My hypothesis after 30 seconds of thinking was that trees evolve independently because height = good for competing for sunlight, while grasses must specialize a ton to ‘afford’ passing up on the height advantage. So once a grass is established somewhere it might be hard for an up-and-coming-almost-grass species to nudge out of its niche. Maybe this is related?
I could imagine lots of plants getting stuck in a local maximum of fitness where they are still pretty tree-like but would need to simultaneously lose some tree features and gain C4 photosynthesis in order to succeed as grasses, so the gap to jump in adaptation-space is too large.
Height is also useful for reducing impact of fires, herbivores, some parasites, etc.; and gives you substantially better volume-of-airflow-over-leaves which can be helpful—a flat sheet of leaf-material would underperform substantially for respiration, even before considering the variable angle of sunlight for photosynthesis.
With some handwaving, we seem to agree that “the absence of trees becoming grass-like indicates that there’s no nice/large path in evolution-trajectory-space which is continuously competitive” and I’m gesturing towards the known-to-be-difficult C3/C4 distinction as a potentially-relevant feature of that space.
Note that while our non-expert speculation might turn up interesting relevant considerations, the space is very complicated and high-dimensional, and I at least have very little data or subject matter expertise. I therefore expect my analysis to be wrong, though I do enjoy and learn from doing it.
My hypothesis after 30 seconds of thinking was that trees evolve independently because height = good for competing for sunlight, while grasses must specialize a ton to ‘afford’ passing up on the height advantage. So once a grass is established somewhere it might be hard for an up-and-coming-almost-grass species to nudge out of its niche. Maybe this is related?
I could imagine lots of plants getting stuck in a local maximum of fitness where they are still pretty tree-like but would need to simultaneously lose some tree features and gain C4 photosynthesis in order to succeed as grasses, so the gap to jump in adaptation-space is too large.
Height is also useful for reducing impact of fires, herbivores, some parasites, etc.; and gives you substantially better volume-of-airflow-over-leaves which can be helpful—a flat sheet of leaf-material would underperform substantially for respiration, even before considering the variable angle of sunlight for photosynthesis.
With some handwaving, we seem to agree that “the absence of trees becoming grass-like indicates that there’s no nice/large path in evolution-trajectory-space which is continuously competitive” and I’m gesturing towards the known-to-be-difficult C3/C4 distinction as a potentially-relevant feature of that space.
Note that while our non-expert speculation might turn up interesting relevant considerations, the space is very complicated and high-dimensional, and I at least have very little data or subject matter expertise. I therefore expect my analysis to be wrong, though I do enjoy and learn from doing it.