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Urban Forest Resiliency: Supporting Southern California’s Trees Through Drought and Extreme Heat

Mina Lai | February 1 2022

Drought and extreme heat challenge the longevity of trees and their ability to offer multiple benefits, from offering nutritious fruits to the shading of mingling humans. A 2017 symposium cohosted by the US Forest Service, Pacific Southwest Region 5, and the USDA California Climate Hub shared that the three most significant factors contributing to tree mortality in the Sierra Nevada were water stress from drought, warming temperatures, and increased beetle activity resulting from weakened trees. While the study was focused on forest trees, drought and heat are still major concerns for urban areas where buildings and hardscape reflect heat and further elevate temperatures. Fortunately, there are tangible steps community members can take to support their neighborhood’s trees through such conditions.

A young tree experiencing drought stress.

Impact of Drought and Extreme Heat

National Geographic defines drought as a prolonged period of below-normal precipitation from rain or snow that causes reduced soil moisture availability. When trees experience water shortage, tiny openings on leaves called stomata close to limit the loss of water. Because these are the same openings the tree use to intake carbon dioxide used during photosynthesis, closing the stomata for too long reduces photosynthetic activity – and thus plant growth. Warm weather increases water loss in the plant through evapotranspiration – think of it as a tree ‘sweating’- which leads to faster drying of soil. Extreme heat (2 days or more of temperatures higher than 90°F) causes stomata closure and drought-like conditions even when precipitation remains the same. Drought and extreme heat both stress and weaken trees, making them more vulnerable to pests/disease with eventual death if left unaddressed.

Choosing a Resilient Tree

Choosing a suitable tree will increase the likelihood of it surviving drought and extreme heat. Trees that tolerate southern California’s drought and heat tend to have small, long-narrow, or needle-like leaves (less surface area to lose water), waxy leaves (shiny coatings to retain water), gray/silver leaves (heat-reflecting) and extensive root systems to draw in more water. Examples are Desert Willow (Chilopsis linearis) with their thin-narrow leaves, Strawberry Tree (Arbutus marina) with a waxy leaf surface, or for larger spaces, the Coast Live Oak (Quercus agrifolia) with its coated leaf curving inward to reduce surface area exposed to sun. Arbor Day Foundation’s Tree Wizard offers more species recommendations: enter your zip code, specify what kind of trees you want, your soil type, sun exposure conditions, and desired traits to get options. The Urban Forest Ecosystem Institute at California Polytechnic State University offers SelecTree which provides supplemental details on the recommended species’ compatible site conditions and considerations including allergy reactions. Some organization offer free trees to homeowners, such as the City Plants program in Los Angeles.

Planting Practices

Planting methods that support a tree’s long-term health will help maintain resilience in the face of drought and heat stress. Sacramento Tree Foundation contains videos and planting practices, many of which I’ve seen practiced by the City of Ontario, California where drought has regularly occurred in the past decade. Recommended practices include:

  • Choose a location at least 5’ clear of any underground pipes and 10’ clear of lighting poles
  • Dig a hole 2 times wider than the container or rootball size
  • Till the soil to break up large chunks before backfilling the soil around the planted tree, to aerate the soil and give it tiny pocket rooms for water to be stored
  • Plant the tree just deep enough to expose the trunk flare (the spot where trunk connects to roots) above the soil; this gives roots access to near-surface oxygen
  • Add a 3” thick layer of mulch surrounding the tree to insulate the soil from extreme heat and reduce evaporation. Leave a 6” mulch-free clearance around tree base to prevent trunk rot. Look for mulch made from organic materials like hay or wood shavings instead of bark, because it decomposes into nutrients that improve soil health for the tree. Check your city for free mulch programs like the one the City of Los Angeles Sanitation offers
Recommended tree planting practice

Watering Trees

Watering needs vary depending on trees, soils, time of the year, and other factors.
A common way to check on soil moisture availability is to insert a screwdriver 8” below the soil and remove it. If the screwdriver pulls out clean due to dry and crumbly soil, water is needed. If soil is sticking to the screwdriver, it means there is sufficient soil moisture (see helpful video).

