A new report out from (*gasp*) the White House (well, their climate change science program) details possible effects of climate change- on agriculture, and biodiversity. From a ScienceNow Daily news article (Climate Change is Bad News for U.S. Agriculture):
"'We risk losing iconic charismatic megaflora such as saguaro cactus and joshua trees,' co-author Steven Archer of the University of Arizona, Tucson, said at a press conference."
I do appreciate the upgrade of cacti to the ranks of such cliche charasmatic fuzzy icons as polar bears. (I wouldn't suggest hugging a cacti though, or a polar bear, for that matter). But such a "doomsday" lead is common in the media reporting of climate change, but the media could do better to explain these effects to the public, albeit about a psychologically taxing subject. I think I know why they predict the decline of desert species, but to the untrained reader, you'd think, well, why would desert species decline if it's going to get hotter? Why does that mean drier necessarily? In fact, these factors are important, but it may be the (unmentioned) carbon dioxide increase that actually matters most here.
This article is apropos today for me, since today is dedicated to studying up on arid-land-adapted grasses. These species have a special photosynthetic mechanism (the C4 photosynthesis pathway) that gives them a competitive advantage in drier areas (they can use water more efficiently than "normal" C3 species). Will this be the rise of C4 plants? I'm sure there are plenty of articles about this out there. Mean temperature during the growing season is highly correlated with their growth, and a higher summer temp should give these an advantage, as long as there is still summer rain to be had (otherwise, the summer rain growing season will be nill and the growing season will be in the winter, when other species have an edge).
However, cacti, like the saguaro also have a special photosynthetic mechanisms, Crassulacean Acid Metabolism, or CAM. Like C4 plants, they are able to save water by using a different pathway. Cacti open their stomata (pores) at night, taking up CO2 when it's cool, and closing them during the day to prevent water loss. The advantage for both of these species is that they are more effective in the uptake of C02 at higher temperatures.
We all know that climate change will have global effects, but it is important to remember that these effects are not linear increases and decreases in rainfall and temperature. Ecosystem feedbacks may cause differential regional effects. For these plants, if they become dominant, evapotranspiration will be less over large areas (they don't lose as much water to the air as others). Therefore, increasing temperature, which may shift the distribution and local floras, may feedback (or forward?) and cause a simultaneous decrease in rainfall due to less moisture coming up and out of the plants.
HOWEVER, the decline of saguaro cacti might actually happen do to a loss of its competitive -C02-capture -advantage. Higher carbon dioxide concentrations actually will favor the "normal" plants over these plants. I won't go into the details... (even I recognize there is a limit to the utility of a full explanation)
Not exactly first page material. But I am glad to have a new buzzword... iconic megaflora!
