Masters Research
Using leaf vein density to trace the emergence of angiosperms in the canopy in the fossil record
What is leaf vein density?
Vein density (Dv) is the total vein length per leaf area. Increasing Dv reduces the overall hydraulic resistance to water flow through the plant (in the mesophyll) by delivering water closer to its site of evaporation (stomata). Thereby, high Dv values favor high transpiration rates and gas exchange capabilities.
Like other leaf traits associated with plant physiology and ecology, Dv varies vertically within a forest.
Vein density (Dv) is the total vein length per leaf area. Increasing Dv reduces the overall hydraulic resistance to water flow through the plant (in the mesophyll) by delivering water closer to its site of evaporation (stomata). Thereby, high Dv values favor high transpiration rates and gas exchange capabilities.
Like other leaf traits associated with plant physiology and ecology, Dv varies vertically within a forest.
A trait preserved in the fossil record
Although high vein density is a unique characteristic of flowering plants, the first angiosperms were characterized by low vein density values. A four fold increase in vein density during angiosperm evolution lead to transpiration rates comparable to the modern ones at least since the end of the Paleocene. Our study suggests that Angiosperms dominate the forest canopy since at least the Paleocene, with high Dv values corresponding to canopy leaves and low density values corresponding to understorey leaves, and matching the Dv distribution in modern stratified forests (figure 2).
Although high vein density is a unique characteristic of flowering plants, the first angiosperms were characterized by low vein density values. A four fold increase in vein density during angiosperm evolution lead to transpiration rates comparable to the modern ones at least since the end of the Paleocene. Our study suggests that Angiosperms dominate the forest canopy since at least the Paleocene, with high Dv values corresponding to canopy leaves and low density values corresponding to understorey leaves, and matching the Dv distribution in modern stratified forests (figure 2).
Vein density (Dv) probability curves
showing high vein density in the canopy (C) leaves (black) and low vein density in the understory (U) leaves (gray) in two tropical forests in Panama (A,C) and a temperate forest in Maryland (B). Black dotted line in C represents vein density of the forest litter spamming from low, understorey-like Dv values to high, canopy-like Dv values. |
Vein density (Dv) probability curves of eight leaf macrofossil assemblages (gray) from the early Cretaceous to the Paleocene compared to Dv density curve of a tropical forest litter (black) (same as dotted curve in figure above). (Fossil Dv data from Feild et al., 2011).
|
