Drought is a meteorological term that means a lack of rain or precipitation over a prolonged period of time. Oftentimes foresters refer to the effects of drought as water stress. Water comprises 85 to 90% of the green weight of trees. When water is limited the equilibrium within the tree is disturbed. Trees under stress from lack of water have a reduced growth rate, a reduction in stem and shoot elongation and leaf expansion. Oftentimes re-hydration aggravates the damage caused by the drought. Hypoxylon canker provides a good example of how drought stress can precipitate a fatal reaction in oaks and other hardwoods. When the moisture content of sapwood of oaks and other hardwoods falls below 60-70%, the Hypoxylon fungus living in the inner bark is activated to invade, removing starch and other nutrients from the tree. As it colonizes and decays sapwood, a stroma of fungal tissue develops beneath the bark eventually causing the bark to slough-off (Figure 134). Symptoms of drought in hardwoods are wilting of foliage, leaf curling and bending, marginal browning and shedding. In conifers, drought is often expressed as needle yellowing and tip browning and needle shedding. Yard and specimen trees should be drip watered 2-3 times a week during droughts to prevent root loss. Trees need an extra one to three inches of water per week when there is little or no rainfall. Drip irrigation or a soaker hose is sufficient for effective watering. The ground should be watered out to the dripline of the tree. A two to four inch layer of mulch over the root system will help to conserve moisture. Pine straw, wood chips, cone mulch, pine bark or other materials can be used. Flooding can cause the air filled pores in the soil to become filled with water. This creates an anaerobic condition from which roots cannot obtain oxygen. Generally soils have 10-30% of the volume composed of air-filled spaces but the percentage decreases as water content increases. Excess soil moisture can cause a “physiological drought” by interfering with water uptake in oxygen-deprived roots. Prolonged flooding during the growing season produces a major stress. Insects and diseases play a major role in determining the survival of water-damaged trees. Insect borers, such as the bronze birch borer, Agrilus anxius, ambrosia beetles, Platypus spp. and bark beetles such as Ips spp., and the hickory bark beetle, Scolytus quadrispinous, often attack flooded trees. Flooded trees are prone to be infected with root pathogens such as Phytopthora and Pythium spp. These fungi are known as water mold fungi that are suited for waterlogged soil conditions. Plant roots stressed by reduced oxygen in waterlogged soils exude more amino acids and ethanol that attract disease spores to root surfaces. Branches and main trunks of trees submerged in floodwaters or those injured by floating debris are prime targets for canker fungi such as Cytospora, Botryosphaeria and Nectria. Salvage is often hindered by waterlogged soils. Foresters need to have a salvage plan in place and be ready to remove all dead and dying trees as soon as possible. It is not uncommon for trees damaged by flooding to die slowly over a period of 2-4 years. Therefore, trees exhibiting chlorotic crowns, numerous cones, crown dieback and leaf fall should be removed along with the insect infested trees. Table 7 lists some common tree species and their relative susceptibility to flooding in the Lower Mississippi Valley and Missouri River Divisions. The tree species listed are common throughout the South and their relative susceptibility shouldn’t change from location to location. Foresters developing forest management plans should consider the relative tolerance of trees to flooding and/or drought to tailor recommendations for certain sites. Only tolerant species should be recommended for planting in areas prone to flooding. |
Forest Pests: Insects, Diseases & Other Damage Agents |
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