to 2014
Historical vegetation datasets offer unique opportunities to assess changes in diversity, structure, and composition of plant communities over long time spans. Studies that focus on historical datasets from diversity hotspots can be especially valuable for both understanding long-term changes for a large number of taxa and for developing management practices aimed at the preservation of diverse species assemblages. In this study, we assessed changes in tree composition and structure in montane, subalpine, and alpine forests in the Klamath Mountains (California, USA) by resampling 77 out of 200 vegetation plots first sampled by John Sawyer and Dale Thornburgh (Humboldt State University) in 1969. The area surveyed, which harbors one of the highest levels of conifer richness in the world (18 species), is threatened by a warming climate, drought, pest outbreaks, and the alteration of disturbance regimes (primarily fire suppression). To accurately compare changes in vegetation, we replicated the Sawyer-Thornburgh 1969 sampling protocol and, in addition, we individually measured and assessed all trees greater than 7.6 diameter at breast height (dbh).
Our results demonstrate that significant changes have occurred over the past 45 years, and that changes in forest composition and structure were the most pronounced in the subalpine zone (2700 to 3500 m elevation). We hypothesize that much of the change is related to decreased snowpack, which makes trees more vulnerable to pathogens including mistletoe and bark beetles. Among the nine conifer species we analyzed, we observed the largest changes among Abies (fir) species, with increases in canopy cover in Abies concolor (white fir) and decreases in cover for Abies lasiocarpa (subalpine fir) and Abies magnifica var. shastensis (Shasta red fir). Furthermore, our results strongly suggest an ongoing outbreak of Scolytus ventralis (fir engraver beetle) which is primarily targeting Shasta red fir, with 50% of all 443 trees surveyed in 2014 dead or currently dying. Our results reveal major changes in this region over the past 45 years and can help inform optimum monitoring and management strategies aimed at preserving this diverse ecosystem.