Concept and classification of coarse woody debris in forest ecosystems

Coarse woody debris (CWD) is generally considered as dead woody material, in various stages of decomposition, including sound and rotting logs, snags, stumps and large branches. CWD is an important functional and structural component of forested ecosystems and plays an important role in nutrient cycling, long-term carbon storage, tree regeneration and maintenance of heterogeneous environmental and biological diversity. However, The definition and classification of CWD have been the subject of long debate in forest ecology. CWD has not been precisely defined. Recently, with the rapid development of landscape ecology and research in CWD, the USDA Forest Service and the Long Term Ecological Research have provided a standardized definition and classification for CWD, which makes data comparison on the landscape scale possible. Important characteristics of their definition include: (1) the minimum diameter (or at equivalent cross-section) of CWD≥10 cm at the widest point (the woody debris with diameter from 1 cm to 10 cm should be defined as fine woody debris, and the rest is litterfall), (2) sound and rotting logs, snags, stumps and large branches, located above the soil, and coarse root debris, larger than 1 cm in diameter. This classification has greatly facilitated CWD studies, helped comparison of different studies and is widely applied in other countries (particularly in North America). However, this classification is not widely known to forest ecologists in China and different definitions and criteria are still used by individual studies which makes many studies incomparable. This article attempts to review the recent progress in classifying CWD with emphasis on introducing the classification system of USDA Forest Service and the LTER. It is expected that the review will help facilitate the development of a standardized definition and classification suitable for forest ecosystems in China.