Evolution of plant phenotypic plasticity in response to grassland management

Abstract

Current environmental change, including climate change, urbanization and land-use change challenge the potential of all living organisms to adapt and survive. Plants, as sessile organisms, experience particular pressure. To cope with changing environmental conditions, plants can 1) migrate to habitats that are more suitable, 2) adapt via changes in trait means, or 3) tolerate environmental variability through phenotypic plasticity. Especially intensified grassland management, contributing to one of the major causes for global change, exerts strong selective pressure on plant populations. While the effects of land use on the evolution of trait means has already received some attention during the last decades, we know much less about the potential evolution of phenotypic plasticity in this context. As the common grassland management practices mowing, grazing and fertilization constitute recurring disturbances and thus create heterogeneous environmental conditions depending on type and intensity of land use, plants should be expected to evolve phenotypic plasticity in functional traits in order to tolerate these. Two contrasting hypotheses emerge from the idea that grassland management creates heterogeneity in environmental conditions. H1: the strength of phenotypic plasticity should increase along a gradient of increasing land-use intensity, representing increasingly heterogeneous environmental conditions (heterogenization hypothesis), and contrary H2: the strength of phenotypic plasticity should decrease along a gradient of increasing land-use intensity, as intensive land use homogenizes environmental conditions (homogenization hypothesis). Earlier studies on the evolution of plasticity in the context of land use were often limited in their spatial extent and level of replication, and they usually only compared few contrasting environments. To advance research on land use-driven evolution of phenotypic plasticity, I investigated relationships between phenotypic plasticity and grassland management using many grassland populations along a broad gradient of land-use intensity in three regions in Germany. Specifically, I designed two common garden experiments with 58 – 68 populations of three common European grassland species - Achillea millefolium, Bromus hordeaceus and Plantago lanceolata – from along the land-use gradient. In one common garden experiment, I clipped half of the plants to study regrowth ability after biomass removal, representing a homeostatic response to challenging conditions. In another common garden experiment, I fertilized half of the plants to investigate opportunistic responses in biomass and nitrogen related traits to favorable conditions. With these experiments, I asked the following specific questions: i) is there genetic variation in responses to biomass removal and fertilization among plant populations? ii) is land-use intensity, especially mowing and grazing intensity, positively (heterogenization hypothesis) or negatively (homogenization hypothesis) associated with the ability to regrow after aboveground biomass removal? iii) is land-use intensity, especially fertilization intensity, positively (heterogenization hypothesis) or negatively (homogenization hypothesis) associated with the strength of opportunistic responses to a nutrient pulse? iv) is the strength of plasticity positively or negatively correlated with increasing inter-annual variation in land use? and v) does the amount of non-structural carbohydrates in the storage root of Plantago lanceolata influence regrowth ability after aboveground biomass removal? The following patterns emerged: i) I found substantial genetic variation in plastic responses to biomass removal and fertilization among populations; ii) and iii) there was little evidence that land-use intensity selected for increased phenotypic plasticity, neither in regrowth ability nor in opportunistic responses to favorable conditions. However, in a few cases the strength of plastic responses was weaker in more intensively managed grasslands, thus supporting the homogenization hypothesis that increased land-use intensity selects for weaker phenotypic plasticity; iv) there was little evidence that inter-annual variation in land-use intensity selected for increased phenotypic plasticity. However, in one case, plants showed a lower regrowth ability with increasing inter-annual variation in land use; v) I found variation in the storage of non-structural carbohydrates among populations of Plantago lanceolata, which was however not associated with the ability to regrow after disturbance. This thesis about intraspecific variation in phenotypic plasticity provides evidence that plastic responses can vary among grassland populations in a land-use context. This genetic variation is an important level of biodiversity as it bears potentially adaptive functions. However, I found only weak evidence for correlations with land-use intensity. Therefore, other environmental variables that still need to be identified might have driven population differentiation.