LEAF VEIN DENSITY AS A PROXY TO CHARACTERIZE FOREST STRUCTURE; POSSIBLE APPLICATIONS TO THE FOSSIL RECORD

Modern angiosperm-dominated forests are emblematic ecosystems characterized by a multistratified structure, with a high light gradient descending from the canopy to the understorey, which strongly influences plant physiology. Neotropical rainforests receive increasing attention from the scientific community because of their incredibly high diversity and significance to the earth-climate balance. Its origin and the development of its modern structure are still debated. Our aims are to explore the variation of leaf vein density (Dv) through the forest, to test if this trait can be used to characterize different forest ecosystems and to study its application to the fossil record. Dv correlates positively with hydraulic conductance; its increase is thought to have allowed an improvement in angiosperms’ productivity through their evolution, during the Cretaceous.

Trees were selected from the canopy and understory of Parque Nacional Metropolitano semi-deciduous dry forest and Parque Nacional San Lorenzo evergreen humid rainforest in Panama, and Edgewater (MA) temperate deciduous forest. Dv variation between species from different strata was compared within and among sites. Our Dv data were also compared to those compiled by Feild et al. (2011) using leaf remains from the early Cretaceous to Paloecene.

Results show a bimodal pattern in Dv distribution through the three modern forests, with significantly higher values for canopy than for understorey trees. This trend shows that the placement of a tree within the forest (strata) is a central driver of leaf eco-physiological adaptations, including Dv, and a convergence in strategy independent of taxonomy or site. The vein density curves derived for the modern forests are not comparable to those of Feild et al., except for the Paleocene where the highest Dv are observed. This can be due to a preservation bias in the Cretaceous (disfavoring high Dv measurements) or to a late development (likely in the Paleocene) of high Dv characteristic of modern angiosperms. The high Dv values, uniquely found in canopy tropical trees and appearing in the Paleocene, suggest that this trait can be used to identify neotropical forest ecosystems in the fossil record. A wider fossil sampling is needed to reinforce this conclusion.

References

T. S. Feild et al., PNAS 108, 8363 (May 17, 2011).