PARENT SESSION
Oral Session 81: Forest Ecology IV: Seeds, Growth, and Recruitment.
Presiding: J Kush
Wednesday, August 4, 1:30 PM to 5:00 PM, Meeting Room C 120.
The influence of epiphytes on the hydrology of old-growth Douglas-fir forests in the Pacific Northwest.
Pypker, Thomas*,1, Bond, Barbara1, Unsworth, Michael1, 1 Oregon State University, Corvallis, Oregon, USA
ABSTRACT- Colonization of forest canopies by epiphytes may significantly impact the hydrology of a forest by altering rainfall interception and storage. Hence, the influence of epiphytes may be of importance in old-growth Douglas-fir forests of the Pacific Northwest because they typically have large epiphyte populations. Traditionally, research in these forests has focused on the impact of microclimate on the distribution of epiphytic lichens and bryophytes; however, these species are not merely a product of their environment. The high water holding capacities of epiphytes may have important implications for the hydrology of individual stands to the entire watershed. We installed weather stations and strain gauges in two Douglas-fir trees at 3, 24 and 45 m to measure rainfall interception by epiphytes in an old-growth forest (>500 y-old) located in Central Oregon. The strain gauges were loaded with branches laden with epiphytes and the weights were continuously recorded. We compared the in situ measurements of rainfall interception by epiphytes to rainfall interception under a rainfall simulator at varying intensities. To quantify the canopy storage of the entire stand we installed an array of tipping bucket rain gauges. We found fruticose lichens, foliose lichens and bryophytes typically found in these old-growth forests to store 323 ±20%, 442 ±35% and 1099 ±48% of their mass in water, respectively. Prior to saturation, lichens and mosses retain >90% of the rainfall they intercept at intensities typical for the Pacific Northwest (1-5 mm h-1). The canopy storage of this old-growth forest is >3 mm and twice the canopy storage of a Douglas-fir forest with a similar leaf area index, but void of epiphytes. Given that this old-growth forest requires >10 mm of rain to saturate the canopy and summer storms are generally less than 15 mm per storm, the high canopy storage may exacerbate the summer drought. The old-growth canopy's high gap fraction and large canopy storage results in spatially variable throughfall; with many locations receiving quantities of throughfall equal to or greater than the gross precipitation and others consistently receiving very little. Therefore, epiphytic lichens and mosses alter the hydrology of a forest by increasing the canopy storage and the spatial variability of throughfall.
Key words: epiphytes, hydrology, old-growth Douglas-fir, rainfall interception