|Desert grassland with mainly tobosa grass located at the top of the White Tank Mountains west of Phoenix, Arizona.|
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.