A key tree species for forest biodiversity, European aspen (Populus tremula), is rapidly declining in boreal old-growth forest reserves

Abstract

The conservation of habitats and ecosystems is widely regarded as the most efficient way to maintain biodiversity. Protected areas are, however, under the continuous influence of natural dynamics that may affect their structure and functioning. In boreal forests, succession in protected forests may change their ecological properties, including tree species composition. European aspen (Populus tremula) hosts high species richness but appears to recruit slowly in old-growth forests. We explored the long-term dynamics of aspen in a network of forest reserves through two full-coverage inventories in 15 forest reserves in Finland, 18 years apart. We found a decline of 37% in the number of living aspen (diameter at breast height ≥ 5 cm) between the two inventories, from an average of 8 trees/ha to 5 trees/ha. The size distribution of living aspen shifted towards a greater proportion of large trees ≥ 35 cm in diameter. In addition, the rate of recruitment was low, despite a large number of saplings. Most of the observed recruitment took place along the old-growth forest edges, whereas saplings were abundantly present in all parts of the forest in conjunction with living aspen, which function as parent trees for vegetative reproduction. We also observed that browsing on aspen saplings by herbivores was greater at the forest edges and was reduced towards the forest interior. The observed decline in aspen, together with a low recruitment rate, seems to reduce the capability of these protected areas to maintain their biodiversity levels. However, and contrary to several previous suggestions, ecological processes near forest edges may mimic the natural disturbances needed to facilitate aspen recruitment.