Scrublands

Lowland Bajadas

Left image
An artist's rendering of scrubland (bajada).

Right image
Lowland bajadas near Guevavi.

 

How It Was

Both thornscrub and desertscrub habitats are generally 3,500 feet or less above sea level, have little annual rainfall which happens irregularly during the year, and are mostly frost-free. The water table is generally much lower than that of the area closer to the river and water tends to flow in washes mainly as a result of summer rains. Because of the arid climate and thin, sandy, rocky soil, plants in these communities have evolved mechanisms which allow them to be drought tolerant. Thorns or spines deter animals. Hairy, narrow, or small leaves conserve water by deflecting sun or reducing surface area. Some plants only germinate or flower in years of abundant rain.

Cultural Connections

The O'odham have a long tradition of knowledge and lifeways connected to these environments. In addition to floodwater farming methods that produced corn, tepary beans, squash, and other foods, the O'odham and their predecesors the Hohokam carried the knowledge and use of wild plants and animals of thornscrub and desertscrub regions to ensure the survival and enrichment of their communities. For example, saguaro fruits and cholla buds are not only important sources of food, they are tied to family, cultural traditions, and ceremonies. Other plants like creosote bush, with its strong antiseptic and anti-inflammatory properties, have been used for treating illness.

After Spanish colonization, grazing activities and a new economic regime began to alter these complex environments, making it more difficult to maintain activities of wild plant use. Grazing decreases the amount and diversity of plant life by increasing soil compaction and runoff and by decreasing the water-holding capacity of many soils, making them more susceptible to erosion and less able to absorb rain.

Disturbance

Thornscrub and desertscrub communities recover from disturbance very slowly. Although intensive grazing activities over the last few centuries have provided a source of meat, dairy products, and income, they have also greatly reduced the diversity and abundance of plants and animals in this unique landscape. The mineral rich geology of some of these areas and the resource extraction-based economy brought by colonization also led to mining for silver in the 18th century, and later for copper and other metals. In the 19th century, increased settlement and expanded ranching and mining had a large impact on the ecology and land use of these environments.

 

How It Is Now

Plant Life

Plant life here consists of widely-spaced shrubs or small trees, cacti, succulents, some grasses, herbs, and wildflowers. Because water and nutrient resources are scarce, some plants like the creosote bush have evolved to produce chemicals in their roots and leaves that are toxic to other plants that might establish themselves nearby.

Common trees and shrubs in these areas include velvet mesquite (Prosopis velutina), desert ironwood (Olneya tesota), blue paloverde (Parkinsonia florida), little leaved, foothill or yellow paloverde (Parkinsonia microphylla), cat-claw acacia (Senegalia greggii), creosote bush (Larrea tridentata), common sotol (Dasylirion wheeleri), ocotillo (Fouquieria splendens,) jojoba (Simmondsia chinensis,) and whitethorn acacia (Vachellia constricta). Mesquite trees are shorter and more shrub-like here because access to water is more scarce than in the mesquite bosque closer to the river.

Cacti are also prevalent, including fishook barrel cactus (Ferocactus wislizeni), several species of cholla (Cylindropuntia spp.), and hedgehog cacti (Echinocereus spp.). These amazing plants have evolved over tens of millions of years to be able to store water in fleshy tissue with pleats that expand and contract in periods of greater or less water. The stored water is protected from evaporation by a waxy coating, and from animals by an array of spines.

Threats

Thornscrub and desertscrub environments are threatened today by increasing housing development, grazing, and continuing demand for minerals obtained through mining. A changing climate foreshadows hotter, drier weather and less predictable rain patterns. These impacts have brought more invasive plant species such as lehmann lovegrass (Eragrostis lehmanniana) and buffelgrass (Pennisetum ciliare) which in turn can increase the threat of wildfires and decrease habitat for native plants and animals that have specifically adapted to this environment.

Road and housing development can also decrease availability of nesting areas for birds and animals that feed on the ground or in low bushes. Recently, researchers have also highlighted the complexity and importance of biological soil crusts in these arid ecosystems. These are formed by algae, fungi, mosses, and lichens and serve in fixing atmospheric nitrogen, holding soil down, and holding moisture in the soil. As soil becomes compacted by grazing and other uses, the impacts are felt througout the system.

Despite these threats, natural systems of this area are remarkably resilient. Community efforts by the Tohono O'odham Nation and the National Park Service highlight the importance maintaining traditional foodways and other cultural practices that are intricately linked to the health of these environments.

 

Learn More

Desert Scrub and Sky Islands
The Fight Against Buffelgrass