By Patrick Parker, RCA #514, PHC Program Director, SavATree, Bedford Hills, NY
As I write this the northeast U.S. is experiencing one of the worst droughts it has seen in over a decade. The U.S. Drought Monitor (http://droughtmonitor.unl.edu/) has classified parts of New England in its second worst category of Severe Drought and all of New England is classified in some stage of drought. Looking at the long range weather forecast, there is no relief in sight. Many arborists in other parts of the country have dealt (and are dealing) with much more severe and extensive droughts than this. According to the Intergovernmental Panel on Climate Change, climate change may increase the duration, severity, and frequency of droughts in many areas (including the U.S.) during the 21st century (IPCC, 2014). Therefore, I thought that I would share my experiences with the mitigation of drought stress/injury in trees here in the northeast and encourage others to share their experiences as well. I am sure that there is a lot that we can all learn from the arborists in the West who are dealing with one of the most extensive droughts in history and those of you in the South that have been battling intermittent droughts over the last few decades (not to mention those of you in the Southwest who deal with permanent drought conditions!).
When considering drought issues and recommendations for mitigation there are two major categories of drought to consider: acute drought and chronic drought. Acute drought conditions can occur when rain or irrigation is withheld for relatively short periods of time or during heat waves in the summer months that increase evapotranspiration rates beyond the capacity of most trees to extract water from the soil. Most established, healthy trees can recover from the effects of drought stress due to acute events with little intervention. Newly planted or unhealthy trees will require more intensive treatments. Chronic drought conditions occur over a longer time frame and may be due to climate, soil conditions, improper tree selection, or incorrect planting practices, among others. Chronic drought conditions usually require more extensive mitigation efforts than acute drought conditions. Trees are more susceptible to drought injury (rather than drought stress) during chronic events resulting in longer recovery times once drought conditions are relieved, even for established, healthy trees. Newly planted or unhealthy trees may not survive at all.
When attempting to mitigate the effects of drought on trees one must consider recommending preventive measures and/or remedial measures. The extent of the recommendations will often be determined by the state of the trees when observed. Preventative measures may include such treatments as providing supplemental irrigation or applying a growth regulator. Preventative measures are often more effective than remedial measures in the mitigation of drought effects but are often not instituted by tree owners due to cost or other factors. Remedial measures are usually focused on providing water to the affected trees along with various treatments to encourage recovery and may or may not be successful depending on the level of damage present when instituted.
RECOMMENDATIONS FOR MITIGATING DROUGHT EFFECTS ON TREES
Supplemental irrigation can be used preventively and remedially to mitigate drought effects on trees. Supplemental irrigation is probably the best solution to mitigating drought effects on trees, when used properly, but its widespread use is limited by several factors.
Irrigation systems are often expensive to install and maintain and watering restrictions due to drought conditions may severely limit the usefulness of an irrigation system at the time when it is needed most. Additionally, in most cases irrigation systems are not programmed to properly irrigate trees. They are usually programmed to run at frequent, short intervals that may be adequate for turf and some shrubs, but do not provide trees with enough water to meet their needs. Frequent, shallow watering can result in shallow rooting leaving trees susceptible to drought damage during dry periods when the irrigation is not being used. Irrigation systems for trees should include separate zones for trees and turf and the tree zones need to be programmed to water less frequently and for longer periods than the turf zones. From a preventive standpoint this will encourage root development deeper in the soil profile and buffer the root system from acute drought conditions.
Another option for supplemental irrigation can come in the form of transporting water from off-site to the tree and either drenching the root zone with it or injecting it below the surface. This method can be cost prohibitive when dealing with chronic drought conditions but can be a good option during acute drought events when an irrigation system is not available.
A general rule of thumb that I have used for watering trees is to provide one inch of water per week (approximately 625 gallons per 1000 square feet) whether through natural rainfall or supplemental irrigation. This should be considered when recommending water to be brought in from off-site as transportation costs can escalate quickly with the large volumes of water required. Of course the amount of water required by trees will depend on soil conditions (texture as well as structure) and the tree size and species involved.
Mulching can be used preventively and remedially to mitigate drought effects on trees. Mulch helps to: retain soil moisture by slowing evaporation from the soil surface, mitigate high (and low) soil temperatures by insulating the soil, reduce competition with other plant roots by excluding weeds and turf, and promote healthy soils through the addition of organic matter and nutrients (when organic mulches are used). Mulching tree root systems helps to recreate the forest floor environment that most trees evolved in and can encourage a healthy soil biology that works with tree roots to help them acquire water and nutrients. Ideally, every tree should at least be mulched out to the dripline. In practice, however, many tree owners are reluctant to give up their turf for a landscape full of mulch beds. Additionally, improper mulching can adversely affect tree health and increase the effects of drought on trees. Volcano mulching and over application of mulch can result in: the development of girdling roots, the development of shallow root systems, death of trunk tissue, the development of hydrophobic layers that resist moisture penetration, and the creation of anaerobic conditions. Anaerobic conditions result in root death from toxic compounds produced by anaerobic bacteria in addition to death of roots from anoxia. Mulching with organic mulches (basically a topical soil application of organic matter) also has some long term benefits for soils and tree roots including: promoting healthy soil biology, improving soil structure, and releasing nutrients in readily available (to tree roots) organic compounds as the mulch decays.
The incorporation of organic matter into the soil in the root zones of trees has many of the same benefits as described for the use of organic mulches and can be used as preventive or remedial treatments. However, results in improving soils often occur much more quickly when large amounts of organic matter are incorporated directly into the soil than when applied topically. Benefits of organic matter in soil include: improving soil structure which increases infiltration and water movement, moisture retention (it has been estimated that organic soils can hold 10 – 1000 times the amount of water as mineral soils), increasing nutrient retention by increasing the cation exchange capacity of the soil, and promoting a healthy soil biology by acting as a food source for various soil microorganisms.
