What do you see on this alpine slope? What you don’t see may be as important as what you do. You might see a rocky substrate with a very sparse cover of plants, all apparently similar. A few grasses and sedges, several asters, and a few other stragglers from other families. They are all short and small, with similar traits reflecting adaptations to the short snow-free growing season. Does anything distinguish them? How can they all coexist next to each other?
The answer may lie in the things you don’t see – in what lies beneath the surface of the soil. Species may coexist when niche differences between them are large enough. Although aboveground differences are minimal, belowground differences may not be. I decided to dig some holes to find out whether these species’ root systems were equally similar.
Consider the example of this buckwheat, Eriogonum umbellatum var. aureum. This individual is about ten centimeters wide and just two centimeters tall. You, like me, might expect it to have a comparably small root system that would be easily extricated.
Unfortunately for me the soil here is a primarily comprised of a blue-gray slate. The surface is a loose gravel, but within a few centimeters of digging, the soil becomes loose rocks, and after a few more centimeters becomes solid rock, only occasionally with a fracture plane present. Soil is a generous description. Yet somehow the plants manage to grow in this matrix, and their roots manage to slip into cracks or themselves make new ones. Digging is a precarious affair where rocks must be carefully removed, and fragile roots must be traced until they disappear into subterranean nothingness. Here you can see the beginning of an extraction operation, with a Phacelia hastata plant also emerging from the soil.
It turns out that this diminutive plant has an impressively large root system, not only covering a wide area but also plunging deep into the rock. What you see here isn’t even the entire root system – I lost the deepest root into a crack that I was not able to fracture with my digging tools. Here you can see my collection effort back in the lab.
And it also turns out that not every species has this same extensive root system. Here is a grass species I brought back to the lab, Achnatherum lettermanii. In this case I dissected it into leaves, stems, roots, and dead tissue. You can see here that the roots are much shorter and smaller – a marked contrast to the buckwheat.
In the end I was able to unearth the full root systems of over seventy plants. The diversity was impressive, and may help to explain why so many species coexist despite such apparent aboveground niche similarity.
We ecologists rarely explore plants’ root systems. They are time-consuming and difficult to dig up. Measuring them often comes at the price of killing the plant. The whole plant measurements I took would be nearly impossible for a large tropical tree, with its roots hopelessly tangled amongst those of its neighbors. But sometimes these extra efforts and costs are worthwhile. They show us things we could not see any other way.