Biomass allocation and shade tolerance in seedlings tree species in a subtropical forest

Background/Question/Methods

Background: Resource availability, such as light, can be an important factor determining plant morphology and niche occupation in tropical and subtropical forests, since species respond to environmental gradients differing in resources investment.

Question: In this study we tested whether seedlings that differ in shade tolerance also differ in the way they partition their resources among stems and leaves. Moreover, we investigated if these morphological characteristics are products of recent adaptations to local conditions or are shared characters inherited from common ancestors.

Methods: The morphology of the eophylls, cotyledons, stems and seeds of fourteen species (five shade-tolerant and nine gap-dependent) from the Atlantic Rain Forest (Brazil) were analyzed. The forest is characterized by a heavily shaded environment and sandy soil with low in nutrients. Twenty seedlings and seeds were analyzed for each species; samples were gathered in the same period for all species, except when the growth of a particular species was limited to specific months of the year. A correlation between the Euclidean distances resulting from the cluster analysis (based on morphological characters of seedling and seeds) and the phylogenetic distances obtained from a phylogenetic relationship between pairs of species was determined in order to test the influence of the historic evolution of the species in relation to the morphology encountered.

Results/Conclusions

Results: Significant differences were observed in the patterns of biomass allocation between the two ecological groups: shade-tolerant species invested more in total biomass, cotyledons and leaves, whereas gap-dependent species allocated more biomass in stems and vertical growth. Furthermore, these characteristics are specific to each group and are not related to the life history of the studied species.  The corresponding characteristics are, thus, considered adaptive strategies to cope with the environment in which they were inserted.

Conclusions: Despite significant importance of abiotic variables, it is possible to identify from morphological characteristics a pattern related to each strategy of settling. Shade-tolerant and gap-dependent species invest more in photosynthesis and vertical growth, respectively. These two strategies possibly define specific niches of these groups in tropical forests.