How the desert survives: keys to nature’s success in California’s vibrant desertscape

Thanks to Toby Maxwell for sharing this piece on the survival strategies of desert plants. The PCT is more than trail tread and the physical effort required to traverse it. The trail is also an introduction to the natural world. Toby’s piece helps to enrich our understanding of what we’re passing by.

Every year as spring comes, the question of how much water will be in the desert is a big concern. For PCT hikers, there can be 20- and even 30-mile stretches without water. And while trail angels try to leave jugs filled with water, these caches are unreliable, making logistics a life and death problem to which hikers much pay the utmost attention.

Nature must survive in this unforgiving landscape as well. Without weather forecasts, friendly neighbors and hitchhikers’ thumbs, plant communities must constantly work with what they are provided by the environment. Plants have evolved with their environment, engineering what may seem like a difficult place to exist into an efficient, nourishing home. The ability of these plants to colonize harsh environments can be used as a landmark for controversy over desertification in the American southwest, as human activity is promoting irreversible change in desert landscapes.

Settling in the Desert Sandbox

If you imagine your backyard garden, and compare it to a children’s sandbox, quite different images come to mind, but the sandbox probably resembles a desert more closely. So, with only a few inches of rain a year, and sand to grow in, how do plants thrive in the desert sandbox? Broadly speaking, plants require a few basic things to live: water, sunlight and nutrients, and desert survivors have a number of interesting ways to conquer such a landscape.

The Search for water

To get water in the desert, plants must be clever. Before we can discuss plant strategies it is important to separate plants into annual plants – or those that grow and die in a single year – and perennial, which live for many years.

Annual plants in the desert take advantage of winter and spring rains to live a very short determined life. Seeds can lie dormant in the ground lasting many dry years until the right conditions appear. With temperature and moisture just right, they pop into life – with very short cycles allowing them to live and die – producing seeds before all of their resources are exhausted. Annual plants grow roots mostly on the soil surface to take advantage of rains that may not infiltrate deeper. In this way, annual plants are opportunistic, meaning that they wait until the time is right, and only go through their life cycle when there is sufficient moisture for them to allow development of roots that will sustain them until they can produce seeds.

Perennials must be more flexible since they live through the dry desert summers. Plants such as the Joshua tree, creosote bush and sagebrush are perennials commonly found in the Mojave Desert, and are not only built for the desert, the also engineer their environment to ensure their survival. The first step to living in the desert is using resources wisely. This means that perennial plants must also be opportunistic. Desert plants often have widespread root systems close to the surface of the soil. Early life for a perennial is about developing a root system that will allow it to access water. After this stage, leaves and branches can grow at large angles which helps direct rainfall towards a plant base rather than allowing it to fall though. This adaptation also helps collect nutrients that have accumulated on leaves and stems from windblown dust and deliver them straight to the plant base. To help accumulation and preservation of both water and nutrients, plant roots, often with the aid of small animals such as beetles and mice, modify the soil. Similar to worms in your home compost pile, they break down, and mix its constituent rocks, minerals, and decomposing leaves into smaller pieces, which gives the area immediately below one of these woody plants a greater ability to store water. This helps draw out the benefits of a rainstorm, preserving soil water instead of allowing it to flow too quickly past the reach of roots. With these adaptations and symbioses, plants have a strategy for growth in the desert that maximizes the amount of water they can acquire before it is out of reach, providing habitat and nutrition for small desert animals that in turn help develop the soil to the plant’s benefit.

Islands of Soil Fertility

Where water flows nutrients go. For plants to move nutrients inside their stems, leaves, and cells, they must be able to be transported in water. Similarly, nutrients also move through the soil dissolved in water. The result is that water and nutrient resources are critically linked, and the processes that are important for getting water also aid in nutrient acquisition. The result is that the area below the canopy of these plants is a tightly regulated ecosystem often referred to as an island of soil fertility. A literal oasis in the desert, these plants build up their own dunes, called coppice dunes, supported by their root structures, and the fine soil particles that are developed by chemical and physical weathering. The formation of coppice dunes also accelerates the accumulation of nutrient rich windblown sediments. Dense root systems throughout the dunes transport water, and dissolved nutrients into the plants, ensuring a tight ecosystem where water and nutrients are conserved, recycling what is accumulated for continued growth.

