Charlotte Møller , Martí March Salas, Jonas Kuppler, Pieter De Frenne, J F Scheepens

Background and aims: Climate-change induced warmer spring temperatures advance tree leaf-out and result in earlier shading of the forest floor. Climate change also leads to more frequent droughts. Forest understorey herbs may respond to these environmental changes by varying functional traits at different hierarchical levels of organisation. While trait variation at the intra-specific level is well-studied, little is known about how variation at the intra-individual level responds to environmental changes.
Methods: We sampled genets of the forest understorey herb Galium odoratum from 21 populations in three regions in Germany, varying in microclimatic conditions. The genets were transplanted into a common garden, where we applied shading and drought treatments. We measured plant height, leaf length and width, and calculated the coefficient of variation (CV) at different hierarchical levels: intra-population, intra-genet, intra-clone and intra-ramet, the latter two representing intra-individual variation.
Key results: Variance partitioning showed that intra-ramet CV explained most of the total variation, followed by intra-clone CV. We found significant variation in CV of plant height and leaf width among populations of origin, indicating that CV is at least partly genetically based. Mean soil temperature at population origins correlated negatively with CV in plant height, suggesting adaptation to local conditions. Furthermore, we observed that early shade led to increased intra-clone CV in leaf length and drought reduced intra-ramet CV in leaf width. Finally, intra-ramet mean leaf width and CV were independent under control conditions but became correlated under drought.
Conclusions: Our experimental results reveal genetically based patterns in CV and correlations with soil temperature, indicating that intra-individual variation can evolve and may be adaptive. Intra-individual variation responded plastically to drought and shading, suggesting functional changes to improve light capture and reduce evapotranspiration. In conclusion, intra-individual variation makes up the majority of total trait variation and can play a key role in plant adaptation to climatic change.