Wallis Robinson | USGS Science

USGS2026

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.

California↗

USGS2023

Drought survival strategies differ between coastal and montane conifers in northern California

Increasingly severe and prolonged droughts are contributing to tree stress and forest mortality across western North America. However, in many cases, we currently have poor information concerning how drought responses in forests vary in relation to competition, climate, and site and tree characteristics. We used annual tree ring evidence of 13 C discrimination (Δ 13 C) and growth metrics to assess drought resistance and resilience for six conifer species at the intersection of several bioregions in northern California. Within each species' range in northern California, we collected competition and tree characteristics from 270 focal trees across sites that varied from wetter to drier habitat conditions (54 sites). Across sites, all six conifer species weathered the severe 2013–2015 drought with reasonably high resistance and post-drought resilience. However, we found important differences in drought responses between coastal and montane species based on annual growth and Δ 13 C metrics. Broadly, the two coastal species showed consistent declines in drought resistance across successive drought years, whereas the four montane species maintained high drought resistance across drought years. More specifically, we found lower Δ 13 C and growth during drought years in coastal species, suggesting stomatal closure during drought with the potential for vulnerability to carbon depletion during long-term drought. Conversely, Δ 13 C and growth were stable in montane species throughout the drought, which may contribute to hydraulic failure under increased drought frequency and/or severity. We also evaluated environmental factors that affect Δ 13 C using data from before and during the drought. These physiological models were consistent for the two coastal species, with a positive relationship between annual precipitation and Δ 13 C and a negative relationship between tree density and Δ 13 C. Conversely, the four montane models illustrated a greater importance of site conditions on drought responses for these species. Our findings show differential risk for drought stress across diverse conifers during severe drought. This work highlights the importance of site and tree characteristics in determining drought responses across cool, annually humid coastal habitats to seasonally dry montane habitats.

California↗