Tuesday, January 8, 2013

White Tank Mountains Goat Camp Trail

Desert grassland with mainly tobosa grass located at the top of the White Tank Mountains west of Phoenix, Arizona.
Goat Camp Trail in the White Tank Mountains west of Phoenix is the longest trail in the park.  It can be done as a 13 mile loop along with the Mesquite Canyon and Bajada trails or a 12 mile hike from trail head to end and back.  The trail head is just past the park entrance on Black Canyon Road.  The first mile and a half of this trail is pretty easy starting at 1600 feet in elevation and hiking up a bajada.  This bajada is fairly interesting.  It has been highly disturbed by some major flood events along the dry washes that come out of the mountain canyons.  This disturbance is evident increasingly as you hike towards the mountains by the presence of large boulders laying on the surface.  Normally, these boulders would be sitting lower in the sediments of the bajada being at least partially buried in the dirt.  Large floods however coming from the canyons washed away a lot of these sediments but were not strong enough to wash away the larger and heavier rocks.  So the rocks remained in place while the finer textured sediments washed downslope.  These unburied boulders become increasingly common upslope where the flood, or floods, were more powerful.  These floods resulted in increasingly variable disturbed soil conditions higher up on the bajada and closer to the mountain.  As a result of the more variable soil conditions plant diversity also increases up slope.  Sections of undisturbed soils typically have triangle leaf bursage while the disturbed sections brittle brush.  Other species such as palo verde and jojoba also seem to like the flood disturbed soils.

After a mile and a half or so, the trail begins to head up the mountain.  This trail has an elevation gain of about 1700 feet, topping out around 3300 feet.  With the elevational gain, average annual temperature decreases several degrees and rainfall increases several inches.  At the mountain base, rainfall averages about 8 inches annually.  At the higher elevations rainfall probably averages around 14 inches annually.  There is a rain gage at the top of the White Tanks but apparently gusty winds around the peak prevent it from collecting rain properly, so it is difficult to get an exact measurement of rainfall at the peak.  Regardless, the vegetation tells us that rainfall is significantly higher and temperatures slightly lower.  In the past decade or so I have seen snowfall above 3000 feet in the White Tanks only one time and never below that level.  The lower temperatures, specifically lower freezing temperatures in winter, make conditions less ideal for cacti such as the saguaro.  Saguaros are not able to survive freezing tempertures for longer than 24 hours.  While the saguaro does grow near the peak it is quite rare in comparison to lower elevations.  This is at least partially due to the increase in the amount of time freezing temperatures occur at the higher elevations.

I think there may be another, possibly better, explanation for the decrease in cacti towards the top of the White Tanks though.  Around 3000 feet the vegetation strongly shifts towards a desert grassland dense with tobosa grass.  Tobosa increases because of the increased rainfall and because it can survive the freezing temperatures quite easily.  Dense tobosa grass is possible out competing the cacti at these higher elevations.  Another indicator of a problem for cacti at these higher elevations is the presence of charcoal.  Obviously charcoal indicates fire has been present in the area at sometime in the past and cacti in general do not survive fire very well.  Grass, such as tobosa, however, are very flammable and actually encourage fire to some extent.  Grass, unlike cacti, are very adapted to fire through.  While I have never seen or heard of a grass fire at the top of the White Tanks, the charcoal is evidence that it has happened at some time in the past.  Even if fire happens only once every few decades, that is enough to severely limit the population of cacti in the area.

A few other plants that are relatively common at the higher elevations include desert agave and crucifixion thorn.  Most of the desert agaves are pretty small and almost all appear to be clones that have grown from root sprouts of older plants.
Crucifixion thorn.

Desert Agave

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