Analysis of sampling methods for coarse woody debris

Abstract

Coarse woody debris (CWD) is an important structural component of forest ecosystems and plays a key role in ecosystem functioning. It includes whole fallen trees and branches, pieces of fragmented wood, stumps, and standing dead trees (snags). A number of sampling approaches have been used to quantify CWD but there has been little systematic analysis of alternative approaches in terms of sampling efficiency. This study analysed sampling methods for CWD in three Australian forest types: eucalypt-dominated woodlands in south central Queensland, eucalypt open forest (dry sclerophyll forest) in south coastal New South Wales and eucalypt tall open forest in southern Tasmania. All downed wood (greater than 10 cm in woodland and 15 cm in other forest types) was measured and mapped on several large plots (1 ha) at each site. Alternative sampling approaches were compared using a computer-based system. These included varying length and arrangement of lines for the line intersect method and varying sizes of fixed-area plots. Variability of CWD volume in forests is relatively high and achieving estimates with a precision (coefficient of variation) less than 50% of the mean requires a high sampling intensity. In analysis of the line intersect method, precision of volume estimates increased as total transect length increased. In denser forests, transects of at least 100 m are required to achieve a precision of less than 100% of the mean. A single long transect resulted in lower coefficient of variation than a number of short transects and is generally more cost effective. There was no improvement in precision by arranging the total transect length in an ‘L’ shape, a triangle or a square. In woodland vegetation (20–50% canopy cover) the amount of CWD is relatively low and variability is high. In these situations fixed-area plots are preferred. 20 m×20 m plots give estimates with a mean coefficient of variation less than 100% in the more dense forest structures but well over 100% in woodlands. Measuring all pieces on a 50 m×50 m plot in woodlands gave estimates with a precision of ±21–73% of the mean.