A renewed interest has emerged in recent years in whole-tree harvesting, a practice that has been scorned by many as dangerous to the health of the forest ecosystem because it depletes the soil of nutrients. While recent evidence suggests that some of those concerns may have been overstated, the long-term impact of the practice is still unknown.
Whole-tree harvesting is a method in which the entire tree – trunk, limbs, branches, and leaves – is removed from the forest. The practice is becoming more common as demand increases for fuel, fiber, and mulch; it’s also a lot safer for the logger, who’s operating from an enclosed cab.
A major concern has been that if the whole tree is removed, there will not be enough nutrients left to replenish the forest, since the decomposition of branches and other debris returns valuable nutrients to the soil. But studies by Syracuse University researcher Chris Johnson and his students in the Hubbard Brook Experimental Forest in New Hampshire have shown that whole-tree harvesting has little effect on calcium, one of the most important mineral nutrients in soil, at least in the short term.
Calcium is critical to many trees and plants, especially sugar maples, which grow best in calcium-rich sites. Acid rain has caused calcium to leach out of soils in many locations, further adding to concerns about the loss of calcium from whole-tree harvesting. Johnson began his study in the mid-1980s, when Watershed five at Hubbard Brook was harvested using the whole-tree method. He measured the availability of calcium from the soils before the harvest and three times in the succeeding 15 years. His data indicate that the harvest had little effect on the total pool of exchangeable calcium, and that the cut did not result in the depletion of pools of available calcium.
“We did an analysis and found that if you estimate the calcium content of the stumps and the root systems left behind, there is enough calcium in those stumps and roots to account for the amount of calcium that has been taken up into the [new] forest vegetation and lost in the stream water,” Johnson explained. “It’s plausible that the decomposition of these root systems and stumps left behind after cutting could supply enough calcium to get the new forest started.”
If that is correct, he said, the effect of whole-tree harvesting may have simply been delayed. “Maybe there is enough calcium for the forest to get started, but there may not be enough calcium in the roots and stumps to grow a new mature forest,” he said. “Over a 25- or 30-year time period we may still see calcium depletion.” Johnson plans to return to Hubbard Brook next year to conduct his analysis again to see if calcium levels have changed since he last visited in 1998.
“If we find that the exchangeable calcium is still the same as in the preharvesting samples, then we’ll know that something more than just the roots and stumps is supplying calcium to the soil,” he said. “We’re either going to see a decrease of calcium in the soil because there is nothing left to replenish it, or we’ll see no change at all, in which case we’ll really have to start digging to find out what processes could be releasing these large amounts of calcium [to sustain the new growth].”