Action: Use thinning followed by prescribed fire
- Three of six studies (including one replicated, randomized, controlled study) in the USA found that thinning followed by prescribed burning increased cover and abundance1 of understory plants as well as the density of deciduous trees. One study found that thinning then burning decreased trees density and species richness. Three studies found no effect or mixed effects of thinning followed by burning on tree growth rate and density of young trees.
- One replicated, controlled study Australia found no effect of thinning followed by burning on the genetic diversity of black ash.
Mechanical thinning of trees followed by prescribed burning is used as a conservation management practice to encourage forest renewal and to increase forest structural diversity.
Supporting evidence from individual studies
A replicated, controlled study in 1999 in temperate mixed forest in Arizona USA (Griffis et al. 2001) found that thinning followed by prescribed burning increased the abundance of native grasses and species richness of exotic herbaceous species. The abundance index of native grass species was higher in thinned and burned (48) than in untreated plots (19). The number of species/375 m2 of exotic herbaceous species was higher in thinned and burned (4) than in untreated plots (2). The abundance index of native herbaceous species (30 vs 26), exotic herbaceous species (6 vs 3) and exotic grass species (6 vs 0), and the number of species/375 m2 of native herbaceous species (19 vs 18), native grasses (6 in both) and exotic grasses (1 vs 0) were similar between thinned and burned and untreated plots. Data were collected in ten 375 m2 plots in each of four thinned and burned forest fragments (30% of basal area removed between 1987 and 1993 and burned between 3-4 years of thinning) and four untreated forest fragments (20-80 ha).
A replicated, controlled study in in temperate eucalyptus woodland in Victoria Australia (Glaubitz, Wu & Moran 2003) found no effect of harvesting followed by burning on genetic diversity of black ash Eucalyptus sieberi. Genetic diversity did not differ between thinned and burned and untreated plots (allelic richness: 7.7 vs 8.0; effective number of alleles: 3.2 vs 3.4; expected heterozygosity:0.49 vs 0.50 respectively). Black ash seedlings were sampled in two 5 ha thinned and burned (cutting to retain ~10% of trees followed by prescribed burning in 1989-1990) and two 5-7 ha untreated plots. Molecular analysis was carried out using 35 Mendelian markers.
A replicated, controlled study in 2003-2005 in temperate coniferous forest in Montana USA (Fajardo et al. 2007) found no effect of selection cutting followed by spring prescribed burning on tree growth rate. Tree basal area increase overten years was not significantly different between thinned and burned (107 cm2) and untreated plots (75 cm2). One thinned and burned plot (selection cutting followed by spring prescribed burning in 1992-1993) and one untreated plot (50 × 50 to 60 × 60 m) were established at each of three sites. Trees were measured in 1992-1993 and again in 2003.
A replicated, controlled study in 2001-2005 in second-growth oak forests in southern Ohio, USA (Albrecht & McCarthy 2006) found that mechanical thinning followed by prescribed fire reduced large sapling density, increased small sapling and large seedling density, but did not affect densities of small seedlings and of oak Quercus spp. saplings. Densities of large seedlings (50-150 cm tall) and small saplings (<3 cm DBH) was higher in thinned and burned (11,000 large seedlings/ha; 3,000 small saplings/ha) than in untreated plots (1,500 large seedlings/ha; 1,000 small saplings/ha). The density of large saplings (3-10 cm DBH) was lower in thinned and burned plots (200 large saplings/ha) than in untreated plots (600 large saplings/ha). The density of small seedlings (<50 cm tall) was similar in thinned and burned (90,000 small seedlings/ha) and in untreated plots (120,000 small seedlings/ha). Three forest areas were divided into treatment units (each approximately 30 ha): untreated, mechanical thinning followed by prescribed fire. Treatments were applied in the inactive season of 2001. Regeneration was sampled in ten 0.1 ha plots/treatment (a total of 40 plots/site) in summer 2004.
