Accounting for size effects and life history to refine analyses of trait variation
The effect of plasticity on reproductive traits increased in our greenhouse experiment when plant biomass was not accounted for in the models. We thus show here that variance in reproductive effort at the individual scale has a “biomass” component that is strongly driven by plasticity, and an “investment per unit biomass” component that is more genetically determined. Our results emphasize the importance of dissecting reproduction into size dependent and independent components. These dependencies among traits have implications for the expectations of demographic buffering and may explain some of the cases contradicting this theory (see McDonald et al . 2017, Hilde et al . 2020), e.g. when reproductive traits are strongly driven by underlying individual biomass.
Our study organism is a short-lived plant, with reproduction having a strong influence on population performance. However, in species with different life histories, other demographic rates and their underlying traits might exert the largest effects on fitness. For example, longer-lived taxa usually depend more on survival rates for population persistence (Silvertown et al . 1996, Morris & Doak 2005), and may display low variance in survival-related traits. In fact, Preiteet al . (2015) found stronger genetic differentiation for survival than reproduction in a long-lived herb. Environmental drivers of trait variation for various taxa with different life histories and ecological strategies should be analysed in order to better generalise the results presented here.
Plant size or biomass is likely to structure the most relevant demographic rates (Harper 1977, Easterling et al . 2000, Biere 1995), and decomposition of trait variability into size dependent and independent components will also help to shed light on the drivers of trait variability, as shown in our study. Accounting for biomass dependency in trait variation across different life histories may refine previous findings of stronger local adaptation in reproduction than in survival across plant and animal species (Hereford 2009), of higher levels of plasticity vs. local adaptation in reproductive traits of invasive plants (Liao et al . 2016), and of an absent relationship between trait plasticity and its proximity to fitness (Acasuso-Riveroet al . 2019). The detection of more common genotype-by-environment interactions in short-lived than long-lived plants (Matesanz and Ramírez-Valiente 2019) could also be evaluated for different trait categories separately. These additional interpretations from functional and demographic perspectives may advance our understanding of trait-environment relationships and improve our predictions of species responses to climate change.