Sunday, May 30, 2021

Fat leaves and thin leaves: How they can tell us where grasses evolved

Correlation between leaf shape and habitat type across 578 grass species (Gallaher et al, 2019)

I hike a lot, and many of the places I hike into are in forests and woodlands, so it's no surprise that I've become somewhat familiar with the grasses that inhabit the shaded interior of these forested environments. 

One thing I have noticed is that the denizens of these habitats tend to have wider leaves compared to species that live out in the open, and I just read a paper that used this fascinating fact to try to resolve the evolutionary origins of the family Poaceae.

The wide leaves of Oplismenus undulatfolius, which inhabit the shady undergrowth

The evolutionary origins of grasses are still under intense study, but differing hypothesis place their original habitat either in forest interiors, in the open, or in forest margins. A new study sought to delineate these options by looking at the various leaf shapes in grass species (Gallaher et al, 2019). Forest grasses it turns out, do tend to have wider leaves than those species in the open, but unfortunately such changes in leaf shape might also be affected by other factors, such as climate and the photosynthetic pathway used by each species (whether C3 or C4).

The broad leaves of a Dichanthelium, another denizen of more shaded areas

In order to determine how various factors influenced leaf shape and size, the study generated phylogenies for 578 grass species representing all the Poaceae tribes and subtribes, and approximately 75% of all grass genera. Leaf shape for the various species in this phylogeny was then correlated with precipitation, solar irradiance, temperature, photosynthetic pathway, and habitat.

They found that leaf shape in the grass family was strongly correlated with the habitat of the species, and that leaf shapes across the entire family converged towards certain shapes becoming associated with specific habitats. So for example, open habitats tended to have grasses with linear leaves, and ovate leaves tended to be present in grass species in the forest understory. Meanwhile, species that lived in forest margins tended to have lanceolate leaf shapes, an intermediate form between the shadier forest interior and the more open plains. They also found out that leaf shape is not strongly related to all other variables, whether it be temperature, precipitation, solar radiation levels, or even photosynthetic pathway. In addition, they noted that grasses have smaller leaves in open and drier areas, and in areas with high solar irradiance.

Ammophila breviligulata with narrow leaves. Habitat is open beach areas.

Interestingly as well, they noticed that the evolution of leaf shape closely tracked habitat changes over the course of grass evolution, and their estimates placed the forest understory as the most likely ancestral habitat of the grasses. Looking within the family, they found that the most probable habitat of the bistigmatic clade, core grasses, BOP clade, Oryzoideae, and Bambusoideae was forest margins, while the crown node of the Pooideae was either derived from open areas or the forest margins,

In addition, they estimated that there were between 12 to 41 transitions by grasses in the move to open habitats over evolutionary time, and all of them were from forest margins. This highlighted the importance of the forest margin ecotone as a launchpad to the eventual diversification and movement of many in the family to having more linear leaves as an adaptation to open environments. Linear leaves are advantageous in these less restrictive habitats because they allow faster heat loss to avoid damage to leaf tissue, as well as help prevent mechanical damage due to wind and other stresses. 

All these changes in leaf shape as grasses moved from the forest margins to open habitats occurred in the Cretaceous or Paleocene, which was long before the first signs of grass-dominated ecosystems in the Eocene, and the eventual dominance of grasslands beginning in the Miocene. 

So the next time you are hiking, take note of the grasses and how their leaf shapes tracks the environment, and marvel at the evolutionary changes that allowed this family to escape the shadier environments of the forest to eventually claim dominance of the wider open spaces.

The narrow leaves of Panicum virgatum, another typical denizen of open areas

Literature Cited:

Gallaher, T.J., Adams, D.C., Attigala, L., Burke, S.V., Craine, J.M., Duvall, M.R., Klahs, P.C., Sherratt, E., Wysocki, W.P. and Clark, L.G. (2019), Leaf shape and size track habitat transitions across forest–grassland boundaries in the grass family (Poaceae). Evolution, 73: 927-946.

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