Fine root growth and mortality in different-aged ponderosa pine stands

Abstract

Root minirhizotron tubes were installed at two sites around three different age classes of ponderosa pine ( Pinus ponderosa Dougl. ex Laws.) to follow patterns of fine root (≤2 mm diameter) dynamics during a 4 year study. Both sites were old-growth forests until 1978, when one site was clear-cut and allowed to regenerate naturally. The other site had both intermediate-aged trees (50–60 years) and old-growth trees (>250 years old). Estimates of fine root standing crop were greatest around young trees and least around intermediate-aged trees. Root production was highly synchronized in all age classes, showing a single peak in late May – early June each year. Root production and mortality were proportional to standing root crop (biomass), suggesting that allocation to new root growth was proportional to root density regardless of tree age. The turnover index (mortality/maximum standing crop) varied from 0.62 to 0.89·year–1, indicating root life spans in excess of 1 year. It appears that young ponderosa pine stands have greater rates of fine root production than older stands but lose more fine roots each year through mortality. The results indicate that soil carbon may accumulate faster in younger than in older stands.