what is the meaning of "dry up and blow away" in these sentences?

"Dry up and blow away" is a phrase that is meant to evoke the passage of time affecting something until it's entirely gone. The drying up portion of the image is meant to illustrate a loss of vitality, as living things shrivel and dry up when they reach the end of their lives. So a slightly different formulation of the phrase, "it looks like it could just dry up and blow away" describes something that is so weak, that it is on the brink of death.

In the first case, Jeremy Hotz is making a joke about the Canadian use of a leaf as a symbol. He implies that Canada is proclaiming that it is as weak as a leaf without a tree, something that is very likely to dry up and blow away. What he's saying is that the Canadian flag, and therefore Canada, doesn't project a frightening image.

The second case is saying that in the environment described, the truth suffered so much that it might as well have disappeared entirely. The simile at play is asking you to imagine the truth as a physical thing left out in a desert, a harsh sun of bad ideas grilling it until it is reduced to dust that blows away.

In the third case, Immermann is implied to be the lifeblood of the settlement. Without it, the settlement is expected to be incapable of surviving, again bringing us to the mental image of something drying up and blowing away.