Despite hopeful report that fungi help trees weather acid rain, not all species are protected, Cornell forest ecologist warns

A discovery reported in the latest edition of the journal Nature (June 13, 2002) — that fungi on the roots of some trees in the Northeastern United States help supply much-needed calcium in forest soils battered by acid rain — would seem to ease worries about the worrisome form of pollution.

But don’t stop worrying just yet, warns Timothy J. Fahey, the Liberty Hyde Bailey Professor of Natural Resources at Cornell University and a co-author of the report, “Mycorrhizal weathering of apatite as an important calcium source in base-poor forest ecosystems.”

“Not all tree species are fortunate enough to be associated with the types of root fungi that supply calcium,” he says, pointing to sugar maples, which in some areas have suffered serious declines in recent years.

“And although our findings suggest that trees with the right fungal associations may be able to short-circuit the loss of calcium in the soil, that may not get them around other problems with acidification of soil,” he adds. For example, when soil pH is lowered (and acidity rises) more naturally occurring aluminum is available to hinder plant growth. Fahey is co-principal investigator in the soil study sponsored by the National Science Foundation (NSF) at New Hampshire’s Hubbard Brook Experimental Forest.

Also contributing to the Nature report were researchers at the University of Michigan, Syracuse University, the Yale School of Forestry and Environmental Studies, the U.S. Department of Agriculture’s Forest Service and the Institute of Ecosystem System Studies in Millbrook, N.Y.

Although forest scientists have known for more than three decades that acid rain causes the essential plant nutrient calcium to leach from forest soils, the role of the “short-circuiting” fungus was not suspected until about three years ago. That’s when electron-microscopy examination of sand revealed tiny tunnels burrowed through the grains; the mini-miners turned out to be ectomycorrhizal fungi that can penetrate micropores in silicates and take up phosphorus, as well as calcium. Living in symbiotic relationships on some tree roots — where fungi obtain sugar needed for life processes — the ectomycorrhizal fungi deliver calcium and phosphorus directly to the trees before the nutrients are lost to acidic soils.

The NSF-sponsored study at Hubbard Brook is testing the long-term result of adding calcium to forested ecosystems to return acid-base ratios to levels that probably existed a century ago, before industrial pollution began to change the chemical landscape of the northeastern United States. The Hubbard Brook researchers were using stable isotope tracing to learn the sources of calcium in plant matter. They found that a significant proportion of calcium in some tree species (particularly conifers, beech and birches) growing in calcium-poor, acidic soil was coming from apatite, a soil mineral mined by fungi on tree roots.

Apatite, pronounced like “appetite,” is a calcium phosphate mineral. The trees also were getting some calcium from the better-understood “soil exchange complex,” in which calcium is replenished by mineral weathering and atmospheric deposition before being absorbed by roots. But without the beneficial “weathering” of apatite by the ectomycorrhizal root fungi, some trees in acid rain-drenched soils probably would not be getting enough calcium, the researchers reasoned.

Benefits of the fungal association are most pronounced in tree species that can sustain the right kind of mycorrhizae on their roots — spruce, fir and most other coniferous varieties, as well as certain hardwoods, such as oaks. And a lesser benefit might accrue to trees species with the “wrong” type of root fungi — including ash, basswood and maples — if they are growing nearby in mixed-species forests, simply because they are close enough to share calcium mined by other trees’ mycorrhizae, Cornell’s Fahey says.

“But trees trying to grow in the center of a single-species stand, like a sugarbush, could be in trouble,” Fahey says, noting that the sugar maple decline in the Northeast has been linked with calcium and magnesium depletion in soils..

And he hopes that anyone who downplays the effects of pollution will not take comfort in the knowledge that obscure fungi are mopping up after acid rain. “There are still numerous deleterious effects of acid rain,” Fahey says, “that have nothing to do with mycorrhizae.”