Assessing changes in bird communities along gradients of undergrowth deterioration in deer-browsed hardwood forests of western Japan

Abstract

The impacts of deer browsing on forest ecosystems, including effects on woodland bird species, are now rapidly being felt in western Japan, and quick assessments regarding the spatial and temporal expansion of deer effects are urgently needed in forest management. We gathered multisite profiles of local bird communities together with information on deer-induced changes in forests’ physical structure and evaluated deer effects on bird communities by using an ordination approach. Forty-two survey sites were established in mountainous cool–temperate forests in the Mt. Hyonosen region, western Japan. Bird abundance at each site was estimated in June 2012 by using the conventional fixed-radius point count method. Deer-induced changes in forests’ physical structure were evaluated by using the shrub-layer decline rank (SDR; ranked D0–D4 based on visual categorization of the shrub-layer vegetation cover). The most recent SDR scores varied from D0 to D4, and by considering previously published scores (5–6years previously), the intensity of deer effects on vegetation during the intervening period were classified as continuously low at 18 sites, increasing at 11 sites, and high at 13 sites. In the nonmetric multidimensional scaling plot of bird community dissimilarity, sites with lower and higher SDR scores were plotted in a mutually exclusive way. SDR scores explained 11.6% of the among-site differences in bird communities over the effects of various microhabitat differences in a partial canonical correspondence analysis. Another advantage of using SDR scores to assess multisite profiles of local bird communities is that the local indicator species for a forest with a lower impact of deer browsing can be roughly estimated without requiring well-defined control data. In the study region, six potential indicator bird species were identified as being closely associated with low-SDR sites based on an indicator species analysis. SDR-guided management of deer density is being considered in western Japan due to its easy application, even at a regional scale. An SDR-guided management strategy might also be preferable for maintaining local bird communities because it would be possible to infer resulting changes in native bird communities using SDR scores.