Variability in fine root decomposition after forest thinning: effects of harvest intensity and root size

Fine root decomposition is an important driver of forest carbon (C) and nutrient cycling. Harvesting operations may affect fine root decomposition rates by altering root properties and environmental conditions, but our understanding of root dynamics is limited. In this study, we investigated the chemistry, mass loss, element release (C, nitrogen (N), and phosphorus (P)), and compound release (lignin and cellulose) of decaying fine roots in a 26 year-old Chinese fir plantation seven years after low- and high-intensity thinning (30% and 70% tree removal) using two root size classes (< 1 mm and 1–2 mm diameter). Low-intensity thinning (LIT) did not affect mass loss in either fine root class or the release of fine root elements or compounds during decomposition. Similarly, high-intensity thinning (HIT) had no effect on the decomposition of large fine roots. However, compared with LIT and no thinning, HIT reduced the decay rates and lignin and cellulose losses of small fine roots. This reduction was related to an increase in the root lignocellulose index (lignin/[lignin + cellulose]) and a decrease in soil invertase activity. Interestingly, thinning did not affect root C, N, or P loss during decomposition. In summary, our results suggest that thinning intensity as well as root size and chemistry should be considered when studying fine root dynamics in managed forests.