Large herbivores control the invasive potential of nonnative Austrian black pine in a mixed deciduous Mediterranean forest

Abstract

The invasive potential of the nonnative Austrian black pine (Pinus nigra subsp. nigra Arn.) was analyzed in a 100-year-old Mediterranean mixed deciduous forest in the Massane Nature Reserve, eastern Pyrenees (France). The reserve holds approximately 120–150 semiferal cattle (Bos taurus L.) that browse and trample the woody regeneration. Tree age structure was assessed by dendrochronology to reconstruct the pine population dynamics in grazed and nongrazed (fenced in 1954) portions of the forested reserve. The age structure of the pine population regenerating before 1960 was similar between the inside and outside of the enclosed reserve area. Since 1960, pine recruitment has occurred only in the nongrazed area. The diameter variability with age changed since the 19th century. For pines less than 20 years old, the diameter variability is low, whereas it is very high for individuals older than 100 years. Diverse forest structural changes (composition, canopy height, density, etc.) likely explain the variability in diameter at a given age. Cattle do not appear to affect tree growth as it is similar inside and outside the fenced area, but they control the regeneration of nonnative Austrian black pines, which can spread in the absence of cattle. If nonnative black pine poses a risk for forest conservation, large herbivores may play a useful role in maintaining this species at low abundance.