What to expect from your trees after a severe drought

With the 2011 pecan harvest in our rearview mirror, some of you probably had a little extra time to enjoy working on a puzzle. Some like jigsaw puzzles. Sudoku is pretty popular, but it’s tough to beat just like a good old-fashioned crossword puzzle. So, for you puzzle enthusiasts, what is a 7-letter word for a natural process that can damage or kill your pecan trees, reduce not only this year’s crop but affect next year’s too, and usually involves El Niño/La Niña interactions?

Unfortunately, too many pecan growers readily know the answer. D-R-O-U-G-H-T. Drought.

Now if you talk to a few growers, you’ll find that most of them consider a pecan tree to be a pretty tough tree. It can take a lot of neglect and still produce a crop. But just because pecan trees have developed a reputation for being a hardy, drought-tolerant, easy-to-care-for plant doesn’t mean it won’t die. The drought of 2011 had an impact on a wide range of plants in the South, including pecan trees.

How should we define drought? Combining information from several sources, the term “drought” denotes an extended period of abnormally low precipitation, leading to a depleted soil water content to a level low enough to adversely affect growing plants, rendering them unable to perform normal life processes.

Basically, the water content of a pecan tree is reduced to a level low enough to disrupt life processes. Periods of drought are not an unusual occurrence in the native range of pecan trees. Over time, trees have developed a prioritized set of responses as drought conditions are recognized. Some of these responses are readily visible, but many can only be analyzed using special scientific equipment.

One of the first reactions to mild water stress is the closure of leaf stomata. Stomates are the small valve-like openings usually active on the underside of the leaf that allows gas exchange and water loss (transpiration). When adequate soil moisture is available, transpiration is lowest during the hottest part of the day, greatest in the morning and late afternoon, and ceases at night. Closure of the stomata is a defense mechanism to reduce transpirational water loss. However, with the stomata closed, carbon dioxide is not absorbed. This closure then causes photosynthesis to stop, and the plant stops growing.

For many species of trees, wilting leaves is a common visible indicator of water stress. The three types of wilting are incipient, temporary, or permanent. Incipient wilting is not visible, but as the stress increases, it will change to temporary wilting. It is characterized by visible drooping of the leaves during the day, followed by rehydration and recovery overnight. As drought conditions increase, temporary wilting can change to permanent wilting where the plant does not recover during the overnight period. Permanently wilted trees may recover only when additional water is added to the soil.

Extensive periods of permanent wilting will result in tree death. Unfortunately, pecans do not exhibit visible signs of wilting. Initially, water stress will result in marginal scorching (browning) of the pecan leaflets. When the supply of water becomes extremely limited, pecan trees will shed leaves to reduce water loss. The oldest leaves are shed initially, with foliage injury and defoliation being most apparent in portions of the crown in full sun. Continuation of the drought can lead to complete defoliation. If water becomes available later in the season, some trees defoliated by drought may produce a second crop of leaves from previously dormant buds.

As was mentioned in the discussion on stomata, a major drought effect is the reduction of photosynthesis. This loss is not only attributed to stomatal closure, but also to a decline in leaf expansion, a reduction of photosynthetic machinery, damage to enzyme systems, and premature leaf loss. This leads to an increase in the use of food reserves for plant maintenance and a reduction in new carbohydrate production. In addition to less carbohydrate production, drought stress inhibits translocation of proteins, enzymes, growth regulators, mineral nutrients and other essential materials needed for normal plant growth. These factors further contribute to reduced growth and development.

Drought-stressed trees may exhibit signs of dieback or decline. This may be the tree’s way of coping with a stressful situation. Leaves in the top-most branches generate the lowest water potentials, and decline and die. Drought affects root contact with the soil and if the roots are unable to supply enough moisture and nutrients to the crown of the tree, the crown will usually begin to die back to bring the tree’s crown and root system into a more favorable balance. This smaller photosynthetic area further impedes the plant’s ability to produce carbohydrates even after the drought has ended. This reduction in food production capacity generally results in a tree requiring several years to fully recover from a severe drought, with the initial year after the drought having the greatest growth reduction.

Drought-induced crown reduction in pecan is minimized to some extent (compared to other tree species), because of its expansive root system. Trees with widely penetrating and branching root systems absorb water more effectively which helps to postpone or prevent drought injury.

