Desert Nurses: How Seedlings Survive Harsh Conditions to Become Full-Fledged Plants
Baby desert shrubs and succulents have it tough. They have to sink new roots into the soil and get their first few leaves into the sun, all using only the little bit of energy stored in their seed. And they're vulnerable while they do it. They haven't had enough time to grow thorns or bad-tasting chemicals to deter hungry animals, a thick layer of wax to keep their water on the inside, or a network of roots to get the water out of the soil in the first place. They're susceptible to drought, chewing and trampling. In order to make it through its first year, a desert shrub seedling usually needs a little help: a bit of protection from the elements and from hungry jackrabbits.
It needs a nurse.
The desert isn't uniform. It has pockets and folds, each with its own microclimate. For each sun-baked south-facing slope there's a north side where there might be a bit more shade and moisture. Though annual seeds can land wherever they want and just wait for enough rain to germinate, even if it takes years, seeds of perennials and shrubs aren't quite as flexible. Their seedlings aren't just focused on making quick flowers and seeds: they're in it for the long haul. They're building infrastructure, and that takes time. Hence the vulnerability.
If a shrub seed is very lucky, it lands in one of these spots where it has a much better chance of survival than all its siblings out there in the sun-baked gravel. It's essentially the same story as the Biblical parable of the mustard seed, though it's generally wise to avoid bringing up mustard seeds when you're talking to desert plant fanciers. Land on the exposed stony ground and you're probably toast. Land in a fertile spot and you have a chance.
Seeds that happen to land beneath an established shrub are very lucky. Under a creosote or a blackbrush or even a cholla, a tender seedling can find a bit of shelter to protect it from the harshest conditions of the desert.
It's called a "nurse plant" relationship, and you see it everywhere in the desert: mature shrubs with little plants of other species in their shade. The term implies a bit of intention, as though the larger shrub is "taking care" of the smaller plant, and a moment's reflection will show that's almost certainly not the case. If the phrase is too anthropomorphic for your tastes, you can always call it "ecological facilitation." But the relationship itself is a real thing. The plants interact and influence each other. It may be the most common relationship between plants in the desert, just one part of the interwoven web of interactions that make of the desert ecosystem.
A younger plant generally derives three main kinds of benefit from its nurse plant. Simle shelter from the elements is the most obvious. Slight shade is a boon even to seedlings of plants that will later withstand the full rigors of desert sun. That little bit of shade allows a plant to control its temperature more easily, and since plants control their temperature by letting water vapor escape from their tissues, less temperature stress means more water conserved.
The space under a nurse plant will often be slightly moister anyway, what with the windbreak its leaves provide and the water those leaves transpire. If the nurse plant has built up a bit of leaf litter beneath it, either by dropping its own leaves or by catching other leaves blowing across the desert, that organic matter holds water far better than most desert soils do. The microclimate beneath a desert shrub is thus often way more amenable to seedling growth than a spot of open desert would be.
Nurse plants can offer protection from another aspect of the environment as well: hungry animals, or clumsy ones. Many desert shrubs are sturdy and prickly enough to make it hard for an animal to get to the plants in their shade. This is especially true of shrubby cacti like chollas. A Joshua tree seedling in the open has a life expectancy measurable in days: baby Joshuas are on the top ten favorite foods list of desert rabbits, as well as several other animals. But few rabbits ever get desperate enough to nose through a thick clump of cholla to see if there are seedlings under it. In parts of the desert where there are lots of young Joshua trees, you'll find many of them with grizzled-looking chollas at their bases, still growing though their young charges have outgrown their shelter.
That works the other way around, too: short clumps of Joshua tree can be pretty good nurse plants themselves, what with all those spines. If Joshua tree and cholla are young enough, it might be hard to determine which one sheltered the other.
Those baby shrubs don't just need protection from teeth, but from hooves as well -- and boots, for that matter. This becomes especially important in desert areas that have been grazed by cattle, though it's a minor issue with native beasts such as bighorn and pronghorn and the like as well, especially around water sources. It doesn't take much to crush or uproot a desert seedling: a casual nick by a stone-sharpened hoof will do. Seedlings protected by a security fence of spiny shrub run a much lower risk of being squooshed.
Nurse plants don't offer complete protection from animals. In fact, some nurse plants seem to increase the risk that a seedling will be eaten by a rodent, bird or other small animal grabbing a convenient snack while taking advantage of the shade. Few things in nature are always clear boons. On the whole, though, it's usually safer for a tender young plant to borrow someone else's defenses for a while.
The final benefit of life under a nurse plant is fertility. As a rule, desert soils don't hold on to nitrogen very well, and nitrogen is the nutrient plants need in the greatest quantities. Plants require nitrogen to make proteins, which are essential for all living things. Proteins are, most importantly here, crucial for the process of photosynthesis. Without nitrogen, the plant can't feed itself on sunlight and carbon dioxide. (That's why your garden plant turn sickly yellow when they need plant food.)
Only a few organisms can capture nitrogen from the air -- it makes up four fifths of our atmosphere -- and turn it into a chemical form they can use to make protein. (We humans are among those organisms, at least since the Haber-Bosch process was invented in the 1940s.) The vast majority of organisms on the planet, get nitrogen by recycling other living things or their leavings. The best place to do this, for a desert shrub seedling, is usually under a mature plant -- especially if that plant happens to be one of the many species of desert trees and shrubs that can fix nitrogen. Plants drop leaves and fruit with nitrogen in them. They attract small animals from insects to birds to eat their leaves, seeds, and sap, and many of those animals will contribute their own nitrogen to the soil beneath while they visit. Even if a seedling is a member of a nitrogen-fixing species, it benefits from the boost in fertility beneath a nurse plant.
The nurse plant relationship does not always end happily for both participants. Competition for resources i the desert is often fierce, especially for water. As a young plant grows, so does its root system, and each plant begins to compete with the other for scarce soil moisture. It's not uncommon to see a youthful, mature saguaro or Joshua tree or other large desert plant rooted in the husk of its late nurse plant. Sometimes you can trace the chain back a few generations: a cholla nurtures a Joshua tree which helps a barrel cactus get its start. You could call it a chain of sacrifices, one each plant in turn giving its life so that the next might live. Though that's probably too anthropomophic.
Chris Clarke is an environmental writer of two decades standing. Author of Walking With Zeke, he writes from Palm Springs regularly at his acclaimed blog Coyote Crossing and comments on desert issues on KCET weekly. Read his recent posts here.
