Brushwood removal from ditch banks attracts breeding frogs in drained forests

Abstract

Forestry drainage has transformed extensive North-European wetlands to homogeneous forests with a dense network of open ditches. In such drained wetlands, small sun-exposed temporary water-bodies – a favoured breeding habitat for amphibians – are replaced by linear ditch corridors that become increasingly shaded by woody vegetation. We explored whether simple removal of woody vegetation from ditch corridors can increase the habitat quality for two frog species, Rana arvalis and R. temporaria, in a drained pine-wetland landscape in Estonia. Such practice is compatible with the forestry purposes of maintaining the ditches and access to the area. In a before–after–control–impact (BACI) experimental design, the mean shade above cleaned ditches decreased from 66% to 35% and appeared to be the main reason for a dramatic increase in frog breeding in the next spring. The change increased numbers of both species and was based on colonization of the ditches from the surrounding landscape. We suggest that keeping forest ditches exposed to the sun may mitigate the overall negative drainage impact on wetland species at least in the short term, and brown frogs could be used as focal species to guide these practices. In the longer term, however, such systems may not be viable without stable source populations in protected or restored wetland patches.