Hiking amid the rugged beauty of Colorado's Rocky Mountains nearly 10 years ago, botanist Herbert Kronzucker was struck by the stark contrast in the type of trees growing on different sides of the same mountain. One side was covered with a dark green blanket of spruce, the other by a shimmering canopy of aspen.
"I asked the rangers about it, and they said the aspen sites had been harvested about 15 years earlier," Dr. Kronzucker recalls. That brief exchange sparked a line of research that may help explain why attempts to replant logged sections of mature forests often fail.
In the process, Kronzucker says, it raises fresh questions about whether such forests can be logged in a sustainable manner. Efforts to replant forests have failed in large areas of North America, he says, noting that in British Columbia alone, 1.5 million hectares (nearly 4 million acres) of once-productive forest have been written off as failed replantings.
According to Kronzucker and his colleagues at the University of British Columbia at Vancouver, a key to the problem is nitrogen, an element vital to plant growth. Kronzucker says he started taking soil samples 10 years ago from under stands of each type of tree. "There were several differences," he says, "but the most noteworthy was the difference in the forms of inorganic nitrogen" that the plants use.
Beneath the aspens, a combination of oxygen and nitrogen known as nitrate was the most abundant form. Beneath the spruce, it was a form of nitrogen and hydrogen known as ammonium. Indeed, in conifer soils nitrate appeared almost nonexistent. Nor was this unique to the Rockies; these nitrogen abundance patterns held true in samples taken from forests elsewhere in North America, Sweden, and Kronzucker's native Germany.
At the same time, other researchers had noted that when fire or logging disturbs a mature forest of conifers such as spruce or pine, the dominant forms of inorganic nitrogen in the soil shift from ammonium to nitrate. When the forest begins to reestablish itself, "pioneer" species, such as aspen, move in to take the conifers' place.
Putting the observations together, the team proposed that replantings may fail because conifers are picky about nitrogen.
To test the notion, Kronzucker formed ammonium and nitrate that included a tracer. He then fed the compounds to white-spruce seedlings. The seedlings, he found, consumed 20 times more ammonium than they did nitrate. Similar tests on other economically important species, such as lodge-pole pine and Douglas fir have shown similar results. The aspen seedlings prefer nitrate.
Botanist Anthony Glass, a member of Kronzucker's team, notes that other factors can influence replantings. For example, spruce seedlings ordinarily grow beneath the canopy of older relatives. When seedlings are planted in a clear-cut zone, they get much more sunlight than normal.
Still, the results imply "that you can't expect a conifer site reforested with the same species to succeed," says Kronzucker, whose research appears in a recent issue of the journal Nature. Replanting can be further hampered by the ease with which nitrates leach from the soil when it rains. This nitrate-laden runoff deprives soil of other vital nutrients.
But why does the soil's makeup change in the first place? New research in the same issue of Nature suggests an answer. According to John Stark at Utah State University in Logan, nitrate plays a larger role in mature-forests than thought, as do the microbes that consume it. These microbes appear to govern the change in soil nitrogen after fire or logging.
Dr. Stark and a colleague studied microbes' effects on nitrogen composition in 11 distinct conifer soils in Oregon and New Mexico. They found that nitrates are much more prevalent in mature forests than most scientists thought, but are taken up by microbes, leaving ammonium for the trees.
Soil microbes have to work hard to turn nitrates into a form of nitrogen their systems can use. But in a mature forest, microbes have plenty of carbon to gorge on, so they're not picky about nitrogen sources. Since there is so much nitrate available, they use it. But after a fire or logging, microbes have less carbon, so they go after the ammonium, which requires less energy to process.
This, Stark says, accounts for the quick decrease in ammonium and the "pulse" of nitrate that appears after mature forests are disturbed. He says Kronzucker's work and his own suggest that "there must be a preparatory tree species that comes in" to increase the carbon so microbes can "draw down the nitrate pools."