Climate-growth sensitivity of a California endemic tree, Pseudotsuga macrocarpa (Bigcone douglas fir)

Anthropogenic climate change is predicted to increase temperatures and reduce precipitation throughout most Mediterranean-climate regions. Projections indicate that species with narrow, limited distributions are at the greatest risk of climate-induced extinction. One such potentially vulnerable species is Pseudotsuga macrocarpa (Bigcone douglas fir), a large coniferous tree that is endemic to the Transverse Mountain Ranges of southern California. Although the species’ geographic extent is limited, it does occur at a wide range of elevations. Trees growing at lower elevations (300-1400 m) are relegated to cooler, mesic micro-habitats composed of steep, north facing canyons. The most common surrounding vegetation in these semi-arid environments is chaparral and coastal sage scrub. Conversely, trees growing at higher elevations (1400-2500 m) are found in continuous habitat and are more spatially extensive. Stands of P. macrocarpa at these elevations are not relegated to north facing exposure or steep slopes. In semi-arid ecosystems, such as where P. macrocarpa is found, precipitation and to a lesser extent, temperature, are the most common factors that limit growth. The dynamic relationship between limiting climatic factors and a tree’s annual radial growth is commonly referred to as climate-growth sensitivity. Research has shown that tree species growing at their dry distribution limit are more sensitive to changes in climate. I hypothesized that the growth of P. macrocarpa individuals had the highest degree of climate sensitivity at low elevations. To address this hypothesis, I used dendrochronological techniques to extract tree core samples from individuals growing at sites of different elevations. Growth chronologies were created from the annual rings of core samples and the growth patterns were then correlated with local climate data. The analysis showed that lower elevation stands had smaller trunk diameters, greater inter-annual growth variability, increased sensitivity to drought, and a higher percentage of missing annual growth rings. If projected climate scenarios hold true, P. macrocarpa populations are at risk of extirpation at these more stressful lower elevations.