Stomates are openings in tree leaves that allow the exchange of carbon dioxide and oxygen, a process that's instrumental in the tree's photosynthetic food making. Leaf stomate closure is one of the first protective measures taken by drought-stricken trees. During the sun's peak on a hot summer day, healthy trees close their stomates but reopen them during cooler evenings. During severe drought, trees close stomates and never reopen them. Different tree species respond differently to drought and rebound at different rates.
University of Georgia foresters note that yellow poplar (Liriodendron), sycamore (Platanus) and buckeye (Aesculus) are several species known to drop leaves during a drought. Other trees such as dogwood (Cornus) have leaves that wilt when exposed to drought. Continued drought causes dogwood leaves to dry out and die. The leaves will not drop until the tree receives enough hydration to complete the shedding process. Pine trees may shed needles on their bottom branches, but they conserve water and resources to encourage growth in the upper crown. If your pine tree has dead needles below but living green needles on higher branches, the tree is reacting normally to drought conditions.
Some trees, such as oaks, are fixed-growth trees, which develop and encourage shoot and leaf growth in early spring. Their growth for the season is predetermined, or fixed. Sustained-growth trees, like redbud, grow rapidly in favorable conditions. The timing of the drought and the tree's growth cycle determine how long leaf regeneration can take. Sustained-growth trees rebound once conditions improve. Early season drought stunts oak leaves in the current growing season. Late season drought damages the potential buds for the upcoming season.Continuous drought conditions stunt tree growth and leaf size and may eventually kill the tree.
Trees benefit from deep watering, so they can suffer drought-like symptoms even in wet years if they receive water in sporadic bursts. Sporadic watering means the tree can't retain the moisture it needs before the ground goes dry again. To help your tree retain water, add mulch around its base. Pile it loosely and allow adequate distance between the mulch and the trunk so air can circulate. Use drip systems and soaker hoses to provide slow, consistent moisture to your tree.
During drought conditions, you might have to change your normal fertilizing and maintenance routines to protect your trees. Horticulturists recommend hand-cutting grass and flowers around tree trunks. Lawn mowers and string trimmers can injure the trunk and surface roots, allowing water leakage and exposure to disease and bacteria. Water naturally moves from an area of greater concentration to an area of lesser concentration. This process -- osmosis -- benefits tree root systems by allowing roots to absorb water from the soil. When soil is salty and dry, reverse osmosis occurs, and water moves from tree roots into the soil. Fertilizers and road salts contribute to salty soils and reverse osmosis. Horticulturists recommend not fertilizing plants or trees during droughts as this action increases the soil's salt content.