For now, the best way to reduce impacts of drought and invasive beetles on wildland trees and shrublands in the Santa Monica Mountains is to plant more trees now and with the RCD on January 20, 2018.
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 invasive shot hole borer/Fusarium disease (ISHB/FD) and a new pathogen carried by the western oak bark beetle (WOBB).
Concerned about the ecological implications of massive native tree loss, the RCDSMM initiated a volunteer-based study of drought impacts in 2015, tagging more than 350 trees in 41 randomly selected 25-meter plots located in critical park areas near the wildland urban interface (WUI) throughout the western Santa Monica Mountains National Recreation Area (SMMNRA).
On Tuesday, December 5, more than 25 people braved the winds to hear the results of a study that began in January 2017 from the Resource Conservation District of the Santa Monica Mountains (RCDSMM) and NASA DEVELOP team at the Topanga library.
Working over two semesters, Kelsey Foster (Project lead), Arianna Nickmeyer, Natalie Queally and Nick Rousseau, under the direction of Dr. Natasha Stovaros, took on the enormous task of correlating Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data with temperature, precipitation, topography and results of the tree condition study coordinated by the RCDSMM.
This effort was augmented in 2017 by the deployment of 46 homemade beetle traps in sensitive riparian areas and upland interface sites to monitor the direction, rate and spread of invasive beetles, and document tree responses. Volunteers contributed more than 2,000 hours to this effort!
WHAT WE LEARNED
While drought was definitely the major stressor causing tree and shrubland mortality, the extreme temperature days (over 95oF) may also be playing a role in tree decline, although that relationship requires additional study. Of the 110,183 acres of vegetation alive in 2013 (not including annual grasslands) in the SMMNRA, only 77,840 live acres remained in 2016. Trees and shrubs also died in 32,343 acres in that same time frame.
The AVIRIS imagery was flown in spring of each year, and the tree conditions were documented each fall. The average rainfall of 2017 was not sufficient to recharge the groundwater basins and restore water tables, so many trees and shrubs continue to suffer and die. The synergistic feedback loops between rainfall, extreme temperature days, and tree life cycles need more examination.
The tree plot data showed that tree condition declined significantly from less than 10 percent in 2015 to more than 20 percent in 2017. If the rain gods so not favor us next year, because of the time lag between environmental conditions and tree response, it is possible that we will continue to see increased tree and shrub mortality.
Of the 299 coast live oaks tagged within the 41 randomly selected study plots, the majority were mature trees, with few young trees observed. This suggests that the pattern of regrowth where all the trees are the same age, is a result of the reduced firewood cutting after electricity became more widely available in the early 1900s, as well as fire management strategies and the limited number of natural pulses when rainfall and acorns were both abundant and able to grow.
Larger trees are more attractive to the invasive shot hole borers, that attack both irrigated and wildland trees. The native western oak bark beetle used to only infest already declining trees as a secondary pest, but now it also attacks healthier trees and kills them with the foamy canker pathogen.
THE BAD BEETLES
The current infestation of ISHB and WOBB in the SMMNRA is patchy, but where it is established, has caused the death of many trees. During the 2017 trap season, 15 traps (32%) detected presence of ISHB and/or WOBB. Plot data on tree condition revealed that in 2015, no invasive beetles were observed on trees and only 29 (22%) trees inspected had borer holes of any kind. However, by 2017, more than 150 (42%) trees showed evidence of borers, both native and invasive.
The Los Angeles County Agricultural Commission deployed GSOB traps throughout the SMMNRA during summer 2017, and, to date, no GSOB has been detected. The US Forest Service also deployed traps for emerald ash borer. Results of that effort have not yet been obtained.
HUMAN ACTIVITY SPREADS THE INFESTATION
Calabasas is the hardest hit, with dying California sycamores and oaks located throughout the city. The beetles are tiny and cannot fly far, so spread is mostly associated with human activities: improper handling of infested trees following removal, contaminated mulch, and green waste facilities are all likely sources. While it is tragic, not to mention expensive to remove trees, that is currently the only sure way to stop the spread of these pests. There are no chemical treatments that have been shown to do more than slow things down, if at all. It is really important to make sure that any infested tree is properly removed, and the wood and slash treated carefully. Most tree companies cannot chip things small enough, so for now, paying the fee to dispose of the material at Lost Hills landfill is our safest option.
THE VALUE OF TREES
More than 9,000 coast live oak and 114,000 riparian trees died in the past four years, a much higher mortality than is common among these usually long-lived trees. While it is always tricky to put a value on living organisms such as trees, there are very real services these trees provide that are measurable.
The amount of stored and sequestered carbon, and the amount of stormwater runoff absorbed are the most common contributions that are translated into economic costs and benefits.
Using our tree species and size data, we calculated the value of these two ecosystem services at more than $22 million/year. The potential loss of the remaining oaks and riparian trees is conservatively estimated at more than $105 million per year.
These estimates do not account for other important contributions of our trees: pollution removal, temperature moderation, habitat value, aesthetic value, real estate values or replacement and removal costs. Since most of our native trees typically live for over 100 years, the long-term contributions quickly reach the billions. These are truly sobering numbers to contemplate.
Another concern about acres of standing dead trees and chaparral shrubs is whether they increase our wildfire risk. Although the NASA DEVELOP team were short on time, the data layers they developed are of great interest to our local fire modeling community. The proof of concept effort to use county-provided Light Detection and Ranging (liDAR), which has one-meter resolution to develop a novel strategy for risk assessment, looked at a small area of Trippett Ranch. Stay tuned for more about those results in the future.
The results of the Landscape Level analysis, combined with both invasive beetle trap data and details from the small research plots provides local jurisdictions, park agencies, and property owners direction on ways to map the spread of impacts, evaluate the potential ecosystem service impacts, develop risk assessment strategies for addressing wildfire risk, identify and prioritize potential management response strategies to make best use of limited resources.
This effort also set the stage for modeling microclimate distribution shifts of oak woodlands and shrublands to projected conditions associated with climate change. Continued coordination to develop an Early Detection Rapid Response plan is critical to preventing severe loss of our important wildland parks.
PLANT 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 wildland trees.
While all of this is rather sobering news, there is hope. Planting new native trees whenever and wherever it is appropriate can help ensure that our wildlands continue to survive into the future.
To learn more, please join us on Saturday, January 20, 2018, in lower Topanga State Park at the old Rodeo Grounds to plant more trees!