I've heard a number of colloquialisms for trees with bent stems like the one above, "pipe-stem tree" is one of the more recent, and "dog-legged tree" is the term I first learned. But what they tell you is the same, regardless of what you call the trees themselves: the soil on this slope is slowly creeping downhill. The tree's roots tunnel deep into the ground, but the top few inches of soil slowly push down the slope (I'm guessing at generally slow rates, in the neighborhood of mm to cm/year.), and slowly push the tree over. But most plants have a geotropic response, and react by delivering growth hormones to the side that's relatively "down." So that side grows a little faster than the side that's "up," which has the net effect of straightening the plant out, to closer approximate vertical. I have been under the impression that with woody trees, once the stem has reached a certain maturity, it no longer grows upward, only outward. You can see a couple of sharper kinks in this Douglas fir trunk, one near the ground, and another about 2/3 of the way to the top of the photo. Above the latter, the stem looks much closer to vertical, though not quite there. This may represent episodic tilting. Or not. I'm not sure. But it definitely indicates soil creep.