If a dew falls in a forest and no one is around, how do we sense it?

Background/Question/Methods

In the seminal 1963 paper, “The ecological importance of dew”, Stone reiterates the “disappointing” realization from over 200 years of research on dew that it is “rarely a limiting ecological factor”. However, dewfall can occupy an important niche in an ecosystem’s water budget between “limiting” and “irrelevant”.  The exact importance that dew has on the water cycle and, ultimately on the carbon cycle, has remained somewhat ambiguous.  This arises, in part, because dewfall generally occurs after the formation of the stable nocturnal boundary layer and is therefore rarely quantified as a latent heat flux using eddy covariance.  Some new techniques such as wetness sensors and in situ stable isotopic measurements of water vapor have the capacity to quantify the timing and magnitude of dew fluxes.  Here, data on dewfall from two forested sites in Colorado are considered by combining these two techniques.  These dewfall fluxes are considered against other more traditional ecosystem quantities such as soil moisture and carbon flux to hypothesize about its potential significance as a moisture source. 

Results/Conclusions

At both sites, dewfall is a surprisingly common phenomenon, occurring during a large percentage of the nights.  Because of the presence of strong humidity and temperature gradients that form in forest canopies under stable atmospheric conditions, its occurrence can be restricted to the ground, to the foliage or throughout the canopy.  Ground-based dew was periodically detected in near-surface soil moisture sensors, which suggest: (1) it may persist within an ecosystem despite its rapid morning evaporation from leaf surfaces and (2) it can be available to shallow-rooting understory plants.   While direct evidence of uptake of dew by leaves was not attained here, its occurrence could nonetheless be relevant in terms of impacting nighttime gas exchange across the stomata and through an effect on leaf temperatures by both warming (during condensation) and cooling (evaporation) of leaves during the night and day, respectively.  Additional studies on nighttime gas exchange under dewfall and leaf temperature dynamics would be useful to generate a more holistic perspective on the ecological significance of dew.