A direct comparison of ecological theories for predicting the relationship between plant traits and growth

Despite long-standing theory for classifying plant ecological strategies,
limited data directly links organismal traits to whole-plant growth rates.
We compared trait-growth relationships based on three prominent theories:
growth analysis, Grime’s competitive-stress tolerant-ruderal (CSR)
triangle, and the leaf economics spectrum (LES). Under these schemes,
growth is hypothesized to be predicted by traits related to relative
biomass investment, leaf structure or gas exchange, respectively. We also
considered traits not included in these theories, but that might provide
potential alternative best predictors of growth. In phylogenetic analyses
of 30 diverse milkweeds (Asclepias spp.) and 21 morphological and
physiological traits, growth rate (total biomass produced per day) varied
50-fold and was best predicted by biomass allocation to leaves (as
predicted by growth analysis) and the CSR traits of leaf size and leaf dry
matter content. Total leaf area and plant height were also excellent
predictors of whole-plant growth rate. Despite two LES traits correlating
with growth (mass-based leaf nitrogen and area-based leaf phosphorus
contents), these were in the opposite direction predicted by LES, such
that higher N and P contents corresponded to slower growth. The remaining
LES traits (e.g., leaf gas exchange) were not predictive of plant growth
rates. Overall, differences in growth rate were driven more by whole-plant
characteristics such as biomass fractions and total leaf area than
individual leaf-level traits such as photosynthetic rate or specific leaf
area. Our results are most consistent with classical growth analysis -
combining leaf traits with whole-plant allocation to best predict growth.
However, given that destructive biomass measures are often not feasible,
applying easy-to-measure leaf traits associated with the CSR
classification appear more predictive of whole plant growth than LES
traits. Testing the generality of this result across additional taxa would
further improve our ability to predict whole-plant growth from functional
traits across scales.