Providing evidence to improve practice

Action: Thin trees within forests: effects on understory plants

Key messages

  • Seventeen of 25 studies (including four replicated, randomized, controlled studies) in Argentina, Brazil, Canada, Japan, Spain and the USA found that thinning trees in forests increased the density and cover of understory plants. Seven studies found no effect or mixed effects. One study found a decrease in the abundance of herbaceous species.
  • Thirteen of 19 studies (including 10 replicated, randomized, controlled studies) in Argentina, Canada, Sweden, the USA and West Africa found that thinning trees in forests increased species richness and diversity of understory plants. Seven studies found no effect.

Supporting evidence from individual studies

1 

A replicated, controlled study in 1984-1985 in dry tropical forest in Ceara state, Brazil (Schacht, Long & Malechek 1988) found that thinning trees increased herbaceous plant biomass. Biomass of herbaceous species that matured late in the season was the lowest in unthinned plots (0% tree cover: 1,649; 25% cover: 1,593; 55% cover: 1,600; unthinned: 221), while total herbaceous biomass was similar between treatments (0% cover: 1,981; 25% cover: 1,845; 55% cover: 1,926; unthinned: 259). Four treatment plots (0.1 ha) were established in 1984 in each of two sites: three thinned (0%, 25% and 55% woody cover retained) and one unthinned (95% woody cover). Data were collected in May 1985 in a subplot protected from grazing (40 × 50 m) in each plot.

 

2 

A replicated, controlled study in 1985-1988 in boreal forest in Ontario, Canada (Reader & Bricker 1992) found that thinninging decreased the number of herbaceous species and the frequency of occurrence of each species. In large plots (0.05 and 0.2 ha), the percentage of herbaceous species that decreased in frequency was higher in uncut (34%-36%) than in 33% tree removal (16-18%) and 66% tree removal plots(18-21%). In contrast, in small plots (0.1 ha), figures were higher in uncut (37%) and 33% tree removal plots (36%) than 66% tree removal plots (12%). The percentage of herbaceous species lost was similar between treatments (uncut: 9-13%; 33% removal: 4-12%; 66% removal: 8-12%). Three plots (0.01, 0.05 and 0.20 ha) of each treatment were replicated five times: uncut, 33% tree removal and 66% tree removal (0%, 33%, and 66% of basal area removed). Treatments were applied in 1985-1986. Data were collected two years after treatments.

 

3 

A replicated, controlled study in 1991-1994 in maritime pine Pinus pinaster woodland in Spain (Pérez & Moreno 1998) found that thinning before wildfire increased post-fire biomass and species richness of herbaceous species, but not of the dominant shrub gum rockrose Cistus ladanifer. Herbaceous biomass (g/m2) (pre-thinned: 37-93; unthinned: 2-10) and species richness (species/plot) (pre-thinned: 6-16; unthinned: 5-7) were higher in pre-thinned plots. Herbaceous cover (pre-thinned: 13%-49%; unthinned: 3%-11%) and gum rockrose cover (pre-thinned: 8%-46%; unthinned: 16%-32%) and density () (pre-thinned: 1-10/m2; unthinned: 2-7/m2) were similar between treatments. Data were collected in six thinned (1975-1991) and six unthinned plots (5 × 10 m), three years after the entire study site was burned by wildfire fire in 1991.

 

4 

A replicated, controlled study in 1996-1997 in Japanese beech Fagus crenata forest in Japan (Nagaike, Kamitani & Nakashizuka 1999) found that thinning increased the occurrence of dwarf bamboo Sasa sp. The percentage occurrence of dwarf bamboo was higher in thinned plots (thinned: 59%; unthinned: 44%). 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).

 

5 

A replicated, controlled study in 1999 in temperate mixed forest in Arizona USA (Griffis et al. 2001) found that thinning increased the abundance of native grasses but did not affect species richness for any under-canopy plant group. Abundance index of native grass species was higher in thinned (33) than in unthinned plots (19). Abundance index of native herbaceous species (23 vs 26), exotic herbaceous species (1 vs 3) and exotic grasses (4 vs 0), and the number of species (/375 m2) of native herbaceous species (17 vs 18), exotic herbaceous species (2 in both), native grasses (6 in both) and exotic grasses (1 vs 0) were similar between thinned and unthinned plots. Data were collected in ten 375 m2 plots in each of four thinned (30% of basal area removed between 1987 and 1993) and four unthinned forest fragments (20-80 ha).

