Thin trees within forests
Overall effectiveness category Awaiting assessment
Number of studies: 5
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 the remaining trees. Thinning has been done historically to maximize timber production but may have ecological benefits. Thinning removes some of the canopy, allowing light in to the woodland floor, replicating the natural creation of woodland clearings. This may benefit butterflies and moths by encouraging the growth of food plants on the woodland floor, or by creating sheltered, sunny sites for basking and territory defence. However, some species spend almost their entire lives in the canopy, and may be adversely affected by the loss of even a few canopy trees, so caution should be taken when applying this action.
For studies that used thinning as part of selective logging methods, see the intervention “Biological resource use – Harvest groups of trees or use thinning instead of clearcutting”.
Supporting evidence from individual studies
A site comparison study in 1998 in two pine forests in Arizona, USA (Waltz & Covington 1999) found that a forest restored by thinning young trees and prescribed burning had a higher abundance and species richness of butterflies than an unrestored forest. Two years after thinning and burning, the restored forest had a higher abundance (6–46 individuals/visit) and species richness (3–11 species/visit) of butterflies than the unrestored forest (abundance: 0–7 individuals/visit; richness: 0–4 species/visit). One species, the checkered white Pieris protodice, was only found in the restored forest, but another, the California sister Limenitis bredowii, was only found in the unrestored forest. In 1996, a 40-acre ponderosa pine Pinus ponderosa forest was thinned (pole-sized trees removed) and burned to reopen the dense understorey. An adjacent forest was not restored. From May–July 1998, butterflies were surveyed six times (every two weeks) along a single 450-m transect in each forest.Study and other actions tested
A site comparison study in 1996 in a logged tropical rainforest in south-east Côte d’Ivoire (Fermon et al. 2000) found that the abundance, species richness and diversity of fruit-feeding butterflies (Nymphalidae) were similar in forest managed by thinning and naturally regenerating forest, but rarer species were caught less frequently in thinned forest. Forest managed by thinning had a similar abundance (54 individuals/trap), species richness (76 species) and diversity (data presented as model results) of butterflies to naturally regenerating forest (abundance: 56 individuals/trap; richness: 71 species). However, species with smaller geographic ranges were caught less frequently in thinned forest (data presented as model results). See paper for individual species results. From 1960–1990, a 216 km2 forest was selectively logged. From 1992 the forest was protected, and two management options were implemented: liberation thinning and natural regeneration (no management). Liberation thinning was designed to promote the growth of commercial timber species, and included cutting of lianas and climbers, and killing some non-commercial trees. Rare trees and important fruit trees were protected. From January–March 1996, butterflies were sampled in 30 ha of thinned forest and 30 ha of naturally regenerating forest, using 28 banana-baited traps in each habitat. Traps were set 1 m above ground, 100 m apart, for six consecutive days, and checked daily.Study and other actions tested
A controlled study in 1997–2001 in two piñon pine and juniper woods in New Mexico, USA (Kleintjes et al. 2004) found that mechanically thinning woodland increased the abundance and species richness of butterflies. Four years after thinning, the abundance (12–20 individuals/transect) and species richness (9–12 species) of butterflies were higher in thinned woodland than in woodland which had not been thinned (abundance: 4 individuals/transect; richness: 5–7 species). The increase in butterflies correlated with an increase in understorey plants in the thinned woodland. In January–March 1997, tree cover in a 40-ha watershed was reduced from 35 to 10%, by removing individual trees <20 cm diameter. Felled trees were applied as rough mulch onto adjacent bare soil. In an adjacent 40-ha watershed, the tree cover was left at 40%. In June–July 1999 and 2001, butterflies were surveyed twice/year on 20 permanent 100-m transects/watershed. Transects were 75–150 m apart, and ran down the slopes, with 10 on each side of each valley.Study and other actions tested
A replicated, randomized, paired, controlled, before-and-after study in 1997–2001 in a pine forest in Arizona, USA (Waltz & Covington 2004) found that forests restored by thinning and prescribed burning had a higher abundance and species richness of butterflies than unrestored forests. One and two years after thinning and burning, restored forests had a higher butterfly abundance (48–132 individuals/unit) and species richness (7–16 species/unit) than unrestored forests (abundance: 10–42 individuals/unit; richness: 4–10 species/unit). Before restoration, there was no significant difference between forest marked for restoration (abundance: 23–50 individuals/unit; richness: 8–12 species/unit) and unrestored forest (abundance: 10–41 individuals/unit; richness: 5–13 species/unit). These results were primarily due to the abundance of species of blue (Lycaenidae) and white (Pieridae) butterflies (see paper for details). In 1997, four blocks within a 5,000-ha ponderosa pine Pinus ponderosa forest were each divided into two units (≤40-ha each). In autumn/winter 1999–2000, one randomly assigned unit/block was thinned and burned. The other units were not restored. From May–August 1997, 1998, 2000 and 2001, butterflies were surveyed six times/year (two-week intervals) along two or three 300-m transects/unit.Study and other actions tested
A replicated, paired, controlled, before-and-after study in 2009–2011 in 15 coniferous forest stands in Vihti and Jokioinen, Finland (Korpela et al. 2015) found that thinning trees near to the edge of woodland, in addition to felling the edge, increased the abundance of specialist butterflies and the total species richness of butterflies, diurnal moths and bumblebees (Bombus spp.) combined. Two years after thinning and felling, the abundance of specialist butterflies (2.3 individuals/plot), and the total species richness of butterflies, moths and bumblebees (7.3 species/plot), were higher in thinned areas than in areas which had not been thinned (butterfly abundance: 0.6 individuals/plot; total richness: 3.8 species/plot). Prior to thinning, both butterfly abundance and total species richness were similar in the plots designated for thinning (butterfly abundance: 0.6 individuals/plot; total richness: 4.9 species/plot) and no thinning plots (butterfly abundance: 0.5 individuals/plot; total richness: 5.3 species/plot). In winter 2009–2010, in each of 15 forest stands, a 50-m-long forest edge was logged. Logging comprised felling a 5-m-wide strip at the forest edge, and behind that a 20-m-wide belt was thinned to a basal area of 8 m2/ha. Trunks were removed, but other debris was left on the ground. A second 50 × 25 m area at each site, within 8–61 m of the logged area, was left unlogged. From late May–August 2009–2011, butterflies, diurnal moths and bumblebees were surveyed seven times/year in each thinned and unthinned area, at two-week intervals.Study and other actions tested