Ya got yer desert trees, most of ‘em legumes, like this ironwood here.
There’s very little usable nitrogen in the soil in the desert. Plants need nitrogen for crucial metabolic processes, chief among them making amino acids and proteins. Without nitrogen plants fail to grow, then die. The desert’s full of nitrogen, of course: most of the air in the desert, like anywhere else on the planet, consist of pure nitrogen — N2. And since there’s a lot of that air in the desert soil, then there’s lots of soil nitrogen. But N2 is chemically inert, almost as much so as the “noble” gases like helium and neon. Plants can’t use it. Plants can only take in soil nitrogen in two flavors: the ammonium ion , NH4, and the nitrate ion, NO3. Both of these are made out of N2 by “nitrogen-fixing” soil microorganisms, which aren’t as abundant in arid soils as they are in wetter places. The nitrate ion carries a negative charge, and so do most soil minerals, so any stray bit of moisture — or in the desert, even a wind that picks up and circulates the soil — will easily carry away nitrate ions. Ammonium ions are positively charged, so they’re attracted to soil particles. But on alluvial fans in the desert, the typical average soil particle is pretty large, which means the total surface area in soil particle is less than in a silt or clay soil, which means the amount of ammonium that the soil can hold isn’t all that great. Down in the flats, where deserts keep their fine-grained soils, there can be a lot more nitrogen — but there’s usually a lot more of every kind of dissolved salt down there as well, in concentrations too high to make most plants truly happy. And even what little ammonium adheres to the soil in the alluvial fan isn’t there for long: each bit of moisture spurs soil microbes to turn that ammonium into nitrite (NO2) and the nitrite into nitrate, which then leaches away.
So plants up on the alluvial fan have to manage with what nitrogen they can get. Other nutrients — phosphorus, potassium, etc. — they can usually get out of the gravel and sand as it weathers. And in fact, here’s a trade secret well known among plant nutrient salespeople, one of which I was, once. In a most settings, like, say, a backyard garden in Iowa, if you put phosphorus and potassium in the soil — the “PK” in the “NPK” rating on the bag of fertilizer — you generally never have to do it again. But nitrogen, the “N” in the NPK, is like water. It’s fungible. It flows through the local ecosystem, flows into plants and from them into animals, and then — eventually — the more chemically reactive compounds of nitrogen with other elements react with the help of “denitrifying” soil microorganisms to form, once more, that old inert N2. Like water, it’s a limiting factor in desert plant growth. You can get a little bit from decaying organic matter, the leavings of animals and plants and such as they metabolize, and, eventually, those animals and plants themselves. But organic matter is in short supply in the desert.
Leguminous trees like palo verdes, mesquites, acacias and ironwoods have an advantage here. Like other legumes, they have evolved a partnership with those nitrogen-fixing microorganisms. They provide habitat in their root nodules and some nutrients, and the microorganisms provide nitrogen, and everyone’s happy, and the trees, if they survive for a few decades — like the one in the photo above clearly has — put out bumper crops of seeds, most of which find their way into the gullets of rabbits, ravens, packrats and k-rats and ground squirrels, which consume the seeds’ fats and the proteins and excrete nitrogen, and the seasons they go 'round and 'round.
But not all those seeds get eaten up. Some of them are lucky enough to get buried under an inch or so of gravel in a flood. Not only does this hide the seeds from bored and hungry critters, but it also generally scratches the seed’s tough coat so that water has an easier chance of soaking in. Which it can do in the next storm, or the one after that.
For instance, the one that dumped a few inches of rain on Joshua Tree National Park a couple weeks ago, washing out miles of road and making the gravel very wet. Water seeps into seed, swells the seed, stirs the seed’s insensate senses. The seed moves, opens, eases a root down through the gravel for a few days, then pushes against that root and propels itself up out of the gravel, its “seed leaves” — the two cotyledons that stored the little tree’s initial boost of nutrient — slowly unfurling.
And most of these get eaten by hungry rabbits within the first few hours of their lives.
But some of them live long enough to put out their first set of true leaves, and perhaps a second set, and a third;
A very few of them will live long enough to grow their own seeds, and the painted ponies go up and down.