Data from: The importance of trees for woody pasture bird diversity and effects of the European Union's tree density policy

1. Recent reforms in the Common Agricultural Policy (CAP) aim for a
greening of the subsidy system with potential improvements for
biodiversity conservation. As part of that process, the tree density limit
for pastures to qualify for European Union (EU) subsidies has been
increased from 50 to 100 trees/ha. However, recent studies show that the
high biodiversity values of these habitats may be threatened by these
limits, highlighting the need for policy improvements. Still, little is
known about the direct effects of tree density limitations on bird
communities in woody pastures. 2. We investigated how bird diversity and
species composition are affected by tree density in 49 Swedish woody
pastures along a gradient of 4 to 214 trees/ha. We recorded bird
communities, tree density and stand structure estimates in the field and
estimated forest cover in the surrounding landscape from aerial photos.
Using generalised additive models (GAM) and redundancy analysis (RDA), we
analysed how bird territorial species richness, bird abundance and species
composition are affected by tree density, taking into account other local
and landscape scale explanatory variables. 3. Tree density had a
significant positive effect on bird species richness at low tree densities
and species richness saturated at approximately 50 trees/ha. Shrub density
had a significant positive linear effect on both bird species richness and
abundance. Tree and shrub density were also the major drivers of bird
community composition, with secondary effects of tree species diversity
and landscape forest cover. 4. Policy implications. These results show
that tree density is not the limiting factor, but rather a driver of bird
diversity and species composition in woody pastures and that tree density
limits may fail to capture the whole range of biological values. To
improve policy recommendations, we therefore stress the importance of
considering additional social-ecological drivers associated to management
quality to preserve biodiversity in woody pastures.12-Jan-2017