This is a complex issue.
Grasslands can foster at least as much biodiversity as forests; in some cases, they can do more, and sequester more carbon. And sea grasses, more so. As is also Dan's point, we need to be sure of the veracity of what we "know."
On the whole, I definitely agree that we can heal the damage civilisation has done, but I think it's incumbent upon us to make that happen. And there are
concrete (pun intended, you'll see what I mean) ways we can go about it.
I like the idea of
biorock artificial reef structures. Current from renewable sources like
solar, wind, or wave/tide can cause the accumulation of sea minerals onto a wire frame, on which coral species
thrive. Based on the fact that carbon is one of the main elements removed from the seawater, sequestering it in a cement-like structure, it should even address ocean acidification, at least locally.
I like the idea of these artificial reefs placed in historic hurricane paths, so that the incoming and outgoing material moved by hurricanes is trapped by them, eventually making sub-surface structures to support aquatic grasses, salt marshes, and mangrove swamps.
As to forests, I love them, but there has been, historically, a tendency to protect trees at the expense of the forest ecosystems. Fire events have been suppressed for decades because of the damage they do to our homes and infrastructure, with little to no consideration for the increasing fire load, or the fact that, when trees become artificially old (artificially senescent), they stop sequestering carbon at the same rate as younger, growing trees.
I say the right way to "let nature heal climate and biodiversity crises" is to make sure whatever steps we take make use of biomimicry. Not intervening might work, but, as in the case of letting naturally-occurring fires burn overburdened forests, we won't like it when our towns and infrastructure burn.
Better to take an active role.
-CK
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein