Where is the best place to replant native trees following the Woolsey Fire? Where will there be suitable habitat to support native trees into the climate-changed future? How can we best protect existing trees from threats like the Invasive Shot Hole Borer and Gold Spotted Oak Borer?
These questions initiated a coordinated effort between local park agencies, Los Angeles County, and Ventura County that was funded by Los Angeles County Supervisor Sheila Kuehl. Over 90 percent of the Santa Monica Mountains National Recreation Area (SMMNRA) was burned in the Woolsey Fire in November 2018.
Everyone wants to help restore the woodlands and native plant communities as well as the homes lost. But guidance on the best places to restore connectivity and habitat, as well as strategically plant to reduce ember ignitions needs to be based on understanding where conditions will be suitable for native trees to survive and mature in the next 100 years under the potential predicted climate changes.
Trees live a long time, and the locations where trees are found now reflect past opportunities for seedling recruitment and survival, as well as anthropogenic impacts. Areas with suitable conditions to allow trees planted now to grow into maturity needed to be identified. In collaboration with the NASA DEVELOP team at the Jet Propulsion Laboratory and more than 100 community science volunteers, we collected and analyzed both ground-based and remote sensing data to find the answers. The results are found in two reports available online at https://www.rcdsmm.org/resources/caring-for-your-oak-trees/.
The Los Angeles County Native Tree Priority Planting Plan (2019) summarizes the results of this analysis. Based on examining the conditions supporting existing mature native trees, and where trees remained following the Woolsey Fire, and existing climate conditions, we modeled how the distribution of the suitable areas that could support native trees might change in the future.
A LONG-TERM EXPERIMENT
Our effort built upon statewide climate change habitat suitability analysis (Thorne et al. 2016), as well as more locally focused surface flow analysis (Taylor et al. 2019) to identify suitable planting locations for each of the 14 most common or rare native tree species. This plan is the basis for a long-term experiment to see whether our climate models were correct, but also will encourage persistence of these important species both now and into the uncertain future. It takes 20-100 years for these trees to reach maturity. Planting now, and then managing and protecting the seedlings for their first five years, is a critical step that can teach us much.
For example, the abundant coast live oak, which is widespread currently, should be able to handle the potential climate changes and could be planted in many locations. The benefits provided by this keystone species not only include providing food and habitat connectivity for over 6,000 associated animals and insects, but also their dense evergreen canopy can catch and slow the spread of wildfires. Strategic placement of coast live oaks along the wildland urban interface (WUI) zone and along the wind corridors could provide multiple benefits.
By contrast, California sycamore trees whose roots need access to a reliable water table, are restricted to wetter canyon areas along the creeks. Climate projections shrink the areas where those conditions are potentially going to occur in the future, although thanks to the rains last winter, volunteer seedlings are sprouting in many burned areas.
Does that mean we avoid planting this species now? No! Individual trees that have survived the seven-year drought, the infestation of invasive shot hole borers, and the fire will be producing seeds again in spring 2020. Selecting seeds from these survivors and planting them in drainages that both now and in the future could support their growth is still recommended.
Another major threat to the future of our native trees is associated with a variety of invasive pests. The Gold Spotted Oak Borer (GSOB) targets oak trees, but the invasive shot hole borer (ISHB) attacks not only many of our important native riparian trees, but also many species of ornamental and urban trees. At this time, the only treatment to contain these pests is removal of the infested tree.
The Early Detection-Rapid Response Plan (EDRR Plan 2019) prepared for the SMMNRA provides a template that can be expanded into all parts of the county. Relying on a coordinated effort to deploy traps followed by visual surveys, the Los Angeles County Agricultural Commissioner’s team will be conducting a baseline survey of the whole county in the spring. The EDRR Plan outlines steps for coordinating regional efforts to identify and share information about the spread of these pests.
“This will be an instrumental tool for officials working to fight against new infestations,” noted Khoa Lam, Los Angeles County Deputy Agricultural Commissioner. “Early detection is a vital aspect to identify hotspots and will be critical to help battle against invasive species that threaten the health of our native trees.”
By setting the stage for coordinated planning by Los Angeles and Ventura Counties, we hope to define the leading edge of the current infestation and develop strategies for containment. Together, these two documents provide a road map for protection, preserving, and enhancing the future of native trees in the Santa Monica Mountains.
The Resource Conservation District of the Santa Monica Mountains (RCDSMM) is located at 540 S. Topanga Canyon Blvd., Topanga, 90290. (818) 597-8627.
By Rosi Dagit, RCDSMM Sr. Conservation Biologist