Effects of forest type and stand structure on coarse woody debris in old-growth rainforests in the Valdivian Andes, south-central Chile

Coarse woody debris (CWD) is an important structural and functional component of temperate forests. There are few studies that have estimated CWD biomass in temperate forests of the southern hemisphere. In Chile, this is the first study of CWD in Andean old-growth forests, where tree-fall is the most common type of disturbance and generates a mosaic of different structures within a forest stand, both conditions that must affect the quantity and quality of CWD. We estimated and analyzed CWD biomass in two differing old-growth forest stands, one composed mainly by Nothofagus dombeyi, Laureliopsis philippiana and Saxegothaea conspicua (Nothofagus-dominated stand) and another composed mainly by L. philippiana, S. conspicua and Dasyphyllum diancanthoides (Mixed-species stand). We set up 80 and 95 circular 0.05-ha plots in these stands, where we measured all live trees >= 5 cm diameter at breast height (dbh, 1.3 m), recorded diameters at the large and small ends of logs and snags, and total length of logs or total height of snags, and classified these within three decay classes. The Nothofagus-dominated forest stand had 88.8 Ma ha(-1) of CWD biomass, compared to 59.6 Mg ha(-1) in the Mixed-species stand, a difference that can be attributed basically to the greater biomass of snags in the former, where most biomass of CWD, logs and snags belonged to N. dombeyi. In both stands we found that most plots had a tree structure dominated by small trees; these plots had greater CWD biomass, of which most was in decay class 111, likely reflecting past tree falls that reduced the numbers of large trees and increased CWD. At the stand level the only significant but low correlation occurred between log biomass in decay class III and mean diameter of the plots. However, there were strong relationships of mean diameter (QMD) of the different structural types and log biomass in decay class III in both stands, thus reflecting that diameter structure (or QMD) can be a good predictor of log biomass. CWD) biomass in these Valdivian old-growth rainforests is in the range of values found for North Patagonian rainforests sampled with similar plot size, greater than biomass in deciduous forests in North America, and lower than temperate rainforests in the Pacific Northwest in North America, in New Zealand and Tasmania. Differences can be attributed to forest type, forest productivity, and successional phase. (c) 2007 Elsevier B.V. All rights reserved.