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Barriers to Establishment of Invading, Non-forest Plants in Deciduous Forest Nature Reserves

A field experiment was conducted to define minimum values of irradiance (i.e. light reaching the forest floor) and exposed mineral soil required for non-forest species to become established in deciduous forest in Southern Canada. Three logging intensities (0%, 33%, and 66%, of tree basal-area removed) were combined with three plot sizes (0.01, 0.05, and 0.20 ha) to create a range of values of irradiance and exposed mineral soil. The total number of non-forest species that became established during the first three years after logging was recorded. Non-forest species invaded experimental plots only when irradiance was at least 8% of incident light. There was no single minimum area of exposed mineral soil required for invasion. A few non-forest species became established even in the absence of exposed mineral soil where penetrating irradiance was 8% or greater. More non-forest species became established as irradiance and the amount of exposed mineral soil increased. Logging increased both irradiance and the amount of exposed mineral soil. Only a small amount of logging (e.g. 33% removal of tree basal-area from a 0.01 ha plot) would be compatible with the conservation objective of preventing invasion by undesirable non-forest species.