Local adaptation primes cold-edge populations for range expansion but not warming-induced range shifts

According to theory, edge populations may be poised to expand species’ ranges if they are locally adapted to extreme conditions, or ill-suited to colonize beyond-range habitat if their offspring are genetically and competitively inferior. We tested these contrasting predictions by transplanting low, mid, and high-elevation (edge) populations of an annual plant throughout and above its elevational distribution. Seed from poor-quality edge habitat had inferior emergence, but edge seeds were also locally adapted. High-elevation plants flowered earlier, required less heat accumulation to mature seed, and so achieved higher lifetime fitness at and above the range edge. Experimental warming improved fitness above the range, but eliminated the advantage of local cold-edge populations, supporting models in which cold-adapted edge populations do not facilitate warming-induced range shifts. The highest above-range fitness was achieved by a ‘super edge phenotype’ from a neighboring mountain, suggesting key adaptations exist regionally even if absent from local edge populations.