Abstract
Tree encroachment is one of the primary conservation issues in Calluna-heathlands, a priority habitat in Europe. Improving understanding of the ecological factors that trigger transitions to woodlands is key to developing strategies for heathlands management. The irrational use of fire has been recognized as one of the key factors that drives the loss of heathlands of north-west Italy. The effect of high frequency pastoral burning on the replacement of heathlands by grasslands has been documented by several studies. The relationship between fire and tree encroachment is less clear. The paper examines the effect of the fire regime on the encroachment of Populus tremula L. and Betula pendula Roth. in the heathland. The study was carried out at the Managed Nature Reserve of Vauda (7° 41’ E, 45° 13’ N), which includes one of the most valuable heathlands of north-western Italy. The experimental design consisted of analysing the age structure, dendrometric variables and the species composition of three aspen and birch stands, circular-shaped and isolated within the heathland matrix. From 1986 to 2009 all stands experienced the same fire regime due to pastoral burning. Wildfires of similar behaviour occurred in 1998, 2003 and 2008 and determined the stand structure observed in 2009. The results evidenced that fire acts as a catalyst not only for seedlings establishment, as previously documented, but also for woodland expansion in the heathland. After initial establishment, stands showed a concentric encroachment dynamic, mainly due to aspen root suckering after post-fire stem mortality, whose steps of expansion coincided with the return interval of wildfires. Moreover, aspen determined the loss of heathland characteristic species, whose relative abundances were inversely correlated to aspen density along a gradient from the stands centre to the surrounding heathland. The regulation of current burning practices by prescribed burning, integrated with rational grazing, presents the next research questions to be addressed.