More To Drought Than Meets The Eye

August 14, 1997

Looking at Different Ways To Diagnose Drought

Albuquerque, N.M. -- Just as you can't judge a book by its cover, you probably can't judge a tree's drought tolerance by its leaf response, according to Penn State researchers.

"It appears that leaf physiology is not the only way to look at the drought tolerance of trees," says Dr. Marc Abrams, professor of forest ecology in the College of Agricultural Sciences.

Conventional measures of drought tolerance in trees include adaptations deemed necessary to conserve or withstand an absence of water, including deep roots and thick leaves that can continue to photosynthesize -- convert sunlight into usable energy. Drought intolerant species tend to have shallow roots and shut down during drought. It is not unusual for drought-intolerant species to drop their leaves in time of drought and stop photosynthesizing.

The researchers decided to look at another measure of a tree's drought response, the amount of energy put into trunk growth.

"It appears that the drought tolerance ranking of trees in Central Pennsylvania based on ecological distribution or leaf physiology during drought is not an indication of a tree's growth response during drought," the researchers told attendees at the Ecological Society of American Conference, today (Aug. 14) in Albuquerque. "When we looked at the tree rings for drought years, some trees that are considered intolerant to drought, had average or above average growth."

The researchers, who include Abrams; Charles M. Ruffner, graduate student in forest resources; and T. A. Morgan, undergraduate in forestry resources, looked at trees growing in four different areas in the ridge and valley province of Pennsylvania -- ridge, valley, barren and riparian.

The ridge tops were the driest areas and the riparian areas near rivers and streams were the wettest. The valley makes up a moist intermediate soil climate and the barrens are moist to dry. Five or six species of tree were investigated in each area with each species appearing in at least two of the zones.

Because trees grow individual rings each year, tree ring dating -- dendrochronology -- is often used to determine the age of trees by looking at the growth pattern and matching all trees on a site using key signature rings, such as the small rings representative of a drought. In this way, the rings corresponding to the drought years of 1985, 1988, 1991 and 1995 were identified.

These rings were then compared to the average of their species growth during non-drought years in the area where they grew. The drought year tree rings were also compared to rings immediately before and after the drought.

Among the four sites studied, the dry ridge sites were most impacted by droughts and had the largest decreases in radial growth. Black cherry, which appeared in all four locations, had below average growth on the ridge and in the barrens, but not in the valley or in the wettest riparian area.

"Species in the valley and the barrens exhibited relatively few significant growth reductions in drought years compared to the ridge site," says Abrams. "Overall, there was not always a clear relationship between species moisture preference and their tree-ring growth during drought."

According to leaf response, pignut hickory and red oak are considered drought-tolerant species in riparian sites, but they had a disproportionate number of drought growth reductions in that area. In the valley, black oak and red maple both suffered large and frequent growth declines during drought, but are considered moderately to highly drought tolerant.

"It appears that there is an inconsistent pattern between leaf-level physiology and radial growth," says Abrams. "More study is needed to see if and how drought tolerant trees actually store energy in deep roots and why some trees are putting energy into trunk growth during droughts."

EDITORS: Dr. Abrams may be reached at by email. Mr. Ruffner may be reached at (814) 865-3049 or by email.

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