Here at the park few visitors are impressed with the Palo Verde Trees. They just think their green bark is weird, and quickly move on to looking at the Saguaros. However, if you tell them why the bark is green, they usually are surprised and comment on what a cool adaptation it is! So why is Palo Verde bark green you might ask?
Photosynthesis! Palo Verde Trees shed their leaves in times of drought to avoid water loss through respiration, which in the desert is usually a rather long period which may extend longer than a year. Yet rather than go dormant like other trees in this situation the Palo Verde trees Photosynthesize with their bark so they have the energy to drill roots deeper in a quest for water. Tunneling to the aquifer during the dry seson is a huge advantage in the race for resources with other plant species!
The saguaro cactus also uses it’s green skin for photosynthesis, but has a different strategy. Rather than drilling it’s roots down toward an aquifer, the cactus fans its roots horizontally outward. Fanning out only a few inches below the soil to maximize rainwater collection. The water it collects is then stored in the soft tissue between its ribs causing the tall slender cactus to swell and fatten after a good rain. If it rains hard for days or weeks the cactus will continue drinking it up, sometimes they even split from reaching maximum capacity!
Scientists now know that the growth of arms on saguaros correlates to Summer rainfall and vertical growth to winter rains. However, no one is certain what causes Saguaro Crests. Crest growths on Saguaros are rare and bizarre in appearance (check out the photo below) Are these deviations from the norm a mutation? Frost damage? The result of a Saguaro failing to transition completely between vertical growth into arm budding from one season to the next? We don’t know yet, but the Resources Department here at Saguaro National Park has been studying the Saguaro Cactus Forest’s seasonal growth patterns for decades. In fact we have the the longest recorded field study of all National Parks!
But the Unsung Hero of Saguaro National Park is the most easily overlooked of all the flora and fauna in these parts. I am a huge fan of Cryptobiotic Soil! Cryptobiotic soil forms a crust that holds loose earth (like sand) in place. This trait makes it essential for preventing soil erosion during times of drought. It not only keeps our biologically diverse Sonoran Desert from dissolving into dunes, but it does so much more! Cryptobiotic soil is NOT a singular organism, but in fact unique symbiotic colonies of mosses, ferns, fungi, unicellular organisms and bacteria! Together they work to draw rain water deeper and faster into the soil around it greatly reducing water loss and storm erosion. In addition, they affix nitrogen from the air into otherwise nutrient poor soils, helping seeds germinate and adult plants thrive! (Here is a good photo of a patch I found just off the trail while working during monsoon season.)
Most of the year Cryptobiotic Soil Crusts simply resemble dried up blackish-gray mud, but once the monsoon rains start they “resurrect” expanding and greening up in a matter of days.Unfortunately, our seldom noticed biological foundation is easily damaged. Cryptobiotic soil is very slow growing and recovering from a single foot step can take 10-20 years! So please stay on the trail, and if nature calls you off it, watch your step.