5.5.1 Changing fire frequency

Trade-off between benefit and harms

Use prescribed fire: effect on understory plants

1Eight of 22 studies, including seven replicated, randomized, controlled studies, in Australia, Canada and the USA found that prescribed fire increased the cover, density or biomass of understory plants. Six found it decreased plant cover and eight found mixed or no effect on cover or density. Fourteen of 24 studies, including 10 replicated, randomized, controlled studies, in Australia, France, West Africa and the USA found that fire increased species richness and diversity of understory plants. One found it decreased species richness and nine found mixed or no effect on understory plants. Assessment: trade-offs between benefits and harms (effectiveness 55 %; certainty 70 %; harms 25 %).

2http://www.conservationevidence.com/actions/1221

Use prescribed fire: effect on young trees

3Five of 15 studies, including four replicated, randomized, controlled studies, in France, Canada and the USA found that prescribed fire increased the density and biomass of young trees. Two found that fire decreased young tree density. Eight found mixed or no effect on density and two found mixed effects on species diversity of young trees. Two replicated, controlled studies in the USA found mixed effects of prescribed fire on young tree survival. Assessment: trade-offs between benefits and harms (effectiveness 45 %; certainty 55 %; harms 23 %).

4http://www.conservationevidence.com/actions/1220

Likely to be ineffective or harmful

Use prescribed fire: effect on mature trees

5Four of nine studies, including two replicated, randomized, controlled studies, in the USA found that prescribed fire decreased mature tree cover, density or diversity. Two studies found it increased tree cover or size, and four found mixed or no effect. Seven studies, including one replicated, randomized, controlled study, in the USA found that fire increased mature tree mortality. Assessment: likely to be ineffective or harmful (effectiveness 25 %; certainty 50 %; harms 50 %).

6http://www.conservationevidence.com/actions/1217

No evidence found (no assessment)

7We have captured no evidence for the following interventions:

• Mechanically remove understory vegetation to reduce wildfires
• Use herbicides to remove understory vegetation to reduce wildfires

5.5.2 Water management

No evidence found (no assessment)

8We have captured no evidence for the following interventions:

• Construct water detention areas to slow water flow and restore riparian forests
• Introduce beavers to impede water flow in forest watercourses
• Recharge groundwater to restore wetland forest

5.5.3 Changing disturbance regime

Trade-off between benefit and harms

Use clearcutting to increase understory diversity

9Three of nine studies, including four replicated, randomized, controlled studies, in Australia, Japan, Brazil, Canada and the USA found that clearcutting decreased density, species richness or diversity of mature trees. One study found it increased trees species richness and six found mixed or no effect or mixed effect on density, size, species richness or diversity. One replicated, randomized, controlled study in Finland found that clearcutting decreased total forest biomass, particularly of evergreen shrubs. Three of six studies, including five replicated, randomized, controlled studies, in Brazil, Canada and Spain found that clearcutting increased the density and species richness of young trees. One found it decreased young tree density and two found mixed or no effect. Eight of 12 studies, including three replicated, randomized, controlled studies, across the world found that clearcutting increased the cover or species richness of understory plants. Two found it decreased density or species richness, and two found mixed or no effect. Assessment: trade-offs between benefits and harms (effectiveness 63 %; certainty 65 %; harms 30 %).

10http://www.conservationevidence.com/actions/1222

Use group-selection harvesting

11Four of eight studies, including one replicated, controlled study, in Australia, Canada, Costa Rica and the USA found that group-selection harvesting increased cover or diversity of understory plants, or the density of young trees. Two studies found it decreased understory species richness or and biomass. Three studies found no effect on understory species richness or diversity or tree density or growth-rate. Assessment: trade-offs between benefits and harms (effectiveness 50 %; certainty 58 %; harms 30 %).

12http://www.conservationevidence.com/actions/1224

Use shelterwood harvesting

13Six of seven studies, including five replicated, controlled studies, in Australia, Iran, Nepal and the USA found that shelterwood harvesting increased abundance, species richness or diversity or understory plants, as well as the growth and survival rate of young trees. One study found shelterwood harvesting decreased plant species richness and abundance and one found no effect on abundance. One replicated, controlled study in Canada found no effect on oak acorn production. Assessment: trade-offs between benefits and harms (effectiveness 78 %; certainty 70 %; harms 28 %).

14http://www.conservationevidence.com/actions/1223

Unknown effectiveness (limited evidence)

Thin trees by girdling (cutting rings around tree trunks)

15One before-and-after study in Canada found that thinning trees by girdling increased understory plant species richness, diversity and cover. Assessment: unknown effectiveness — limited evidence (effectiveness 58 %; certainty 13 %; harms 0 %).

16http://www.conservationevidence.com/actions/1226

Use herbicides to thin trees

17One replicated, controlled study in Canada found no effect of using herbicide to thin trees on total plant species richness. Assessment: unknown effectiveness — limited evidence (effectiveness 5 %; certainty 13 %; harms 0 %).

18http://www.conservationevidence.com/actions/1225

Unlikely to be beneficial

Use thinning followed by prescribed fire

19Three of six studies, including one replicated, randomized, controlled study, in the USA found that thinning followed by prescribed fire increased cover or abundance of understory plants, and density of deciduous trees. One study found it decreased tree density and species richness. Three studies found mixed or no effect or mixed effect on tree growth rate or density of young trees. One replicated, controlled study Australia found no effect of thinning then burning on the genetic diversity of black ash. Assessment: unlikely to be beneficial (effectiveness 35 %; certainty 40 %; harms 15 %).

20http://www.conservationevidence.com/actions/1227

No evidence found (no assessment)

21We have captured no evidence for the following interventions:

• Adopt conservation grazing of woodland
• Coppice trees
• Halo ancient trees
• Imitate natural disturbances by pushing over trees
• Pollard trees (top cutting or top pruning)
• Reintroduce large herbivores
• Retain fallen trees.