 

6 

A replicated, randomized, controlled study in 1993-1996 in temperate coniferous forest in Washington State, USA (Thysell & Carey 2001) found that variable density thinning increased plant species richness and diversity and the proportion of exotic plant species. Total species richness (thinned: 24-27; unthinned: 16-17 species/100 m2 plot), native species richness (thinned: 21-22; unthinned: 15-17), Shannon's index of diversity (thinned: 2.5-2.7; unthinned: 1.9-2.0) and the percent of exotic species (thinned: 12%-17%; unthinned: 2%) were higher in thinned plots. Two thinned (variable density thinning to a 2:1 ratio of >4.75 and <4.75 residual trees/ha respectively) and two unthinned treatment units (13 ha) were established in 1993 in each of four sites. Data were collected in 1994 and 1996 in 15 plots (25 m2) in each treatment unit.

 

7 

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 plant species richness. The number of plant species/treatment unit was similar between treatments (thinned: 22; unthinned: 23). Data were collected in 1998 in thinned (targeted to retain 1,000 stems/ha) and unthinned treatment units (1.8-12.6 ha) established in 1993 in each of three study areas.

 

8 

A replicated, controlled, before-and-after study in 2000-2003 in temperate broadleaf forest in Sweden (Götmark et al. 2005) found that thinning trees increased species richness of herbaceous species.  The increase in herbaceous species richness was higher in thinned (18.3%) than in unthinned plots (1.2%). Average numbers of species/25 m2 section was 13-27 before vs 14-29 after treatment in thinned plots, and 13-28 before vs 13-26 after treatment in unthinned plots. Thinned (25-30% of basal area cut) and unthinned treatments were applied to six pairs of 1 ha plots in winter 2002-2003. Data were collected before (2001-2002) and after treatment (summer 2003) in eight sections (25 × 1 m) within each plot.

 

9 

A replicated, randomized, controlled study in 2001-2004 in temperate coniferous forest in Montana, USA (Metlen & Fiedler 2006) found that thinning increased understory plant species richness. Numbers of species/0.1 ha plot for all species (unthinned: 57; thinned: 66) as well as for native species (unthinned: 53; thinned: 59), exotic species (unthinned: 4; thinned: 7) and forbs (unthinned: 34; thinned: 40) was higher in thinned plots. Numbers of species of grasses graminoids (12-14) and shrubs (9-10) were similar between treatments. Numbers of species/1 m2 was higher in thinned plots for forbs (unthinned: 5.5; thinned: 6.4) and similar between treatments for all species (10.8- 12.2) and for the other plant groups (native species: 10.5-11.8; exotic species: 0.3-0.4; graminoids: 2.4; shrubs: 2.4-2.9). Cover of all plants (28-32%) was similar between treatments. In 2001, ten plots (0.1 ha) were established in each of three replicates of thinned (retaining 11 m2/ha basal area) and unthinned treatment units (9 ha). Species composition was determined in 2004 in 12 quadrats (1 m2) in each plot (total of 720 quadrats).

 

10 

A replicated, controlled study in 1992-2004 in Ponderosa pine Pinus ponderosa forest in Arizona, USA (Montero-Solís et al. 2006) found that thinning increased herbaceous biomass. Herbaceous biomass (kg/ha) was higher in thinned (270-280) than in unthinned plots (~10). Data were collected in 2004 in four circular subplots (2.5 m radius) in each of 10 thinned (thinned from below in 1993, retaining trees 40.6 cm DBH) treatment plots (0.2-0.3 ha), and in three subplots in each of five unthinned treatment plots (total of 55 subplots).

