Climates on the move: Implications of climate warming for species distributions in mountains of the northeastern United States

Abstract Mountains contain steep but constricted climate gradients that can provide climate warming refugia often overlooked in coarse-scale models of species migrations. With continued climate warming, the potentially important role of mountains in maintaining suitable climate for migrating species is still not clear. To determine if mountains in the northeastern U.S. can continue to serve as refugia for species in high-elevation spruce-fir forests under warming climate, we studied climate and climate-vegetation relations along elevational gradients across 76 sites on 11 mountains in four states of this region. We calculated (a) fine-scale temperature lapse rates using in situ climate loggers on each mountain, and (b) regional long-term temperature trends using 36 meteorological stations, in order to determine (c) recent and expected future shifts in species temperature envelopes along elevational gradients by linking lapse rates with regional temperature trends and climate warming scenarios for 2100 (+1, 3, and 5 °C). Since 1960, temperature regimes have shifted upslope on average by 377 m and 133 m for the monthly mean of daily minimum (T min ) and maximum (T max ) temperatures, respectively, although climate did not warm equally for all months. By 2100, mid-range warming of 3 °C may shift monthly temperature regimes upslope relative to their 1960s locations on average by 986 m for T min (580 m for 1 °C, and 1393 m for 5 °C scenario) and 588 m for T max (285 m for 1 °C, and 891 m for 5 °C scenario). We confirmed that spruce-fir forest distribution in the northeastern U.S. is strongly related to temperature, particularly October T max that has surprisingly differed from the overall warming trend as it cooled slightly since the 1960s and thus possibly contributed to the recent downslope shifts in some species ranges documented across the region in other studies. However, the vast majority of monthly temperature variables suggest considerable climate warming since the 1960s, and, given the expected future warming, the temperature regimes characteristic of the lower range margin of spruce-fir forests are unlikely to be present on many mountains in the region by 2100. Consequently, mountains in the northeastern U.S. may not provide long-term climate refugia for species dependent on the climate currently found in spruce-fir forests unless they can adapt to warmer temperatures.