VEIN DENSITY: WHY SHOULD WE ACCESS THE FOREST CANOPY? (Invited Presentation)

Isolated fossil leaves might provide important clues about plant taxonomy, ecology, physiology, and paleoclimate, but they reveal little information about the structure of past ecosystems. Here, we present how leaf venation density (D_v, a proxy for leaf gas exchange) can be used to infer the placement of a plant in a forest stratum. We apply this knowledge to assess the presence of fossil angiosperms as members of the canopy in past ecosystems.

Plants growing in the lowermost forest stories have lower gas exchange rates compared to canopy members. We explored whether this well supported difference is reflected by D_v (a trait measurable on fossils) and performed tests for the generality of this trend (and its applicability to ancient scenarios). We compared venation density between canopy and understory plants of one temperate and two tropical forests. Our comparison shows significant differences in D_vdepending on the canopy placement (with higher values in the upper canopy). The observed trend remains unaffected by taxonomic affinity, ecological strategy, phylogenetic position of the plant, and site/climate. Furthermore, venation density values of a standing forest are reflected in its leaf litter (the best analog to a fossil flora).

A confidence interval for canopy position (derived from the data on sampled leaves) allowed us to assess the presence of canopy taxa; this was applied to a previously published sample of Angiosperm D_vderived from Hauterivian-Paleocene floras to infer when early angiosperms first emerged into the forest canopy. We provide statistical evidence that supports the presence of angiosperms as canopy trees at least since the Paleocene, (and possibly since the Maastrichtian).

Our results show that the inclusion of D_v can inform on the structure of past forests, and has a mostly unexplored potential for paleoenvironmental studies. Further exploration of finer structural aspects of a forest (such as canopy openness) become potentially accessible with the aid of D_v and a subject to study. The difficulty of accessing canopy leaves in modern forests is the main limit to these kinds of studies but it might allow us to access otherwise hard to gather - but fundamental - information about the structure of past ecosystems.