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Riding in cars with ecologists

I have always loved the landscapes of the western United States, and after driving all over them with Heather, I have come to see them with somewhat different eyes. On our trips to New Mexico and Flagstaff in the last couple of weeks, we talked a lot about the state of these regions—the desert grasslands, juniper savannahs, and so forth.

As you climb in elevation from the true sonoran desert, you quickly reach arid grassland; this is the sort of place where grazing has taken place for many decades in Arizona, Utah, Idaho, New Mexico, Colorado—much of the West. Next time you drive from Tucson to Flagstaff or Santa Fe, look carefully at the rolling hills that you pass. The first observation that an ecologist like Heather makes is that activity like grazing (combined with pumping groundwater for development) really has had a profound effect on the landscape: Areas that once were lush with grasses are now covered with shrubs, oaks, and junipers, and the occasional swatch of bunch grass. How does this happen?

Grazing is almost always accompanied by fire suppression, and both of these activities increase with human occupation. With settlement comes increased use, higher levels of grazing, and more fire suppression. This is in areas of the country where the historical record and contemporary fire ecology shows a general fire return interval of ten to fifteen years; that is, every decade or so, shrubby plants and small trees would be burned up by fires that could wash their way through twenty-mile wide valleys in a few hours. Importantly, these fires were carried by the small fuels provided by grass. If a grassland is grazed, and grazed hard enough that the grass doesn’t return or returns very slowly, as has happened all over the West, then the shrubs and trees move in. If fire is suppressed, as happens just about everywhere, then growing trees and shrubs eventually choke out the grass, so that the fuel that once would have carried a fire through the area disappears. It then become next to impossible, without extensive (and extremely expensive) mechanical clearing, to restore grass to the region.

(Note that this is a very different process than that which produces massive forest fires. Unlike in the large forest fires that we have seen in the past decade, former grasslands now full of shrubby trees are dense enough to choke out grass but not dense enough to carry fire on their own.)

There are a few ways to think about this transformation. As you drive thirty miles or so north of Phoenix, the hillsides that are thick with junipers were once grasslands; on one hand, well, they’re really pretty, and the same goes for lowland pinion/juniper savannahs in New Mexico and Colorado. We might say,”Okay, the landscape has changed, but it’s all good; after all, we like forests.”

On the other hand, we also know that the changing landscape is partly a consequence of our actions in these places. The trick I asked Heather to explain is if and why we should acknowledge this as important. That is, are there any arguments against this chain of events that aren’t based on bird-loving luddite hippies? Turns out, yes: Water.

Grasslands recharge the water tables that are critical to our lives in the arid west, and they prevent erosion of the soil on which our ranchers make their livings. When a gentle hillside or valley is covered in grass, water is held and slowly absorbed into the ground. The same hillside without grass becomes a funnel, so that water just drains away—taking with it the topsoil and leaving behind a dry hillside. See the cycle? Next time you cross a lowland valley filled with scrub, look at the ground. Chances are good that you’ll mostly see bare ground. This is because, after drying out, topsoil has blown away. Cows can no longer graze there because grass will never grow there again, at least in the time horizon of most of our lives.

Another bit of roadside ecology: Everywhere you drive in the West, you’ll see steep-sided dirt canals that cut through pastures and fields. These aren’t necessarily irrigation ditches or old rivers; they’re the direct result of water moving across denuded grasslands; rather than seeping gradually, water in these places flows, and eventually cuts out a path. It’s tempting to think of these cuts as being healthy—hey, water runs here—but they’re potentially indicative of long-term damage.