Tracking Drought Factors and Effects on Plants

Written by Jeff Osborne, Forest Stewardship Program Associate, James C. Finley Center for Private Forests at Penn State

In Pennsylvania, the Department of Environmental Protection (DEP) 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. Let’s look at these tracking systems, plant responses to drought, and plants that can tolerate drought and improve water infiltration and soil for future plant species.

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.

 The USGS map above shows the status different drought factors in a circular color-coded dial. Starting in the upper left section and going clockwise it shows 90-day precipitation, surface water, groundwater, and Palmer Index. This map is from June 14, 2023. Green indicates normal conditions, yellow indicates drought watch, orange indicates drought warning and red indicates drought warning levels. Since June 14, many factors have improved, especially in the east and the southwestern corner of Pennsylvania, but we remain in a drought watch. https://pa.water.usgs.gov/apps/drought/

Lack of rain for a period of less than 90 days, say 45 to 89 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.

The above map, created by the National Oceanic and Atmospheric Administration and the National Integrated Drought Information System, shows most of Pennsylvania categorized as D4, drier than 98% of past measurements for the same period in past years, or in D3, drier than 95-98% of past measurements for the same period in past years, according to the Multi-Indicator Drought Index. The Multi-Indicator Drought Index (MIDI) looks at current drought conditions across the U.S. by integrating several key drought indices on precipitation and moisture into one objective, computer-generated map. The Short-Term MIDI approximates drought impacts from changes in precipitation and moisture over a short-term timeframe (looking back up to 90 days), such as impacts to non-irrigated agriculture, topsoil moisture, and range and pasture conditions. Long-term droughts (lasting months to years) can have different impacts. This experimental map is based on methodology from the NOAA National Weather Service’s Climate Prediction Center. Learn how this map is made. Source(s): UC Merced, via Climate Engine. https://www.drought.gov/states/pennsylvania

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.