Thanks for your reply. I'm not sure I agree with all of your beliefs though.
And I remain interested in anyone who can answer my original question: what minerals tend to get depleted first when sheep overgraze a particular patch of land?
First off, a photo as requested:
Black cherry (the crooked one) under a canopy of pine. Most of the thinner trunks in the background are crooked cherries, most of the thicker trunks are pine from circa 1960. The cherry looks dead but all its foliage is at the top, outside the picture. The green haze against the sky is the foliage of the cherries farther back.
In answer to your questions:
I am trying to grow trees with straight trunks, for timber. The section I'm talking about is part of 50 hectares of forest that we own. This particular section has been a pine monoculture since around 1900, with periodic invasions of black cherry that we have fought back with lots of labor and glyfosate.
Our problem with black cherry is its unhappy combination of crooked growth (meaning no hardwood value) and its tendency to shade out other tree species. The only value they have is that they are willing to grow on this poor soil and that their leaf litter improves the quality of the soil over the course of several decades.
The crooked growth is not a wind thing. The 'hill' we're on is extremely shallow and fully forested, so it's not a windy place, certainly not below the canopy. The black cherry grows crooked everywhere, regardless of wind or light conditions, and the crookedness goes in all directions, and differently so in each individual plant.
In part that could be because these trees are descended from a shrub variety that was planted on purpose in the mid-20th century to improve the soil. They turned out not to stay quite as shrubby as expected (they grow to about 15 meters), so that was a big bummer for my great-grandparents and for foresters all across Western Europe who tried the same cheat.
Getting the cherries to be more tree-like is most certainly dependent on the quality of the soil: they can't grow straight unless it is moist and rich in minerals. That's why the one individual with garden waste heaped below it is growing so nice and straight, while all the others are higgledy-piggledy. So this is why I ask my question: what soil minerals get depleted when sheep overgraze the soil and their manure is taken away?
You suggest compaction: that could certainly be a good explanation, but it doesn't apply here. It's been forest for over a hundred years, which is well over the threshold that soils need to recover from any compaction (80 years). Besides, when it was first planted, the soil was plowed down to 50cm, which would have taken care of any compaction by sheep trampling right at the start. Harvesters may compact some soil, but certainly not all of it. Being mostly sand, the soil there is not prone to much compaction at all.
I disagree with your belief that minerals can't be depleted. Check out Australia: it's got some of the oldest surface on the planet, and most of its soils are dirt poor, because all the minerals have been leached out over millions of years. It's not a matter of acidic conditions locking out the minerals: they've simply been removed. Though to be fair to Australia's soil, large parts of it could be a lot less dry and shrubby if it were better managed by sheep-herding humans.
Minerals needed by trees don't ususally come from deep underground. Trees prefer to get them in the easiest place they can: the topsoil, where most of their roots and most of their fungal support system sit. If the topsoil doesn't have the minerals, some trees may grow deeper roots. This is why our forest was originally mostly pine: they root deeper, they need less calcium, and they don't need good topsoil.
A topsoil poor in minerals will support little micro life, which means it won't have much water-retaining capacity. Which means trees will have a harder time too. Now black cherry is a kind of zombie in this respect: its seeds will germinate anywhere, and some individuals will survive even in thin and poor and dry soil. But though they will survive, they won't thrive. They will stay shrubby and crooked, and part of the blame is with the soil.
So this is why I'm into the soil mineral thing.
If grasses and shrubs keep extracting new minerals from the soil for their own growth, and sheep keep eating that growth and pooping elsewhere, then eventually the part of the soil that's accessible to roots will run out of certain minerals. I'm sure fungal activity and weathering can unleash new minerals from stone and bedrock over the long run, but that's going to take millennia, and those fungi need a moist soil to live in while they do their work.
So forgive me for rudely re-asking my question: what minerals do sheep deplete first when a soil is overgrazed?