Cloud water interception by different types of montane vegetation in Hawai‘i

Abstract:

Cloud water interception is thought to be an important water source for fog-affected ecosystems, especially tropical montane cloud forests (TMCF). Liquid cloud droplets near the ground surface are captured by plants, wetting the canopy and increasing net precipitation under the vegetation canopy. This moisture input is separate from ordinary rainfall and is often overlooked, but studies have suggested that cloud water interception not only improves plant survival during drought, but also can increase recharge and enhance downstream water resources. Both the atmospheric conditions and the canopy characteristics affect cloud water interception. However, it is hard to evaluate the effects of different vegetation because cloud water availability is highly variable through space and time, making site comparisons difficult. To compare between sites to examine how different vegetation types influence cloud water interception, atmospheric conditions must be controlled for.

In this study, we compare cloud water interception at five TMCF sites on three of the main islands in Hawai‘i. These sites cover a range of different native ecosystems including wet montane cloud forest sites with canopy heights of approximately 20-30 meters, leeward dry forest, high-elevation sub-alpine shrub, and elfin cloud forest bog. At each site, cloud water interception is estimated based on observations of rainfall, throughfall, stemflow, and potential evaporation, using a single layer canopy water balance model which keeps track of the canopy water storage at each time step. The atmospheric factors, including cloud water flux based on a Juvik-type fog gauge, leaf wetness, and other meteorological variables, were also measured at each of the five study sites. We examine the variation in cloud water interception due to vegetation structural differences by comparing the cloud water interception at each site normalized by the meteorological conditions.