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.
I suspect it’s related to the distinction between C3 and C4 photosynthesis—both are common in grasses and C4 species tend to do better in hot climates, but trees seem to have trouble evolving C4 pathways even though that happened on 60+ separate occasions.
(also IMO monocots top out at “kinda tree-ish”—they do have a recognisable trunk, but more fibrous than woody)
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.