Trees and Dry Conditions

The grass is brown in central Pennsylvania. I don't mind too much. I'd rather not mow. But we're also seeing some trees starting to shed leaves. The soil is dry, and plants are starting to suffer.

Many areas across the northeastern US are experiencing dry and drought conditions, Pennsylvania included. As of July 28, two-thirds of Pennsylvania was in an abnormally dry to moderate drought condition. Yet, tallied across the state, we are very close to the total predicted rainfall for an average year to date. Depending on where you look in Pennsylvania, the regions are somewhere between 25% below, to as much as 26% above annual precipitation. These numbers don't sound like they would result in two-thirds of the state be in abnormally dry conditions. So, how does that work?

Climate change models for Pennsylvania have consistently predicted the pattern that we're seeing this summer. Less frequent, but more intense rainstorms, with extended drought periods occurring between those large rain events. The result is highly variable and uneven conditions across the state. Conversations with landowners in the south central region have shared their recent 4-inch rain storm events and subsequent flooding. In many other Pennsylvania regions, rains came early, and we've entered a dry summer period with rather infrequent rain events. What does this all mean for the trees?

Pennsylvania is known for its waterways - over 86,000 miles of streams, creeks, and rivers. We usually have ample water during the growing season. We rarely have the water conservation requirements that the US West and Midwest implement to conserve water for human consumption.

Healthy, established trees can normally withstand relatively long periods of drought, so long as there are intervening months or years that are more favorable. However, recently planted trees, or trees with small soil footprints (rootprints?) are more susceptible to decline under these dry conditions. Consider the area of soil occupied by a tree's roots is often equal to or greater than that of the above ground crown.

The site details also matter. Trees in sandy soil - with large openings between soil particles - are more likely to experience drought conditions as water moves rapidly through these systems (down to the water table). Clay soil - with smaller spaces between soil particles - is more likely to be wet as these small particles more tightly bind to water molecules. However, this same process also makes it more difficult for the roots to draw water away from the clay. For most tree species, loamy soils (a mix of sand and clay), present the most favorable growing conditions with regard to water and nutrient availability.

Trees move water from the soil through the roots and up to the food production factories that are the leaves. (Or, more accurately, solar-powered evaporation pulls the water through the trees). As the leaves absorb the carbon dioxide necessary for photosynthesis, they also lose oxygen and water vapor through the stomata on the underside of the leaves (a process known as transpiration). Stomata are basically pores in the leaf that can open or close based on the balance of conditions between the leaf and the environment. When water is scarce, trees close their stomata and slow down or shut off photosynthesis and accompanying transpiration. If water remains scarce, trees may cut their water losses by shedding these leaky leaves. But this presents a tradeoff: forego food production to keep your water, or risk dehydration in an attempt to keep the food factories open. Variations in the drought-related loss of leaves in different species, and different individual trees, reflect the "decisions" inherent to these trade-offs.

Trees that endure long periods of drought can shed all their leaves and enter winter dormancy early. Established trees can generally survive without photosynthesizing for up to a year, but continued annual drought conditions can kill water-stressed trees, or make them more susceptible to pest insects and disease.

Signs of water-stressed deciduous (hardwood) trees include wilted, curled, or warped leaves, leaves that look scorched around the edges, early "fall" coloration, and ultimately leaf drop. In conifers, the older growth of needles will turn yellow or red and potentially drop early (conifers do regularly drop their needles, but only a few at a time, and over a longer timeline than deciduous trees). Unfortunately, other tree diseases can result in similar signs of leaf stress, so if you're unsure of the cause, it's best to ask an expert.

What to do?

Sometimes trees can lose many or most of their leaves and recover. Sometimes not. If a tree loses all its leaves, it may be unable to produce enough photosynthate to harden off and survive the winter. The presence of buds and flexible stems at the end of the growing season is a good sign. Brittle branches (with no underlying green tissue) and lack of buds is a less promising prognosis.

In the meantime, what can you do? For less established trees, water them - but not too often. You want to encourage deeper root growth - and too much water may only encourage the proliferation of shallow roots. A mulch layer at no more than an inch think, and not touching the stem of the tree (never, ever create mulch volcanoes!), will help the soil retain moisture.

Pay attention to leaves as they are an early indicator of stress. Wilted leaves are easy to identify. Ask for help if you think your trees are potentially being impacted by more than water stress. International Society of Arboriculture (ISA) certified arborists have the skills and knowledge to identify tree threats and tree risks. You can find one near you at www.treesaregood.org.

In Pennsylvania, we're fortunate to have a climate and landscape that usually provide abundant fresh water nourishing the forests which, in turn, help protect these water resources. Pay attention to frequency, duration, and amounts of rainfall, and watch for its impacts on the trees in your woods.