Providing evidence to improve practice

Action: Thin trees within forests: effects on mature trees Forest Conservation

Key messages

  • Eleven of 12 studies (including two replicated, randomized, controlled studies) in Brazil, Canada, and the USA found that thinning trees in forests decreased the density and cover of trees. One study found no effect of thinning on tree density.
  • Five of six studies (including one replicated, controlled, before-and-after study) in Australia, Sweden and the USA found that thinning trees in forests increased tree size. One found mixed effects of thinning on tree size.
  • One replicated, controlled study in the USA found that thinning trees in forests decreased tree species richness and diversity.
  • One replicated, site comparison study in the USA found that thinning reduced the number of conifers killed by beetles. Two replicated, controlled studies in the USA found no effect of thinning on bark-beetle caused tree mortality.

Supporting evidence from individual studies

1 

A replicated, controlled study in 1984-1985 in dry tropical forest in Ceara state, Brazil (Schacht, Long & Malacheck 1988) found that thinning decreased woody biomass. Woody biomass (kg/ha) was the highest in unthinned compared to thinned plots (0% cover-retention: 966; 25% cover-retention: 1,058; 55% cover-retention: 1,003; unthinned: 1,891). Four treatment plots (0.1 ha), three thinned (0%, 25% and 55% woody cover retained) and unthinned (95% woody cover) plots were established in 1984 in each of two sites. Data were collected in May 1985 in a subplot protected from grazing (40×50 m) in each plot.

 

2 

A replicated, controlled study in 1993-1998 in temperate lodgepole pine Pinus contorta forest in British Columbia, Canada (Sullivan et al. 2002) found no effect of lodgepole pine thinning on total tree density. The number of trees/ha was similar in thinned (3,259) and unthinned plots (7,648) Data were collected in 1998 in only three thinned (targeted to retain 1,000 stems/ha) and three unthinned treatment units (1.8-12.6 ha) established in 1993 in each of three study areas

3 

A replicated, controlled study in 1952-1995 in temperate coniferous forest in Maine, USA (Sendak, Brissette & Frank 2003) found that thinning decreased trees volume growth rate (final volume plus harvested volume plus mortality minus initial volume) but not their net growth (discounting mortality). Annual gross growth (m3/ha) was lower in thinned (2.51-4.27) than unthinned plots (4.08). Annual net growth was similar between treatments (unthinned: 1.59; thinned: 2.01-3.4). Two unthinned and 18 thinned (thinned in different time intervals following different procedures for 40 years since 1951) treatment units were established inside a 1,619 ha study area. Data was collected every five years from 1951, in a total of 307 plots (0.8 ha).

 

4 

A replicated, controlled study in 1992–1998 in silvertop ash Eucalyptus sieberi forest in Victoria, Australia (Connel, Raison & Jenkins) found that thinning increased growth rate of the retaining trees. Annual basal area increase of retained trees in thinned plots was higher (2.2 m2/ha) than in unthinned plots (1.2 m2/ha). Data was collected in 1998 in four replicates of two treatment plots (0.16 ha): unthinned and thinned (retaining 45 of the largest trees/plot). Treatments were established in 1992.

 

5 

A replicated, controlled study in 2000-2005 in temperate coniferous forest in California, USA (Stephens & Moghaddas 2005) found that thinning decreased tree density, basal area and canopy cover and increased tree height. For all trees >2.5 cm DBH density (thinned: 429/ha; control: 1,109/ha), basal area (thinned: 41 m2/ha; unthinned: 56 m2/ha) and canopy cover (thinned: 58%; unthinned: 75%) were lower in thinned plots, while height was higher in thinned plots (thinned: 23 m; unthinned: 16 m). Data were collected in 2005 in 25 plots (0.04 ha) in each of three thinned (to retain 28-34 m2/ha basal area in 2001, large trees removed and small trees shredded) and three unthinned treatment units (14-29 ha).

 

6 

A replicated, paired sites study in 2005 in Mediterranean type woodland in Oregon, USA (Perchemlides, Muir & Hosten 2008) found that thinning decreased woody canopy cover. Cover of trees and shrubs >0.3 m tall was higher in unthinned (97%) than thinned (25%) transects. Data was collected in 2005 using 30 pairs of thinned (for fuel reduction between May 1998 and June 2001) and unthinned transects (50 m). Tree cover was measured in five plots (3 m2) along each transect.