The Arbor Day Foundation recommends 10 gallons of water for every inch of a tree’s diameter, but in many cities like Claremont, California, the guideline is 15 gallons per week for young trees (less than 3 years in the ground.) This is usually divided into 2 or 3 times of the week depending on season. For example, if watering 3 times a week for summer months, then you might water 5 gallons on Monday, 5 gallons on Thursday, and 5 gallons on Saturday – spread out the days to avoid rot.

Mature trees may require watering just once or twice a month. These trees prefer occasional deep watering instead of frequent light watering that only saturates the upper soil and ends up encouraging roots to stay at shallow depths where soil moisture can easily be lost to extreme heat. Deep water by using a hose around the root zone but away from the trunk, with water running on slow trickle; soak until water reaches the ground covered by the furthest reach of the canopy – this should represent soaking of 12-18” below surface.

California ReLeaf offers helpful videos on watering, in both English and Spanish.

If using an irrigation system, consider tree bubblers to make more efficient use of limited water. These spray heads let water out gently at lower pressures in narrow stream patterns. This avoids sudden flooding that can result in wasted runoff, and because those streams are directed at the rootball, water infiltrates to areas the tree roots can actually reach for use. Unlike sprinklers which fan out water, these precise streams minimize the amount of water wasted from being carried away by wind.

Irrigation bubblers efficiently deliver water to trees.

The Environmental Protection Agency offers other water efficiency tips, such as using weather or soil-moisture based controllers that automatically adjust your irrigation schedule to avoid watering right after it has rained: this prevents valuable water from being used when it is not needed and reduces water bills. The overall aim of choosing more efficient equipment is to ensure that although water is being used for a worthy purpose of keeping trees alive, is is being used for its intended purpose and no unnecessary amounts are wasted.

You may also want to consider alternate ways of obtaining water during droughts. Reusing graywater – the resulting waste water from washing hands at sink, showering, or laundry – to irrigate trees is a safe and beneficial way of recycling valuable water. Greywater Action is a great place to begin learning how to implement graywater systems safely in your homes. Another option is rainwater harvesting, which is the action of collecting or storing water from rain instead of letting them run off into the storm drain. “Rainwater harvesting can be as simple as directing [roof gutter] runoff to basins around your [trees] or collecting it in a rain barrel, or more complex systems may include gutters, storage tanks, pumps, and a delivery system” (Water Use it Wisely 2021.) Public agencies like SoCalWater$mart or San Diego County have incentive programs you can make use of to help cover for rainwater harvesting barrels as well as water efficient irrigation controllers.

Addressing Drought Symptoms

Even with appropriate tree selection, planting practices, and care, drought stress may still impact trees. Early action can help trees recover before their condition worsens. Signs of drought stress include wilting leaves, loss of their deeper green color, early leaf dropping, and smaller than usual leaves. When trees are undergoing drought stress, ensure you are continuing to periodically deep water and supplement mulching to conserve soil moisture as needed around the tree while leaving clearance around the trunk. Avoid using fertilizers during drought, because this is when tree roots are susceptible to burning from fertilizers’ salts or chemicals. Furthermore, avoid pruning the tree during this period when it has been weakened to minimize the chances of pest/disease infections at open cuts.

Drought and extreme heat conditions may be a burden, but there are ways to help trees thrive for years to come, enhancing neighborhood values, cooling hotter days to reduce air conditioning needs, and supporting human and wildlife’s well-being. Choosing a suitable tree species, planting it to support its growth, watering efficiently, and minimizing damage during drought stress are all effective ways communities can prolong the longevity of their trees.

Mina Lai is a Landscape Architecture graduate student at Cal Poly Pomona. She has an interest in intermixing urban and natural realms together for multiple benefits for humans (physical and mental well-being), animals, plant/fauna, and the overall environment living beings share. She hopes to build various technical skills and insight necessary to harmonize a sustainable and healthy balance of the numerous factors affecting the outdoor environments.