Biomass allocation strategies shaping woody species adaptations to shade and drought

Optimal partitioning theory predicts that plants allocate a greater proportion of biomass to the organs acquiring the most limiting resource when different environments challenge a given species (acclimation). Results are disputed when testing how biomass allocation patterns among species with contrasting tolerance of abiotic stress factors (adaptation) conform to optimal partitioning theory.

We tested the optimal partitioning theory by analyzing the relationships of proportional biomass allocation to leaves, stems and roots with species tolerance of shade and drought at a global scale including ~7000 observations for 604 woody species. The dataset spanned three plant functional types. In order to correct for ontogeny, differences among plant functional types at different levels of shade and drought tolerance were evaluated at three ontogenetic stages: seedlings, small trees and big trees. Adaptation and acclimation responses were also compared.

We did not find overarching biomass allocation patterns at different tolerance values across species even if tolerant and intolerant species rarely overlapped in the trait space. Biomass allocation mainly varied among plant functional types due to phenological (deciduous vs. evergreen broad-leaved species) and broad phylogenetical (angiosperms vs. gymnosperms) differences. Furthermore, the direction of biomass allocation responses between tolerant and intolerant species was often opposite compared to that predicted by the optimal partitioning theory.

Plant functional type is the major determinant of biomass allocation patterns in woody species at the global scale. Finally, interactions between ontogeny, plant functional type, species-specific stress tolerance adaptations (i.e. changes in organs surface area per unit dry mass), phenotypic plasticity or convergence in plant architecture can alter biomass allocation differences. All these factors permit woody species with different shade and drought tolerances to display multiple biomass partitioning strategies.