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

Abstract

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 Mg 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 III, 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.

Introduction

Coarse woody debris (CWD) is an important structural component of many forest ecosystems in temperate forests (Harmon et al., 1986, Spies et al., 1988, Stewart and Burrows, 1994, Carmona et al., 2002). CWD provides habitat for many vertebrate and invertebrate species, an important substrate for plant germination and growth and, a sink and source of nutrients and carbon that can be recycled within the ecosystem (Harmon et al., 1986, Hunter, 1990, Donoso, 1993, Takahashi et al., 2000, Christie and Armesto, 2003). Logs (fallen dead trees) and snags (standing dead trees) are the major types of CWD and their relative contribution to total ecosystem biomass varies greatly in the landscape depending on forest types, disturbance regimes, topography and stand age (Spies et al., 1988, Harmon and Hua, 1991, Goebel and Hix, 1996). Harmon et al. (1986) and Spies et al. (1988) determined that in a forest chronosequence the amount of CWD follows a U-shaped pattern, where the maximum stock of CWD occurs following stand disturbance and in old-growth forests, in the latter due to small-scale disturbances (mainly tree falls) that generate a new pulse of CWD input.

Valdivian temperate rainforests (Veblen et al., 1983, Veblen and Alaback, 1996) in the southern cone of South America are highly productive (Donoso, 1989, Armesto et al., 1995), and in old-growth forests tree-fall gaps are a common type of small-scale disturbance (Veblen, 1985a, Veblen, 1985b, Veblen, 1989, Armesto et al., 1992, Veblen et al., 1996). This is one reason why these forests should accumulate large amounts of CWD. Another reason is that the two major forest types of the region, the evergreen and the Coigüe–Raulí–Tepa (after the common name of its three major tree species, i.e. Nothofagus dombeyi (Mirb.)–Nothofagus nervosa ((Phil.) Dim. et Mil.)–Laureliopsis philippiana ((Looser) Schodde)), usually have a high degree of canopy stratification, including emergents among which the most common species are members of the Nothofagus family (Veblen et al., 1981, Parada et al., 2003, Donoso and Lusk, 2007).

The only reported study of CWD biomass in Chile (Carmona et al., 2002) was conducted in old-growth forests of the evergreen forest, with CWD biomass values similar to those for standing live tree, and even higher than values found for coniferous temperate rainforests of the Pacific Northwest of North America. There is no report of CWD in the Coigüe–Raulí–Tepa forest type, which is simpler in tree species composition than the evergreen forest type. In this study we intensively sampled two old-growth forest stands of this forest type, one strongly stratified due to the presence of emergent N. dombeyi trees (Nothofagus-dominated stand) above a canopy tier of shade-tolerant species, and another comprised only of shade-tolerant species (Mixed-species). From the point of view of biomass dynamics in forest ecosystems, the Nothofagus-dominated forest stand represents a transition phase, and the Mixed-species forest stand a steady-state phase (sensu Borman and Likens, 1979) within old-growth forests. The working hypotheses of this study were that (a) the Nothofagus-dominated forest stand has greater CWD than the Mixed-species forest stand, and (b) within each old-growth forest matrices, patches dominated by small-sized living trees would have greater CWD due to more recent past disturbances, particularly tree falls that allowed for CWD accumulation. The associated objectives were (a) to compare biomass of two old-growth forests stands in the transition and steady-state phases of biomass accumulation (with and without an emergent tree component); (b) to analyze if the current structure and species composition of live trees can reflect the abundance of CWD; and (c) to compare CWD biomass in these forests with other temperate old-growth forests of North America and of the southern hemisphere.