1.6 THE ROLE OF EPIPHYTES IN THE INTERCEPTION AND EVAPORATION OF RAINFALL IN OLD-GROWTH DOUGLAS-FIR FORESTS IN THE PACIFIC NORTHWEST T G Pypker 1 , B J Bond and M H Unsworth Oregon State University, Corvallis, Oregon 1. INTRODUCTION Rainfall interception loss (I n ) accounts for 10 to 40% of rainfall entering a forest canopy (Zinke, 1967). The size of I n depends on two variables: the canopy water storage capacity (S) and evaporation during the storm (E) (Gash, 1979). Changes in either S or E will impact the quantity of water available for soil recharge, plant water uptake and the discharge of streams and rivers. Structural changes in a forest canopy may influence the size of S. For example, S is very large in old-growth Doulglas-fir forests in the Pacific Northwest relative to a young Douglas-fir forest forests (Pypker, unpublished data; Link et al., in press). Old-growth Douglas-fir forests have a high leaf area index (LAI) and large epiphyte populations (McCune, 1993; Thomas and Winner, 2000). Could the large S associated with old-growth Douglas-fir forests result from high LAI and/or large epiphyte populations? Researchers frequently use the LAI of a forest to estimate S (e.g. Flerchinger et al., 1996). This may be a reasonable assumption for some forests because of the high surface area associated with leaves and needles. However, LAI is not always appropriate for estimating S. For example, the use of LAI to predict S in some tropical and temperate forests has been shown to be inadequate (Herwitz, 1985; Link et al., in press). To properly assess the magnitude of S, we must incorporate other factors. The use of LAI to estimate S is inappropriate for Douglas-fir forests in the Pacific Northwest (Link et al., in press). Young (25-y-old) and old growth (>400- y-old) forests can have similar LAI but very different S. For example, a young and an old-growth Douglas- fir forest in South Central Washington have nearly identical LAI (old-growth 9.6; young – 10.1), but S in the old-growth forest is more than double that of the young forest (old-growth 3.32 mm; young 1.26 mm) (Pypker, unpublished data; Link et al., in press). One factor that may explain the high S in temperate old- growth Douglas-fir forests is epiphytes. Epiphytic lichens and bryophytes have high water-holding capacities and are abundant in temperate old-growth Douglas-fir forests (Kershaw, 1985; McCune, 1993; Shaw and Goffinet, 2000). Furthermore, lichens and bryophytes can store between 200 to 1500% of their dry biomass in water (Kershaw, 1985; Shaw and Goffinet, 2000). The large populations of epiphytes in conjunction with their 1 Corresponding author address: T G Pypker, Department of Forest Science, Oregon State University, Corvallis, OR, 97331, USA; E-mail: pypkert@oregonstate.edu high water-holding capacity may be sufficient to explain the elevated S in old-growth Douglas-fir forests. However, epiphytes may also indirectly influence S. Lichens and bryophytes may further affect S by altering the time required for the canopy to dry. Old-growth Douglas-fir forests typically exceed 60 m in height (Shaw et al., in press). The tall trees may increase the time required for branches to dry lower in the canopy by diminishing the quantity of light and wind available to drive evaporation. If water storage by lichens and bryophytes primarily occurs lower in the canopy, S may be affected because the canopy will remain wet for longer periods. The influence of lichens and bryophytes on forest hydrology may be very important for old-growth Douglas-fir forests. The purpose of this paper is to assess the influence of lichens and bryophytes on the: • size of the canopy storage (S) in old-growth canopies • time required for the canopy to dry 2. METHODS AND INSTRUMENTATION 2.1Study site The study area is located in the Western Cascades within the boundaries of the H J Andrews Experimental Forest (44.2 °N, 122.2 °W). The study site is approximately 2 ha in size, is comprised of old- growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and western hemlock (Tsuga heterophylla (Raf.) Sarg.) and has an LAI of 12.1 (Moore et al., 2004). The rainfall occurs primarily in the winter and averages 2300 mm annually. 2.2 Biomass and distribution We estimated epiphytic lichen biomass using an established 1:100 relationship between the quantities of epiphytic lichens littered on the forest floor to the biomass of epiphytic lichens in the canopy (McCune 1994). In brief, we randomly established 27 circular plots (4 m diameter) within the study area and collected all epiphytic lichen fragments found in the plot. The lichens were sorted into two functional groups: foliose lichens (plate-like structure) and fruticose lichens (hairy structure). The lichens in each plot were oven dried at 70°C for 72 h and the mean biomass for all plots were multiplied by 100 to estimate the epiphytic lichen biomass. Epiphytic bryophyte biomass is more difficult to estimate and generally requires destructively harvesting trees (McCune, 1993). We were not permitted to harvest trees in this study area, so we used estimates of epiphytic bryophytes from nearby old-growth Douglas- fir forests within the H J Andrews Experimental forest