The role of epiphytes in rainfall interception by forests in the Pacific Northwest. II. Field measurements at the branch and canopy scale

Abstract

To determine how epiphytes affect the canopy hydrology of old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests, we measured rainfall interception by individual branches and an entire stand from March 2003 to May 2004. Epiphyte-laden branches at heights of 3.1, 24.8 and 46.5 m remained partially saturated for most of the wet season and required more than 30 mm of rainfall to become saturated. We used the mean, minimum, and individual storm methods to estimate canopy water storage capacity. Canopy water storage capacity averaged 3.1–5.0 mm, but these are probably underestimates of the maximum canopy water storage capacity, because the canopy was partially saturated prior to most storm events and the saturation of the canopy was delayed by preferential flow through the epiphyte-laden branches. Contrary to expectation, the water stored on epiphyte-laden branches after exposure to natural rainfall increased with rainfall intensity because the rough three-dimensional structure of the lichen and bryophyte mats limits water loss from raindrop splash and impedes the drainage of water from the branch. We conclude that epiphytic lichens and bryophytes increase canopy water storage capacity, prolong the time required for the canopy to saturate and dry, and alter the transfer of water through the canopy.