 

7 

A replicated, controlled study in 2000-2003 in temperate mixed forest in Georgia, USA (Brockway et al. 2009) found that mechanical thinning decreased tree density and diversity. In thinned plots the following were lower than in unthinned plots: number of trees/ha (thinned: 212; unthinned; 793), the number of tree species/100 m2 (thinned: 4.0; unthinned: 8.7) and diversity of trees (Shannon’s index in 100 m2; thinned: 0.78; unthinned: 1.13). Four blocks were established in 2000, each containing thinned (mulching of all broadleaf trees regardless of size, and all pines <20 cm diameter at breast height) and unthinned treatment plots (110 × 110 m). Data were collected in 2002-2003 in five subplots (10 × 10 m) within each treatment plot.

 

8 

A replicated, controlled, before-and-after study in 2000-2006 in temperate broadleaf forest in Sweden (Götmark 2009) found that thinning trees increased oak Quercus spp. regrowth rate. Relative basal area increase of oak trees, i.e. increase/initial area (cm2/cm2), was higher in thinned (3.8%) than in unthinned plots (3.2%).  Oak tree basal area increase was 106 cm2 in thinned and 81 cm2 in unthinned plots. Data were collected before (2002) and after treatment (2006) in 25 pairs of thinned (25% of basal area cut in winter 2002-2003) and unthinned 1 ha plots.

 

9 

A replicated, controlled study in 2002-2006 in temperate coniferous forest in Washington State, USA (Harrod et al. 2009) found that thinning decreased tree density and basal area and increased their average stem diameter and canopy height. Number of trees/ha (thinned: 205; unthinned: 530) and tree basal area (thinned: 17 m2/ha; unthinned: 34 m2/ha) were lower in thinned than in unthinned plots. In contrast, the average diameter of trees (thinned: 36 cm; unthinned: 30 cm) and height of the base of the canopy (thinned: 9 m; unthinned: 5 m) were higher in thinned plots. Six thinned (retaining 10–14 m2/ha basal area) and six unthinned treatment plots (10 ha) were established in 2002-2003. Data were collected 2-4 years after thinning in six 20 × 50 m plots within each treatment unit.

10 

A before-and-after trial in 2003-2006 in temperate coniferous forest in California, USA (Vaillant et al. 2009) found that mechanical thinning decreased tree density and cover, and increased their diameter and canopy height. Number of trees/ha (before: 427-1,201; after: 183-587) and canopy cover (before: 38%-71%; after: 28%-60%) decreased after thinning. In contrast, the height of the base of the canopy (before: 1.2-4.0 m; after: 3.4-7.6 m) and average diameter (before: 18-38 cm; after: 25-46 cm) increased. Data were collected in 40 plots (0.1 ha) before (2003) and after (2006) thinning followed by tree residue removal. Thinning was carried out in 2003.

11 

A replicated, paired-sites study in 2001-2008 in temperate coniferous forest in Colorado USA (Battaglia et al. 2010) found that thinning trees decreased the number of trees and the bulk density of the canopy, and increased canopy height. The density of trees >10 cm diameter at breast height (trees/ha) was higher in unthinned (1,691-580) than in thinned plots (383-55). Canopy bulk density of the trees in lodgepole pine Pinus contorta, ponderosa pine P. ponderosa and mixed conifer forests was greater in unthinned (0.15, 0.12 and 0.14 kg/m3 respectively) than in thinned plots (0.04, 0.04 and 0.01 kg/m3 respectively). However, canopy density was similar between thinned and unthinned in pinyon pine/juniper Pinus edulis/Juniperus sp. forests (0.02 vs 0.007 kg/m3). The height of the base of the canopy of the trees in lodgepole pine and mixed conifer forests was higher in thinned (7.7 and 5.1 m respectively) than in unthinned (5.8 and 2.5 m respectively). However canopy height was similar between thinned and unthinned in ponderosa pine (5.4 vs 2.3 m) and pinyon pine/juniper forests (3.9 vs 3.3 m). Trees were measured in 2007-2008 in three 50 m transect in each thinned (mulched with Hydroax© or Morbark© chipper in 2001-2006) and unthinned plot. Plots were replicated within five lodgepole pine, four ponderosa pine, six pinyon pine/juniper and three mixed conifer forests.

 

12 

A replicated, randomized, controlled study in 1998-2005 in boreal forest in south eastern Alaska, USA (Cole, Hanley & Newton 2013) found that thinning decreased canopy cover of conifers. Canopy cover of conifers was similarly lower in all thinning treatments (50-67%) than in unthinned plots (95%). Two 0.2 ha plots of each of four conifer thinning treatments (retaining 250, 370, 500, and 750 trees/ha) and unthinned plots were replicated in seven 16-18 year old forest sections. Treatments were applied in 1999, data were collected in 2005.

