Coarse woody debris density and carbon concentration by decay classes in mixed montane wet tropical forests

Abstract

The role of coarse woody debris (CWD) in the global carbon (C) cycle is growing under increasing tree mortality driven by climate variability and disturbances. Quantifying C in CWD critically depends on accurate estimates of CWD density and C concentration in CWD. This study considered the main decomposition pathways (the proportions of CWD decomposed by fungi vs. invertebrates; fungal decay types) and the relationships between decay class, wood density, moisture, and C content of CWD in old-growth mixed monsoon montane tropical forests in Vietnam based on the inventory of 359 CWD pieces. The bulk density of wood of the 1st, 2nd, 3rd, 4th, and 5th decay classes averaged 0.56, 0.49, 0.37, 0.28, and 0.15 g cm−3, respectively. The density reduction across decay classes did not significantly differ for snags, stumps, branches, leaning, and lying logs. Wood density was negatively related to wood moisture. The wood mass loss averaged 0%, 20%, 37%, 54%, and 74% in the CWD of the 1st, 2nd, 3rd, 4th, and 5th decay classes, respectively. Signs of invertebrate activity were recorded on 3% of CWD pieces. Among CWD samples with identifiable decay type, 88% were decomposed by white-rot, 8% were decomposed as brown-rot, and 4% of pieces contained both white- and brown-rot patches. The mean C concentration in wood was 46.5%. It did not change with decay class. Our research provides an empirical basis for future inventories of CWD carbon stocks in Asian montane tropical mixed forests.