A ten-year decomposition experiment of woody roots in temperate coniferous forests within the Pacific Northwest

Background/Question/Methods

: The decomposition process is a key component of the biogeochemical carbon (C) cycle. A large proportion of the work on decomposition of woody roots is based on chronosequences or short-term time series studies. Most of these time series studies of decomposition only lasted for a few years (< 3 years). There is few longer-time decomposition experiment of woody roots available. The primary objective of this presentation will focus on the mass loss pattern of a ten-year decomposition experiment of woody roots in temperate coniferous forests within the Pacific Northwest. The secondary objective of this presentation will examine key factors (e.g., species, size of roots, sites) that influence the decomposition of woody roots in the region. We used litter-bag techniques to conduct a ten-year decomposition experiment of woody roots of dominant tree species in three sites (Cascade Head Experimental Forest, H.J. Andrews Experimental Forest, and Pringle Falls Experimental Forest) of temperate coniferous forests within the Pacific Northwest. Five different sizes of woody roots, including fine (< 2 mm in diameter), small (2~10 m in diameter), medium (10~50 mm in diameter), large (50~100 mm in diameter), and jumbo roots ( > 100 mm in diameter), were used in this study.

Results/Conclusions

: Species significantly influenced fine-root mass loss during the 10 years of decomposition. In general, the fine roots of deciduous species decomposed faster than those of coniferous species. However, species effect disappeared after a decade long decomposition. The percent (%) mass remaining of fine roots was around 25, regardless of conifer or deciduous species. Over the same period, site showed significant effects on woody root decomposition. Among the three sites, woody roots (2~100 mm in diameter) decomposed slowest at Cascade Head site (the coastal site) and fastest at H.J. Andrews site. The only exception to this pattern was jumbo roots, which the fastest decomposition occurred at Pringle Falls site and the slowest decomposition was observed at H.J. Andrews site. After a decade long decomposition, the average percent (%) mass remaining of woody roots increased with root sizes. Our study demonstrates the importance of incubation sites besides temperature and moisture condition as well as the long-term experiment study on woody root decomposition. Our study also challenges current leaf-based decomposition theory about the patterns of mass loss of woody roots over time.