STATE COLLEGE — In Pennsylvania, the Department of Environmental Protection declares drought watches, warnings and emergencies based on deficits in stream flow, precipitation, groundwater level, and soil moisture during periods of at least three months.

On June 15, the DEP declared a drought watch for the entire state. With a drought watch, there are suggested water-saving measures for people to voluntarily follow. The effect of these measures is monitored for their impact on the human water supply. There are also factors, including precipitation and soil moisture, which are monitored which have significant impacts on unirrigated crops, lawns, and newly established woody plants.

The National Integrated Drought Information System tracks these factors and provides many different maps.

On the DEP’s website, you can access the status of 90-day precipitation, surface water and groundwater levels, and ratings on the Palmer index, an index that estimates relative dryness by comparing temperature and precipitation. You can also access maps showing declarations of drought watches, warnings, and emergencies since 1980. The last time any Pennsylvania counties were in a drought emergency was November 2002, although, since then, there have been parts of the state in drought warnings where trees and shrubs have been significantly stressed during the growing season.

Lack of rain for a period of less than 90 days during the growing season can wreak havoc on annual crops, most lawn grasses, and trees and shrubs that are just a few years old or recently transplanted. Drought.gov provides maps that track shorter term drought factors that are more relevant for annual or newly established plants.

HOW ARE PLANTS AFFECTED BY DROUGHT STRESS?

When plants cannot draw moisture from the soil, they start to dry out. Foliage can wilt as the plant opens stomas to increase evaporation to regulate temperature. This may be more noticeable later in the day, and foliage can rehydrate overnight. Plants use one of two different stoma-regulating strategies, isohydric or anisohydric.

Isohydric plants close stomas to maintain higher leaf water potential, which makes their water use efficiency greater. Anisohydric plants keep stomas open, which prolongs photosynthesis but reduces leaf water potential.

Studies have shown that anisohydric plants are more productive than isohydric plants during short to moderate periods of drought, but they may not recover as well as isohydric plants after prolonged drought. Some species of woody plants, like some grapevine and poplar species, have individuals of both isohydric and anisohydric genotypes.

Tulip poplar and sugar maple are considered isohydric and oaks are considered anisohydric. When plants reach a point at which moisture is unavailable to them to continue normal functions, they may drop their leaves to preserve the moisture they have left in their roots and stem. If they lose their leaves earlier in the growing season, trees may re-leaf, but if leaf loss occurs later in the growing season, that might not happen.

If trees and shrubs do part with their leaves after August, do not count them a complete loss until they do not don leaves the next spring.

WHAT HELPS PLANTS REDUCE DROUGHT STRESS?

One attribute that helps plants reduce drought stress is having deeper roots. During periods with little precipitation, soil moisture available for plants tends to drop faster in the upper root zone and is more stable in the lower root zone.

The onset of a functional drought for the plants in a mowed lawn, which have most of their roots within six inches of the soil surface, could happen several weeks before trees and shrubs that have roots deeper than three feet.

Woody plants can transfer water from the heartwood to the sapwood and use it for biological functions. Many plants have symbiotic associations with mycorrhizal fungi which increase the roots’ ability to uptake moisture and nutrients.

Trees put on most of their annual growth within the first two months of their growing season; so, an acute lack of rain in late spring can reduce tree growth more than a lack of rain at other times. Now that most of Pennsylvania has received rain in the last week, the worst effects of the early season rain deficit for many established trees and shrubs may be lower-than-average growth rates for this year.

The early lack of rain may have been more detrimental in areas affected by spring insect and fungi afflictions. There isn’t much we can do to alleviate drought stress on the landscape, but when we are planting trees and shrubs, or deciding which ones to remove in a timber harvest, we can favor species which are more drought tolerant on drought prone sites like southern or western slopes, areas with sandy, rocky, or clayey soil.

You can find many lists online of drought resistant plants like the list provided by Penn State Extension and the list provided by the University of Tennessee. But pay close attention, many of these lists contain non-native species which shouldn’t be preferred species.

Many of the drought resistant species, like Kentucky coffeetree, honey locust, nine bark, and hackberry, can also be found on riparian species lists. If you are working to improve a droughty area with plantings, there are many interesting plants to choose from.

The Pennsylvania Forest Stewardship Program provides publications on a variety of topics related to woodland management. The Pennsylvania DCNR Bureau of Forestry, USDA Forest Service, Penn State Extension, and the James C. Finley Center for Private Forests at Penn State, in Partnership through Penn State’s Department of Ecosystem Science and Management, sponsor the Forest Stewardship Program in Pennsylvania.