Late Season Drought in Trees

For the most part, 2019 has been a very good year for tree growth in Frederick County, with abundant rainfall up until the end of July. Since the beginning of August, though, rainfall has been scarce and spotty, at best. This late season dry spell, though not as severe as a prolonged drought, can impact tree health and development.

A tree experiencing water stress initiates a number of physical and chemical changes to conserve water.

The tree may close the stomatal openings to conserve water, limit the production of chemicals, nutrients, and food reserves, drop its leaves early, or curtail its growth altogether. Will these changes have any affect on trees? There could be, according to experts. One of the immediate changes is that leaf drop may occur early and fall colors may not be as brilliant. Another change that might occur is that the tree’s height and diameter growth may be reduced, along with root development and possible seed production. Reduced growth rate is probably more sensitive in young trees and newly planted individuals than it would be in a well established individual. Drought damage may also impact a plant’s ability to ward off attacks by insects and disease, since drought stressed plants are unable to produce defensive chemicals or simply outgrow the attack. Desiccated leaves and cracked bark are more easily compromised by pathogens than healthy tissue. Therefore, less drought hardy plants or plants that already harbor infections are less likely to keep these pathogens at bay during dry periods. There have been reports of premature leaf browning of large white oaks across the region. This leaf browning may be due to the very wet conditions we had this spring, or the late season drought. Some think the tree has died— others do not think so. The real test is to see if they will leaf out next spring. Other pathogens which more easily impact drought stressed trees include pine bark beetle, pine nematode, armillaria root rot, Dutch elm disease, bark cankers, Verticillium wilt, and bacterial leaf scorch. The late season dry spell may also be setting the region up for a more severe forest fire season, because the lush herbaceous vegetation that grew when water was abundant is now drying up and contributing to light, flashy fuels.

Article by Frederick County Forest Conservancy District Board

Nature Note for 11/3/2019