Some herbvivores also eat the roots, fruits, and the flowers, but most herbivores mainly eat the green leaves right? I understand animals not eating the whole tree trunk, but if some animals can eat roots, surely they can eat the branches too right? I tried googling but I couldn’t find anything other than insects specialized in eat through tree barks.
In: Biology
Bark is *incredibly* difficult to digest. As far as I know, even insects that “eat” bark are just trying to dig through the bark to get to the living, nutritious phloem underneath. That’s what beavers are eating – the phloem, not the bark. The only organism that really eats bark is fungi, which *very slowly* digest the bark with special enzymes. Living trees shed the bark constantly so that few things can digest all the way through before that piece falls off and is replace by a new piece.
Under the phloem, the tree goes back to being pretty worthless, nutritionally. The cells there are just full of water to hold the tree up, and that’s about it – if the cells are alive at all. It’s all structural, made to be extremely tough to hold up the weight of the tree. Once the tree has grown beyond that part – either taller or wider – those cells don’t need to replicate anymore, so there’s no energy going to them and no real active proteins doing anything.
Even just chewing through the bark requires tough mouthparts. Beavers, for example, have evolved to incorporate extra iron in their teeth, making them much stronger than ours. Even then, beaver teeth have to grow continuously because despite being reinforced with iron, they *still* wear down.
Roots tend to be a lot less tough than bark, but they’re also buried. Digging them up takes work, and chewing them requires teeth that can handle chewing some dirt along with the roots. Some plants are fairly easy to uproot, like many grasses, and grazers will often pull the whole plant up. But woody bushes send their roots deep and wide, and it’s generally not worth the effort to dig them all up.
Leaves, on the other hand, are *relatively* soft and extremely nutritious. That’s where the sugars are being made, where most of the business of *being alive* happens in a plant. The leaves still aren’t *easy* to chew, compared to something like meat. They can still wear down teeth. Many plants, like grasses, have evolved to incorporate silicate (essentially glass) into the structure to make them harder to chew. If humans tried to eat a diet of only [raw] grass, our teeth would wear down to nubs and we would starve.
That’s a *lot* of effort to eat the most nutritious part of the plant. It generally is not worth that effort to eat the less nutritious parts. It’s all the same work, all the same wear on mouthparts, for very little reward.
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