Drought and Beetle Impacts to Native Trees in the Santa Monica Mountains

Rosi Dagit

Published May 2019 in International Oaks No. 30: 169–176

Abstract

Since 2014, hundreds of native trees (alders, oaks, sycamores, willows) in the Santa Monica Mountains have died, mostly due to the drought, but many are also victims of the Euwallacea sp. (invasive shot-hole borer)/Fusarium complex (ISHB) and a pathogen newly carried by Pseudopityophthorus pubinpennis (western oak bark beetle; WOBB). Concerned about the ecological implications of massive native-tree loss, the Resource Conservation District of the Santa Monica Mountains (RCDSMM) initiated a citizen-science-based study of drought impacts in 2015 using 41 randomly selected plots measuring 625 m2, tagging over 350 trees in critical park areas. The trees are located near the urban/wildland interface throughout the western Santa Monica Mountains National Recreation Area. This effort was augmented in 2017 by the deployment of 46 homemade beetle traps in sensitive riparian areas to monitor direction and rates of spread of invasive beetles, and to document tree responses. Concurrently, this data provided on-the-ground information used in a NASA DEVELOP Project using remote-sensing tools and satellite data to help understand the landscape-level impacts over time. Results show that extensive drought impacts occurred in 2015, followed by increased loss associated with invasive pathogens in 2016-2017. To date, infected tree removal is the only recommended way to reduce impacts from reproductive host trees. The RCDSMM is working with regional and local parkland managers and other concerned stakeholders to develop a more appropriate and realistic management strategy for urban/wildland interface trees to meet the forecasted challenges of a changing climate.

Keywords

Quercus agrifolia, coast live oak, drought, invasive shot-hole borer, western oak bark beetle

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