You know how we pull these alders up, and cut them down, and yet they continually come again.

Now, if we and all human beings were to leave this pasture for a few hundred years, would not those alders increase into a wood?
Would they not kill the grass, and spread right and left, seeding themselves more and more as the grass died, and left the ground bare, till they met the oaks and beeches coming down the hill?
And then would begin a great fight, for years and years, between oak and beech against alder and willow.
But how can trees fight?
Could they move or beat each other with their boughs?
Not quite that; though they do beat each other with their boughs, fiercely enough, in a gale of wind; and then the trees who have strong and stiff boughs wound those who have brittle and limp boughs, and so hurt them, and if the storms come often enough, kill them.

But among these trees in a sheltered valley the larger and stronger would kill the weaker and smaller by simply overshadowing their tops, and starving their roots; starving them, indeed, so much when they grow very thick, that the poor little acorns, and beech mast, and alder seeds would not be able to sprout at all.

So they would fight, killing each other's children, till the war ended--I think I can guess how.
How?
The beeches are as dainty as they are beautiful; and they do not like to get their feet wet.

So they would venture down the hill only as far as the dry ground lasts, and those who tried to grow any lower would die.
But the oaks are hardy, and do not care much where they grow.