By Gary Paul Nabhan

Welcome to Book of the Week – a weekly feature offering you a glimpse between the pages of an Acres U.S.A. title. Get the Book of the Week email newsletter delivered directly to your in box! This week’s Book of the Week feature is Growing Food in a Hotter, Drier Land by Gary Paul Nabhan, produced by Chelsea Green Publishing. The following excerpt is reprinted with permission from the publisher.

From Chapter 5: Breaking the Fever

One ecological principle fundamental to reducing heat stress in plants and animals is that of establishing a boundary layer between the sun and an organism vulnerable to excessive temperatures and damaging solar radiation. In most cases, the thermal buffer does not lie immediately on the skin of an animal or the surface of leaf, but it creates a layer of air between the “inner surface” of the organism and an “outer surface”—a leaf tree canopy, a latticework of feathers, hairs, thorns, spines, or fibers.

Think for a moment of the black robes worn by Bedouin nomads of the Sinai and Saharan deserts. Although you would at first guess that a dark robe would make a nomad in the desert hotter rather than cooler, the cloth itself is not pressed against the person’s skin but forms an air space layered between the nomad’s skin and the surface of the robe, which ab-sorbs some heat but insulates and deflects much of it away from the body. In a similar manner, black ravens, crows, vultures, and buzzards thrive in deserts, for they have a shiny latticework of feathers that reflects the sun’s rays before they reach the birds’ skin. Similarly, some black-skinned cattle or black-fleeced sheep create a boundary layer that keeps solar radiation from driving them toward heat stress.

palo verde tree growing in the desert
Palo verde growing wild in the desert.

A nurse tree does the same, cooling the organisms in its understory during the summer and warming them in winter by establishing its own microclimate within the boundary layer beneath the tree canopy. And yet buffering underlings from extreme temperatures is not the sole service performed by nurse plants. Since the 1930s, desert ecologists have determined that particular nurse plants like mesquite, palo verde, ironwood, hackberry, and acacia provide a wider range of benefits than just thermal buffering to plants and animals sheltered by their canopies:

  • Seeds are readily dispersed and accumulate beneath nurse plant canopies in soils where seedling recruitment will be favored.
  • Canopy shade creates a boundary layer of humidity. This in turn creates a moist microclimate that buffers the underlings from death by lingering drought, catastrophic freezes, or intense heat spells.
  • Organic matter under the nurse plant nurtures seedbeds with greater moisture- and nutrient-holding capacity than the surrounding desert floor, thereby fostering higher levels of germination and seedling survival.
  • Shallower roots of nurse plants often inoculate the seedlings of associated underlings with mycorrhizal fungi or, if both the nurse tree and the underling are legumes, with nitrogen-fixing Rhizobia bacteria.
  • Deeper nurse tree roots often pump up or lift water, macronutrients, and trace minerals used for building the plant canopy. In addition, as they shed their leaves, branchlets, and surface roots, they deposit them in the litter below the tree, where they are composted and utilized by other plants.
  • Spines, thorns, or bristles on nurse trees repel browsers and grazers that might otherwise eat or trample developing seedlings, creating prey refugia where underling plants are protected from herbivores.
  • Nurse trees provide ideal nesting or roosting sites for frugivorous birds that not only carry seeds or fruits with them but defecate them out in nitrogen-rich packages of manure.
mesquite tree growing near cacti in the desert
A mesquite tree growing near cacti in the desert.

Some nurse plants, such as honey locust and carob, function well in semi-arid temperate zones, while others provide the greatest benefits when situated in true deserts or in the arid subtropics. A few premises will help you select nurse trees for your specific locale and your own guild of food crops:

  • Not all nurse plants are created equal. Some are better than others at meeting the needs of particular crops placed in their understory. Because some nurse plants are drought- or cold-deciduous, while others are evergreen, the nurse plant guilds, or micro-communities beneath them are not simply an amalgam of randomly selected parts. Studies in the Mediterranean of 11 species planted under 16 different kinds of nurse trees indicate that some underlings experience different survival rates depending upon the nurse. The deciding factor for which nurse trees may be best in any case is whether the understory plant needs protection from heat, catastrophic freezes, low soil fertility, low moisture-holding capacity, or from grazing damage by herbivores.
  • The hotter and drier the environment, the greater the need for using older, well-established nurses with dense evergreen canopies that provide continuous shelter for underlings.
  • The higher the risk of wild or domesticated herbivores browsing, trampling, or damaging the plants in the understory, the more important it is that the nurse plants selected have spines, thorns, barbs, or bristles to repel the animals.
  • The poorer the soil, the more important it is that the nurse tree provides abundant leaf litter, fixes nitrogen, attracts mycorrhizae, and pumps water to higher levels of the soil. The permaculture concept of stacking many functions into a cohesive set of plants makes abundant sense when thinking about nurse plant guilds.

Read more about reducing heat stress in crops and cattle in the book Growing Food in a Hotter, Drier Land by Gary Paul Nabhan.

About the Author:

Gary Paul Nabhan is an internationally-celebrated nature writer, agrarian activist and ethnobiologist who tangibly works on conserving the links between biodiversity and cultural diversity. Listen to his interview on the Acres U.S.A. Tractor Time podcast here.

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