COS 141-4 - Into thin air: Impacts of global change drivers and species interactions on range expansions

Abstract: Climate change is predicted to drive shifts in species ranges as populations respond to changing environmental conditions. Range expansion may be lagged in comparison to environmental change, if limited dispersal capacities or slow demographic rates do not allow populations to track their niche through time and space. This may ultimately create novel biotic conditions as some populations lag behind their environmental optimum and others expand their range limits by invading new locations. Our objective was to determine how species interactions and global change drivers interact to jointly determine range expansion and species persistence of a subalpine plant expanding into the alpine. Alpine ecosystems are an excellent system in which to experimentally test the regulators of range expansion, as distinct subalpine and alpine environmental conditions occur in close proximity due to steep elevation changes along mountainsides. We set up a range manipulation experiment in 2021 at Niwot Ridge LTER, CO, to test if a subalpine buttercup species, Trollius albiflorus, could persist in the alpine under multiple interacting global change drivers. We leveraged a 17-year experiment with fully factorial global change manipulations of nitrogen addition, day-time warming up to 2 degrees Celsius, and increased snow pack. We manipulated half the transplants to experience reduced below-ground biotic interactions using PVC pipe, and we recorded neighborhood cover as a proxy for shading. The first year after transplant, we found that below-ground and above-ground interactions promote range expansions of subalpine species into the alpine, increasing the probability of survival by 25% with biotic interactions. Warming with no biotic interactions hindered Trollius growth the most, with an average transplant leaf width of 1.87 cm (± 0.42 95% confidence interval) which was approximately half the size as in the warming treatment with biotic interactions (mean width = 3.91 cm ± 0.91 cm). Trollius seed production was also higher with biotic interactions (mean = 4.82 ± 0.26 seeds, n = 11), compared to without biotic interactions, where only two transplants set seed (mean = 2 ± 5.64 seeds). Our results suggest that Trollius experiences facilitation from the surrounding alpine plant community. This may be because water was retained through above-ground shading and below-ground water flow, buffering Trollius from desiccation. Our results emphasize that both biotic interactions and global change drivers need to be jointly considered, and that facilitation from the current community in some cases may buffer the effects of global change and promote range expansion.