Cooked spinach

The New Scientist has just published an article (A forest is born, Vol 196, no 2631, p38, 24 Nov 2007; www.newscientist.com) which addresses the evolutionary emergence of large-leaved trees, which can be said to be the majority of deciduous trees we see today (my northern hemispheric interpretation). The important point to note is that while large leaves are very efficient at photosynthesis, they are very difficult to keep cool, which they manage by release of water vapour through the stomata. Apparently, the number of stomata is regulated by a genetic switch in response to CO₂ levels in the atmosphere. Unfortunately, it appears that this is an inverse relationship: the number of stomata decrease as the CO₂ level rises. Extrapolating from this article, it may just be that we are about to render our forest canopies just so much cooked spinach.

Equally interesting is that the article notes that it was plants’ rooting ability (and thus underground C₂ sequestration) which brought about the climatic upheaval which allowed large leaves to evolve.

Perhaps disappointingly, from my point of view at least, is that the article does not suggest a temperature at which declining numbers of stomata will cause “spinach-fall”, nor whether the CO₂ levels we are expected to approach this century will cause stomatal numbers to approach this critical point. Heat and aridity certainly cause leaf drop in our young walnuts, but perhaps I have been misinterpreting the cause. The leaflets are typically yellow at the point they fall. Were they still green, I might be able to provide the data required for the “spinach” test.