Radiative cooling and latent dew formation in plant canopies
Latent, or occult, pathways of water inputs often occur in coastal arid systems and rainforests. Dewfall and fog inputs in arid systems— while not large— can be a proportionally large fraction of total water inputs. In temperate rainforests, regular fog events relieve what would otherwise be significant summer droughts. Tropical montane rainforests receive a significant proportion of plant water from direct interaction with clouds that pass over mountain ridges. In all of these systems, plant canopies are wetted by this occult precipitation and the direct uptake of water by leaves and/or branches has been shown to occur.
We propose that foliar uptake of water, and the subsequent amelioration of water stress, may be much more widespread than previously thought. Previous work at the leaf and stand level has shown both foliar uptake capacity and latent dew formation in tree species that are not typically thought to experience occult precipitation. From canopy-energy-balance considerations, we hypothesize that the natural tendency of plant canopies to radiatively cool at night could lead to latent dew formation in plant canopies; dew formation that occurs in the canopies alone and may not be measured by standard leaf-wetness sensors.
We show through energy balance modeling that tree canopy temperatures can often reach dew point temperatures at night, at which point dew forms on the canopy only. Through empirical work, we show that tree-canopy temperatures are typically lower than ambient air temperatures at night and, importantly, often remain near or below the ambient dewpoint temperature. Using infrared thermometers and thermocouples, a Balsam Fir canopy (Whiteface Mountain, NY) was nearly always below ambient nocturnal air temperatures, and within 0.5 °C of dewpoint temperatures for 50 percent of nights during the growing season. Using thermal imaging cameras, we show similar support for Douglas Fir trees in the Pacific Northwest and we will have new data from coniferous and deciduous broadleaf trees for the early summer of 2015.