Diameter-related variations in root decomposition of three common subalpine tree species in southwestern China

- The roots of broadleaf tree decayed faster compared to those of two conifer trees.
- Decay rate tended to decrease with increasing root diameter.
- The diameter-related effects are dependent on species and season.

Despite the importance of root quality for its decomposition, the effects of forest conversion and diameter size on root decomposition still remains poorly understood. A two-year field experiment was conducted to examine the mass loss and nutrient release of three root diameter classes (0-2 mm, 2-5 mm and 5-10 mm) in three subalpine tree species (Abies faxoniana, Picea asperata and Betula albosinensis) using a litter-bag method on the eastern Tibetan Plateau of China. The roots of B. albosinensis decayed faster compared to those of two conifer trees (A. faxoniana and P. asperata) in the 2-5 mm and 5-10 mm roots. Root diameter and decay rate exhibited a significantly negative correlation for two conifer trees. Regardless of tree species and diameter classes, all root litters experienced significant net nitrogen (N) and phosphorus (P) immobilization during the first winter. Both 2-5 mm and 5-10 mm roots tended to release more N than fine roots after 2-year incubation. Almost no obvious P release was observed for all root litters over the experimental period. Irrespective of tree species, both C:N and C:P ratios followed a trend of 0-2 mm roots < 2-5 mm roots < 5-10 mm roots during the experimental period. The root decomposition and N release were strongly associated with initial root quality (e.g., N and C:N). Our results suggest that diameter-associated variations in substrate quality could be an important driver of root decomposition and nutrient dynamics. Moreover, the diameter effects are dependent on tree species and decomposition period.