Trees hog nitrogen to survive on tough turf.

Plants have a variety of tricks enabling them to survive on infertile, acidic soils. To conserve their nutrients, evergreens, for example, grow slowly and shed their foliage infrequently. Now, scientists studying a unique Northern California forest have uncovered another bit of their survival strategy: how they get by on small amounts of the all-important nitrogen.

Like many other kinds of vegetation, Bishop pines (Pinus muricata) produce compounds called polyphenols; many ecologists believe these substances ward off pathogens and herbivores. "But we show that polyphenols have a function that has nothing to do with defensive chemistry," says Robert R. Northup of the University of California, Davis.

Instead, the compounds have a lot to do with ensuring that nitrogen remains useful to pine trees and worthless to most other types of vegetation, he and his colleagues report in the Sept. 14 Nature.

The scientists studied Bishop pines in a coastal forest known as the Ecological Staircase, where the trees grow close together but on three different soils: one very old and extremely infertile, another moderately acidic and infertile, and the third slightly acidic and fertile. In earlier work, the researchers found that the concentration of polyphenols in the pines' foliage increases as soil fertility declines.

In their new study, Northup and his colleagues examine the relationship between nitrogen release and polyphenols in the pine needles littering the ground under the trees.

They find that litter containing large amounts of polyphenols also has a high ratio of dissolved organic nitrogen (DON) to inorganic nitrogen, the form used by most plants. The least fertile soil had, on average, twice the concentration of polyphenols and one-third more DON than the most fertile soil but less than one-tenth of the inorganic nitrogen, says Northup.

Polyphenols bind the organic nitrogen and prevent a process called mineralization, which turns organic nitrogen into its inorganic form, other studies indicate.

The Bishop pines benefit from the polyphenols' ability to bind with organic nitrogen, the authors suspect. Mycorrhizal fungi living on the trees' roots get enough carbohydrates from the trees to make enzymes that break down the nitrogen-polyphenol compound, Northup explains. That enables the pines to use the organic nitrogen--a feat most plants cannot perform.

Other studies have shown that vegetation growing on infertile soil absorbs more nitrogen than mineralization produces, suggesting that such plants must have another source of the nutrient, notes Northup.

In an accompanying comment, F. Stuart Chapin III of the University of California, Berkeley, contends that the new research adds "to the developing body of evidence that organic nitrogen is a major and direct source for plants in nitrogen-limited ecosystems."

The new report also "shows for the first time that a pine...can strongly influence [the] release of DON in soils through the production of polyphenols in leaf litter," Chapin adds, although he cautions that the authors'assumption that the trees absorb DON and benefit from it still needs to be demonstrated.