Impact of historic drought on the size and age structure of a Bishop pine (Pinus muricata) forest on Santa Cruz Island, California
The ongoing California drought offers a rare opportunity to address how extreme drought impacts growth, structure, and population dynamics of a coastal tree species, Bishop pine (Pinus muricata D. Don) near the southern and driest edge of its range, on Santa Cruz Island (SCI). Scientific and anecdotal evidence indicates that drought induces widespread mortality of Bishop pines on SCI; however, it is unknown how age classes of trees differ in their vulnerability to drought. Our research addressed the following questions: 1) How does severe drought impact the stand structure and age classes of Bishop pines on SCI? 2) How well do spatial and temporal factors affecting water availability correlate with Bishop pine growth and survival? We randomly established 24 forest inventory plots on SCI and quantified the sizes and densities of all live and dead trees, saplings and seedlings. Increment cores were collected from a subset of live and dead trees at each plot, from which tree rings were measured and cross-dated. We also extracted values from spatially explicit data layers of topographic, geomorphic, and climatic variables for each plot, and used these variables in a multivariate regression analysis to explain variability in mortality across the stand.
Of the trees that died recently (approximately 35% of all trees), the highest mortality rates occurred for those individuals that established between 1980 and 1986, which was just prior to the last severe drought period (1987-91) that drove widespread Bishop pine mortality on SCI. Approximately 75% of the seedlings that likely established during the 2012-2014 drought survived. These results suggest that established seedlings and saplings are relatively resistant to drought, whereas older trees are more prone to drought-induced mortality. Preliminary results suggest that fog inundation and a topographic wetness index, a measure of water availability, have the greatest explanatory power of tree mortality across plots. Tree growth rates correlated positively with winter rainfall (r = 0.47) and negatively with winter vapor pressure deficit (r = -0.49), indicating strong growth sensitivity to winter soil water recharge. The strength of these correlations is more comparable to tree species from the arid Southwestern U.S. than Bishop pines from coastal northern California. Our study provides baseline information about the vulnerability of Bishop pines at different life stages to drought-induced mortality. Only with these types of datasets can we accurately predict the likelihood that Bishop pine, and other drought-sensitive coastal species, will persist in a warmer, drier future.