The 2012 drought is having obvious impacts on crops and landscaping plants, but what about forests? I recently visited a state forest in southern Indiana and noted several obvious signs of drought stress: Some tree and shrub leaves are showing colors like yellow and orange that normally are not seen until Fall. Tuliptree leaves are yellowing, dying, and falling from the trees. Some shrubs like spicebush and maple-leaf viburnum are wilting. I saw numerous blackberry canes in a forest opening drying up with fruit still on the canes. Similar symptoms could be observed in forests around the state.
An organism as long-lived as a tree may face several droughts of varying intensity through its long lifespan, so many of our native trees are well-adapted to occasional drought. However, extended, severe, or multi-year droughts may test even the hardiest plants. Past droughts in Indiana and surrounding states have given us some guidance for expectations related to this drought. Tuliptree (the state tree of Indiana) is a common tree in many forests across the state that has experienced decline or death following severe drought. The earliest signs of stress are yellowing leaves that may later turn brown and drop during the growing season. In subsequent years upper branches may die back partially or completely. In some cases the entire tree will die. These symptoms of branch and tree death may emerge over a period of several years after the drought. Tuliptree growing on upland sites where soil moisture and depth may be limited even in the best of times can be particularly at risk. Drought can damage or kill almost any tree growing on a site where the species is not well-suited to grow. Trees that love full sunlight, grow quickly, and have seeds easily spread by wind or wildlife can start growing on disturbed sites of all types, from bottomlands to hilltops. They may do very well so long as normal rainfall is available, but once a severe drought hits, species like tuliptree, sycamore, silver maple, and cottonwood may be killed on the drier sites, leaving behind the slower growing oak and hickory that are more tolerant of drought.
Drought can harm trees by means other than water stress. Trees under stress are more susceptible to attack by insects, fungi, and other pathogens. Wood boring beetles and some fungi can cause significant damage and death to trees already weakened by drought. Some of these pests are native to our forests, but can create much more damage when trees are stressed. Some are exotic species that are already causing great damage, like emerald ash borer or Asian longhorn beetle. The Purdue Plant Pest Diagnostic Lab can assist with identification and treatment recommendations for forest pests.
Extended drought may also make our forest more susceptible to wildfires. The eastern hardwood forest is generally not at great risk from the explosive type of fires seen in the west, but substantial fires have happened during dry, low humidity conditions, similar to the weather we have seen recently. In most years our woodlands are too green, moist, and humid to carry a large fire, but as the wood becomes drier, the chance for a grass fire moving into and burning through the woods increases. Many grasslands are already extremely dry and flammable. Areas with planted or natural conifers like white pine or eastern red cedar can burn rapidly and may be able to carry a crown fire where entire trees burn and ignite neighboring trees. The Hoosier National Forest has a site recounting significant forest fire events in Indiana: http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5298750.pdf
What can a forest landowner do?
Watering forest trees is impractical or impossible. Drought has been and will continue to be a natural part of forest processes that sort trees onto the sites where they are best suited. Scout your woods for signs of drought damage or death. Look for trees with recently dead or dying upper limbs, areas of bark dying and peeling, particularly on the upper trunk, evidence of wood borer damage to trunks or branches, and sprouting of new limbs and leaves at the base or on the trunk of larger trees. Remember that these symptoms may appear during a drought or up to several years following the drought. If you experience tree decline or death, you may want to salvage useable wood for personal use or for sale to the wood using industry here in Indiana. Consult a professional forester for advice and assistance with timber harvests and other forest management. A listing of professional foresters in Indiana can be found at www.findindianaforester.org. You can contact a Purdue Forestry and Natural Resources Extension Specialist at firstname.lastname@example.org
Two publications to assist you with decisions related to selling timber are:
Tips on How to get the Most from Your Timber Harvest
To protect your forest from wildfire, observe all outdoor burn bans. Be aware of ignition sources that might not be obvious – exhaust systems on vehicles in contact with dry vegetation can start fires. Tilled or bare soil can provide firebreaks to prevent wildfire spread.
It is too late to prepare your forest for the drought of 2012, but some forest management practices may help prepare your woodlands to better cope with future weather extremes. Correctly planned and executed thinnings, harvests, and control of grapevines and invasive plants can all improve the vigor and resilience of individual trees and the forest as whole.
The Forest Improvement Handbook provides information on management practices to improve forest health.