Eco-evolutionary responses to environmental change
Plant species persistence in a changing environment depends on three possible responses: migration to suitable environments, plastic responses to local conditions, or evolutionary adaptation. Key open questions concern the potential speed of such responses relative to the pace of environmental change, and how abiotic and biotic factors interact. Here I summarize results from projects in my lab addressing these questions. Range shifts along an elevational gradient were studied via re-surveys of “legacy” vegetation plots across a span of 40 years on Mont Mégantic, Québec. The influence of soil properties and seed predators in limiting upslope migration of one species, sugar maple (Acer saccharum), was studied via manipulative experiment. Responses of intraspecific trait variation (ITV) of forest understory species were assessed in the same site for specific leaf area (SLA), flowering phenology, and maximum height. Finally, evolutionary responses to environmental change have been assessed in the arctic tundra and in temperate grassland by conducting reciprocal transplant experiments between treatment and control plots of experiments manipulating temperature and CO2 concentration, respectively.
Over the past 40 years, distributions of both trees and herbs have shifted an average of 35m upslope at Mont Mégantic, which is <20% of the upslope shift in temperature isotherms, indicating a substantial time lag. However, this is not only due to demographic inertia: experiments with sugar maple indicate that >95% of seeds dispersing beyond the upper elevational range limit are consumed by small mammals, compared to <50% within the range. Established seedlings perform equally at different elevations. For understory herbaceous plants, intraspecific trait variation is often correlated with environmental conditions, but different species show highly variable responses both in terms of direction and magnitude. Interestingly, the contribution of ITV to community-level trait turnover was greatest along gradients varying at small scales (e.g., light and soil properties) compared to larger-scale gradients (e.g., climate). Finally, experiments both in the arctic tundra and temperate grassland reveal evolutionary adaptation to abiotic environmental change in just 15-20 years, although the magnitude of temporal responses is quite subtle compared to genetic differences among populations, again indicating important time lags or unknown evolutionary constraints. In sum, we see empirical evidence of all three responses to environmental change – migration, plasticity, and evolution – but with substantial time lags, and unexpected constraints on such responses that can only be revealed via detailed, system-specific studies.