Leaf variation in a tree of Pourouma tomentosa (Cecropiaceae) in French Guiana

Abstract

In the flora of French Guiana we find considerable within-plant variation in leaf form. We observed entire, two-lobed, and three-lobed leaves within five separate levels (tiers) of the canopy of a single individual ofPourouma tomentosa subsp.maroniensis. Five branches from each of the five tiers of the tree were collected around the axis of the trunk. From these branches five secondary branchlets were selected and all leaves excised with information recorded as to nodal position, number of leaf nodes, and fertility status of the main branch. This design produced 1015 leaves representing about 20 m² of foliar area and about 2.4 kg of blade dry weight. Our objectives were to determine if statistically significant patterns exist for leaf variation and to suggest improvements for future, general collections. The four lower tiers had 62% entire, 10% 2-lobed, and 28% 3-lobed leaves, in contrast to the top tier with 38% entire, 11% 2-lobed, and 51% 3-lobed leaves. The top tier had no fertile branches. in the lower tiers, fertile branches produced 68% entire leaves whereas nonfertile branches produced only 46% entire leaves. In the top tier, lobed leaves made up 73% of surface area, while in the lower tiers, lobed leaves made up only 48% of total surface area. We selected a random subset of 75 leaves from the 1015, for morphometric analysis using two-way ANOVA (tier×leaf type). The boundaries of leaf images were digitized and rendered into Fourier coefficients, yielding leaf surface area and two variables that quantify aspects of shape: dissection index and leaf complexity. The Fourier coefficients were averaged by tier and by leaf type to reconstruct synthetic, average leaf images. Logistic regression was used to predict the position of leaves on the tree and to provide visualization of the relationships between leaf position on the tree and leaf morphological variables. Within the tree crown, leaf surface area and leaf specific mass (LSM) increases with height, although leaf shape does not change with height. LSM does not vary with leaf form; and sun leaves are larger than shade leaves on this tree. We conducted computer sampling experiments based on exact randomization to simulate the process of obtaining all leaf shapes present in an individual tree when making field collections of varying numbers of duplicates. This also points out the importance of noting the presence of within-tree variation in leaf form on herbarium labeds. Failure to recognize leaf variation can lead to incorrect delimitation of species as well as cause overestimates of the number of species in diversity studies.