 

11 

A replicated, controlled study in 2000-2002 in boreal forest in Alberta, Canada, (Powell & Bork 2006) found that removal of trembling aspen Populus tremuloides canopies increased the biomass of understory vegetation and cover of herbaceous species. Biomass  (kg/ha) of understory vegetation was higher in partial  (1,300-2,200) and complete removal plots (2,100-2,700) than control plots (700-850) at the parkland site and differed between all treatments at the boreal site (control: 400-750; partial removal: 1,100-1,150; complete removal: 2,100-2,900). Cover of non grass herbaceous plants at the boreal site (control: 29-45%; partial removal: 33-38%; complete removal: 46-66%) and of grasses at the parkland site (control: 8-20%; partial removal: 15-37%; complete removal: 52-79%) was higher in complete than in partial removal and control plots. Cover of tall shrubs (>1 m) at the boreal site was lower in partial (3-8%) and complete removal (8-20%) than in control plots (15-42%). There was no difference between treatments for the following: cover of tall shrubs at the parkland site (control: 4-10%; partial removal: 5-8%; complete removal: 0-3%), low shrubs (<1 m) at the parkland (control: 25-31%; partial removal: 12-25%; complete removal: 17-38%) and at the boreal site (control: 24-51%; partial removal: 35-40%; complete removal: 46-52%), forbs at the parkland (control: 7-10%; partial removal: 3-4%; complete removal: 4-7%) and grasses at the boreal site (control: 1-3%; partial removal: 0-2%; complete removal: 2-9%). Three replicates of complete removal (all aspen canopies removed), partial removal (half of aspen canopy area removed) and control plots (10 × 10 m) were established in 2000 in a 'boreal' site (16,319 stems/ha) and a 'parkland' site (13,194 stems/ha). Data were collected in 2002.

12 

A replicated, randomized, controlled study in 1998-2004 in temperate coniferous forest in Oregon, USA (Youngblood, Metlen & Coe 2006) found no effect of thinning on understory species richness and diversity. Numbers of species/400 m2 plot (thinned: 26; unthinned: 30) and diversity (Shannon's index thinned: 0.12; unthinned: 0.12) were similar between treatments. Data were collected in 2004 in 10-28 plots (400 m2) in each of four thinned (thinned in 1998 to reduce trees basal area from 26 to 16 m2/ha) and four unthinned experimental units.

 

13 

A replicated, controlled study in 2001-2005 in temperate mixed forest in Washington State, USA (Devin, Harrington & Peter 2007) found that conifer cutting increased cover of non-native, but not of native plants under Oregon white oak Quercus garryana canopies. Under oak canopies cover of non-native forbs (conifer cut: 10%; uncut: 7%), grasses (conifer cut: 24%; uncut: 12%) and woody plants (conifer cut: 20%; uncut: 9%) was higher under conifer cut oaks. There was no difference between treatments for cover of native forbs (conifer cut: 30%; uncut: 35%), grasses (conifer cut: 7%; uncut: 5%) and woody plants (conifer cut: 127%; uncut: 128%), or total plant cover under Oregon white oak canopies (99% under both conifer cut and uncut oak trees). Data were collected in 2005 under six conifer cut (all conifer covering the oaks cut in 2001) and six control Oregon white oak trees (average height: 16 m, average crown diameter 7.5 m) at each of four forest sites.

 

14 

A replicated, randomized, controlled study in 2001-2004 in temperate conifer forest in Montana, USA (Dodson, Metlen & Fiedler 2007) found that thinning increased native plant species richness. Species richness (in 1,000 m2) for common (thinned: 34; unthinned: 32) and uncommon (thinned: 15; unthinned: 12) native plant species was higher in thinned plots. Data were collected in 2004 in 10 thinned (in 2001, 11 m2/ha retained) and 10 unthinned plots (1000 m2) in each of three blocks.

 

15 

A replicated randomized, controlled study in 2000-2004 in temperate broadleaf forest in Ohio, USA (Schelling & McCarthy 2007) found no effect of thinning on soil seed-bank species richness or diversity. Total numbers of species (thinned: 37; unthinned: 38) and Shannon's index of diversity (thinned: 3.03; unthinned: 3.11) were similar between treatments. In autumn to winter 2000-2001, ten plots (20 × 50 m) were established within each thinned (retaining ~13.75 m2/ha basal area) and unthinned treatments (20 ha) replicated at each of two sites. Species richness and diversity were determined by monitoring emerging seeds in 10 soil samples (1000 cm3) extracted from each plot in summer 2004.

