Clear-cuts and warming summers caused forest bird populations to decline in a southern boreal area

Abstract

Land use and climate change are the main drivers of biodiversity change. However, their relative roles in the decline of biodiversity are still insufficiently known. Climate change is expected to be globally most pronounced in the boreal biome, where another key driver affecting boreal species is forestry. In NW Europe almost all forest land outside protected areas is subject to systematic forest management practices including clear-cutting, thinning and replanting. In addition, a human-caused increase in herbivores, such as ungulates, may affect the forest vegetation structure and thereby also bird species. We studied population changes of a bird community in a southern boreal area in Finland over a period of 30 years and related changes in the bird community concerning both land use and climate variables. During the study the total bird density declined by 26%, i.e., 1%/year. Long-distance migrants and residents declined the most, both by 44%, and short-distance migrants by 29%, while partial migrants increased by 29%. Clear-cuts were the most important variable explaining the decline in the total bird density and the density change of long-distance and short-distance migrants and residents. In addition, the higher summer mean temperature was related to the decline of the total bird density and short-distance migrants, which were also moderately negatively affected by the ungulate density. The increase in partial migrants was mostly due to an increase in the numbers of two bird species, the great tit Parus major and the blue tit Cyanistes caeruleus, which have benefitted from human actions in many ways. As a whole, the bird community did not recover to the level that prevailed before the clear-cuts in the study period. This indicates that recolonisation and the recovery rate by birds in maturing young stands cannot fully compensate for the loss of birds resulting from the clear-cuts. Forest management practices should adopt more biodiversity-friendly methods, such as continuous cover forestry, to better maintain populations of forest birds.