Fine roots are an integral part of plant functioning but remain less understood than leaves and other plant organs, both in terms of their functional tradeoffs and how these may be influenced by the abiotic environment. We were interested in how environmental variation associated with tropical forest age (secondary vs. primary) affects above and belowground functional traits. We investigated how leaf and fine root functional traits varied with their soil environment along a 6 km transect spanning secondary and primary tropical forest in Jianfengling, Hainan Island, China. Along the transect, leaves and entire fine-root systems containing at least three rootorders were collected from >400 saplings of >70 species. Seven leaf and 14 root morphological traits were measured, and leaves and roots were subject to tissue nutrient analyses, measuring several macronutrients along with Carbon and Nitrogen and their stable isotopes. Rhizosphere soil from 300 saplings was collected and subjected to a combination of soil analysis methods to quantify soil texture, pH, organic content, and cation exchange capacity. We expected that within taxa traits would become more acquisitive in older forest, as competition for resources (e.g., light, water and soil nutrients) increases.
Results/Conclusions
Soils along the gradient were similar: slightly-acidic, nutrient-poor, humic latosols. Secondary forest areas tended to have coarser soils with more available nitrogen, but primary forest soils had greater carbon content. Leaves and roots across species were different in their functional traits, aligning along known axes of life history variation. For example, specific root length, root tissue density, and root branchiness averaged 45 m/Kg, 0.6 g/cm3 and 2.1 tips/length for 11 Fagaceae species and 39m/Kg, 0.41 g/cm3 and 1.7 tips/length for 19 species in the Lauraceae. Partitioning of trait variance via the relative Rao method showed that <1% of functional trait variation in either leaves or roots could be attributed to forest age. Using the same method, we found most of the leaf and root trait variability, 93% and 94% respectively, was interspecific, suggesting relatively little plasticity in functional traits within species across the gradient. Multiple regression of distance matrices revealed that soil nutrients were more related to leaf and root morphological traits than were nutrients in the tissues themselves. Despite inherit variability in root and leaf morphologies of tropical saplings, much of which is interspecific related to plant strategy, our results demonstrate how environmental influence on morphological traits can be independent of plant tissue nutrient concentrations.