Retain undisturbed patches during thinning operations
Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 2
Background information and definitions
Thinning is a forestry practice that involves the selective removal of trees to reduce tree density and improve the growth rate, health and timber quality of remaining trees. Thinning has been done historically to maximize timber production but may have ecological benefits, such as opening up the canopy and allowing more light in, which may benefit some species. However, some species may benefit from the shelter available within retained undisturbed forest patches.
Supporting evidence from individual studies
A randomized, replicated, controlled, before-and-after study in 2001–2003 of a coniferous forest in Montana, USA (Ausband & Baty 2005) found that snowshoe hares Lepus americanus used retained undisturbed patches more than they used thinned forest. More hare tracks were counted in undisturbed patches than in thinned areas when patches comprised 8% (undisturbed: 106; thinned: 25 tracks/km) and 35% (undisturbed: 107; thinned: 15 tracks/km) of the stand. The same was found for faecal pellet counts in 8% (undisturbed: 1.0; thinned: 0.2 pellets/tray) and 35% (undisturbed: 1.4; thinned: 0.1 pellets/tray) retention patches. After treatments were applied, hares increased use of undisturbed (before treatment: 29; after: 144 tracks/km) and mature (before treatment: 64–80; after: 88–181 tracks/km) stands, suggesting movements into these areas. Five conifer stands (10.5–14.0 ha), regenerating naturally after felling in 1985, were selected. Treatments were applied in June 2002 and comprised: thinning with five 0.2-ha unthinned patches (8%) retained (two stands), thinning with five 0.8-ha unthinned patches (35%) retained (two stands) and one undisturbed stand. Conifer density was 5,350–7,050/ha before and 656–750/ha after thinning. Two adjacent mature stands represented pre-harvest conditions. Hare-track density was assessed from December–March in 2001–2002 (prior to thinning) and 2002–2003 (after thinning). Faecal pellets were surveyed each winter within 50 trays in each stand, into which pellets accumulated during April snowmelt.Study and other actions tested
A randomized, replicated, controlled study in 2005–2007 of a ponderosa pine Pinus ponderosa forest in Northern Arizona, USA (Loberger et al. 2011) found that tassel-eared squirrels Sciurus aberti made greater use of undisturbed than thinned forest. In winter 57% and during the rest of the year 51% of squirrel home range areas fell within undisturbed forest compared to 39% availability by extent in the study area. Squirrels also showed a preference for dense canopies. In winter, canopies with 51–75% cover accounted for 53% of squirrel use compared to 44% of resource availability. Thinning was carried out from 1998–2000. Seventeen-hectare blocks within a 10-km2 area were randomly assigned to no thinning and to low, medium and high-intensity thinning. A combination of these managements was applied to four additional blocks of approximately 40 ha each. Squirrel locations were monitored by radio-tracking from December 2005 to July 2007.Study and other actions tested