Size Matters

I'm sorry.  This was one of those posts that I needed to add to my blogger notes for my Tucsonan gardening friends either new to the area or new to gardening. This rule applies to all of our desert friends out there.  One of the things that you may have noticed already about these tree postings is that leaf size matters in our desert.  The larger the leaf; the more maintenance the plant requires.  That's why on these previous posts about the Desert Willow, Palo Verde, Jacaranda, and Mesquite, you may have noticed that the leaves are tiny or pinnite in nature.  The trees don't have to work as hard to survive in our extreme heat here when they don't have to supply water to large size leaves.  Of course, there's that infamous mulberry tree out there who defies our desert rules, but that's a fruit tree and not native to our desert....here's an article from our incredible Arizona Sonoran Desert Museum.  If you visit Arizona this Spring, it's a "Must See".  I promise you....it's worth the trip.

"Drought tolerance or drought dormancy refers to desert plants’ ability to withstand desiccation. A tomato plant will wilt and die within days after its soil dries out. But many nonsucculent desert plants survive months or even years with no rain. During the dry season the stems of brittlebush and bursage are so dehydrated that they can be used as kindling wood, yet they are alive. Drought-tolerant plants often shed leaves during dry periods and enter a deep dormancy analogous to torpor (a drastic lowering of metabolism) in animals. Dropping leaves reduces the surface area of the plant and thus reduces transpiration. Some plants that usually retain their leaves through droughts have resinous or waxy coatings that retard water loss (creosote bush, for example).  Microphylly (the trait of having small leaves) is primarily an adaptation to avoid overheating; it also reduces water loss. A broader surface has a deeper boundary layer of stagnant air at its surface, which impedes convective heat exchange. A leaf up to 1/2 inch (10 mm) across can stay below the lethal tissue temperature of about 115°F (46°C) on a calm day with its stomates closed. A larger leaf requires transpiration through open stomates for evaporative cooling. Since the hottest time of year is also the driest, water is not available for transpiration. Non-succulent large-leafed plants in the desert environment would overheat and be killed. Desert gardeners know that tomatoes will burn in full desert sun even if well watered; their leaves are just too big to stay cool. Desert plants that do have large leaves produce them only during the cool or rainy season or else live in shaded microhabitats. There are a few mysterious exceptions, such as jimson weed (Datura wrightii) and desert milkweed (Asclepias erosa). Perhaps their large tuberous roots provide enough water for transpiration even when the soil is dry."
 Source: http://www.eoearth.org/article/Adaptations_of_desert_plants?topic=58074
Further Reading
Plant Ecology of the Sonoran Desert Region, Arizona-Sonora Desert Museum.
Desert Plant Survival, Desert USA.
Phillips, S.J. and P.W. Comus (eds.) 2000. A Natural History of the Sonoran Desert. Arizona-Sonora Desert Museum Press, Tucson, and University of California Press, Berkeley.