 

16 

A replicated, randomized, controlled study in 2000-2003 in temperate mixed forest in California, USA (Wayman & North 2007) found no effect of thinning on understory plant species richness and cover. Numbers of species/10 m2 plot (unthinned: 4; understory thinning: 4; canopy thinning: 3) and cover (unthinned: 8%; understory thinning: 5%; canopy thinning: 6%) were similar among treatments. Three replicates of unthinned, understory thinning (removing trees 25–76 cm DBH, retaining at least 40% canopy cover) and canopy thinning (removing trees >25 cm DBH leaving 22 large trees/ha) treatment units (4 ha) were established in 2000-2001. Data were collected in 2003 in 9-49 plots (10 m2) in each treatment unit.

 

17 

A replicated, controlled, before-and-after trial in 2004-2005 in temperate broadleaf forest in Ontario Canada (Falk et al. 2008) found that thinning increased the species richness of herbs. The increase in number of herbaceous species/plot was higher in thinned (3.6 to 4.5) than in unthinned plots (4.3 to 4.8). Overall percent of plant species lost (15% and 11% unthinned and thinned respectively) and of plant species gained (29% and 42%) was similar among treatments. Two thinned (leaving basal area of 20 m2/ha) and two unthinned blocks (average 33 ha) were established between November 2004 and April 2005. Sampling of herbs that grew mid-spring was in April 2004 (pre-harvesting) and in April-May 2005 (post-harvesting) in 45 regeneration growth plots (4 m2) in each block.

 

18 

A replicated, controlled study in 1992-2005 in temperate coniferous forest in Arizona, USA (Laughlin et al. 2008) found no effect of thinning on plant species richness or on changes in species composition. Numbers of species/2 m2 (unthinned: 6; thinned: 8) and the change in species composition between 1992 and 2005 (unthinned: 0.36; thinned: 0.44) were similar between treatments. Complete species lists were collected in two 1 m2 quadrats in each of 35 subplots (2.5 m2), four in each of five thinned (thinned from below in 1993, retaining all trees >37.5 cm DBH) and three in each of five unthinned plots (0.2-0.3 ha). Data were collected between 1992 and 2005.

 

19 

A replicated, paired sites study in 2005 in Mediterranean type woodland in Oregon, USA (Perchemlides, Muir & Hosten 2008) found that thinning trees increased the cover of herbs and the number of regenerating shrubs. Cover of herbs (thinned: 103%; unthinned: 69%) and number of the shrubs sticky whiteleaf manzanita Arctostaphylos viscida and buckbrush Ceanothus cuneatus regenerations/transect (thinned: 1.7; unthinned: 0.3) were higher in thinned transects. Plant species richness/transect (thinned: 29; unthinned: 28) and diversity (Shannon's index thinned: 2.3; unthinned: 2.4), as well as number of regenerations/transect of oak Quercus spp. (thinned: 1.7; unthinned: 2.1) and conifer (thinned: <0.1; unthinned: <0.1) were similar in thinned and unthinned transects. Data was collected in 2005 using 30 pairs of thinned (thinned for fuel reduction between May 1998 and June 2001) and unthinned transects (50 m). Shrub cover was measured in five plots (3 m2) along each transect. Cover of herbs was measured in two quadrats (1000 cm2) within each plot.

 

20 

A replicated, controlled, before-and-after study in 2000-2004 in Piedmont forest in South Carolina, USA (Phillips & Waldrop 2008) found that thinning increased plant species richness. Changes (after minus before treatment) in number of plant species/0.1 ha plot were higher in thinned plots (thinned: 39; unthinned: 32). Changes in cover of shrubs (thinned: 0.27%; unthinned: -0.41%), vines (thinned: 0.09%; unthinned: -2.73%), forbs (thinned: 0.29%; unthinned: 0.22%) and grasses (thinned: 0.52%; control: -0.48%) were similar between treatments. 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.

