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Providing evidence to improve practice

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

Key messages

  • Six of twelve studies (including two replicated, randomized, controlled studies) in Japan and the USA found that thinning trees in forests increased the density of young trees. One study found that thinning decreased the density of young trees. Five found no effect or mixed effects on the density of young trees. One replicated, controlled study in the USA found no effect of thinning on the density of oak acorns.
  • One controlled study in Peru found that thinning increased the growth rate of young trees.

Supporting evidence from individual studies

1 

A replicated, controlled, paired site study in 1993–1995 in temperate coniferous Douglas-fir Pseudotsuga menziesii forest in western Oregon, USA (Bailey & Tappeiner 1998) found that thinning increased conifer seedling density. Tree seedling density in thinned forest segments (1,433/ha) was greater than in unthinned forest segments (233/ha). Monitoring was in 1993–1995 in 32 pairs of thinned (between 1969 and 1984) and unthinned sites that had regenerated naturally following harvest between 1880 and 1940. Undisturbed old-growth Douglas-fir stands (>200 years) were present for comparison on 20 of the 32 paired sites.

 

2 

A replicated controlled study in 1996-1997 in Japanese beech Fagus crenata forest in Japan (Nagaike, Kamitani & Nakashizuka 1999) found that thinning increased the number of new tree stems. The number of new stems/ha (thinned: 686; unthinned: 413) was higher in thinned than unthinned plots. Data were collected in 1997 in 60 quadrats (5 × 5 m) in each of 17 thinned (30–70% by volume of the trees cut 10 years before measurements) and five unthinned plots (10 × 150 m).

 

3 

A controlled study in 1989-2000 in tropical rainforest in Peru (Dolane, Gorchov & Cornejo 2003) found that five years after strip-clearing, thinning enhanced annual growth increase of new tree stems. In one cleared strip, annual growth increase for stems of three groups: recruits, stump sprouts and commercial species advance regeneration, was higher after thinning (0.13-0.19 cm) than in control plots (0.04-0.08 cm). In the other strip, annual growth increase for stems of commercial recruits, commercial stump sprouts, other recruits and other stump sprouts, was higher after thinning (0.20-0.28 cm) than in control plots (0.09-0.16 cm). Two 30×150 m strips were clear-cut in 1989. Each strip was divided into twenty 15×15 m plots. In 1996 all trees were thinned in two 30×45 m blocks in each strip. Data were collected in 2000.

 

4 

A controlled study in 1997-2001 in temperate coniferous forest in Japan (Zhu et al. 2003) found no effect of thinning on Japanese black pine Pinus thunbergii seedlings density. Density (seedlings/m2) was similar in all thinning treatments (unthinned: 14; 20%: 16, 30%: 13; 50%: 17). Four treatments: unthinned and 20%, 30% and 50% of the area thinned in a patch pattern were applied each to a 40 × 50 m forest section in December 1997. Japanese black pine seedlings were monitored four growing seasons after thinning in five 2 × 2 m plots in each treatment.

 

5 

A replicated, controlled study in 1999-2003 in temperate mixed forest in California, USA (Jones et al. 2005) found that thinning by removal of all conifers increased trembling aspen Populus tremuloides density. Total aspen density (stems/ha) was higher in thinned (16,000) than in unthinned plots (6,000). Data were collected in 2003 in 2-4 transects (30.5 × 1.8 m) in each site (~1.7 ha) of four thinned (all conifers removed in 1999) and unthinned pairs.

 

6 

A replicated, controlled study in 2001-2005 in second-growth oak Quercus spp. forests in southern Ohio, USA (Albrecht & McCarthy 2006) found that mechanical thinning reduced small seedling density and increased large seedling and small sapling densities. Density (individuals/ha) of small (<50 cm tall) seedlings was lower in thinned plots (unthinned: 135,000; thinned: 70,000). In contrast, the density of large seedlings (40-150 cm tall) (unthinned: 2,000; thinned: 7,000) and small saplings (<3 cm DBH) (unthinned: 1,000; thinned: 2,400) was higher in thinned plots. Thinning had no effect on density of large saplings (3-10 cm DBH) (unthinned: 600; thinned: 500). Three forest areas were divided into unthinned and thinned (mechanical-thinning) treatment units (30 ha). Treatments were applied in the inactive season of 2001. Regeneration was sampled in ten 0.1 ha plots in each treatment (a total of 40 plots/site) in summer 2004.

