Responses of Juglans californica to extreme drought with respect to slope

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Southern California experienced its most severe recorded drought from 2011 through 2016. During the 2013-2014 rainfall season, much of the region received record low precipitation and the average temperature was one of the highest ever recorded; these conditions led to extensive crown dieback and resprouting of shoots by many Southern California black walnuts (Juglans californica) by spring 2014. By summer 2017, mortality was extensive for J. californica on the campus of Cal Poly Pomona. The primary goal was to determine the relationship between tree size and mortality. Because J. californica occurs on different slopes (east-southeast and north-facing) at the study site, mortality was also described with respect to aspect and walnut population density. It was hypothesized that smaller trees would experience the greatest mortality, due to their smaller storage capacity, and that mortality should have been lower on the on the north-facing slope than on the east-southeast-facing slope, because north-facing slopes are the preferred habitat for the species. Mortality increased with decreasing tree basal diameter and diameter at breast height. Trees on the east-southeast-facing slope experienced lower mortality than those on the north-facing slope, which may be related to the fact that tree density was far greater on the north-facing slope, suggesting greater competition for resources on this slope. In conclusion, competition for water during the drought led to extensive mortality and crown dieback for trees, especially those with small basal diameters, on the north-facing slope.