Air

Continuing with my elemental theme of recent posts, when air is mentioned then the atmosphere ought to be one of the first things to come to mind. And it is. But more than just providing essential nutrients /elements /chemicals such as oxygen (for respiration) and carbon dioxide (for photosynthesis), it also provides nitrogen (which is made available to plants after its fixation and oxidation to such compounds as nitrate) and oxides of sulphur that help to provide essential sulphates (when the acid rain rains down). But the air contains more than those compounds and in some environments can be an important source of water, too, e.g. cloud forests. Almost in an act of faith – well, I knew I’d read it somewhere, but couldn’t remember where – and over several years, I have told my students that clouds (colloids of liquid water in a gaseous phase) in cloud forests were a major – and direct – source of water for the trees in that ecosystem. Fortunately, that claim was never challenged by the students (which I attribute to their humbling belief that their older and wiser plant biology mentor would not deliberately mislead them…). But, and thanks to the work of Greg Goldsmith et al., fears that any botanical sleight of hand would be discovered under even the mildest of interrogations have now been allayed because they show that cloud-based ‘leaf wetting’ does permit the direct uptake of water accumulated on leaf surfaces into the leaves themselves. So much for the ‘physiology’, as for the ‘ecology’ they also indicate that such foliar uptake improves plant water status during the dry season. At least that is the case in ‘tropical montane cloud forests’ in Costa Rica, which I’m almost certain is the particular ecosystem I had – no doubt presciently – in mind in my lectures…