When first exposed to drought, the tree allocates more food to root growth to improve the root absorptive area per unit area of foliage ratio. Extended drought leads to roots being suberized to prevent water loss to the soil. A high water-absorbing ability coupled with a low transpiration rate give pecan trees a better chance to survive drought conditions.

Now that we’ve discussed what happened to the trees in 2011, what can we expect and how can we help the trees in 2012? According to the NOAA, La Niña conditions returned during the fall of 2011 and are expected to persist throughout the current outlook period (January-March). This means that drought conditions will persist or intensify in areas already suffering extreme drought conditions.

Many of you have asked the question, “Are my trees still alive?” It can often be difficult to determine if a tree has died from drought stress or has simply become dormant and appears to be dead. There are a couple of simple tests to help determine if a drought-stressed tree is alive or dead.

One test is to collect some small twigs about one-eighth inch in diameter and try to break the individual twigs. Live branches should be flexible and bend considerably without breaking, and when they do break they generally will not snap in two cleanly. If they snap and break like dead, dry twigs then at least part of the tree is dead. A second test is to use your fingernail or knife blade to scrape bark from a small twig or branch. A live tree will have green, moist tissue under the bark. If you are still not sure, wait until the next spring to see if it sprouts a new crop of leaves.

I’ve also received several questions on the impact of the recent rains in December and January. Just because your orchard may have received a few inches, don’t think that the problems due to last year’s drought are going to disappear. Trees weakened and invaded by insects and diseases may not show symptoms immediately because of the food reserves stored within the tree. Many pecan trees were forced to use precious carbohydrate reserves to stay alive during the dry summer when sugars couldn’t be produced through photosynthesis. Trees carrying a pecan crop, especially a large crop, are probably under even greater stress. These trees will have a reduced ability to defend themselves against opportunistic insect and disease organisms looking to take advantage of their weakened condition. These trees will also be susceptible to freeze damage during the winter.

Drought predisposes trees to pests because of lower food reserves,  poorer response to pest attack, and poorer adjustment to pest damage. Attacks on trees by boring insects that live in the inner bark and outer wood can be more severe in dry years than in years when there is minimal water stress. Flat-headed apple tree borers, ambrosia beetles, and shothole borers are attracted by stressed trees. Armillaria mellea and A. tabescens  are soil-borne rootrotting fungi which can attack the roots of stressed trees. The large mycelia mats produced by the fungi clog the vascular system of the tree. Insect pests and disease organisms further weaken trees by defoliation or by causing stem and root damage that impedes absorption and translocation of water and nutrients.

Properly managing soil fertility will help prevent nutrient stress and minimize the effects of drought. Several nutrient deficiencies can intensify the effects of drought stress. A deficiency of nitrogen or micronutrients can result in a reduction of photosynthates produced in the leaves. Phosphorus deficiency can restrict root growth, and a potassium deficiency can interfere with normal functioning of the stomates that affect internal water relations. For best results, fertilizer application rates should be based on soil and leaf analysis. Avoid the use of agricultural grade fertilizers that have a high salt content, as these can intensify drought stress. Applying nutrients during a drought will have little impact on plant growth because water is the limiting factor. Therefore, fertilizer should be applied after drought conditions have subsided and soils are recharged by rainfall or irrigation.

During drought conditions, vegetation around your trees will strongly compete for available moisture and nutrients. One of the best management strategies for a pecan orchard floor is to use a grass alley with a vegetation-free strip in the tree row out to the dripline of the trees. Several different herbicides can be used to establish and maintain the vegetation-free strip. Mulches can be used around small trees to help conserve soil moisture and reduce competition for water from weeds. The addition of organic matter to the soil promotes root development and improves the soil’s moisture-holding capacity. The mulch can be composed of many different organic materials including wood chips, shredded bark, bark nuggets, pine straw, leaves, etc. Mulches should be applied to a depth of 2-4 inches around young trees.

In summary, moisture stress from drought periodically affects pecan orchards. The impact of drought on tree growth and production varies with the severity and duration of the drought, as well as other factors including tree age, soil conditions, insects, diseases, and other stresses. To improve the trees’ chance of a rapid recovery when drought conditions improve, routinely monitor the health of your trees, and provide them with the best possible conditions to minimize damage from severe drought. A comprehensive plant health care program featuring proper irrigation, weed management, soil nutrient management, and integrated pest management is recommended to minimize the effects of drought on pecan trees