The role of epiphytes in the interception and evaporation of rainfall in old-growth Douglas-fir forests in the Pacific Northwest

Large populations of epiphytes influence the hydrology and micrometeorology of old-growth Douglas-fir forests in the Pacific Northwest (PNW). Epiphytes have high water holding capacities that result in increased canopy storage of rainfall. To monitor the effect of epiphytes on canopy hydrology and micrometeorology we monitored rainfall interception and evaporation at the branch and stand scale. We installed weather stations and strain gauges in two Douglas-fir trees at 3, 24 and 45 m in an old-growth forest (>500 y-old) in Central Oregon (HJ Andrews LTER). 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 and interception loss of the entire stand we installed an array of tipping bucket rain gauges.

We found the epiphytes that are typically found in old-growth Douglas-fir forests to store 323 ±20% (fruticose lichens), 442 ±35% (foliose lichens) and 1099 ±48% (bryophytes) of their biomass in water. The old-growth canopy has 1272 ±457 kg ha-1 of lichens and an equivalent amount of bryophytes. Therefore, this old growth forest has the capacity to store approximately 1.9 mm of rainfall in epiphytes alone. The high water holding capacities of the epiphytes resulted in the canopy storage exceeding 3.5 mm of rainfall.

The high water holding capacities of the epiphytes combined with the tall canopy (>65 m) retards the loss of water from branches lower in the canopy. Subsequent to a rainfall event the canopy frequently required more than 60 h to dry and the rate of evaporation depended on canopy position. The upper branches dried rapidly (45 m) and are typically dominated by lichens and the lower branches (3 m) dried slowly and were typically dominated by bryophytes. Hence, the time required for branches lower in the canopy to dry was prolonged by the diminished energy (windspeed and solar radiation) available for evaporation deeper in the canopy and the higher water holding capacity of the bryophytes. Therefore, the high water holding capacities of epiphytes in old-growth Douglas-fir forests increase the canopy storage and extend the period the canopy is wet.