Why do plants in shallow soil end up so short?

This was a real question I fielded last night.  My brother had observed that in areas where the soil is thin -- say, up in the mountains -- the plants there are shorter than their deep-rooted counterparts, sometimes dramatically so.  He said that it seemed to affect all sorts of plants, too, like there was some sort of "as below, so above" rule.  He wasn't sure why.

Photo courtesy of the blogging nurseryman, via Flickr.

He's quite correct; dwarfing is common in plants for various reasons, and one of the places where you really notice it is in areas with shallow topsoil.  Donner Pass in California is a good example: you can look out your car window and see old weathered pines that are only four feet tall.

Just having no room to send roots down doesn't dwarf a plant, though.  In marshy areas, plants can get by with a cupful of soil and send out abundant top growth.  Palms grow tall in lots of places, but their root balls are naturally small (look at pictures of uprooted palms in hurricane areas).  So why the difference?

It's a question of resources.  Plants all need two things: water and nutrients.  In fact, water is the carrier for those nutrients, which get dissolved and taken up into the rest of the plant.  If the soil is shallow, there is no deep reserve of water that the plant can rely on between rains; the soil starts to dry out on the surface, and it's stuck.  As the soil gets drier, it gets harder and harder to pull water out of it, and what the plant can get goes only so far.  Literally: like sucking on a straw stuck into a corked bottle, the plant can get water only so many inches above the ground before it just won't go any further.  Any growth above that wouldn't survive in dry spells.  Some plants can "suck" harder than others, or get by with less water and nutrients overall, which is why some of them end up a bit taller.

Thin soil also tends to be poor in nutrients, as there aren't many things to refresh it (like river silt) and rain easily leaches out what's already there.  Without the basic building blocks like nitrogen and magnesium, plants have to live within their means, and build small.  Add wind whipping around mountain peaks or across deserts, and there are more advantages to hunkering down.  Blowing wind evaporates water from leaves that may be hard to come by, and staying low and dense can help deflect it.

All of this together means that plants which normally grow tall and full, like pine trees, turn into wizened dwarfs where the soil is only a few inches deep.  It's not universal, either among regions or among species, but it's enough of a trend that it's hard not to notice.

Oh, and in case you were wondering, the little pots bonsai trees are kept in aren't what keep the trees short.  Proper bonsai care does involve pruning roots periodically to help them fit in the pot -- but the pruning and repotting leads to a very dense, healthy root system buried in extremely rich soil.  They are kept well-watered, and without diligent pinching up top, most of them would double in size in short order, even with such tiny pots.  Bonsai trees are like miniature athletes in the plant world: fit, vigorous, and pampered.