 

21 

A replicated, randomized, controlled study in 1994-2003 in savanna woodland in West Africa (Savadogo et al. 2008) found no effect of cutting on species richness or diversity of herbs. Numbers of species/0.25 ha (uncut: 13-16; cut: 14-16) and diversity (Shannon's index uncut: 2.5-2.9; cut: 2.6-2.9) was similar between treatments. Data were collected in 2003 in two uncut and two cut (50% of merchantable tree volume removed in 1994) treatment plots (0.25 ha) replicated in eight blocks, at each of two sites (18 ha).

22 

A replicated, controlled study in 2000-2006 in temperate mixed forest in Vermont and New York, USA (Smith et al. 2008) found that thinning increased species richness, diversity and cover of understory plants. Changes in number of species/0.04 plot (control: -1; group: 0; single tree cutting: 4; complexity enhancement cut: 9) and cover (control: -5%; group cut: 2%; single tree cutting: -4%; complexity enhancement cut: 8%) were higher in complexity enhancement than control plots. Change in diversity was higher in complexity enhancement cuts (Shannon's index: 3) and single tree cutting (2) than control plots (-0.5). Eight control (unthinned), four single tree cutting (cutting in dispersed pattern, retaining 18.4 m2/ha basal area), four group cut (cutting in aggregated pattern, retaining 18.4 m2/ha basal area) and four complexity enhancement cuts (cutting trees to a target typical diameter distribution, retaining 34 m2/ha basal area) treatment units (2 ha) were established in 1999-2003. Data were collected three years after treatments in eight plots (0.04 ha) in each treatment unit.

 

23 

A replicated, randomized, controlled study in 2000-2007 in temperate broadleaf forest in North Carolina and Ohio, USA (Waldrop et al. 2008) found no effect of tree thinning on herbaceous cover. At a 'cool temperate climate' site the number of hardwood tree saplings (>1.4 m tall) and cover of herbs (thinned: 3-19%, unthinned: 5-13%) were similar between treatments. Three pairs of thinned (in 2000-2002) and unthinned treatment units (10-26 ha) were established at each of two sites. Data were collected 4-5 years post-treatments in ten plots (0.1 ha) in each treatment unit.

24 

A replicated, controlled study in 2000-2003 in temperate mixed forest in Georgia, USA (Brockway et al. 2009) found that mechanical thinning increased the cover of understory plants. Understory plant cover was higher in thinned than unthinned plots (thinned: 112%; unthinned: 71%). Four blocks, 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) were established in 2000. Data were collected in 2002-2003 in five subplots (10 × 10 m) within each treatment plot.

 

25 

A replicated, controlled study in 1988-2005 in temperate coniferous forest in Arizona, USA (Sabo et al. 2009) found no effect of thinning on  understory plant biomass. Above ground biomass (kg/ha) of native grasses (unthinned: 600; thinned: 1,100) and forbs (unthinned: 300; thinned: 250) was similar between treatments. No non-native grasses or forbs were found in control or thinned plots. Data were collected in 2005 in 10 plots (20 × 50 m) in each of three unthinned and four thinned (>30% of basal area removed between 1988 and 1995) forest units (20-80 ha).

 

26 

A replicated, controlled study in 1994-2005 in temperate coniferous forest in Colorado, USA (Wolk & Rocca 2009) found that thinning increased understory vegetation cover and the proportion of non-native species. Understory vegetation cover (unthinned: 3.9%; thinned: 6.1%; thinned and chipped: 7.1%) was higher in thinned and chipped plots than unthinned plots. The proportion of non-native understory species was higher in the thinning treatments (18% in both) than the unthinned treatment (14%), while the total number of species/1,000 m2 was similar between treatments (unthinned: 53; thinned: 47; thinned and chipped: 48).  Data was collected in 2005-2006 in 31 plots (1,000m2) established in 1994. Six plots were unthinned, 13 thinned (harvested matter removed from the site) and 12 were thinned and chipped (harvested matter chipped and distributed on the site).

