Earthworks & Systems

Of Woods and Trees and Water

by Adam Downing
February 2009

“Drought” - a word that conjures up images of dust storms, dying corn plants, and dry rivers. While you may not witness all of these occurrences, you no doubt considered the word last summer. Trees, both in the woods and in our yards are no strangers to the effects of drought.

Trees occur in basically two different settings. The most natural setting is in “the woods.” By design, trees are naturally part of a forested ecosystem where they do very well. The second distinct setting where trees are common is the urban/residential area; a very different environment for trees. Amazingly, many trees also do quite well in these areas. The environment surrounding an individual tree differs drastically between forested and urban/residential areas. For this reason, the effect of drought is different.

Trees in Forests

Let’s first take a look at a tree in a forest. A mildly astute observer will notice several environmental changes when they enter a forest. The environment of a closed canopy forest (where tree crowns touch each other) is rather mild in comparison to other settings. For starters, temperature fluctuations are less extreme and it is less windy. You may also notice a difference under foot. In low disturbance areas, the ground is soft and spongy. It’s also perfectly covered by the best compost & mulch material ever designed. Annual leaf fall and other plant parts, along with natural aerators such as worms, millipedes, and moles create a perfect interface between below and above ground environments. Moisture falling on a forest is collected and stored very effectively & efficiently. (So much so that some metropolitan areas compensate landowners to keep large properties in forest to assure a continued supply of clean water from the watershed.)

The mild environment of a forest serves to lessen the likelihood of drought damage. Non-the-less, extended periods of drought can gradually take a toll on an otherwise healthy forest. Extended drought weakens otherwise healthy trees. Weakened trees become more susceptible to insect and disease damage, which further weakens the tree, setting the stage for a downward spiral. Eventually, the cumulative effects may result in death. More than likely, however, a healthy forest will survive several drought years.

Survival is less of a sure thing, however, in very young and very old forest. Just like humans, the young and old are more fragile to environmental stress. Before planting trees to re-forest or afforest (establish a new forest on land that was not previously forested) an area, the threat of drought damage can be minimized by preparing the site properly, controlling competing vegetation and taking care to handle and plant seedlings correctly. Older stands with mature or over-mature trees should be tended to increase vigor. Forest management tools such as thinning, timber-stand-improvement, and timber harvesting are useful to reduce drought damage susceptibility.

Trees in Our Yards and Communities

Similarly, very old and newly planted trees in urban and residential settings are also at risk for drought damage. Additionally, the wrong tree in the wrong place is more susceptible to drought damage. If you are planting a tree, be sure to fit the tree to the site. For example, you should know if your site is dry or wet, sandy or clay, sunny or shade and find a tree that grows well in that environment. Doing a little bit of homework will save many problems down the road.

Drought in residential or urban areas only accentuates the many stressors already present in these environments. Many of these areas are fairly hot and windy (in comparison to wooded settings). Heat and wind both work to “dry-out” trees. Perhaps the main factor to consider when evaluating susceptibility to drought however, is soil volume and structure. Many trees in urban settings, such as street trees or trees grown in containers along sidewalks are limited by soil volume. As a tree grows bigger, it needs more soil volume for uptake of nutrients and water. Even in residential settings, desirable soil volume can be limiting. When soil structure is compacted (as a result of repeated traffic by humans, dogs, and vehicles) space for air and water is limited. Ideal soil is 50% pore space (filled with a fluctuating mixture of air and water). When soil is compacted, pore space decreases which lessens the soil's ability to absorb and store water.

There are two simple things you can do to help trees in danger of drought damage:

1. Mulch! Recall the forest floor of a tree in the woods… you can mimic that excellent interface by properly applying organic mulch (such as wood chips) under your tree’s canopy. It only needs to be 2-3 inches deep (deeper is bad) and at least 3 feet in radius, wider is even better. Organic mulch not only conserves moisture in the soil but also increases soil porosity over time, keeps roots cooler, reduces weeds, and protects from lawn mower damage (a leading cause of death for yard trees).
2. Water deeply. Newly planted trees may need regular irrigation during dry spells. A good deep soaking once or twice a week is better for the tree and conserves more water than frequent light sprinklings.

“Drought” – it can be a scary thing but there are steps we can take to minimize the damage it does.

Virginia Cooperative Extension has publications on the topics of “Forest Management Strategies to Minimize Drought Damage,” “Moisture Stress and Trees: The Hidden Problems,” “Creating a Water-Wise Landscape,” and “Trees for Problem Landscape Sites – Wet and Dry Sites.”

Go to http://www.ext.vt.edu/resources/ or call your local extension office for printed copies.