Reduce Heat Stress in Crops and Livestock: Arid Agriculture

While I was spending a month in Guatemala in the early 1990s, I became aware of the fact that the concept of nurse plant guilds was well known not only in the Sonoran Desert where I was from, but in the hotter tropical reaches of Central America as well. I had spent a week trying to sleep in the sweltering heat of a multistory concrete apartment in a crowded colonia of a Guatemalan city before gaining access to a palm-thatched palapa hut in the middle of a coffee plantation.

Although the two dwellings were located less than two miles apart and shared the same macroclimate, the microclimate in the coffee plantation was at least a dozen degrees cooler by noon each day. The air-conditioning there was not built into the wall of the cone-shaped hut, but came from the palm thatch of the roof, the coffee trees around the hut, and the dense tree canopy above us.

The Impact of Nurse Plant Guilds

The cultivated tree that provided us such relief from the heat is known throughout Latin America by the name madre de cacao, mother of cocoa. It is a nitrogen-fixing legume known to scientists as Gliricidia sepium. It may now be the woody tropical plant most widely used across the world to provide “shade for cacao, coffee, and other shade-loving crops.”

Although scientists have fostered the spread of madre de cacao as a nurse plant for coffee to the many parts of the Americas, the Caribbean, Africa, Asia, and the Pacific Islands, its first use as a buffer against heat stress likely began in the Mayan-dominated reaches of Central America, where madre de cacao was first recruited to provide shade for tender young cacao plants. This must have occurred anciently, for madre de cacao is linked to both chocolate and arboreal monkeys in the ancient Mayan epic the Popul Vuh.

As historians of the Mayan agricultural landscape have written, the cacao plant became dependent upon madre de cacao trees “because it requires a fine-tuned ecosystem to survive: it is wind sensitive, sun sensi- tive, drought sensitive, and nitrogen dependent.” Were it not for the tall, shade-producing, nitrogen-fixing windbreaks and nurse plants of madre de cacao and a close cacao relative (balam-té, protector tree), we might never have enjoyed the pleasure of eating chocolate or drinking hot cocoa.

Diversity of Nurse Plants

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 absorbs some heat but insulates and deflects much of it away from the body.

Why Some Species Thrive in Deserts

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.