Ecological trade-offs of mechanical thinning in temperate forests

Mechanical thinning, the partial removal of stand basal area using tracked or wheeled machines, is a standard silvicultural treatment for whole tree extraction in many temperate forest ecosystems worldwide. It is also increasingly recommended to reduce fire risks in, and increase water yields from, temperate forests. In firesuppressed forest types, reduced fire risk has been documented in thinned forests under certain conditions. Thinning also may enhance drought resilience in some forest types and assist in habitat restoration for some species. However, we argue there are substantial costs to ecosystems associated with large-scale forest removals from mechanical thinning. Depending on environmental context, scale, frequency, and type of removals, impacts include: (1) loss of ecological integrity in a key successional stage and degradation of habitat suitability for associated taxa; (2) increased susceptibility of thinned stands to fire spread from higher sub-canopy wind speeds and increased fine fuels (slash); (3) soil compaction from heavy machinery; (4) reduced resilience where thinning from above removes naturally competitive dominant trees that may contain adaptive gene complexes; (5) release of carbon stored in forest biomass and soils; and (6) an increase in flammable understorey species, including the spread of invasive plants. Thinning requires an extensive road network that alters hydrology, especially where roads intersect streams and on steep erosive slopes. In addition, the relatively high financial costs of thinning may divert resources from other activities that are more cost-effective and restorative. Forest managers need to carefully consider the multi-facetted costs and environmental trade-offs of thinning operations and not just the benefits in silviculture and restoration projects.