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

Abstract

According to theory, edge populations may be the best suited to initiate range expansions and climate-driven range shifts if they are locally adapted to extreme edge conditions, or the worst suited to colonize beyond-range habitat if their offspring are genetically and competitively inferior. We tested these contrasting predictions by comparing fitness of low, mid, and high-elevation (edge) populations of the annual Rhinanthus minor, transplanted throughout and above its elevational distribution under natural and experimentally-warmed conditions. Seed from low-quality edge habitat had inferior emergence across sites, but high-elevation seeds were also locally adapted. High-elevation plants initiated flowering earlier than plants from lower populations, required less heat accumulation to mature seed, and so achieved higher lifetime fitness at high elevations. Fitness was strongly reduced above the range, but adaptive phenology enhanced the relative fitness of high-elevation seeds. Experimental warming improved fitness above the range, confirming climates importance in limiting R. minors distribution, but eliminated the advantage of local cold-edge populations. These results provide experimental support for recent models in which cold-adapted edge populations do not always facilitate warming-induced range shifts. The highest fitness above the range was achieved by a super edge phenotype from a neighboring mountain, suggesting key adaptations exist at the regional scale even if absent from local edge populations. Our results demonstrate that assessing the value of edge populations will not be straightforward, but suggest that a regional approach to their conservation, potentially enhancing gene flow among them, might maximize species ability to respond to global change.\n\nSignificanceIndividuals from range-edge populations are the most likely to disperse to habitat beyond the species current range, but are they best suited to colonize it? Our multi-year transplant experiment throughout and above the elevational range of an annual herb in the Canadian Rocky Mountains found that adaptive flowering phenology enhanced the fitness of high-edge seeds above the range, outweighing detrimental effects of poor seed quality. However, only one edge population maintained its advantage over central populations under experimental warming. While edge populations were most likely to drive range expansion, adaptation to cold climates may not help them initiate range shifts in response to climate warming, unless superior genotypes spread among isolated edge populations.