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Providing evidence to improve practice

Action: Thin trees after wildfire Forest Conservation

Key messages

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  • Five replicated, controlled studies examined the effects of thinning trees in burnt forest areas. Two studies in Spain found that thinning increased plant species richness. One in Canada found that it increased the cover of aspen saplings. One study in the USA found thinning decreased plant biomass and one in Israel found it decreased mortality of pine seedlings.
  • One paired-site study in Canada found that logging after wildfire decreased species richness and diversity of mosses.


Supporting evidence from individual studies


A replicated, controlled study in 1989-1992 in Aleppo pine (Pinus halepensis) forest in Israel (Ne'eman, Lahav & Izhaki 1995) found that thinning decreased the mortality of pine seedlings. Mortality was higher in control (79%) than in pine thinned (52%) and rockrose (Cistus spp.) thinned plots (49%), and lowest in plots where both pine and rockrose were thinned (0%). Data were collected in 1992 in four treatment plots (14 × 70 m): no thinning, pine thinned (removing all pine seedlings less than 20-25 cm apart, leaving the tallest ones), rockrose thinned (removing all rockrose seedlings less than 20-25 cm apart, leaving the smaller ones) and pine and rockrose thinned (combined pine and rockrose thinning) in each of five blocks. All blocks were totally burnt down in September 1989.  Burned trees were cut down and trunks and smaller twigs removed from the plots in September-November 1990). Thinning was carried out in February 1991.



A replicated, controlled study in 1994-2001 in a Mediterranean Aleppo pine Pinus halepensis forest in south east Spain (De Las Heras et al. 2004) found that thinning of five year old seedlings increased the number of plant species in one of two study sites but did not affect the total cover of shrubs. Two years after thinning, in one of the study sites the number of species in thinned (27) was higher than in control (21) plots, while in the other site numbers of species were similar (22 in both). Shrub cover was not affected by thinning at either site (control vs thinning: 85 vs 99%, 60 vs 60%, at each site respectively). Data were collected in June 2001 in three replicates of thinning (leaving a final density of 1,600 trees/ha) and control plots (10 × 15 m). Plots were established in August1999, at each of two sites that were burned by wildfire in August 1994.



A replicated, paired-sites study in 2002-2004 in boreal mixed-wood forest in Alberta, Canada (Bradbury 2006) found that logging after wildfire decreased species richness and diversity of bryophytes. On burned wood substrate in the first and second years after fire, numbers of species were higher in unlogged (2.6 and 4.6 respectively) than in logged areas (1.6 and 2.6 respectively). Species diversity was higher in unlogged (Shannon’s index of diversity index: 0.79 and 1.26 respectively) than in logged areas (0.51 and 0.88 respectively). On scorched soil substrate in the first year the number of species and diversity were higher in unlogged (4.6 and 1.36 respectively) than logged areas (3.4 and 1.08 respectively). In the second year results were similar for both number of species: 4.9; diversity: 1.47) and unlogged areas (species: 5.0; diversity: 1.46). Logged and unlogged treatments were applied in each of 24 landscape units of 625 ha in an area burned by wildfire in 2002. Bryophytes were sampled in 72 plots within each treatment in 2003 and 2004.



A replicated, controlled study in 2002-2004 in boreal forest in Alberta, Canada (4) found that thinning in burned forest increased the cover of trembling aspen Populus tremuloides saplings but did not affect plant species richness. Cover of trembling aspen saplings was higher in thinned (9-11%) than in control plots (4%) while total plant species richness was similar between treatments (16-18 species/plot). Data were collected in 2004 in 40 unthinned control and 74 thinned plots (8 × 8 m), all burned by wildfire in May 2002. Treatments were applied in autumn 2002.



A replicated, controlled study in 1999-2005 in temperate coniferous forest in Spain (Moyas et al. 2009) found that after wildfire some but not all pruning and thinning treatments increased shrub species richness, but treatments had no effect on shrub species cover. At one site, the number of shrub species/was lower in untreated (4/10 m transect) than in two out of nine treatments (7) and similar to the other seven treatments (4-7).  At the second site numbers of shrub species was lower in untreated (4) than in one out of seven treatments (10) and similar to the other seven treatments (6-8). Shrub cover was similar between treatments at both the first (untreated: 40%; treatments: 30-70%) and second site (untreated: 8%; treatments: 6-30%). In 1999, three untreated and 27 treatment plots (10 × 15 m) were established at one site, and three untreated and 21 treatment plots of similar size were established at a second site. All plots were burned by wildfire in summer 1994. Treatments included different combinations of pruning and thinning (reducing density to 800-1,600 trees/ha) in 1999 and 2004. Data were collected in June 2005 along a 10 m transect in each plot.



A replicated, controlled study in 2004-2006 in temperate mixed forest in Oregon, USA ( Donato et al. 2013) found that thinning decreased the biomass of live and dead plants in burnt forest areas. Total dead organic matter was higher in unlogged than in moderate and high-intensity logged plots in both moist (unlogged: 709; moderate-intensity: 355; high-intensity: 244 kg x 103/ha) and dry forest units (unlogged: 435; moderate-intensity: 182; high-intensity: 161 kg x 103/ha). Total live biomass was higher in unlogged and moderate-intensity than in high intensity treatments in moist forest units (unlogged: 5.6; moderate-intensity: 7.3; high-intensity: 1.6 kg x 103/ha). Total live biomass was similar in all treatments in dry forest units (unlogged: 4.9; moderate-intensity: 4.9; high-intensity: 2.8 kg x 103/ha). The whole study area was burnt by wildfire in 2002. A 1 ha plot was established in each of eight unlogged, seven moderate-intensity logged (25-75% basal area cut) and six high-intensity logged (>75% basal area cut) moist forest treatment units, as well as three unlogged, three moderate-intensity and three high-intensity logged dry forest treatment units (average 8 ha). Logging occurred in 2004-2006. Data were collected 3-9 months after treatments.


Referenced papers

Please cite as:

Agra H., Schowanek S., Carmel Y., Smith R.K. & Ne’eman G. (2019) Forest Conservation. Pages 331-347 in: W.J. Sutherland, L.V. Dicks, N. Ockendon, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2019. Open Book Publishers, Cambridge, UK.