Drought resistance is greater in montane conifers compared to coastal conifers in northern California

Frequent and intense droughts are rapidly altering stand dynamics in western North American forests. The ecological and geographical diversity of northern California provides a unique opportunity to measure these responses across species, habitat types, and levels of competitive pressure. This study used dendrochronological techniques and linear mixed-effects models to assess growth responses to drought in four montane and two coastal conifer species across 54 study sites (nine sites per species, 526 trees total) in northern California. Montane species included Abies magnifica var. shastensis, Picea breweriana, Pinus lambertiana, and Pinus monticola; coastal species included Picea sitchensis and Tsuga heterophylla. Growth was evaluated from 2002 to 2018 and the drought period was from 2013 to 2015. There were significant differences among species and environments (coastal vs montane) in growth, drought resistance and resilience, and annual latewood proportion. Growth in montane species was generally positively related to moisture availability (Palmer Drought Severity Index) and negatively related to tree competition. The four montane species maintained relatively stable drought resistance, resilience, and latewood proportion across the study period. In contrast, growth in the two coastal species was influenced more by tree size and crown ratio than moisture availability or competition. As the 2013–2015 drought proceeded, coastal species showed marked reductions in drought resistance and resilience and increases in latewood proportion. Across the six conifer species, mean reductions in growth during and after the drought were generally less than 20% and never exceeded 40%. Compared to montane species, the lower resistance measured in coastal species suggests greater risk for increased stress and mortality in the event of more severe, prolonged, and/or frequent droughts.