Phenotypic plasticity masks evolutionary change in grasslandplant traits in response to land use abandonment

Abstract

Traditionally managed grasslands are among the most biodiverse habitats in Europe, but are threatened by land use abandonment. While the negative impacts of grazing and mowing cessation on species richness are well documented, little is known about potential evolutionary changes within species. Yet, intraspecific functional diversity is critical for successful grassland restoration and ensuring adaptive potential to future climate change. To disentangle the heritable and non-heritable components of population-level response to land use change in semi-natural grasslands, we examined 22 populations of a common grass, Briza media, from grazed sites and sites where grazing was abandoned, with resulting tree and shrub encroachment. We measured a range of traits under field conditions and following clonal propagation under common garden conditions. Field surveys revealed that abandonment resulted in litter accumulation, greater shading by woody and herbaceous vegetation, and lower temperature and moisture fluctuations compared with grazed sites. Plants responded to conditions at abandoned sites with phenotypic plasticity in traits that enhanced competitive ability for light (greater height and specific leaf area) but reduced tissue protection against stress (lower dry matter content). However, when the same genotypes were measured after clonal propagation in common conditions, counter-gradient variation was apparent, with heritable shifts towards increased tissue protection in populations from abandoned sites. When measured under field conditions, trait diversity was higher in sites with higher levels of shading and productive sites with higher variation in light conditions. However, high heritable trait diversity was instead characteristic of grazed sites with high densities of flowering B. media individuals. Synthesis. These findings demonstrate that land use change can cause evolutionary shifts and changes in heritable trait diversity that are masked in the field by phenotypic plasticity. Hence, the assessment of functional trait variation based on field observations is not a reliable way to assess the genetic variation essential for population adaptive potential.