Global change at the landscape level: Relating regional and landscape-scale drivers of historical climate trends in the Southern Appalachians

Organisms in montane environments are sensitive to fine-scale climatic variation associated with highly dissected topography, yet few studies have examined the sensitivity of different landscape positions to climate change. We downscaled biologically significant temperature variables to below-canopy 30m resolution and assessed temporal trends from 1980 to 2011 across elevation and topographic gradients in Great Smoky Mountains National Park (GSMNP; Tennessee and North Carolina, USA) using a previously developed empirical model derived from a 120-sensor temperature network. Additionally, we assessed GSMNP climate trends from 1900 using six historical climate records from the region and an additional eight records from 1980, spanning the Park's elevation gradient. Regional temperatures increased through the 1980s and 1990s, but currently remain at or below those recorded in the early to mid-20th century and are strongly associated with different phases of the North Atlantic Oscillation. In contrast, annual and growing season precipitation steadily rose during the past century. Landscape-scale analysis showed that rates of change for maximum seasonal temperatures, frost-free days (FFD), and growing degree days were strongly mediated by topographic position, with high-elevation ridges having greater rates of maximum temperature increases, whereas high-elevation near-stream positions showed the least amount of increase in FFD and growing degree days. Most importantly, we show how modelled differences in rates of climatic change based on landscape position could have significant ecological effects in this biologically significant region, depending on how organisms respond to particular climate factors. Organisms that depend on growing season length may experience the largest climate effects at the lowest elevations, while those that depend on warm days in spring and autumn for particular phenological processes will experience the largest shifts at high-elevation ridges.