 

27 

A replicated, randomized, controlled study in 1997-2008 in temperate coniferous forest in western Oregon, USA (Ares,Neill & Puettmann 2010) found that thinning increased the number of understory species. The number of species/80 m2 was higher following fixed (high or moderate) density thinning (76 and 86 respectively) than following variable (high or moderate) density thinning (54 and 55 respectively) and unthinned (48). It was not different than the other five treatments following variable low density (60). A set of six thinning regimes, each comprising 20–44 ha, was applied in 1997 at each of three forest sites: unthinned; fixed high density treatment (300 trees/ha); fixed moderate density treatment (200 trees/ha); variable high density treatment (300 trees/ha); variable moderate density treatment (200 trees/ha); variable low density treatment (100 trees/ha). Between four and 20 permanent 0.1 ha plots were located randomly in each treatment (total of 77 plots/site). Four 20 m2 sub-plots were installed in each plot. Monitoring was carried out in summer 2003 and 2008.

 

28 

A replicated, controlled study in 2002-2005 in an oak Quercus spp. savanna in Iowa, USA (Brudvig 2010) found that cutting all non-oak trees increased species richness. Species richness/1 m2 (non-oaks cut: 18; uncut: 10) as well as species richness of grasses (non-oaks cut: 3; uncut: 1) and woody plants (non-oaks cut: 8; uncut: 4) were higher in non-oaks cuts than in uncut plots. Diversity (Simpson's index non-oaks cut: 8; uncut: 5) and forb species richness (non-oaks cut: 7; uncut: 4) were similar between treatments. The percentage of native species was higher in uncut plots (non-oaks cut: 94%; uncut: 99%). Data were collected in 2004-2005 in 11-21 plots (1 × 1 m) at each of four non-oaks cut (all non-oak trees >1.5 m tall removed in 2002-2003) and four uncut sites (1.5-3.3 ha).

 

29 

A replicated, randomized, controlled study in 1998-2005 in boreal forest in south eastern Alaska, USA (Cole, Hanley & Newton 2013) found that thinning trees increased the cover of understory vegetation. The total cover of understory plants was similarly higher in all thinning treatments (62-72%) than in unthinned plots (30%). 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 was collected in 2005.

 

30 

A replicated, controlled study in 2004-2008 in temperate broadleaf forest in Pennsylvania, USA (Huebner et al. 2010) found that tree thinning increased the cover of bramble Rubus spp. and fern as well as tree saplings density, but did not affect fruit production and cover of some herbaceous species. Cover of bramble (thinned: 0%-27%; unthinned: 0%-3%) and hay-scented fern Dennstaedtia punctilobula (thinned: 0%-70%; unthinned: 0%-33%), as well as number of tree saplings/m2 (thinned: 0.0-1.8; unthinned: 0.0-0.4) were higher in thinned plots. Total number of fruit/plot for three herbs: painted trillium Trillium undulatum, sessile bellworth Uvularia sessilifolia, and Indian cucumber root Medeola virginiana (0-430) as well as their relative cover (0-3%) were similar between treatments. Data were collected in 2008 in three blocks of 16 thinned (10-30% of basal area removed in 2001-2002) and eight unthinned plots (50 × 80 m) each.

 

31 

A replicated, randomized, controlled study in 2002-2008 in temperate coniferous forest in Alabama, USA (Outcalt & Brockway 2010) found that thinning decreased the density of understory shrubs and trees and increased the cover of grasses. Density (stems/ha) of hardwood trees <3 cm DBH (thinned: <50; unthinned: >1,500) and cover of shrubs >1.4 m tall (thinned: <1%; unthinned: 33%) were higher in control plots, while cover of grasses (thinned: 20%; unthinned: 7%) was higher in thinned plots. Cover of shrubs <1.4 m tall (~55%) and forbs (3%-8%) were similar between treatments. Unthinned and thinned (leaving 11.5–13.5 m2 basal area of longleaf pine Pinus palustris, removing hardwoods and other pines) treatment units (12 ha) were replicated in three blocks. Thinning was in April 2002. Data were collected in 2005 in ten 20 × 50 m subplots within each treatment unit.

