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 and health of remaining trees. Thinning has been done historically to maximize timber production but may have ecological benefits. The retention of large old trees may provide roosting sites for bats, and opening up the canopy may provide favourable foraging habitats.
For studies that used thinning as part of selective logging methods, see the intervention ‘Use selective or reduced impact logging instead of conventional logging’.
Supporting evidence from individual studies
A replicated, site comparison study in 1993–1994 of 24 forest sites in the Cascade mountains, USA (Erickson & West 1996) found that thinned tree stands of two different ages had higher bat activity than young unthinned tree stands, but lower bat activity than clearcut stands. Significantly more bat passes were recorded in 10–13 year old thinned stands (average 2 bat passes/night) and mature thinned stands (4 bat passes/night) than in young unthinned stands (no bat passes). However, fewer bat passes were recorded in both thinned stands than in clearcut stands (8 bat passes/night). At least five bat species were recorded (see original reference for data for individual species). Six replicates of tree stands in four post-harvest stages were sampled: thinned stands (10–13 year old Douglas fir Pseudotsuga menziesii), mature thinned stands (51–62 year old Douglas fir or western hemlock Tsuga heterophylla), young unthinned stands (30–40 years old, high tree density with varied tree diameter), clearcut stands (2–3 years post-harvest, 1–2 m high Douglas fir seedlings). At each of 24 sites, bat detectors recorded bat activity for six nights in July–September 1993 and 1994.Study and other actions tested
A replicated, paired sites and site comparison study in 1994–1995 in 11 pairs of forest stands and nine old growth forests in the Oregon Coast range, USA (Humes et al 1999) found that thinned tree stands had higher bat activity than unthinned tree stands, and there was no difference in bat activity between thinned stands and old growth forest. Overall bat activity (of at least nine bat species) was significantly higher in thinned (average 10 bat passes/night) than unthinned stands (6 bat passes/night). There was no significant difference in bat activity between thinned stands and old growth forest (average 13 bat passes/night). Surveys were carried out in 11 pairs of stands (10–63 ha, 50–100 years old) that were thinned (in 1971–1985) or unthinned, and in nine old growth forest stands (20–70 ha, >200 years old). All 31 tree stands were dominated by Douglas fir Pseudotsuga menziesii. Bat detectors recorded bat activity at one random location in each of 11 pairs of tree stands and in a nearby old growth forest stand simultaneously for two consecutive nights on four occasions in June–September 1994 or May–September 1995.Study and other actions tested
A replicated, controlled, site comparison study in 1999–2000 of 36 deciduous, coniferous and mixed forest sites in Alberta, Canada (Patriquin & Barclay 2003) found that thinned tree stands had similar activity for three bat species to unthinned tree stands, but one bat species was recorded less often in thinned stands than in clearcut patches. The activity of little brown bats Myotis lucifugus, northern long-eared bats Myotis septentrionalis and silver haired bats Lasionycteris noctivagans did not differ significantly between thinned and unthinned tree stands in any of the three types of forest (data reported as statistical model results). In all three types of forest, silver-haired bat activity was significantly lower in thinned tree stands than in clearcut patches. Experimental forest patches (10 ha) had four tree density treatments (0% clearcut, 20% and 50% thinned, 100% unthinned) in three types of forest (deciduous, coniferous and mixed, all >50 years old). Three replicates of each treatment in each forest type were surveyed. At each of 36 sites, bat activity was recorded with bat detectors at the centre and edge of each patch in June–July 1999 and June–August 2000 for a total of 11–14 nights.Study and other actions tested
A replicated, paired sites study in 2001 in 13 managed red pine Pinus resinosa forests in Lower Michigan, USA (Tibbels & Kurta 2003) found that thinned tree stands had similar bat activity to unthinned stands. Total bat activity (of at least five bat species) did not differ significantly between thinned (16 bat passes, 0.3 feeding buzzes) and unthinned stands (8 bat passes, 0.5 feeding buzzes). At all sites, bat activity was significantly higher in nearby openings within the forests (thinned: 788 bat passes, 5 feeding buzzes; unthinned: 725 bat passes, 5 feeding buzzes) than within tree stands. Thirteen paired tree stands (one thinned, one unthinned) were surveyed on two occasions. All stands were >10 ha and 52 years old on average. Thinned stands had been thinned 5–11 years prior to the study. Openings in stands were either cleared for wildlife or sites used by loggers. Bat surveys were carried out simultaneously at groups of four sites (interior and openings in a pair of thinned and unthinned stands). Bat detectors recorded bat activity for one full night/site in May–June and July–August 2001. Bats were captured using mist nets during six nights in May–August 2001 at half of the thinned sites and half of the unthinned sites.Study and other actions tested
A replicated, controlled, site comparison study in 2001–2002 of nine pine forest sites in South Carolina, USA (Loeb & Waldrop 2008) found that thinned tree stands had higher activity for two of three bat species than unthinned control tree stands. Activity of big brown bats Eptesicus fuscus and eastern red bats Lasiurus borealis was significantly higher in thinned tree stands (big brown bats: average 1.2 bat passes/night; eastern red bats: 0.7 bat passes/night) than in unthinned control stands (big brown bats: 0.1 bat passes/night; eastern red bats: 0.5 bat passes/night) or burned stands (big brown bats: 0.3 bat passes/night; eastern red bats: 0.3 bat passes/night). Activity of eastern pipistrelles Perimyotis subflavus did not differ significantly between thinned (0.4 bat passes/night), unthinned (0.1 bat passes/night) or burned stands (0.1 bat passes/night). Nine 14 ha stands (loblolly pine Pinus taeda and shortleaf pine Pinus echinata) were surveyed with three replicates of three treatment types: thinning to 18 m2/ha (in winter 2000–2001), prescribed burning (burned in April 2001 with strip head fire and flanking fires), and a control with no treatment. Bat activity was sampled with two bat detectors at random points in each of 12 stands for two full nights/month in May–August 2001 and 2002.Study and other actions tested
A replicated, site comparison study in 2012–2013 at 24 eucalypt forest sites in southeastern Australia (Blakey et al 2016) found that thinned forests had greater total bat activity than forests with unthinned regrowth, but bat activity was similar between thinned and natural forests, and 10 of 11 bat species were recorded in all forest types. Total bat activity was lowest in unthinned regrowth (average 140 bat passes/night) and similar in forest thinned 0–4 years ago (318 bat passes/night), forest thinned 5–10 years ago (344 passes/night) and natural forest (350 bat passes/night). The same 10 bat species were recorded in all four types of forest, except for the chocolate wattled bat Chalinolobus morio, which was not recorded in forests with unthinned regrowth. Six sites were surveyed for each of four thinning categories: unthinned regrowth (even-aged, 1,253 stems/ha), thinned 0–4 years ago (even-aged, 280 stems/ha), thinned 5–10 years ago (patchy structure, 419 stems/ha), natural forest (mature, open forest with mixed-age, large trees, 295 stems/ha). Bat activity was recorded with bat detectors at two locations/site for 2–6 full nights between December 2012 and January 2013.Study and other actions tested