Rapid Assessment Monitoring Detects Early Threats to Drought-Stressed Native Oaks

Single oak tree in a grassy field at sunset

California’s extended drought has put native oaks into survival mode as soils dry out at greater depths and water becomes less accessible. The trees conserve water use by decreasing transpiration and reducing the amount of tissue requiring water.

At this stage, native oak trees are more susceptible to secondary pests and disease, invaders that attack trees already stressed from predisposing factors. These pests and diseases can include Gold Spotted Oak Borer (Agrilus auroguttatus), flat-headed wood borers, ambrosia beetles, polyphagous shot hole borer, root rot, scales, cankers and opportunistic fungal pathogens. The susceptibility may result in an infestation or infection that can push the trees past the “tipping point” of being able to recover.

Oak Woodland Monitoring in Southern California

Responding to an increased likelihood of pest and disease outbreaks, Dudek arborists and oak specialists worked with the Rancho Mission Viejo Land Trust (RMVLT) to develop an early-detection program for 1,423 acres of conserved coast live oak woodland, forest, and riparian forests in the Southern Subregion Habitat Reserve in south Orange County.

According to Laura Coley Eisenberg, RMVLT Executive Director, the trust instituted the early detection program as part of its adaptive management and monitoring program in response to the continued spread of Gold Spotted Oak Borer from San Diego County.

“We view pests such as the Gold Spotted Oak Borer and polyphagous shot hole borer as significant threats to the health of our protected oak woodland and forest,” Eisenberg said. “Being able to detect pests such as these early, is key to effective management.”

The oak woodland rapid assessment monitoring (OWRAM) program collects and tracks detailed data on the health of oak woodlands in a cost-effective, efficient way, and is used to make recommendations on maintaining healthy trees.

Woodland Monitoring Methods

The program uses two monitoring methods: vantage point reconnaissance and plot-centered data collection.

Vantage point reconnaissance evaluates general woodland health and canopy decline on a wide scale from accessible, elevated vantage points. Data collected includes the woodland location, canopy decline observations, canopy photographs, and suspected issues. “This monitoring provides a filter for targeting woodlands that may be experiencing change,” said Christopher Kallstrand, a Dudek urban forestry specialist. “From our observations, we are able to better identify stressed woodlands to be further assessed using more detailed plot-centered data collection.”

The plot-centered method evaluates individual trees within randomized 1/10th acre plots throughout a portion of the identified stressed woodlands. Data is collected consistent with United States Forest Service woodland and the California Air Resources Board compliance sampling protocols, Kallstrand said. For each tree, Dudek collects:

  • Site location
  • Tree location
  • Diameter at Breast Height (D.B.H)
  • Tree Height)
  • Canopy Spread
  • Crown ratings (healthy, moderate thinning, severe thinning, dead)
  • Structure (good, fair, poor, dead)
  • Woodpecker damage (none, low, moderate, high)
  • Leaf symptoms
  • Foliage/branch pests
  • Bark and wood borers
  • Borer damage level (none, low, medium, high)
  • Disease
  • Additional observations

Plot-centered evaluation is completed following the region’s wet season (typically April/May) and summer season (typically September/October).

“Monitoring oak woodlands during these periods allows arborists to evaluate the woodland’s overall health in relation to increased or decreased water availability and trees’ recovery,” Kallstrand said. When summer season monitoring is completed, Dudek analyzes the data for overall woodland and individual tree health. “We look for trends and for the absence or presence of secondary pests and disease,” Kallstrand said. “The data allows us to fully understand the woodland’s overall health, detect regional trends that may be occurring, and recommend tree management activities.”

Oak Woodland Management and Continued Monitoring

Recommendations can cover a broad range of activities including continued monitoring, pest trapping, tree removal, tree replacement/restoration, supplemental irrigation, and more. Kallstrand said pesticide use is in the toolkit, but rarely recommended.

Biannual monitoring throughout the drought will continue to be critical to early detection of pests and diseases, Kallstrand said.

The OWRAM methodology used by RMVLT was developed to meet the changing needs of oak woodlands. The program can easily be adapted for oaks in urban landscapes such as streetscapes, medians, parks, parking lots, and open space. Aerial imagery review can be used in place of vantage point reconnaissance, and plot-centered data can be redeveloped to meet the structure of urban forests.

“The continuing drought places native oaks in woodland and urban settings at risk,” Kallstrand said. “Monitoring has proven effective as a way to identify pest and disease outbreaks early and respond with the appropriate treatment.”

Christopher Kallstrand is an urban forestry specialist for Dudek’s urban and community forestry group, with over 8 years’ experience with arboricultural assessments, large-scale oak management plans, and biological resources monitoring. For more information, contact us.