Gardening: How trees respond to stress

Landscape trees planted along the Front Range of northern Colorado face challenges in their fight for survival in a sometimes harsh environment. Our trees are subject to drought, temperature extremes, high winds and poor soils.

By Nika Reininger
Colorado State University Extension Master Gardener in Larimer County

As living organisms, trees react to stress with a variety of mechanisms aimed at plant survival and reproduction. Here are a few examples of changes that can occur to a tree in response to common stressful growing conditions:

Drought
Drought and water restrictions are commonplace across our state, with wide seasonal differences in water availability for plants. Water plays a critical role in a plant’s many chemical reactions and gives structure to plant cells. Water evaporates through tiny “holes” in leaf surfaces known as stomata, through a process known as transpiration. Loss of water from aboveground tissues (such as leaves) leads to uptake of water and minerals through the roots. This cycle creates an evaporative cooling effect in the tree’s surrounding environment. Under drought conditions, growth slows as the plant struggles to maintain existing tissues. Stressed plants may direct energy towards flower and seed production instead of root or shoot growth. Drought can lead to early fall color displays. Tree growth rates and overall vigor may be slowed for years following a drought.

Wind
Windy conditions lead to increased transpiration rates, resulting in increased water usage. If soil moisture levels are low, or if the soil is frozen, a tree in windy conditions may transpire water faster than its roots can take up soil moisture. This is especially problematic for delicate evergreens, which retain their foliage through the winter months as well. When a tree loses water faster than it can be replaced, wilting and eventual tissue death can occur.

Overwatering and Compacted Soils
Excess water can be as disastrous as drought for trees. Root systems require oxygen as well as moisture. Compacted or overwatered soils contain little space for air around a tree’s roots. As roots begin to rot in response to lack of soil oxygen, they can no longer support aboveground plant tissues. Symptoms of overwatering can be deceptively similar to symptoms of underwatering, including leaf wilting. Leaves may appear scorched as water is lost through respiration faster than the roots can take up soil moisture. In contrast to the “top-down” death of plant tissues under drought conditions, plants suffering from excess soil moisture are often characterized by lower foliage that yellows and eventually dies from in the “inside-out”. Proper soil drainage is important for overall tree health. Trees growing in compacted or overly wet soils often have roots growing closer to the soil surface than trees planted in well-drained soil, as the roots seek oxygen necessary for growth.

Temperature Extremes
Tender young buds are vulnerable to dieback from spring frosts, especially after emerging during a period of unseasonable warmth. Rapid temperature changes can damage plant cells, resulting in problems such as sunscald and frost cracks on a tree’s trunk. These problems can be exacerbated by reflected light and/or heat from the surrounding hardscape. Tissue or plant death may occur when a plant is exposed to temperatures below its hardiness zone. While rates of transpiration and photosynthesis generally rise with increasing temperatures, under very hot conditions a plant may not be able to produce food for itself (in the form of sugars through the process of photosynthesis) quickly enough to keep up with transpiration rates, resulting in cessation of plant growth.

Mechanical or Chemical Injury
Mechanical trauma to a tree, such as pruning cuts, hail damage, lawnmower injury to the trunk, or limbs breaking under heavy snow, leave the plant vulnerable to secondary disease problems or insect infestations. Nearby chemical applications can interfere with tree growth, even if not sprayed directly on the tree in question. Incorrect pesticide or herbicide use, or even simple over-fertilization, may result in color changes to a plant’s leaves, leaf curling, abnormal growth or tissue necrosis.

Insects and Pathogens
Insects and pathogens are most often seen in association with trees that are already weakened from improper cultural management. For example, fungal root rots are problematic in waterlogged soils, as overly moist conditions favor fungal growth. Healthy trees have natural defenses; stressed trees are vulnerable to fungi, bacteria, insects and other organisms. Remember, choosing the right plant for the right place, utilizing the correct planting technique, and providing appropriate cultural care will keep the tree healthy and naturally disease-resistant.

The following Colorado State University Extension links offer a deeper understanding of the science behind tree health:
Fact Sheet #2.926, “Healthy Roots and Healthy Trees.” http://www.ext.colostate.edu/pubs/garden/02926.html
Fact Sheet #2.932, “Environmental Disorders of Woody Plants.” http://www.ext.colostate.edu/pubs/garden/02932.html
CMG GardenNotes #103, “Diagnosing Root and Soil Disorders on Landscape Trees.” http://www.ext.colostate.edu/mg/gardennotes/103.html
CMG GardenNotes #331, “Plant Pathology.”
http://www.ext.colostate.edu/mg/gardennotes/331.html
CMG GardenNotes #633, “The Science of Planting Trees.” http://cmg.colostate.edu/gardennotes/633.pdf