Montane birds shift downslope despite recent warming in the northern Appalachian Mountains
Montane regions support distinct animal and plant communities that are widely viewed as communities of high conservation concern due to their significant contribution to regional biodiversity. These communities are also thought to be particularly vulnerable to anthropogenically caused stressors such as climate change, which is generally expected to cause upward shifts and potential range restrictions in montane plant and animal distributions. In the northern Appalachian Mountains of North America, not only is it becoming warmer at mid-elevations but the ecotone between the northern hardwood and the montane coniferous forests is also shifting. Therefore, species that are limited by climate or habitat along the elevational gradient of mountains may also be experiencing distributional shifts. We studied birds along replicate elevational gradients in the White Mountains of New Hampshire, USA, from 1993 to 2009 and used mixed effects models to estimate the rate of elevational change to test the hypothesis that northern hardwood forest- and montane forest-dependent birds are shifting upslope, consistent with climate change predictions. As predicted, the upper elevational boundary of 9 out of 16 low-elevation species showed evidence of shifting upslope an average of 99 m over the course of the study period. Contrary to our expectations, 9 out of 11 high-elevation species had lower elevational boundaries that shifted downslope an average of 19 m. The opposing elevational shifts of two distinct and adjacent bird communities is, to our knowledge, unprecedented and highlights the need for caution when applying conventional expectations to species' responses to climate change.