Assessing climate change tolerance and the niche breadth-range size hypothesis in rare and widespread alpine plants

Species range limits often reflect niche limits, especially for ranges constrained along elevational gradients. In this study, we used elevational transplant experiments to test niche breadth and functional trait plasticity in early life stages of narrow-range Nabalus boottii and broad-range N. trifoliolatus plants to assess their climate change vulnerability and the applicability of the niche breadth-range size hypothesis to explain their range size differences. We discovered that the earliest life stage (seed germination) was the most vulnerable and the two alpine taxa, N. boottii and N. trifoliolatus var. nanus, were unable to establish at the warm low elevation site, however non-alpine N. trifoliolatus established at all three elevations, including at the high elevation (beyond-range) site. Niche limits in seed emergence may therefore contribute to range size in these taxa. In contrast, when seedlings were planted we found substantial functional trait plasticity in later life stages (average 44% across ten traits) that was highly similar for all Nabalus taxa, suggesting that differences in plasticity do not generate niche differences or restrict range size in the focal taxa. While this substantial plasticity may help buffer populations faced by climate change, the inability of the alpine taxa to establish at lower elevation sites suggests that their populations may still decline due to decreased seed recruitment under ongoing climate change. We therefore recommend monitoring alpine Nabalus populations, particularly globally rare N. boottii.