Action: Mechanically remove understory vegetation after tree planting
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- Five studies (including three replicated, randomized, controlled studies) in Canada, the USA, France, Panama and Sweden found no effect of controlling understory vegetation on the emergence, survival, growth rate or frost damage in planted seedlings. However, one found removing competing herbs increased seedling biomass.
- One replicated, controlled study in Canada found that removal of sheep laurel shrubs increased the growth rate and height of planted black spruce seedlings.
Mechanical removal of understory vegetation can reduce the competition for resources and help the establishment of planted trees.
Supporting evidence from individual studies
A randomized, replicated, controlled study in 1989–1990 in a former arable field, in New Jersey, USA (Facelli 1994) found that removing competing herbs from plots did not increase the emergence or survival of tree of heaven Ailanthus altissima seedlings, but did increase seedling biomass. Seedling emergence did not differ between removal and untreated plots (removal: approx. 10; untreated: approx. 9 seedlings/plot). Similarly, seedling mortality was similar between treatments (removal: approx. 10%; untreated: approx. 7%). However, seedling biomass was greater in plots where competing herbs were removed than in untreated plots (no data provided). In 16 plots (0.8 × 1 m), all herbs were clipped at surface level and root connections were severed by driving a spade 30cm in the ground, around the plot’s perimeter. The other 16 plots were not clipped. In all plots, 20 seeds had been planted to ensure regeneration.
A replicated, randomized, controlled study in 1988-1995 in boreal forest in Sweden (Langvall, Nilsson & Örlander 2001) found no effect of mowing treatment on frost damage to Norway spruce Picea abies planted seedlings. Percentage of seedlings with frost injuries was similar between treatments (6-11% in site 1, 25-35% in site 2). Five blocks of four mowed (ground vegetation cut to <20 cm height when necessary 1989-1993) and four control plots (4×4 m) were established in 1988 in each of two sites. Data were collected in each plot two growing seasons after planting of spruce seedlings.
A replicated, controlled study in 1996-1997 in degraded tropical forest in Panama (Hooper, Condit & Legendre 2002) found no effect of mowing of invasive grass wild sugarcane Saccharum spontaneum on the survival of planted native tree seedlings in abandoned farmlands. Seedling survival was similar between treatments (once mown: 62%; three mows: 39%; control: 44%). Data were collected in July 1997 in three subplots (1×8 m): once and three times mown (wild sugarcane was hand-cut once or three times during the experiment) and control (untreated), in each of five plots, replicated in five sites. Each subplot was planted with 10 seeds of each of 20 native tree species in July 1996-March 1997.
A replicated, controlled study in 1999-2006 in boreal forest in Quebec, Canada (LeBel, Thiffault & Bradley 2008) found that removal of the shrub sheep laurel Kalmia angustifolia increased growth rate and height of planted black spruce Picea mariana seedlings. Seedling annual relative growth (removal: 10.3-11.1%; control: 4.7-4.9%) and height (removal: 147-167 cm; control: 57-76 cm) were higher in removal plots. Data were collected in 2005 and 2006 in six removal (sheep laurel removed in August 1999 using glyphosate herbicide and re-sprouting manually clipped from 2000 to 2006) and six plots with no removal within a 0.2 ha area. Each plot was planted with 20 black spruce seedlings at 1 m spacing in June 2000.
A replicated, controlled study in 1996-2000 in boreal forest in Saskatchewan, Canada (Milakovsky et al. 2011) found no effect of ground vegetation control treatments on survival or growth rate of planted white spruce Picea glauca seedlings. Survival (75-78%) and height increase (20-26 cm) were similar between treatments. In 1996, fifteen plots (4 × 8 m) of each cutting (all vegetation cut to ground level), crushing (all vegetation and rootstock ground up) and control treatments were established in each of eight blocks. Data were collected in 2000 in four subplots (2 × 2 m) planted with white spruce in June 1996. All plots were harvested (trees >2 m height removed by a feller-buncher) before treatments.
A replicated, randomized, controlled study in 2005 in Mediterranean Aleppo pine Pinus halepensis woodland in France (Prévosto et al. 2011) found no effect of mechanical cutting of ground vegetation and scarification treatments on survival of planted downy oak Quercus pubescens and holly oak Q. ilex seedlings, or on grass cover. There was no difference between treatments for survival of downy oak (control: <0.1; chopped: 0.2-0.3; one scarification: 0.2-0.6; double scarification: 0.3-0.5 seedlings/sawing point) and holly oak (control: 0.7-1.1; chopped: 1.1-1.6; one scarification: 0.8-1.4; double scarification: 1.2-1.3), or grass cover (control: 16%-17%; chopped: 24%-27%; one scarification: 17%-27%; double scarification: 17%-24%). Data were collected in 2006 in eight replicates of control, chopped (ground vegetation mechanically chopped), one scarification (vegetation chopped, forest floor and top soil loosened in one direction) and double scarification (forest floor and top soil loosened in two directions) plots (14 × 14 m). Treatments were applied in 2005. All plots were thinned in 2004 (from 410 to 210 trees/ha) and seeded in November 2006 with holly oak and downy oak at sowing points of three acorns spaced 1 m apart.
- Facelli J. (1994) Multiple indirect effects of plant litter affect the establishment of woody seedlings in old fields. Ecology, 75, 1727-1735
- Langvall O., Nilsson U. & Örlander G. (2001) Frost damage to planted Norway spruce seedlings—influence of site preparation and seedling type. Forest Ecology and Management, 141, 223-235
- Hooper E., Condit R. & Legendre P. (2002) Responses of 20 native tree species to reforestation strategies for abandoned farmland in Panama. Ecological Applications, 12, 1626-1641
- LeBel P., Thiffault N. & Bradley R.L. (2008) Kalmia removal increases nutrient supply and growth of black spruce seedlings: an effect fertilizer cannot emulate. Forest ecology and management, 256, 1780-1784
- Milakovsky B., Frey B.R., Ashton M.S., Larson B.C. & Schmitz O.J. (2011) Influences of gap position, vegetation management and herbivore control on survival and growth of white spruce (Picea glauca (Moench) Voss) seedlings. Forest ecology and management, 261, 440-446
- Prévosto B., Monnier Y., Ripert C. & Fernández C. (2011) Diversification of Pinus halepensis forests by sowing Quercus ilex and Quercus pubescens acorns: testing the effects of different vegetation and soil treatments. European journal of forest research, 130, 67-76