It is well established that tropical forests have much higher plant diversity and a more complex forest structure than temperate forests. Although this fundamental contrast has given rise to decades of ecological research, a subtle link between the forest structure and diversity has gone unnoticed: the physiognomy of the forest. Here we integrate the plant life form characterization scheme of Raunkiær and the forest profile diagrams of Richards to reveal patterns of plant distribution among the vertical layers of tropical and temperate forests. Specifically, we tested the hypothesis that physiognomy, or the dominance and vertical distribution of plant life forms, constitutes another fundamental yet unquantified way that Neotropical forests differ from temperate forests. In 18 well-described mature forest research sites across the Americas, we characterized the distribution of relative species richness among (1) five major life forms and (2) five vertical forest strata where the species reproduces. We used the USDA PLANTS database and TROPICOS online herbarium specimens to classify each species into one of the five major life forms of vascular plants (trees, shrubs, herbs, lianas, and epiphytes) and into one of five vertical forest strata in which it becomes reproductively mature (forest floor, understory, subcanopy, canopy, or emergent).
Results/Conclusions
Overall, we found strong support for our Physiognomy Hypothesis, wherein Neotropical and Eastern Deciduous forests are fundamentally different in which plant life forms dominate the forest as well as where in the vertical strata plants reproduce. We found that terrestrial herbaceous plants make up 81% of the vascular plant species richness in Eastern Deciduous Forests. In Neotropical forests, terrestrial herbaceous plants represent 26 % of the overall species richness, and epiphytes add another 18% (44% total). Accordingly, 83% of the species in Eastern Deciduous forests reproduce on the forest floor (<2 m high), and 47% of the species in Neotropical forests reproduce in the understory (2-15 m high). To compare, trees are merely 7% of the Eastern Deciduous vascular plants and in Neotropical forests, lianas and trees together make up fewer species than the herbs and epiphytes. This should come as a surprise since the majority of ecological studies from these biomes including the permanent 50-ha plots only take woody plants into consideration. Our results highlight the need for more ecological research focused on herbaceous plants both in the temperate and tropical forests.