One way to incorporate organic matter into soils without disturbing the landscape is to use a liquid source of organic matter. Several products are available using humic acids in concentrations up to 12% as a source of organic matter in liquid formulations. These products can be mixed with water and injected below the soil surface or applied to the soil surface as a drench.
Solid sources of organic matter in granular formulations of humic acids (with concentrations up to 85%), can be broadcast on the soil surface (with no disturbance to turf) or incorporated more deeply into the soil profile. Other solid sources of organic matter such as composted leaf litter (leaf mold), composted cow manure, or other forms of composted waste are best incorporated into the soil profile as they can be messy if applied to the soil surface. Soil incorporation also speeds up the intended results as the organic matter is distributed more evenly throughout the soil profile.
Means of soil incorporation can include the use of mechanical devices such as augers or rototillers but when working around tree root systems, supersonic air tools are the preferred method to avoid physical damage to roots. Soils to be treated are broken up using the air tool. Organic matter is then spread over the surface of the area and the air tool is used to mix the organic matter throughout the loosened soil profile. The use of organic mulches is preferred to cover the treated area, but many tree owners insist that turf be reestablished after treatment.
Other soil amendments can be applied to tree root zones to aid in mitigating drought effects preventively or remedially. Many of these amendments can be classified as plant biostimulants. Plant biostimulants are substances or microorganisms applied to plants or to soils in order to improve: nutrient acquisition, abiotic stress tolerance, and/or crop quality, regardless of their nutrient content. (For a comprehensive review of biostimulants please see du Jardin, 2015.)
I have used various biostimulants such as kelp based products, humic and fulvic acid (humate) based products, mycorrhizae, and beneficial soil inhabiting fungi and bacteria to aid in the mitigation of drought effects on trees. The biostimulant products that I have used have been mixed with water and applied as a drench to the soil surface or injected below the surface into the root zone but many biostimulants may be applied effectively to the foliage as well.
The exact mechanisms of the kelp and humate based products in protecting trees from abiotic stresses are not well known but hormone and micronutrient effects are considered important as is the promotion of beneficial soil microorganisms. My experience using kelp based products is that fine root growth is generally increased after treatment and that physiological changes allow trees to use water more efficiently thus reducing their overall water requirements.
Some mycorrhizae and beneficial soil inhabiting fungi and bacteria form sheaths of mycelium or colonies that cover root surfaces and can reduce moisture loss from roots as well as aid in the acquisition and transport of water from the soil to the root. Mycorrhizae and beneficial soil bacteria also provide many other benefits to trees that can help them to recover from drought effects including facilitating nutrient acquisition from the soil and protecting roots from attack by pathogens.
Application of tree growth regulators preventively may also help to mitigate drought effects particularly in the long-term by changing certain physiological attributes of trees. Growth regulators containing the active ingredient paclobutrazol can result in smaller, thicker leaves, thicker leaf cuticles, smaller stomatal apertures, increased frequency and size of trichomes and other leaf appendages, increased production and retention of abscisic acid (ABA), and in some cases increased fine root growth. (For a review of the effects of paclobutrazol on trees please see Chaney, 2005.) Smaller, thicker leaves reduce the leaf surface to volume ratio and therefore slow water loss through transpiration as do smaller stomatal apertures. Thicker cuticles reduce moisture loss directly through the cell walls of epidermal cells. Increased size and number of leaf trichomes or hairs can also slow moisture loss through transpiration by slowing air movement and increasing the thickness of the boundary layer surrounding the leaf. ABA is important in stomatal regulation and water uptake in roots and can therefore increase whole plant water use efficiency. Increased root growth provides more opportunity to acquire water from the soil. The magnitudes of these effects are tree species dependent and therefore results will be variable between species. Paclobutrazol based growth regulators are usually applied to the soil at the base of the trunk where they are root absorbed and can provide drought related benefits for years after the application. I believe that these products have great potential for the long-term management of trees subjected to frequent or chronic drought conditions.
Protection of trees from drought effects should begin well before drought conditions are present. Proper selection, siting, and planting are very important to the process as well as ongoing management of the site including mulching and irrigation practices. In reality we are often called in after the damage has been done. The first step in addressing existing damage is to assess the health status of the tree to determine whether it is a good candidate for treatment. Once it is determined that the tree is likely to survive, any number of methods can be instituted to aid in its recovery. The choice of methods will often be determined by the extent of the damage, site conditions, and the experience of the arborist. I have heard of other treatments used for mitigation of drought effects on trees such as the application of biochar and hydrogels (polyacrylamide) to soils to improve water retention. Other biostimulants such as protein hydrosylates, botanical extracts, and chitosan also show potential to mitigate drought effects in trees. I know turf managers who use wetting agents (surfactants) to manage droughty conditions in turfgrass and it seems that they would work to mitigate drought effects in trees as well. Having no first-hand experience with these treatments, I would appreciate hearing anyone else’s experiences with these and other treatments. I encourage other arborists to share their experiences in mitigating drought effects on trees so that we can all benefit from our shared knowledge.
Chaney, William R., 2005. Growth Retardants: A Promising Tool for Managing Urban Trees. Purdue University Extension Bulletin FNR-252-W.
Du Jardin, Patrick, 2015. Plant Biostimulants: Definition, Concept, Main Categories and Regulation. Scientia Horticulturae 196, 3 – 14.
IPCC, 2014. Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp.