Thin trees within forests: effects on non-vascular plants
Overall effectiveness category Likely to be ineffective or harmful
Number of studies: 4
Background information and definitions
Thinning is the removal of trees to control the development or enhance the future condition of a forest, by adjusting its density, structure and species composition.
Studies looking at tree removal with the aim of removing biomass are discussed in 'Logging/tree removal within forest'.
Supporting evidence from individual studies
A replicated, site comparison study in 1994-1997 in boreal forest in Finland (Kuusinen & Siitonen 1998) found that thinning decreased the number epiphytic lichen species. The total number of epiphytic lichen species/ha was lower in early cut (69) than old growth (88) and similar to both treatments in late cut forest (78). Numbers of epiphytic lichen species/ha occurring on Norway spruce Picea abies was lower in early cut (47) than late cut and old growth forest (54-56). Data were collected in 15 sample plots (1 ha) classified according to the age of the dominant tree Norway spruce and recent signs of cutting, there were five replicates of the three treatments: early-cut (age 102 years, 465 cut stumps/ha), late-cut (age 135 years, 247 cut stumps/ha) and old-growth forest (age 161 years, 3 cut stumps/ha).Study and other actions tested
A replicated, controlled, before-and-after study in 2000–2004 in boreal forest in Sweden (Paltto, Nordén & Götmark 2008) found that thinning increased the number of lichen species but also increased the extinction rate of some bryophytes species. Numbers of lichen species/stump increased more in thinned (76%) than in unthinned plots (26%). The increase in number of species/stump for mosses (thinned: 10%; unthinned: 50%) and liverworts (thinned: -50%; unthinned: -10%), and number of species/log for lichens (thinned: 35%; unthinned: 0%), mosses (thinned and unthinned: 30%) and liverworts (thinned: -15%; unthinned: -10%) was similar between treatments. Extinction rate (number of species lost after thinning/total number of species before thinning) for generalist species (living on at least two substrate types) was higher in thinned (43%) than in unthinned plots (16%). Extinction rate was similar between treatments for species living on bark or on both wood and bark (thinned: 75%; unthinned: 65%) and species living on dead wood (thinned and unthinned: 65%). Sites were 15 pairs of thinned (conifers and medium-sized trees removed in October 2002) and unthinned plots (1 ha) situated at least 20 km apart from each other. Data were collected before (September-November 2000) and after (October 2004) thinning.Study and other actions tested
A replicated, randomized, controlled study in 1998-2004 in boreal forest in Alberta, Canada (Caners, Macdonald & Belland 2010) found that thinning treatments decreased species richness and abundance of non-vascular plants that grow on other plants. Numbers of species was the lowest in 10% and 50% canopy retention sites (5/tree), intermediate and similar to the other treatments in the 75% canopy retention sites (6) and highest in unharvested sites (8). The abundance was lower in 10, 50 and 75% canopy retention sites (present in 19, 21 and 25% of sampling points respectively) than in unharvested sites (50% of sampling points). In 2004, six to eight trees were sampled in each of four harvesting treatments (10 ha): 10%, 50% and 75% canopy retention and unharvested, randomly applied in 1998-1999 in each of three sites (a total of 80 trees).Study and other actions tested
A replicated, controlled, before-and-after study in 2000-2009 in boreal forest in Sweden (Nordén et al. 2012) found that thinning prevented a decrease in the number of lichen species, but not of mosses. The change in total number of epiphyte species/plot was negative in unthinned (-3.3) and different than in thinned (1.0), as well as number of lichen species (unthinned: -3.3; thinned: 0.0). The change in the number of bryophytes species was similar (0.1-0.8 species/plot) between treatments. Epiphytes were recorded before and after treatment (2001 and 2009) on five oak trunks in each plot (1 ha). There were 24 pairs of thinned (25% of tree basal area removed in 2002-2003) and unthinned plots.Study and other actions tested