When times are tough in drought years, plants will shed leaves and stems, reducing their canopy size, and the amount of water they require to survive. Additionally, stored water and nutrients in the stems and leaves are siphoned back into the living part of the plant. With this flexibility, the demand of the plant can be highly reduced, and resources that were acquired in years of plenty are used as a safety net for survival. Deposited leaves then become part of the soil system, and are broken down by insects and other invertebrates, mixed into the soil, and become part of the island.

Even plants need sunscreen

Sunlight may be the only resource found in excess in the desert. However, similarly to humans, UV radiation can damage a plant, and intense sunlight only increases the fleeting nature of desert precipitation. To resist this stress, desert plants have developed an array of protective physical features that limit their exposure, and prevent stress. You may notice in the desert that plant leaves are often dull in color compared to other places. This is because of one of several adaptations to deflect sunlight. While some plants may even be red or purple, most have coatings of opaque waxes, growth of furry coatings, or dull color. Each adaptation is targeted at mitigating the amount of sunlight actually absorbed by the plant. Light is very important as a main ingredient for photosynthesis, which is the process plants use to remove carbon dioxide from the atmosphere to produce compounds needed to grow roots, shoots, and leaves. For most plants, this occurs in the leaves through small openings called stomata. At the same time as carbon dioxide enters, the plant opens the door for water to leave. Losing water in this way is necessary, as it helps to draw up nutrients from the soil, however if water is limited this is a costly practice. Physical adaptations like waxy coatings above help prevent too much photosynthesis from occurring, which could result in exhaustion of soil water resources.

A Special Case: Cacti

Everyone associates cacti with the hot deserts of the world, but what allows them to survive there? They can store excess water in their structure, a property common to all succulent plants, giving them a greater buffer to resist drought. However, even more importantly this water storage allows them to do photosynthesis in a unique and inventive way that conserves water. Rather than collecting carbon dioxide from the atmosphere during the day, they do so at night, and store it until there is light during the day. This leads to the least possible water loss, because during the day they do not have to open up their leaves and lose water. This clever adaptation stretches every drop of water, allowing for cacti to populate some of the driest environments on earth. Interestingly the fact that cacti store water also relates to why they grow spines. Thirsty animals often get significant portions of their water through their diet, so desert plants must protect themselves from herbivores.

Controversy: Desertification and Grazing Cattle in the Southwest

Can native plants be invasive? This is a tricky question, but certainly changes in vegetation are an indicator of how landscapes change with human involvement. Desert systems are fragile because it takes a long time for environmental and biological conditions to converge, allowing a plant to grow. For this reason, disturbances in the ecosystem can be catastrophic, and irreversible. The Mojave and Chihuahuan deserts of the southwestern United States have grassland prairies that are often on government land, and leased for cattle grazing. When cattle pass through, they move the grass, compact the soil by walking over it and leave urine and manure behind in isolated patches. A natural grassland covers most of the land surface in vegetation, protecting the soil from wind erosion, keeping nutrients locked in place. However, after a disturbance, erosion increases because the vegetation is no longer able to stabilize soil with its roots. Once the grass is gone it very difficult for these ecosystems to return to their old state. What patches remain, in a subsequent year actually tend to grow very well, attracting the attention of grazers, after which they may reach a state which is permanently altered, rendering these lands a spent resource. This primes the land for shrubs to invade, by creating a heterogeneous environment out of what used to be more homogenous grassland. This transition from grassland to the sparse cover of a shrubland is a factor of major concern in global desertification because cattle grazing is about the only lucrative use for arid grasslands. However, there is a significant threat of permanent alteration of the landscape with little hope for natural processes to return the system to its original state. Does this mean that we should eliminate grazing from the desert? Not necessarily, but it may be important to be cautious moving forward with the practice.

1. Asner, G. P., Elmore, A. J., Olander, L. P., Martin, R. E. & Harris, A. T. Grazing Systems, Ecosystem Responses, and Global Change. Annu. Rev. Environ. Resour. 29, 261–299 (2004).
2. Went, F. W. Ecology of Desert Plants . I . Observations on Germination in the Joshua Tree National Monument. Ecology 29, 242–253 (1948).
3. Whitford, W. G., Anderson, J. & Rice, P. M. Stemflow contribution to the ‘fertile island’ effect in creosotebush, Larrea tridentata. J. Arid Environ. 35, 451–457 (1997).

Toby Maxwell is a PCT hiker, PCTA volunteer and Ph.D. Candidate in the Agricultural and Environmental Chemistry Graduate Group of the Department of Land, Air, and Water Resources at U.C. Davis.