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 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. A greater number of 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 paper 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 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, average 184 trees/ha) or unthinned (average 418 trees/ha), and in nine old growth forest stands (20–70 ha, >200 years old, average 155 trees/ha). 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 1998–2000 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 (bat passes/hour) 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 lower in thinned tree stands than in clearcut patches. Experimental forest patches (10 ha, average 974–1,210 stems/ha) were created in winter 1998–1999 with three replicates of four treatments (clearcut with no trees retained, thinned with 20% or 50% of trees retained, unthinned with 100% of trees retained) in each of the three forest types (all >50 years old). 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 33–42 nights/site.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. Overall 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 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: 12 stems/100 m2: one unthinned: 22 stems/100 m2) 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 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 an average of 576 live trees/ha (in winter 2000–2001), prescribed burning (burned in April 2001 with strip head fire and flanking fires, average 532 live trees/ha), and a control with no treatment (average 755 live trees/ha). 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 overall 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. Overall bat activity was lowest in unthinned regrowth (average 140 bat passes/night) and similar in forest thinned 0–4 years previously (318 bat passes/night), forest thinned 5–10 years previously (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 (see original paper for data for individual species). Six sites were surveyed for each of four thinning categories: unthinned regrowth (even-aged, average 1,253 stems/ha), thinned 0–4 years previously (even-aged, average 280 stems/ha), thinned 5–10 years previously (patchy structure, average 419 stems/ha), natural forest (mature, open forest with mixed-age, large trees, average 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
A replicated, randomized, site comparison study in 2013–2014 of 10 hardwood tree stands in Tennessee, USA (Cox et al 2016) found that thinned and burned tree stands had higher overall bat activity for three of four treatment types than untreated tree stands. Overall bat activity was higher in tree stands thinned to 14m2/ha and burned in the spring (average 656 bat passes) or autumn (292 bat passes) than untreated control stands (95 bat passes). However, tree stands thinned to 7m2/ha had higher bat activity than control stands when burned in the autumn (280 bat passes) but not in the spring (123 bat passes). Six groups of bat species were recorded (see original paper for data for individual species groups). The study does not distinguish between the effects of thinning and burning. Each of four treatments (thinning to 7 or 14 m2/ha with burning in the autumn or spring) was randomly applied to two tree stands (20 ha, 80–100 years old). Two tree stands were untreated controls (average 20 m2/ha). Overstorey thinning was carried out in June 2008 and prescribed fires in October 2010 and 2012 (autumn) and March 2011 and 2013 (spring). Each of 10 stands was surveyed with a bat detector for seven full nights on three occasions in May–July 2013 and 2014.Study and other actions tested
A replicated, site comparison study in 2006–2010 of 12 tree stands in two upland hardwood forests in Ohio, USA (Silvis et al 2016) found that overall bat activity was higher in thinned and burned tree stands than in untreated tree stands. Overall bat activity was higher in tree stands thinned with 50% of the overstorey retained and burned (average 16–30 bat passes/night) and tree stands thinned with 70% of the overstorey retained and burned (14–24 bat passes/night) than in untreated control stands (3–4 bat passes/night). Four bat species or species groups were recorded (see original paper for data for individual species). The study does not distinguish between the effects of thinning and burning. In each of two forests, four tree stands (10 ha) were treated with thinning (commercially thinned between June 2005 and March 2006 with 50% or 70% overstorey retained) and prescribed fire (backing and strip fires in autumn 2009 or spring 2010) and two tree stands were untreated controls (tree density not reported). In each of 12 tree stands, eight points were sampled with bat detectors for 3 h/night over a total of six nights in May–August 2006 and June–September 2009 and 2010.Study and other actions tested
A replicated, controlled, before-and-after study in 2012–2015 of 10 forest sites in south-eastern Australia (Gonsalves et al 2018) found that thinning increased the activity of six of nine bat species/species groups. For six bat species/species groups (Gould’s wattled bat Chalinolobus gouldii, free-tailed bats Mormopterus spp., inland broad-nosed bat Scotorepens balstoni, large forest bat Vespadelus darlingtoni, southern forest bat Vespadelus regulus, little forest bat Vespadelus vulturnus), activity was higher after thinning than before (data reported as statistical model results), whereas activity in control plots was either similar (five species) or decreased (one species). For two bat species/species groups (long-eared bats Nystophilus spp., yellow-bellied sheath-tailed bat Saccolaimus flaviventris), activity was similar before and after thinning, but decreased at control plots. For one bat species (white-striped free-tailed bat Austronomous australis), activity decreased after thinning and in control plots. Two plots (0.1 ha) were sampled in each of 10 forest sites (dominated by river red gum Eucaluptus camaldulensis). Five treatment sites were thinned in 2012–2015 (average 434 stems/ha). Five control sites were left unthinned (average 1,150–1,300 stems/ha). One bat detector recorded bat activity at the centre of each of 20 plots for three consecutive nights in December 2012 (before thinning) and December 2015 (after thinning).Study and other actions tested
A replicated, site comparison study in 2015 of six forest sites in New South Wales, Australia (Gonsalves et al 2018) found that recently thinned sites had higher bat activity and diversity than unthinned sites, but results varied for sites thinned more than eight years previously. Overall bat activity and diversity were higher in sites recently thinned (<8 years previously) than in unthinned sites (data reported as statistical model results). Sites thinned 8–20 years previously had similar bat activity to unthinned sites, but higher bat diversity. Sites thinned >20 years previously had higher bat activity than unthinned sites, but similar bat diversity. Bat activity did not differ significantly between thinned sites and undisturbed forest but was lower at unthinned sites than undisturbed forest. Twelve bat species were recorded in total (see original paper for data for individual species). Five treatments were sampled at each of six forest sites (20–30 ha, dominated by white cypress pine Callitris glaucophylla): unthinned (∼6,500 stems/ha); recently thinned (<8 years previously); thinned 8–20 years previously; thinned >20 years previously; undisturbed forest. All thinned sites had a similar density of stems (∼1,600 stems/ha). One bat detector recorded bat activity for 2–3 nights at each of 30 sites in November 2015.Study and other actions tested
A replicated, controlled study in 2016–2017 at eight forest sites in New South Wales, Australia (Law et al 2018) found that thinned tree stands had higher overall bat activity and activity of little forest bats Vespadelus vulturnus than unthinned tree stands, and long-eared bats Nyctophilus spp. had higher activity in thinned tree stands in the spring but not in the autumn. Overall nightly bat activity (of 10 species/species groups) was higher in thinned (183 bat passes) than unthinned tree stands (97 bat passes) as was the activity of little forest bats (data not reported). Average nightly activity of long-eared bats was higher during spring in thinned (7 bat passes) than unthinned tree stands (1 bat pass), but the reverse was true in autumn (thinned: 3 bat passes; unthinned: 6 bat passes). All eight sites (12 ha) were dense white cypress pine Callitris glaucophylla regrowth separated by ≤200 m. Four sites were thinned in June–July 2016 (average 358 stems/ha) and four sites were left unthinned (average 463 stems/ha). Each pair was surveyed simultaneously with 1–2 bat detectors/stand for two nights in November 2016 (spring) and March 2017 (autumn).Study and other actions tested