Controls on Mountain Plant Diversity in Northern California: A 14,000-Year Overview

A network of eight Holocene paleoenvironmental records from lakes in the Klamath Mountains of Northern California provides insights on how diverse coniferous forests are maintained in the face of climate change. Pollen data suggest that in most cases plants kept pace with climate change. The steep costal-to-inland precipitation gradient resulted in asynchronous responses to climate change with coastal forests responding before inland sites. This was likely due to the proximity to oceans, warm valleys, and the differential responses to changes in ocean upwelling. Plants growing on soils with heavy metals showed little response to Holocene climate variability, suggesting that they experienced stability during the Holocene, which helps explain the localized plant diversity on the harsh soils. Plant communities on soils without heavy metals adjusted their ranges along elevational gradients in response to climate change, however. Fires were a common occurrence at all sites and tracked climate; however, sites that were more coastal experienced fewer fires than inland sites. Fire severity remained similar through the Holocene at individual sites; however, it was low to moderate at southern locations and higher at more northern locations. The article highlights historical factors that help explain the diversity of plant species in the forests of Northern California and provides insights for managing these complex ecosystems.