Drought effects on tree mortality and regeneration in northern California

The 2012–2016 California drought was the most severe in the state’s recorded history, contributing to the death of millions of trees. Through sampling 54 (0.25 ha) plots in northern California and employing standard dendrochronological techniques this study compared tree mortality and regeneration patterns before, during, and after California’s recent record-setting drought in both montane and costal environments. This study evaluated 1) the influence of habitat and competitive covariates on mortality and regeneration trends using ridge regression analysis; and 2) tree death and seedling/sapling establishment dates using dendrochronology and Superposed Epoch Analysis to explore the influence of climate on forest demographics. Results showed two trends related to climatic patterns in montane environments: (1) years with high rates of mortality were positively associated with climatic water deficit (CWD) in the 1–2 years preceding and during tree death dates; and (2) regeneration dates were significantly associated with below-average CWD in the preceding year. In montane sites, pre-drought mortality was greater at wet sites than dry sites, and drought-related mortality was positively related to canopy openness. In coastal environments, drought-related tree mortality was positively associated with maximum temperature and topographic position (e.g., greater on upper slope sites). Drought-related tree mortality occurred primarily in trees smaller than 40 cm diameter at breast height (DBH, 1.37 m) in montane forests, and exclusively in trees 80 cm DBH or smaller in coastal environments. Our findings also indicate that current demographic trends will likely reduce forest diversity in the future, especially in montane environments. For example, in montane environments mortality was greater in white pine species (Pinus lambertiana and P. monticola) than other species and regeneration was weighted towards advanced regeneration of shade-tolerant fir (Abies) species (median age of 34 years). These findings highlight the effects of fire exclusion, and the need for targeted management, including reducing tree density and returning fire process, aimed at decreasing drought related mortality, and increasing regeneration of shade-intolerant species (e.g., pines). Management should also preferentially retain medium to large trees, which demonstrated less vulnerability to mortality, to enhance the resilience and diversity of northern California forests. •Montane tree mortali