Biomass allocation strategies shaping woody species adaptations to shade and drought

<p>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.</p><p>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.</p><p>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.</p><p>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<strong> </strong>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.</p>

Short Note: Root Restriction Hindered Early Allometric Differentiation Between Seedlings of Two Provenances of Canary Island Pine

Based on the optimal partitioning theory, the comparative assessment of seedling allometry is a common task in retrospective genetic tests and early testing of forest reproductive material. Our hypothesis was that root restriction imposed by the container might hinder or rule out genetic differences in biomass allocation. We grew seedlings of two contrasted provenances of Canary Islands pine in mini-rhizotrons, 60 and 90 cm deep, and alternatively in standard bottom-open 200 cc forest containers. In the mini-rhizotrons, plants from the drier provenance allocated more biomass to roots, especially to the tap root and invested less in needles, both in a biomass and leaf area basis, and this morphological divergence increased between two harvests, undertaken at 57 and 115 days after planting. By contrast, confirming our hypothesis, at the 115 days harvest, the plants grown in standard containers did not exhibited significant differences between provenances for Leaf Mass Fraction, Root Mass Fraction and Leaf Area Ratio. We conclude that the physical constraint for root development imposed by small containers increases the probability of dismissing the genetic effect in biomass allocation when assessing forest reproductive materials at the short term, even when the whole plant growth (total dry weight) might be unaffected.