Authors: Colin Tudge
Nitrogen is the mineral needed in greatest amounts. As a chemical element, it is tremendously common: it accounts for nearly 80 percent of all the gas in the atmosphere. But in gaseous, elemental form, nitrogen is of no use to plant or beast. For plants to absorb it, it must first be converted into some soluble form—of which the commonest by far are nitrate and ammonia. Of course, organic gardeners contrive to supply their plants with nitrogen in organic form—which, broadly speaking, means nitrogen in the form of protein (or the broken-down products of protein) in manure and rotting vegetation, and so on. Fair enough. But the plants cannot absorb the nitrogen in the organic material, and hence make use of it, until it, too, is broken down (by soil bacteria) to ammonia or nitrate. Those two simple compounds are the ultimate currency of nitrogen.
In nature (unassisted by nitrogenous car exhaust), soluble nitrogen comes from four sources. Some may come from ground rock. Organic material—the rotting corpses of animals, plants, fungi, and bacteria, and the feces of animals—is also important. (But the dead leaves that form most of the leaf litter on the forest floor are typically low in nitrogen, for the trees withdraw the nitrogen from them before shedding them.) Then there is “nitrogen fixation,” in which nitrogen gas is chemically combined with hydrogen (derived from water) to form ammonia, which may then be oxidized in the soil to form nitrate. This happens in two ways. First, lightning fixes a surprising amount of nitrogen: the necessary chemistry is brought about by the electric flash, and the ammonia thus formed is carried into the soil by rain. But a wide range of bacteria can also pull off this trick, albeit with somewhat less drama.
These nitrogen-fixing bacteria live in a variety of ways. Many cyanobacteria are nitrogen fixers. You often see them on the boughs of trees as a dark bluish slime (hence the misleading soubriquet of “blue-green algae”); but you won’t see the ammonia (converted to nitrate) that they produce, which is carried down the trunk in solution when it rains, runs into the soil, and so nourishes the tree. Many nitrogen-fixing bacteria live free in the soil, and to a large extent (it seems) they are nourished by carbohydrates that the tree “deliberately” exudes to keep them happy. This is symbiosis of the mutually beneficial kind, known as “mutualism”: the tree provides the bacteria with sugars, which they absorb like any other heterotroph; and the bacteria in turn provide soluble nitrogen, which the tree would otherwise lack.
But about seven hundred species of tree are known to form much more intimate, mutualistic relationships with nitrogen-fixing bacteria (and another three thousand tree species are suspected of doing so). In these, the bacteria lodge in custom-built nodules on the roots.
Most of the plants that have such nodules on their roots are in the Fabaceae family, the legumes—like acacias, mimosas,
Robinia,
and the tropical American angelim. It comes as no surprise to any gardener that these leguminous trees are nitrogen fixers—for so, too, are peas and beans, from the same plant family. In all of the legumes, the nitrogen-fixing bacteria in the roots are from the genus
Rhizobium
(though there are many different species of rhizobia). Most gardeners would be surprised to learn, however, that various species from ten other families of flowering plants are also known to fix nitrogen. Like the Fabaceae, all of those families come from rosid orders. Among the nitrogen-fixing, nonleguminous trees are the she-oak,
Casuarina,
and the alder,
Alnus.
In the nonlegumes, the nitrogen-fixing bacteria are not rhizobia but from a quite different genus,
Frankia.
Whatever the details, nitrogen-fixing trees in general can grow on particularly infertile soil, since they make their own fertilizer: and thus we find alders on dank and impoverished riverbanks. Nitrogen-fixing trees also tend to provide particularly nutritious leaves, for fodder. The leguminous trees, especially, are the arborescent equivalents of clover, alfalfa, and vetch, which enrich the world’s grasslands and are much favored by livestock farmers. Since the nitrogen-fixing nodules are leaky, they release surplus nitrogen—and so they serve to enrich the soil at large. For all of these reasons, nitrogen-fixing trees are often of particular use to foresters—and especially to agroforesters, who seek to raise other crops, or livestock, among the trees. Thus acacias and
Robinia
are highly favored the world over not simply on their own account but also to help all else that grows.
Clearly, close cooperation (via root nodules) between plants and nitrogen-fixing bacteria has evolved more than once: once in the Fabaceae with
Rhizobium,
and also in other rosid groups with
Frankia.
