|Two Saguaro cacti in the White Tank Mountains Regional Park.|
When it comes to viewing Saguaro Cacti and an unusual bajada or alluvial fan formations, the South and Bajada Trails in the White Tank Mountains are about as easy and as they come, at least for the western portion of Phoenix, Arizona. The trail begins at the South Trail near the entrance of the regional park, heading towards a short section of Goat Camp trail, and then to the Bajada trail, entire length about 3.5 miles and all pretty flat for some relaxed hiking. The entire length of trail transects the bajadas, or alluvial fans, that were deposited by ancient erosion and avalanches descending from the surrounding mountainsides to the north and south of the trails and Goat Camp Canyon to the west of the trails. Different types of deposits and differing types of drainage patterns has resulted in two major alluvial fans forming a bajada that is seemingly backwards in formation. One of these alluvial fans though is ideal habitat for the iconic Saguaro Cacti as I will explain below. This post will be in two parts, this first post will cover mainly South Trail. The second post on Monday will cover mainly Bajada Trail.
|Desert dry wash along the South-Goat Camp-Bajada Trails in the White Tank Mountains.|
Beginning at the South Trail trailhead near the entrance to the park, you immediately hike down into a dry wash. This wash is one of at least a few washes that are responsible for the unusual bajada formation along the trail further along. As most people would expect, a dry wash, also known as an ephemeral drainage, is where runoff water concentrates and flows for short periods of time following heavy rainfall. The water only flows a short period of time because it is quickly absorbed into the loose sand in the channel where it can infiltrate very deep. Deep in these wash sediments the water is protected from the extreme heat on the surface and is therefore a reservoir during long dry periods. Only deep rooted plants, however, can access these reservoirs and that is why there is a concentration of ironwood and palo verde trees. Even shallow rooted plants such as cacti and triangle-leaf bursage are in higher concentration here through, simply because there is more moisture in these washes then the surrounding soils. Along with transporting water, washes similar to this one also transport sediments eroded from further upslope. Washes, as they transport sediments down slope, are the major creators of alluvial fans. Many alluvial fans were once at the end of a dry wash where the wash sort of spewed out over the surface all the sediments it transported from upslope during short periods of water flow. Washes also are the major destroyers of alluvial fans being they erode into and carry away the sediments that compose the fans. Because of these things, if you are familiar with the washes on a Sonoran Desert alluvial fan or bajada you can know a lot about the history of these landforms. We will see this later on in the hike towards the end of the South Trail.
|The plant community along the South Trail is mainly Cacti, Bursage, and some Palo Verde.|
Once hiking out of the wash the trail crosses onto another alluvial fan surface with a different soil type. This soil type continues for most of South Trail. The vegetation is characterized by shallow rooted plants, mainly cacti and triangle-leaf bursage. This tell me that the soil is well developed, probably with a caliche horizon and maybe with a weak argillic. Caliche is simply a rock-like calcium layer that forms about 20 inches below the soil surface in deserts and argillic horizons are deposits of clay just above the caliche. Both layers prevent deep roots from penetrating and help shallow rooted plants become established, the most important of which is triangle-leaf bursage. This small shrub grows everywhere along most of South and Bajada Trails and because of its abundance there is also an abundance of cacti. Bursage functions as a nurse plant for many desert plants. Odd as it might sound, most desert conditions are too hot, dry, and sunny for most desert plants to establish themselves without the aid of another plant. Being bursage can establish without help of other plants, it provides the shade and slightly cooler and moister conditions required for cacti to germinate and grow under their small canopy. The presence of bursage, along with the soil conditions, is therefore why there are so many cacti present along the South and Bajada Trails.