Most plants experience frequent
canopy wetting events in the form of dew, rain, or fog condensation. Yet, the
influence of these wetting events on the movement of water along the
soil-plant-atmosphere continuum (SPAC), as well as on physiological
performance, are poorly understood. We investigated how canopy wetness
influences plant water status and photosynthetic gas exchange by comparing
pre-dawn and midday leaf water potential, leaf-level gas exchange, and
chlorophyll fluorescence of sapling Sequoia sempervirens trees. Plants exposed to dry soil, with and without
overnight canopy fog interception while preventing direct watering via drip
showed increased predawn and midday leaf water potential above the well-watered
and dry-down treatments. Further,
midday carbon assimilation and stomatal conductance to water vapor were
higher in saplings receiving intercepted fog water. We found no differences in dark-adapted leaf chlorophyll
fluorescence among treatments. The effect of canopy interception on water
relations of mature S. sempervirens trees
along a gradient in fog intensity was also evaluated and indicated that canopy
interception of fog increased leaf water potential above the maximum leaf water
potential predicted by the gravitational potential gradient of 0.01 MPa m-1.
These results indicate that canopy interception and direct water absorption
represents an important and quantifiable water subsidy that may temporarily
decouple plants from their soil water resource. Future models of plant and
ecosystem hydrology in habitats receiving regular canopy wetting events should
incorporate this phenomenon.