| Summary: | Abstract We ask whether climate warming will be likely to trigger rapid northward range expansion for understory plants in the northeastern forests of North America at the temperate–boreal forest ecotone. We answer this question through a detailed analysis of contemporary occupancy patterns at cold range edges and by looking for patterns consistent with climate or other environmental (e.g., edaphic) limitations. We first identify each species' realized growing degree‐day niche to sample zones central, at the tail, or beyond the edge of this niche. We model habitat suitability, based on 11 nonclimate variables, for each species, and compare the availability of, and occupancy on, suitable sites across zones, with the assumption that a limiting climate is likely to result in decreased occupancy or abundance on otherwise suitable sites toward the range limit. We also check for evidence of shifts to warm microclimate conditions at the range limit as a potential indication of climate limitations. Based on our habitat models, we find that the availability of suitable sites declines across these three zones for most species, while occupancy increases on suitable sites at range edge, and abundance does not vary significantly. Some species shifted to sites with warm microclimate conditions; of these, several also declined in abundance suggesting climate limitations. We conclude, however, that for most species studied, suitable site availability appears to determine the northern edges of distribution and, consequently, warming may not necessarily lead to near‐term range expansions.
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