Plant fossils consist mostly of isolated organs that are poorly informative of plant life form and ecosystem structure. Therefore, questions like “when did the first Neotropical forest originated?” are still debated. We used leaf vein density (a trait visible on leaf compressions) as a tool to reconstruct the occurrence of stratified forests with a canopy dominated by angiosperms. In fact, this trait (similarly to others) is particularly variable in flowering plants, whereas it is scarcely plastic in all other plant groups. Leaf physiological traits are know to vary within a forest depending on the strata where a leaf is located, reflecting ecological adaptation to different microenvironments. The main trigger of these variations is light availability, especially in Neotropical forest, where no other factor is limiting plant growth.
Leaf vein density is positively correlated with conductance and water vapor; increasing vein density allows more efficient transpiration. Previous studies had reported higher vein density in leaves from top tree branches (fully sun-exposed) and lower vein density in leaves from bottom tree branches (shaded). However this trend had never been studied before on a large spatial scale.
Using a 40-meter-tall canopy crane equipped with a gondola, we were able to collect leaves from the very top of the tree. We measure leaf vein density of 132 species from two Panamanian tropical forests and one temperate forest in Maryland (US), and we compared the values of canopy-top and forest-bottom leaves. We show that venation density is higher in the leaves located in the forest canopy and decreases in the lower levels. Furthermore leaves from the forest litter conserve this pattern. Because the forest litter is the closest analog to fossil floras, leaf vein density can be used in the fossil record to reconstruct the emergence of flowering plants in the upper forest strata. Vein density data from the literature from the Hauterivian (132.5 Ma) to the Paleocene (58 Ma) show that vein density values similar to present ones appeared at least 58 Ma. Therefore, the emergence of flowering plants in the canopy occurred at least in the Paleocene.
I plan in the future to extend this study to other environments (e.g. more open, gymnosperm-dominated), and to herbaceous plants.
~Camilla
Read the paper here! Excerpts of this came from a review written here.
