Quantifying Multi-Decadal Drought Effects on Mixed Oak Savannas at Sedgwick Reserve

Kaili Brande, University of California, Santa Barbara, Bren School of Environmental Science and Management

Frank W. Davis, Bren School of Environmental Science and Management, University of California, Santa Barbara

Dan Sousa, Jet Propulsion Laboratory, California Institute of Technology

Olivia d’Arezzo, University of California, Santa Barbara

Understanding mortality patterns and trends among California oaks is critical to oak woodland management efforts throughout the state. Yet examinations of these patterns rarely span longer than a few years or decades. We reconstructed trends in overstory oaks for the 80-year period, 1938-2018, to better understand long-term demographic trends in mixed oak savannas in the foothill landscapes of Sedgwick Reserve. We were especially interested in comparing mortality rates during the 2012-2016 severe drought compared to the earlier record for coast live oak (Quercus agrifolia) and valley oak (Quercus lobata). 

We analyze rates and trends of mortality by size, species, and location using field data and high resolution digital archival air photos. Field data, collected for blue oak (Quercus douglasii) in addition to coast live oak and valley oak, include measurements of diameter at breast height and crown diameter (short and long axis) as well as visual assessments of tree condition and vital status.

While mortality rates appear relatively constant for all species from 1938-2004, preliminary results suggest higher mortality rates for coast live oaks than for valley oaks in recent years, especially since the most severe drought period of 2012-2016. Consistent with these preliminary findings, we expect that further analysis will reveal higher mortality rates in drought periods versus non-drought periods, and higher levels of mortality for coast live oaks than valley oaks.

With climate change, increasing climate variability could lead to higher probability of severe multi-year droughts, posing a greater challenge to these species than the occasional drought year. Understanding these climatic effects and their implications for sustaining oak woodlands is an important research priority.