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.