A replicated, randomized, controlled study in 2000-2003 in temperate mixed forest in Georgia, USA (Brockway et al. 2009) found that thinning followed by burning decreased tree density and species richness, and increased the cover of understory plants. The number of trees/ha (winter: 215; spring: 305; summer: 258; untreated: 793) and the number of tree species/100 m2 (winter: 4.3; spring: 6.0; summer: 3.3 untreated: 8.7) were lower in all thinned and burned treatments than in untreated plots. Understory plant cover (winter: 130%; spring: 113%; summer: 114%; untreated: 71%) was higher in thinned and burned treatments than in untreated plots. In 2000, three thinned and burned (mulching of all broadleaf trees regardless of size, and all pines <20 cm diameter at breast height followed by winter/spring/summer prescribed fire) and one unmanipulated treatment were randomly assigned to four plots (110 × 110 m) in each of four blocks. Data were collected in 2002-2003 in five subplots (10 × 10 m) within each treatment plot.
A replicated, controlled study in 2000-2007 in temperate conifer forest in Oregon USA (Endress et al. 2012) found that mechanical thinning followed by prescribed burning increased the density of deciduous tree species. The density of deciduous species (trees/ha) was higher in thinned and burned (84) than untreated plots (20). Two 1 ha plots were established in each of three thinned and burned sites (mechanical thinning followed by prescribed burning between 2000 and 2003) and three untreated sites. Data were collected from 2005 to 2007.
A before-and-after study in 2003-2005 in temperate coniferous forest in California, USA (Walker et al. 2012) found that prescribed fire following thinning increased seedling density and decreased sapling density of conifers. The change in density (after minus before individuals/ha) of seedlings <1.37 m tall (thinned and burned: -98; control: -2,303) was lower (more negative) in control plots. In contrast, the change in density of saplings >1.37 m tall and <10 cm diameter at breast height was lower in burned plots (burned: -740; control: 74). Data were collected in 2003 (before) and 2005 (after) in five plots (0.04 ha) in each of two thinned and burned (thinned to residual 30 m2 basal area in June 2003 and burned in June 2004) and two control treatment units of approximately 1 ha each.
- Griffis K.L., Crawford J.A., Wagner M.R. & Moir W. (2001) Understory response to management treatments in northern Arizona ponderosa pine forests. Forest Ecology and Management, 146, 239-245
- Glaubitz J.C., Wu H.X. & Moran G.F. (2003) Impacts of silviculture on genetic diversity in the native forest species Eucalyptus sieberi. Conservation Genetics, 4, 275-287
- Fajardo A., Graham J.M., Goodburn J.M. & Fiedler C.E. (2007) Ten-year responses of ponderosa pine growth, vigor, and recruitment to restoration treatments in the Bitterroot Mountains, Montana, USA. Forest Ecology and Management, 243, 50-60
- Albrecht M.A. & McCarthy B.C. (2006) Effects of prescribed fire and thinning on tree recruitment patterns in central hardwood forests. Forest Ecology and Management, 226, 88-103
- Brockway D.G., Outcalt K.W., Estes B.L. & Rummer R.B. (2009) Vegetation response to midstorey mulching and prescribed burning for wildfire hazard reduction and longleaf pine (Pinus palustris Mill.) ecosystem restoration. Forestry, 82, 299-314
- Endress B.A., Wisdom M.J., Vavra M., Parks C.G., Dick B.L., Naylor B.J. & Boyd J.M. (2012) Effects of ungulate herbivory on aspen, cottonwood, and willow development under forest fuels treatment regimes. Forest ecology and management, 276, 33-40
- Walker R.F., Fecko R.M., Frederick W.B., Johnson D.W. & Miller W.W. (2012) Seedling recruitment and sapling retention following thinning, chipping, and prescribed fire in mixed Sierra Nevada conifer. Journal of Sustainable Forestry, 31, 747-776