Background information and definitions
Thinning of trees (i.e. removal of, trees to reduce density) may be undertaken as a timber management practice e.g. within forestry plantations where saplings may be planted at unnaturally high densities, or as a deliberate conservation management practice to reinstate more natural open woodland conditions that have been lost due to active fire suppression and/or loss of populations large mammal grazers and browsers. Whilst mechanical thinning may be aimed at benefitting certain target species (flora or fauna), impacts on non-target species inevitably occur.
Many studies (especially in pine forests and savannas) simultaneously thin forests whilst introducing a prescribed fire regime. These studies are described in detail in ‘Use prescribed burning’.
Supporting evidence from individual studies
A controlled before-and-after study in 1973-1983 in pine- Pinus spp. oak Quercus spp. woodlands in Arizona, USA (Brawn & Balda 1988), found that over 90% of nests on two managed plots were in nest boxes, compared to 30% on an unmanaged plot. This study is discussed in more detail in ‘Provide artificial nesting sites’.Study and other actions tested
A replicated before-and-after study in four National Forests in Texas, USA (Conner et al. 1995), found that red-cockaded woodpecker Picoides borealis populations increased at all four sites in the early 1990s after management, including reducing pine tree basal area to 14 m2/ha, was intensified in 1989. This study is discussed in detail ‘Provide artificial nesting sites’, ‘Translocate individuals’ and ‘Use prescribed burning’.Study and other actions tested
A replicated controlled study in 1992-1993 in 33 pine-grassland stands in Ouachita National Forest, Arkansas, USA (Wilson et al. 1995), found that overall bird species richness and abundances were similar in stands with tree thinning, compared to control stands. This study is discussed in more detail in ‘Use prescribed burning’.Study and other actions tested
A study in mixed pine forests in 1985-1996 in South Carolina, USA (Franzreb 1997) found that a population of red-cockaded woodpeckers increased following the thinning of trees, reducing basal area to 14-18 m2/ha, amongst other interventions. The results of this study are discussed in more detail in ‘Use prescribed burning’.Study and other actions tested
A replicated, controlled study in 1992-1994 in oak-hickory Carya forests in the Ozark Mountains, Arkansas, USA (Rodewald & Smith 1998), found that three of 14 species analysed were more abundant in plots with both thinning and understorey control, compared to control plots or those with just understorey control. This study is discussed in detail in ‘Manually control/remove understorey and midstorey vegetation’.
A replicated study in 1995-1996 in pine savanna in South Carolina, USA (Krementz & Christie 1999), found that there were fewer scrub-successional species in stands managed for red-cockaded woodpeckers (including tree thinning) than in stands which were clearcut to remove non-native pines and replanted with longleaf pines Pinus palustris. This study is discussed in detail in ‘Clearcut and re-seed forests’.Study and other actions tested
At Piedmont National Wildlife Refuge, Georgia, USA (Powell et al. 2000), a replicated controlled study in 1993-1996 found no impact of thinning and prescribed burning on wood thrushes Hylocichla mustelina. This study is discussed in detail in ‘Use prescribed burning’.Study and other actions tested
A replicated study in four oak-hazel Corylus avellana woodlands (average size 5.3 ha) in 1996-1999 in Uppland and Åland, Sweden (Hansson 2001), found that sites that were subject to brush cutting and tree thinning had similar numbers of migrant and breeding birds as grazed sites, and more than some abandoned sites. Sites under traditional management (cleared in spring, mown in mid-late summer and grazed in autumn) had higher abundances of migrant birds. This study is discussed in detail in ‘Employ grazing in natural and semi-natural habitats’.Study and other actions tested
A replicated, controlled paired sites study in 1995-1997 in pine forests in Angelina National Forest, Texas, USA (Conner et al. 2002), found that spring bird species richness and abundances were significantly higher in plots managed for red-cockaded woodpecker, compared to unmanaged plots. This study is discussed in detail in ‘Use prescribed burning’.Study and other actions tested
A replicated study across 40 shortleaf pine Pinus echinata-hardwood stands in Ouachita National Forest, Arkansas, USA, in 1999-2000 (Cram et al. 2002) found that northern bobwhite Colinus virginianus abundances were higher in thinned stands, compared to controls. This study is discussed in more detail in ‘Use prescribed burning’.Study and other actions tested
A controlled before-and-after study in 1993-1996 in loblolly pine Pinus taeda forests in Piedmont National Wildlife Refuge, Georgia, USA (Lang et al. 2002), found that habitat management for red-cockaded woodpecker (largely prescribed burning and thinning) had little effect on wood thrushes. This study is discussed in detail in ‘Use prescribed burning – pine forests’.
A replicated, randomised, controlled study in 1998-2005 in 12 ponderosa pine stands (15-20 ha) in the North Cascade Range, Washington, USA (Gaines et al. 2007), found that there were no differences in bird densities between four stands with low-retention thinning and prescribed burning and those with high retention thinning and burning (averages of 13 birds/ha for both). This study is described in detail in ‘Use prescribed burning’.Study and other actions tested
A controlled before-and-after study in 2000-2006 at three ponderosa pine Pinus ponderosa forest sites in northern Arizona, USA (Hurteau et al. 2008), found that species richness and evenness did not differ between thinned forest blocks and controls. In addition, none of five common species were more abundant after thinning, but two (yellow-rumped warbler Dendroica coronata and mountain chickadees Poecile gambeli were less abundant in thinned plots. This study is discussed in more detail in ‘Prescribed burning’.Study and other actions tested
A replicated, paired site study from April-June in 2006 in 20 conifer plantation sites in Moray Firth, Scotland (Calladine et al. 2009), found that bird species richness and abundance was similar between thinned and unthinned plantations. Sites were in first-rotation and 18-32 years since planting and consisting of ten thinned sites paired with ten unthinned (average of 11 and 16 trees within a 5 m radius around count stations). Average species richness was 19 (range 13–24) at the thinned sites and 19 (range 15–22) at control sites. No significant differences between treatments were found in occurrence rates or abundance for any bird species. As the authors did not find any difference in species richness, they concluded that thinning within the study areas was also unlikely to have influenced the breeding populations of the scarcer species. No significant differences in ground cover, the presence of shrubs or stem diameter at breast height were found between treatments.Study and other actions tested