Summer Heat Stresses Trees

Washinto n Post, Setember 2002

While forest fires in the West have captured the nation's attention, a similar but less visible disaster has been wreaking havoc with East Coast trees: an arboreal broil caused by this summer's record-breaking drought and high temperatures. From Georgia to New Jersey, extreme water shortages and heat waves have placed even drought-resistant trees under severe stress, causing early leaf loss, increased susceptibility to disease and premature death.

In metropolitan areas such as the District and its Virginia and Maryland suburbs, trees are taking an even bigger hit because city roads, sidewalks and buildings radiate extra heat and because sparse urban rainfall typically gets diverted into storm sewers before trees have a chance to sate their thirst.

"I don't mean to sound alarmist, but this is the worst crisis for trees along the eastern coast of the United States since the chestnut blight at the beginning of the century," said Kim D. Coder, a professor at the University of Georgia's Warnell School of Forest Resources in Athens. That fungal disease all but wiped out the American chestnut, which until 1900 was a dominating presence in East Coast hardwood forests. It's not just this year's scorching weather that's bringing Eastern trees to their knotty knees, Coder said.

Much of the region has been parched for three or four summers in a row. So instead of putting on inches and pounds during the summer growth season, trees have been raiding their own precious carbohydrate food stores. Many East Coast trees have just about used up those reserves and are now putting their last bursts of energy into especially large batches of acorns or cones -- a classic response to extreme environmental stress that says, in effect, "I may not make it, but the next generation might." "Especially in urban centers," Coder said, "we are at the end of what these trees can handle." That's bad news for people, as well as for trees, urban foresters say. In populated areas, trees do much more than give residents shady respite. They cleanse the air of pollutants such as nitrogen dioxide, sulfur dioxide, carbon monoxide and ozone. And evaporation from tree leaves pulls significant amounts of heat from the urban atmosphere, lowering air temperatures and reducing the need for air conditioners at night.

Perhaps because trees seem so stoic and impervious to stress -- and perhaps because urban citizens overestimate the level of care local governments can offer their trees -- residents too often fail to help these woody giants in their time of need, experts say. Water is the key. Although last week's rain provided some short-term relief, the showers had no significant impact on the overall drought, weather service officials said. Trees can lose hundreds of gallons of water on a single hot day, so they need a fairly regular, if modest, supply. About 90 percent of water-absorbing tree root hairs are in the top foot of soil, so as little as an inch of water dripped or sprinkled around a tree once a week can save it from becoming next year's chain saw fodder. And although scorched leaves are already hitting the ground in many neighborhoods, it's not too late to help, said Bonnie Appleton, a Virginia Tech extension specialist with the Hampton Roads agricultural research center in Virginia Beach.

Many trees are just now making their leaf and flower buds for spring. "If people water their trees now," Appleton said, "it can have a big effect next year." A couple of inches of mulch can also do a lot to maintain soil moisture between waterings. But this is not the time to add fertilizer, Appleton said, which can be harmful to leaves and roots during periods of water stress.

Trees face a central problem during hot and dry periods: They need carbon dioxide from the atmosphere to perform energy-capturing photosynthesis, yet there is no way to let that gas into the plant without at the same time losing precious water to the atmosphere. Gas intake and water loss occur through tiny leaf pores called stomata. During midday temperature highs, stomata close to minimize water loss -- putting a crimp in photosynthesis that the plant can tolerate for limited periods. During long days and weeks of extremely high temperatures, however, stomata stay shut to save water, and trees find themselves in an energy crunch.

Photosynthesis slows or even stops and, gradually, leaves are dropped. If leaf loss is scattered throughout a tree, it can usually be saved with added water, experts said. But if a tree is losing major hanks of foliage, especially around the crown, then energy stores have probably been emptied, and the tree is dying. Even when water is available, extremely high temperatures can harm trees directly. The enzymes that facilitate the chemical reactions that keep trees alive operate best between 70 degrees and 80 degrees Fahrenheit. When temperatures hit the mid-90s, photosynthesis starts to shut down. In the 100-degree range, fatty elements of tree cell membranes start to "melt," and chlorophyll molecules break down. At about 115 degrees, enzymatic molecules literally fall apart inside cells, and tree tissues suffer irreparable damage that can spread with each new day of unrelenting heat.

City trees get even hotter than their country counterparts, because solar energy bounces off concrete and brick surfaces as infrared radiation, adding 9 degrees to 12 degrees to the air well into the night. Passing cars and trucks, whose surfaces in summer can exceed 120 degrees, add to the heat load on leaves. And roots, which work best at temperatures between 60 degrees and 80 degrees and generally stop functioning when soil temperatures exceed 95 degrees, can find themselves roasting, because summer soil temperatures can exceed 100 degrees. Street cuts to accommodate utility work and road resurfacing take further tolls on urban trees by disturbing delicate root hairs.

Mark Buscaino, the District's chief forester said that although 4,000 streetside trees are being planted annually in Washington, losses have long been outpacing gains. A study of Washington tree cover over the 25 years ending in 1997, conducted by American Forests, found that tree canopy had declined by about 44 percent. "Every tree out there is a miracle," Buscaino said. A miracle that this year could use a helping hand.