How can I help you?

Saguaro cactus (Carnegeii gigantea) are rarely seen in the middle of wide open space. Instead, in the Pinnacate lava flows where this photo was taken, plants cluster, growing up under an adult which shelters delicate seedlings.

With all the conflicts in Egypt, Libya, and Wisconsin these last few weeks, I figure it’s time to briefly bid farewell to the little doom cloud floating over me and write about something entirely different:  positive interactions.

The Sonoran Desert is a pretty harsh place. I myself am already cursing my stupidity in setting up a field study that requires me to spend summers in Arizona instead of somewhere pleasant, like Colorado or Montana.

The heat and dryness is not so nice for little plants, or little mammals, like the furry-tailed woodrats or docile and bouncy kangaroo rats. Unfortunately for the Sonoran seedlings, the way rodents deal with their thirst is to eat off the juicy, green stalks. So between the sun, the drought, and the animals, desert plants have things pretty tough.

Okay, none of this has sounded like anti-doom cloud so far. Maybe because it’s 10 pm Friday night and I’m in my underground office, taking a break from work to write this blog.

But this is where nurse plants come in, and they are nice. If you have ever seen a saguaro cactus in its natural habitat, you will notice they are seldom growing out in the open by themselves. No, there usually lies near their majestic base the remains of a smaller shrub that sheltered them from the sun, increased the moisture in the soil, and hid the tiny, soft-spined cactus from the creatures that would devour it.

Some scientists named Brad Butterfield, Julio Betancourt, Ray Turner, and John Briggs recently studied how plants help one another out as nurse plants in Arizona. They found out that this is actually kind of a big deal for how populations grow or shrink here: adult plants shelter juveniles, helping them get established. The more it rains, the more these nurse plants help the little babies.

Of course, the downside is that all these little plants that germinate and establish then compete to the death with one another and their former nurse. But the facilitation – the positive interactions of plants sheltering one another – helps to protect the populations from the extreme variability of the weather and climate.

Want to know more about facilitation? Check out this paper by Jan Bowers and Elizabeth Pierson on how saguaro and other Sonoran Desert superstars get their start, or this more mathy one by R. Diaz-Sierra, M.A. Zavala, and M. Reitkurk.

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