 

13 

A replicated, site comparison study in 2001-2007 in mixed conifer forest in California USA (Egan et al. 2010) found that thinning reduced the number of conifers killed by fir engraver beetles Scolytus ventralis. The density of ponderosa pine Pinus ponderosa and white fir Abies concolo trees (>10.2 cm) killed from 2001 to 2007 was lower in commercially thinned (23.8 trees/ha) and salvage-thinned plots (16.4 trees/ha) than in unthinned forest units (44.5 trees/ha). Monitoring was carried out using 20 clusters of four 20 × 100 m transects, in commercially thinned (residual basal area ~37 m2/ha), salvage-thinned (salvage harvesting of bark beetle-killed trees and live-tree thinning to reduce basal area to ~25 m2/ha) and in unthinned forest units. Thinning occurred between 1988 and 1998.

 

14 

A replicated, controlled study in 1998-2003 in temperate coniferous forest in California, USA (Fettig, Borys & Dabney 2010) found no effect of thinning on tree mortality caused by bark-beetle. The cumulative percentage of trees killed by bark beetles was similar in thinned (1%) and unthinned plots (3%). All trees killed by bark beetles were recorded in six treatment units (10 ha): three unthinned and three thinned (thinning from below and selection harvest leaving all stems <76.2 cm diameter at breast height and all sugar pine Pinus lambertiana, incense cedar Calocedrus decurrens and ponderosa pine Pinus ponderosa). Thinning was in 1998-1999. Data were collected in 2003.

 

15 

A replicated, controlled study in 2001-2005 in temperate coniferous forest in Montana, USA (Fiedler, Metlen & Dodson 2010) found that thinning decreased trees density and basal area. Density (thinned: 157 trees/ha; unthinned: 400 trees/ha) and basal area (thinned: 12 m2/ha; unthinned: 25 m2/ha) of trees >10 cm diameter at breast height was lower in thinned than in unthinned plots. Data were collected in 2003-2005 in ten 0.1 ha plots in each of three replicates of thinned (low thinning and improvement/selection cutting) and unthinned 9 ha treatment units. Thinning was conducted in winter 2001.

 

16 

A replicated, randomized, controlled study in 1976-2008 in boreal forest in Maine, USA (Weiskittel et al. 2011) found that thinning decreased tree density and basal area. Number of trees/acre was higher in unthinned (2,962) than row-thinned (2,279) and the lowest in tree-released (1,699) and combined treatments (1,716). Basal area (ft2/acre) was higher in unthinned (229) than tree-release (188) and combined treatments (173), and higher in row-thinned (206) than combined treatments (173). Data was collected in 2008 in four replicates of unthinned, row-thinned (5 ft. wide row removal with 3 ft. wide residual strips), tree-release (cutting selected trees at 8 ft. intervals) and combined (row-thinned plus tree-release) treatment plots (64 × 64 ft.) established in 1976.

 

17 

A replicated, controlled study in 1997-2010 in Douglas-fir Pseudotsuga menziesii forest in Oregon, USA (Dodson, Ares & Puettmann 2012) found that thinning decreased the density of mature trees. Density of trees >5 cm diameter at breast height (unthinned: 531; thinned: 261-329 individuals/ha) and their basal area (unthinned: 61; thinned: 35-45 m2/ha) were higher in unthinned than in thinned plots. One unthinned and three thinned (retaining 100-300 trees/ha) treatment units (14-58 ha) were replicated in seven sites. Mature trees were monitored in 14-21 plots (0.1 ha) in each treatment unit. Treatments were applied in 1997-1999. Monitoring was 11 years after treatments.

 

18 

A replicated, controlled study in 2001-2003 in temperate coniferous forest in California, USA (Stark et al. 2013) found no effect of thinning and mulching on tree mortality casued by bark beetle. Mortality of trees 11-25 cm DBH (thinned: <0.1%; unthinned: 0-0.2%), trees 25-45 cm DBH (thinned: 0%; unthinned: <0.1%) and trees >45 cm DBH (thinned: <0.1%; unthinned: <0.1%) was similar between treatments. Mortality caused by bark beetle for white fir Abies concolor, sugar pine Pinus lambertiana and ponderosa pine Pinus ponderosa trees was monitored in 2003 in 20 subplots (0.4 ha) in each of three unthinned and three thinned (crown thinning followed by thinning-from-below and mulching in 2001) treatment plots (14-29 ha).

 

Referenced papers

Please cite as:

Agra, H., Schowanek, S., Carmel, Y., Smith, R.K. & Ne’eman, G. (2017) Forest Conservation. Pages 285-328 in:W.J. Sutherland, L.V. Dicks, N. Ockendon & R.K. Smith (eds) What Works in Conservation 2017. Open Book Publishers, Cambridge, UK.