 

32 

A controlled study in 2001-2005 in temperate Nothofagus pumilio forest in Argentina (Lencinas et al. 2011) found that thinning increased plant cover, biomass and species richness. Cover (thinned: 36-40%; unthinned: 20%) and biomass (thinned: 1,000-1,251 kg/ha; unthinned: 200 kg/ha) of understory plants were higher in the three thinning treatments. Numbers of plant species/1 m2 was higher in aggregated retention plots (8.2) than in unthinned plots (6.1), and similar to both in dispersed (7.1) and combined retention plots (7.0). In 2001, three thinning treatments (11-24 ha): dispersed retention (20–30% of green tree retention); aggregated retention (28% of trees retained, one aggregate of forest/ha); combined retention (40–50% of retention, one aggregate/ha and dispersed retention among them), and unthinned (9 ha) were established within a 61 ha area. Data were repeatedly collected 1-4 years after treatments in 10 permanent plots (1 m2) in each treatment.

 

33 

A replicated, randomized, controlled study in 1998-2006 in temperate coniferous forest in Arizona USA (Stoddard et al. 2011) found that thinning increased plant species richness. The number of observed species was higher in thinned (34-38) than unthinned plots (20), while plant cover was similar between treatments (thinned: 9-16%; unthinned: 4%).  Monitoring was carried out in 2006 in three thinned and one unthinned 14 ha forest units that were randomly assigned in 1998 in each of three blocks.

 

34 

A replicated study in 1975-2006 in temperate coniferous forest in Oregon USA (Berger, Puettmann & McKenna 2012) found that a second thinning treatment increased the cover and abundance of some understory plant groups. The percentage cover of ferns (43 vs 30%) and exotic plant species (1.0 vs 0.1%) was higher in twice thinned than in once thinned plots, while percentage cover of all forest understory species was similar between treatments (95% in both treatments). Frequencies were higher in twice thinned than in once thinned plots for ferns (2.0 vs 1.6 respectively), grasses (2.2 vs 1.3), open site species (4.1 vs 2.2) and exotic species (0.7 vs 0.1).The frequency of all forest understory species was similar between treatments (10.3 vs 9.9). Two treatments: once thinned (thinned from below in 1975-1982 to densities of 270-590 trees/ha) and twice thinned (re-thinned in 1997-2000 to 100-150 trees/ha) were replicated in four sites. Understory vegetation was monitored six years after the second thinning in 6-12 once thinned and 12-13 twice thinned 0.1 ha plots at each site.

 

35 

A controlled study in 2007-2009 in Piñon-juniper woodland in Utah, USA (Ross, Castle & Barger 2012) found that thinning increased understory vegetation cover. Cover of understory plants was higher in the two thinning treatments (piled and burned: 16%; woody debris: 21%) than control plots (4%). Three treatment sites (0.4-1 km2): piled and burned (trees manually cut with debris placed in discrete piles that were later burned), woody debris (trees manually removed and debris scattered across the site) and control (untreated) were established in 2007. Data were collected in 2009 along 10 transects (35 m) in each site.

 

36 

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 understory vegetation cover. The changes (after minus before) in cover (thinned: 0%; unthinned: -2%) were similar between treatments. 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 debris mulched in June 2003) and two unthinned treatment units (~1 ha).

 

37 

A replicated, controlled study in 2004-2011 in temperate coniferous forest in Arizona, USA (Huffman et al. 2013) found that thinning increased plant cover but not species richness. Total plant cover was higher in thinned plots (thinned: 5.4%; unthinned: 3.1%), while species richness (33-37 species) and diversity (Simpson's index 0.8-0.9) were similar between treatments. Four thinned (pinyon pine Pinus edulis trees <25.4 cm diameter at root collar, Utah juniper Juniperus osteosperma <30 cm diameter at root collar and ponderosa pine Pinus ponderosa trees <22.9 cm diameter at breast height cut) and four unthinned treatment units (1 ha) were replicated in six blocks. Thinning was in 2005. Data were collected in 2011 in one 0.04 ha plot in each treatment unit (total of 48 plots).

 

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.