Several candidate size metrics explain vital rates across multiple populations throughout a widespread species' range

Individual plant size often determines the vital rates of growth,
survival, and reproduction. However, size can be measured in several ways
(e.g., height, biomass, leaf length). There is no consensus on the best
size metric for modelling vital rates in plants. Demographic datasets are
expanding in geographic extent, leading to choices about how to represent
size for the same species in multiple ecological contexts. If the choice
of size variable varies among locations, inter-population comparative
demography increases in complexity. Here, we present a framework to
perform size metric selection in large-scale demographic studies. We
highlight potential pitfalls and suggest methods applicable to diverse
study organisms. We assessed the performance of five different size
metrics for the perennial herb Plantago lanceolata across 55 populations
on three continents within its native and non-native ranges, using the
spatially replicated demographic dataset PlantPopNet. We compared the
performance of each candidate size metric for four vital rates (growth,
survival, flowering probability, and reproductive output) using
generalized linear mixed models. We ranked the candidate size metrics
based on their overall performance (highest generalized R2) and
homogeneity of performance across populations (lowest total magnitude of,
and variance in, population-level error). While all size variables
performed well for modelling vital rates, the number of leaves (modelled
as a discrete variable, without transformation) was selected as the best
size metric, followed by leaf length. We show how to interrogate potential
trade-offs between overall explanatory power and homogeneity of
predictions across populations in any organism. Synthesis: Size
is an important determinant of vital rates. Using a dataset of
unprecedented spatial extent, we find a) consistent size-based models of
growth, survival, and reproduction across native and non-native
populations of this cosmopolitan plant species and b) that several tested
size metrics perform similarly well. This is encouraging for large-scale
demographic studies and for comparative projects using different size
metrics, as they may be robust to this methodological difference.