Every day is Dewsday in the forest…

The dew point is ‘the temperature to which a volume of humid air must be cooled… for water vapor to condense into liquid water’, and is usually an early-morning or early-evening phenomenon in nature. OK, but, ‘what is the point of dew’? That is a different question, but one which might have been answered by Michael Latakos et al. – at least in a botanical context. In their intriguing study they demonstrate that dew – ‘condensed water that forms on a solid surface’ – is generated on the bark of understorey trees in a lowland forest in French Guiana until early afternoon, because of the thermal properties of the trunks. This extensive window of hydration – up to 0.69 mm of dewfall a day – is instrumental in prolonging photosynthesis, of epiphytic crustose lichens in particular. The team propose that this phenomenon may be a more general feature of forest habitats worldwide, and that this hitherto unrecognised mechanism of midday dew formation contributes to the water supply of most corticolous (bark-dwelling) organisms. Nice work!

In addition to the article, I also recommend Michael Proctor’s thoughtful commentary thereon. Coincidentally, though, similar conclusions about the importance of dew were reached by Khumbudzo Maphangwa et al., who examined an altogether drier environment where ‘differential interception and evaporation of fog, dew and water vapour and elemental accumulation by lichens explain their relative abundance in a coastal desert’. Just as new hydrobotanical discoveries are made above ground, news of another, down below. Using neutron tomography, Ahmad Moradi and co-workers have quantified and 3-D visualized the water content in situ in the rhizospheres of chickpea (Cicer arietinum), white lupin (Lupinus albus) and maize (Zea mays). Finding that – counter-intuitively – soil water content increased towards the root surface for all three species, the team propose that plants modify the hydraulic properties of the rhizosphere’s soil in a way that improves water uptake under dry conditions. This ‘reservoir’ of water can be viewed as a reserve that helps the plants overcome short periods of drought. Hydraulic lift (sorry, redistribution – ; Rebecca Neumann and Zoe Cardon), anyone?