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.

物种范围限制通常反映生态位限制，特别是对于沿高程梯度限制的范围。在本研究中，我们使用海拔移植实验来测试窄范围Nabalus boottii和广范围三叶草植物早期生命阶段的生态位宽度和功能性状可塑性，以评估它们的气候变化脆弱性和生态位宽度范围的适用性。大小假设来解释它们的范围大小差异。我们发现最早的生命阶段（种子萌发）是最脆弱的，两个高山分类群N. boottii和N. trifoliolatus var. nanus无法在温暖的低海拔地区建立，但非高山N. trifoliolatus建立在所有三个海拔处，包括高海拔（超出范围）地点。因此，种子出现的生态位限制可能会影响这些分类群的范围大小。相比之下，当种植幼苗时，我们发现生命后期阶段的大量功能性状可塑性（10 个性状平均为 44%）对于所有Nabalus都高度相似分类群，表明可塑性的差异不会产生生态位差异或限制焦点分类群的范围大小。虽然这种巨大的可塑性可能有助于缓冲气候变化面临的种群，但高山类群无法在低海拔地区建立，这表明由于持续的气候变化导致种子补充减少，它们的种群可能仍会下降。因此，我们建议监测高山Nabalus种群，尤其是全球罕见的N. boottii。