Leave coarse woody debris in forests
Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 3
Background information and definitions
Coarse woody debris consists of fallen dead trees and cut branches that are left after tree harvesting. Coarse woody debris increases the structural diversity at the forest floor. Sometimes, debris may be removed as part of forestry operations, such as for use as biofuel. However, retained coarse woody debris may provide resources on the forest floor that benefit woodland species.
This intervention covers studies where coarse woody debris is left evenly distributed. See also Gather coarse woody debris into piles after felling.
Supporting evidence from individual studies
A replicated, controlled, before-and-after study in 1993–1996 of a boreal forest area in Alberta, Canada (Moses & Boutin 2001) found that retaining woody debris following harvesting did not enhance numbers of three small mammal species, relative to those in cleared areas. This was the case for estimated annual peak populations of red-backed vole Clethrionomys gapperi (debris: 53–91 voles/plot; no debris: 91–99 voles/plot), deer mouse Peromyscus maniculatus (debris: 71–115 mice/plot; no debris: 79–151 mice/plot) and meadow vole Microtus pennsylvanicus (debris: 36–118 voles/plot; no debris: 7–146 voles/plot). In a 6 × 6-km study area, trees across four plots were clearfelled during winter 1993–1994. In two plots, woody brash was spread by bulldozer to form a strip, approximately 50 m wide and 0.5 m deep, generally along block centres. Woody debris was removed entirely from the other two plots. Small mammals were surveyed using 60 or 120 Longworth live traps/6 ha block. Traps were operated for three nights and two days, at fortnightly or longer intervals, from May or June to August or September in 1993–1996.Study and other actions tested
A replicated, controlled study in 2007–2008 of three stands of loblolly pine Pinus taeda in South Carolina, USA (Davis et al. 2010) found that increasing coarse wood debris quantity increased the abundance of two of three shrew species compared to removing debris, but not compared to leaving debris as it fell. More southeastern shrews Sorex longirostris were caught in plots with increased coarse woody debris quantities (0.057 shrews/m of drift fence) than in plots cleared of fallen debris (0.013). Numbers in neither treatment differed significantly from those in unmanipulated plots (0.026). The same pattern was seen for southern short-tailed shrew Blarina carolinensis (increased debris: 0.105 shrews/m of drift fence; debris cleared: 0.051; unmanipulated: 0.058). However, there were no differences between treatments for North American least shrew Cryptotis parva (increased debris: 0.012 shrews/m of drift fence; debris cleared: 0.014; unmanipulated: 0.015). Three plots, each 9.3 ha, were located in each of three loblolly pine stands planted in 1950–1953. In each stand, woody debris quantities were increased fivefold in one plot in 2001 by felling trees, decreased in one plot by annually removing woody debris ≥10 cm across and ≥60 cm long from 1996 and left as it fell in one plot. Shrews were sampled across plots for 14 days, during seven seasons, from January 2007 to August 2008. Shrews were caught in 19-l plastic buckets connected by drift fencing.Study and other actions tested
A replicated, site comparison study in 2013 of a tropical forest in Malaysia (Yamada et al. 2016) found more small mammal species groups, but not individual small mammals, where woody debris was left after selective logging than in areas lacking woody debris. On average, six small mammal species groups were recorded at sites with debris compared to four at sites without. No significant difference was detected for average numbers of small mammal recorded at sites with debris (43) compared to sites without (39). Sites were compared with respect to tree density, canopy openness, understorey vegetation cover, distance to road and slope and no differences in these measures were detected between sites with and without debris. Trees were selectively logged, within a 200-ha area, in 2010–2011. Single camera traps were set, around two years later, for 10 days each at 17 locations with logging woody debris and 17 without. Camera locations were ≥50 m from logging roads and were baited.Study and other actions tested