Biomass and dynamics of attached dead branches in the canopy of 450-year-old Douglas-fir trees

Abstract

The amount, physical characteristics, and spatial distribution of attached dead branches in the canopy of 450-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) trees were studied over a 5-year period (1998–2003) to quantify their contribution to the canopy woody detritus pool of an old-growth Douglas-fir – western hemlock (Tsuga heterophylla (Raf.) Sarg.) forest in the western Washington Cascades. We developed a five-class decay rating for attached dead branches. From the size distribution and relative amounts and vertical distribution of attached dead branches in the five decay classes, we inferred that death of large original branches had occurred in the recent past, followed by the production and death of epicormic branches. Tree height was an important variable for predicting branchwood dry mass per tree. We estimated that attached dead branches of Douglas-fir contributed 63.8% of the total canopy woody detritus pool of the stand. During the study period, 0.24%·year –1 of live branchwood mass died attached, and 0.37%·year –1 and 4.34%·year –1 , respectively, of live- and dead-branch mass were lost due to branchfall and fragmentation. Because branch death and branchfall are stochastic processes, long-term monitoring in the canopy and on the ground is needed to understand the dynamics of canopy woody detritus.