The role of epiphytes in rainfall interception by forests in the Pacific Northwest. I. Laboratory measurements of water storage

Abstract

Old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests frequently contain large populations of epiphytic lichens and bryophytes. To determine the effect these epiphytes have on canopy hydrology we measured the maximum water fraction (f(x)max; maximum mass of internal and external water stored by an epiphyte divided by its tissue dry mass) of common lichens, bryophytes, and dead branches in the laboratory and the water storage and interception efficiency (pi) (the rainfall stored on a branch divided by the rainfall intercepted by the branch) of whole epiphyte-laden branches under a rainfall simulator at three intensities (11.3, 16.1, and 39.8 mm·h–1). The f(x)max values for epiphytic fruticose lichens, foliose lichens, and bryophytes were 2.2 ± 0.4, 3.4 ± 0.6, and 10.0 ± 0.5, respectively. The water stored by an epiphyte-laden branch during and after exposure to simulated rainfall could be predicted if the biomass of epiphytic lichens and bryophytes on the branch was known (R2 = 0.8, p value < 0.0001). For all three rainfall intensities, the branches required >6 mm of rainfall to saturate. Values of pi averaged between 0.5 and 0.7 after 2 mm of rainfall and did not differ among the three intensities (all p values > 0.05). We conclude that epiphytes increase the canopy water storage of a typical old growth Douglas-fir forest by >1.3 mm.