As extremely dry conditions stifle much of the nation this summer – especially in California, which is in the midst of a historic drought – trees in these areas are likely to continue to suffer.
“While it’s impossible to keep every tree in good health in times of severe drought, taking a proactive approach for a prized or sentimental tree can prevent damage,” recommends Tchukki Andersen, CTSP, board certified master arborist, certified tree care safety professional and staff arborist with the Tree Care Industry Association. “A tree that is supported with good cultural practices, proactive monitoring for pests and disease and responses to warning signs will have an increased chance of survival.”
According to a recent report in the Los Angeles Times, a Los Angeles parks department survey ending in April showed that as many as 14,000 trees in city parks – about 4 percent of the total – may have died during the last year. The death of so many trees is likely to drive up temperatures, wreak havoc on habitats and limit the capture of water. Experts predicted that even more trees will wither as Californians struggle to comply with new conservation rules.
Clearly, this is an important issue for tree health in affected areas, and it’s not going away any time soon.
So, what can we do to protect these prized trees? Drought damage starts beneath the soil line, in the form of root damage, long before any outward signs of trouble. After a tree’s unsuccessful attempt to conserve water by closing leaf stomates, its feeder roots die back, sometimes so drastically that the tree is unable to absorb enough water to support itself. Such drought stress may lead to tree death, but usually the signs of stress are much less dramatic.
“Leaves are undersized and may wilt, yellow, curl or crinkle, and may be marginally scorched or even turn brown and drop early,” explains Andersen. “Emergent shoots are short. In an effort to right the imbalance caused by root-loss, crown dieback or a general thinning of the canopy occurs.”
Opportunistic pests and diseases
Boring insects are drawn to the chemical and acoustic signals of stressed trees. The sound of the tree’s breaking water columns cues the borer to invade the tree and lay eggs. Andersen recommends applying a 3-inch layer of composted wood chip mulch on the ground over the root zone at least out to the drip line (the ground under the outer edge of the branch spread).
This will hold moisture longer for stressed roots to access, and will provide a long-term nutritional source for the soil. Prized or important trees may be protected from wood-boring insects with spray or injection chemicals, but they should be treated before becoming drought-stressed.
While all trees are at risk during long periods of drought, some are more susceptible to its effects. New transplants are highly vulnerable to drought stress; supplemental watering for the first few years of establishment is necessary, to the extent that it is allowed. Trees already under stress, such as those on dry slopes, surrounded by pavement or improperly planted, are at high risk of decline.