We have already seen many times how nature has constantly reinvented the same kinds of structure or modus operandi, so this need not surprise us. Indeed, such associations seem so good for the plant—it gets free fertilizer—that we may wonder why all plants don’t do it. But nothing is for nothing. The nitrogen-fixing bacteria are not altruists. They want something in return—that something being sugars. Hence legumes and alders and the rest must use some of the organic molecules that they create by photosynthesis to feed their nitrogen-fixing lodgers rather than directly for their own growth. Clearly, it’s often worth it. Worldwide, the Fabaceae are a particularly successful family. Leguminous trees are a huge presence throughout the tropics, where soils are often low in nitrogen. There are many places, not least the cold, dank woods of Latvia, where alders flourish. She-oaks, too, find their special niches. Equally clearly, it is sometimes just as easy to do without bacterial residents, and get your fertility from elsewhere (for instance, from neighboring legumes).
Far more common and widespread than such arrangements with nitrogen-fixing bacteria are the associations between trees and fungi that invade their roots—not as parasites but as useful and in some cases essential helpmates. These associations are called mycorrhizae, which means “fungus-root.” Most forest trees and many other plants, too, make use of mycorrhizae: some, like oaks and pines, seem particularly reliant on them.
Fungi in general consist of a great mass of threads (known as “hyphae,” which collectively form a “mycelium”) and a fruiting body that typically appears only transiently, and often manifests as a mushroom or toadstool. Many of the toadstools that are such a delight in autumn, and are avidly collected by gourmet peasants in France and Italy and elsewhere, are the fruiting bodies of fungi, which, below ground, are locked into mycorrhizal associations with the roots of trees and help them grow. Thus the fungi are even more valuable than they seem. The wild mushrooms and toadstools are often only a tiny part of the whole fungus. The whole subterranean mycelium, including the mycorrhizae, sometimes covers many acres and weighs many tons. Forest fungi, mostly hidden from view, include some of the largest organisms on earth.
Mycorrhizae take various forms. Some simply seem to ensheath the fine roots of the trees. Sometimes the hyphae penetrate between the cells of the root, and often, in various structural arrangements, they invade the cells themselves. The relationship, in short, can be extremely intimate. Often, a tree will form mycorrhizal associations with more than one fungus at once, each with a different invasive strategy. Leguminous trees such as acacias, which harbor bacteria in root nodules, commonly have various mycorrhizae as well. Their roots are a veritable zoo.
The arrangement between trees and fungi, like those between trees and nitrogen-fixing bacteria, is extremely advantageous for both participants. The fungi gain because they take sugars from the tree, the products of photosynthesis. The fungal hyphae in turn are functional (and indeed anatomical) extensions of the roots, and hugely increase their efficiency. The hyphae commonly spread far beyond the normal limits of the roots, and vastly increase their effective absorptive area. They also function in the way that fungi always do—by producing enzymes that digest surrounding materials and then absorbing them. Thus they make direct use of organic materials in the soil and may also break down phosphorus-containing rock—and lack of phosphorus (in the form of phosphate) is often a huge problem for growing plants. Then again, fungi are heterotrophs—they live by breaking down organic material; and so an oak or a pine or an acacia whose roots are fitted with mycorrhizae has the advantage both of autotrophy (through photosynthesis) and of heterotrophy (via its fungal helpmeets). Furthermore, a single fungal mycelium, sometimes covering several acres, may interact with many different trees. Thus all the trees in a forest, even of different species, may be linked to others; and each may therefore share to some extent in the benison of all the others. Trees collaborate one with another in several ways, as we will explore in the next two chapters. Here is one: cooperative feeding.
Many trees, including pines, are as successful as they are largely because they have evolved particularly advantageous relationships with mycorrhizal fungi. Astute foresters commonly supply young trees with cultures of mycorrhizae to set them off. Many tropical trees prefer to grow as far away as possible from others of their own species (for reasons discussed in the next chapter), but young temperate oaks are said to grow best when close to others of their kind. Close together, they gain from one another’s mycorrhizae.
