Species are the unit of analysis in many global change and conservation biology studies; however, species are not uniform entities but are composed of different, sometimes locally adapted, populations differing in plasticity. We examined how intraspecific variation in thermal niches and phenotypic plasticity will affect species distributions in a warming climate. We first developed a conceptual model linking plasticity and niche breadth, providing five alternative intraspecific scenarios that are consistent with existing literature. Secondly, we used ecological niche-modeling techniques to quantify the impact of each intraspecific scenario on the distribution of a virtual species across a geographically realistic setting. Finally, we performed an analogous modeling exercise using real data on the climatic niches of different tree provenances. We show that when population differentiation is accounted for and dispersal is restricted, forecasts of species range shifts under climate change are even more pessimistic than those using the conventional assumption of homogeneously high plasticity across a species' range. Suitable population-level data are not available for most species so identifying general patterns of population differentiation could fill this gap. However, the literature review revealed contrasting patterns among species, urging greater levels of integration among empirical, modeling and theoretical research on intraspecific phenotypic variation.

Review SummaryPhenotypic plasticity is considered the major means by which plants cope with environmental heterogeneity. Although ubiquitous in nature, actual phenotypic plasticity is far from being maximal. This has been explained by the existence of internal limits to its expression. However, phenotypic plasticity takes place within an ecological context and plants are generally exposed to multifactor environments and to simultaneous interactions with many species. These external, ecological factors may limit phenotypic plasticity or curtail its adaptive value, but seldom have they been considered because limits to plasticity have typically addressed factors internal to the plant. We show that plastic responses to abiotic factors are reduced under situations of conservative resource use in stressful and unpredictable habitats, and that extreme levels in a given abiotic factor can negatively influence plastic responses to another factor. We illustrate how herbivory may limit plant phenotypic plasticity because damaged plants can only rarely attain the optimal phenotype in the challenging environment. Finally, it is examined how phenotypic changes involved in trait-mediated interactions can entail costs for the plant in further interactions with other species in the community. Ecological limits to plasticity must be included in any realistic approach to understand the evolution of plasticity in complex environments and to predict plant responses to global change.New Phytologist (2007) 176: [749][750][751][752][753][754][755][756][757][758][759][760][761][762][763]

Global change drivers create new environmental scenarios and selective pressures, affecting plant species in various interacting ways. Plants respond with changes in phenology, physiology, and reproduction, with consequences for biotic interactions and community composition. We review information on phenotypic plasticity, a primary means by which plants cope with global change scenarios, recommending promising approaches for investigating the evolution of plasticity and describing constraints to its evolution. We discuss the important but largely ignored role of phenotypic plasticity in range shifts and review the extensive literature on invasive species as models of evolutionary change in novel environments. Plasticity can play a role both in the short-term response of plant populations to global change as well as in their long-term fate through the maintenance of genetic variation. In new environmental conditions, plasticity of certain functional traits may be beneficial (i.e., the plastic response is accompanied by a fitness advantage) and thus selected for. Plasticity can also be relevant in the establishment and persistence of plants in novel environments that are crucial for populations at the colonizing edge in range shifts induced by climate change. Experimental studies show taxonomically widespread plastic responses to global change drivers in many functional traits, though there is a lack of empirical support for many theoretical models on the evolution of phenotypic plasticity. Future studies should assess the adaptive value and evolutionary potential of plasticity under complex, realistic global change scenarios. Promising tools include resurrection protocols and artificial selection experiments.