 

7 

A replicated, controlled study in 2000-2005 in temperate broadleaf forest in Ohio, USA (Lombardo & McCarthy 2008) found no effect of thinning on numbers of black oak Quercus velutina and chestnut oak Q. prinus acorns. The density (acorns/ha) of black oak (20,000-30,000) and chestnut oak (30,000-40,000) was similar between treatments. Data were collected in 2005 in nine thinned (thinning from below retaining 70% of tree basal area in 2000-2001) and nine unthinned plots (0.1 ha) at each of two sites.

 

8 

A replicated, randomized, controlled study in 2000-2005 in temperate forest in California, USA (Moghaddas, York & Stephens 2008) found no effect of thinning followed by mulching on conifer or California black oak Quercus kelloggii seedling densities. The combined density (trees/m2) of conifer and oak seedlings (thinned: 2.8; unthinned: 1.5) and the density of oaks (thinned: 0.40; unthinned: 0.45) were similar between treatments. Data were collected in 2006 in 10 sets of four plots (1 m2) in each of three thinned (trees >25 cm DBH removed in 2001, followed by mulching of the remaining trees) and three unthinned treatment units (14-29 ha).

 

9 

A replicated, controlled, before-and-after study in 2000-2004 in Piedmont forest in South Carolina, USA (Phillips & Waldrop 2008) found that thinning increased tree seedling density. Changes in density of tree seedlings <1.4 m tall was higher in thinned plots (thinned: 19,400/ha; unthinned: 8,550/ha). Changes in density of tree saplings >1.4 m tall and <10 cm DBH were similar between treatments (thinned: 515; control: 243). Ten plots (0.1 ha) were established in 2000/2001 in each of three unthinned and three thinned (basal area reduced to 18 m2/ha) treatment units. Data were collected three years after treatment.

 

10 

A replicated, randomized, controlled study in 2000-2007 in temperate broadleaf forest in North Carolina and Ohio, USA (Waldrop et al. 2008) found that thinning trees increased the cover of seedlings and the density of tree saplings. At a 'cool temperate climate' site the number of hardwood-tree saplings (>1.4 m tall) (thinned: 800/ha, unthinned: 370/ha) and cover of shrub and tree seedlings (< 1.4 m tall) (thinned: 53%, unthinned: 27%) were higher in thinned than unthinned plots. At a 'warm continental climate' site, cover of shrub and tree seedlings was higher in thinned plots (thinned: 28%, unthinned: 18%), while numbers of tree saplings was similar between treatments (thinned: 1200/ha, unthinned: 1800/ha). Three pairs of thinned (in 2000-2002) and unthinned treatment units (10-26 ha) were established at each of the two sites. Data were collected 4-5 years post-treatments in 10 plots (0.1 ha) in each treatment unit.

 

11 

A replicated, controlled study in 2001-2005 in temperate coniferous forest in Montana, USA (Fiedler, Metlen & Dodson 2010) found that thinning decreased tree sapling density. The density of tree-saplings >0.1 and <10 cm diameter at breast height was lower in thinned (5,293 stems/ha) than in unthinned plots (11,483 stems/ha). 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.

 

12 

A replicated, controlled study in 1997-2010 in Douglas-fir Pseudotsuga menziesii forest in Oregon, USA (Dodson, Ares & Puettmann 2012) found that thinning increased the density of new tree stems. The density of saplings >137 cm tall and <5 cm diameter at breast height (unthinned: 114; thinned: 527-815) and seedlings 15-136 cm tall (unthinned: 502; thinned: 2,719-4,594) was higher in thinned than in unthinned plots. One unthinned and three thinned (retaining 100-300 trees/ha) treatment units (14-58 ha) were replicated in seven sites. Saplings and seedlings were monitored in four subplots (0.002 ha) within each plot. Treatments were applied in 1997-1999. Monitoring was 11 years after treatments.

 

13 

A before-and-after study in 2003-2005 in temperate coniferous forest in California, USA (Walker et al. 2012) found no effect of thinning on the density of conifer seedlings and saplings. There was no difference between treatments for the change in density (individuals/ha after minus before) of seedlings <1.37 m tall (thinned: -539; unthinned: -2,303) and saplings >1.37 m tall and <10 cm DBH (thinned: -222; unthinned l: 74). Data were collected in 2003 (before) and 2005 (after) in five plots (0.04 ha) in each of two thinned (thinned to retain 30 m2/ha basal area with slash mulching in June 2003) and two unthinned treatment units (~1 ha).

 

Referenced papers

Please cite as:

Agra H., Schowanek S., Carmel Y., Smith R.K. & Ne’eman G. (2018) Forest Conservation. Pages 285-328 in: W.J. Sutherland, L.V. Dicks, N. Ockendon, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2018. Open Book Publishers, Cambridge, UK.