Indeed, although we often think of fungi as pests of plants (and they often are: mildews, rusts, wood rotters, and the rest) they often emerge as key allies. Lichens are associations of fungi with algae: and lichens are found everywhere, on rocks and as epiphytes, in thousands of forms. Indeed, many botanists now feel that the association of plants with fungi is intrinsic to the success of both. Both groups evolved initially in water. It seems at least possible that neither could have invaded the land except by cooperating with the other. There is indeed some fossil evidence that the very first algae that came onto land had fungal companions. Since then, fungi have evolved in all directions, not least to produce the magnificent creatures that we now know as toadstools; and plants have evolved this way and that, and now include the world’s wonderful inventory of trees. But the old habits persist. Plants and fungi still stick together to their mutual advantage, as, apparently, they always did.
All soils are different, but this, in broad-brush detail, is how all trees cope with all of them: the ground rules. Some soils, however, are more different than others. Some are positively weird. But there are trees to cope with some of the weirdest.
STRANGE SOILS: MANGROVES AND OUTRIGHT TOXICITY
Around the shallower shores of the tropics and subtropics, in 114 countries and territories, are the forests known as mangrove swamps, named for the trees and shrubs that live within them. The mangrove swamp is typically low, but some trees within it may grow to a height of fifty or sixty meters. Mangrove swamps generally extend only a few miles inland and cover only 71,000 square miles worldwide, yet they are supremely important. Like any forest, they are habitats for a huge array of land creatures—insects, spiders, frogs, snakes, birds, squirrels, monkeys—plus a host of epiphytes; and they provide local people with food, fuel, and timber for shelter. Also like any forest, they lock up a significant amount of carbon and so help to protect the world’s climate (of which more later). Unlike most forests, they lack an understory of specialist shade-loving plants: at ground level, there are just roots, water, and mud. Unlike other forests, too, they serve as the breeding ground for a long inventory of marine creatures, including fish, crustaceans, and mollusks, and around their roots are trails of marine algae. Thus the mangroves link the food webs of land and sea. For good measure (in a natural state), they filter the silt that may flow from the land, and so they protect the beds of sea grass that generally lie farther out, permanently submerged, and the coral reefs that often lie beyond the sea grass. They also protect the land from excessive seas—the tsunami that struck so devastatingly at the end of 2004 might well have been less devastating if some of the shores had retained more of their mangroves. If we take away the mangroves, all the creatures that they are home to, and all the sea grass beds and reefs and coastal lands they protect, are liable to disappear as well.
Most plants, like most creatures of any kind, are killed by too much salt; but mangroves spend much of their time with their roots in pure seawater. This is sometimes diluted by rain, but at other times it evaporates to become as saturated with salt as water can be, until the salt begins to crystallize out; and for good measure, much of the tree roots are intermittently exposed as the tides rise and fall. In addition, the mud in which the trees are rooted is often shallow and is invariably starved of oxygen except for the top few millimeters—yet roots need oxygen. In temperate latitudes, willows, poplars, and alders are among the trees that cope with waterlogged soils that may similarly be deprived of oxygen—but at least in their cases the water is fresh. Salt water raises a whole new raft of problems.
To be sure, tropical seashores are in many ways favorable for growth (nutrients, water, sunshine), but conditions overall are as tricky in their way as in any desert, or on any frozen tundra. So you might expect that only a very few, extremely specialist plants could live there. Yet mangrove forests worldwide contain many species of trees and shrubs, and there is a core group of thirty or forty that occur in most of them, and these come from several plant families. The core group includes various “white mangroves” from the Combretaceae family; the red mangrove,
Rhizophora,
and others from the Rhizophoraceae family;
Xylocarpus
from the mahogany family, the Meliaceae;
Avicennia
from the somewhat recondite Avicenniaceae family in the order Lamiales (the order of teak and mint); and palms of the genus
Nypa.
Independently of one another, the different tree families of the mangroves have evolved a series of physiological tricks to cope with the otherwise disastrous conditions in which they find themselves. Thus, the tissues that form a sheath around the xylem of the roots provide “ultrafiltration,” preventing the salt from entering the conducting vessels and thus polluting the rest of the plant. All mangrove species can do this, but some are particularly good at it, including the red mangrove. Some that are less efficient at filtering out the salt, like
Avicennia,
do absorb at least some salt; but then they excrete it actively (by processes that use energy) from special glands in their leaves, where it may dry in the sun to form palpable crystals. Many seabirds do the same thing through their beaks.