Canopy structure has a significant impact on the canopy hydrology of
Douglas-fir forests in the Pacific Northwest (PNW). Whole canopy rainfall
interception was measured for young Douglas-fir forest and compared to an
old-growth Douglas-fir forest. The old-growth forest had significantly greater
canopy water storage capacity (5) and direct throughfall fraction (p). However,
the interception loss (la) for the old-growth forest was only slightly larger than
the young forest due to the similar ratios of evaporation to rainfall intensity
(E/R). The spatial distribution of throughfall was more right-skewed in the
old-growth forest; with many locations receiving throughfall in excess of gross
precipitation (PG). Despite differences in the spatial distribution of throughfall,
the spatial distribution of soil moisture did not differ between the two forests.
Because the S was significantly greater in the old-growth forest, the influence
of epiphytic lichens and bryophytes on S was examined. The maximum water
content (MWC) of individual samples of fruticose lichens, foliose lichens, and
bryophytes from an old-growth Douglas-fir forest were measured in the
laboratory and the field. The laboratory results indicate that typical epiphytic fruticose lichens, foliose lichens and bryophytes for old-growth Douglas-fir
forests in the PNW could store 2.23, 3.42 and 9.99 times their dry weight in
water, respectively. Although these values could be used to predict the
maximum epiphyte-laden branch water storage under laboratory conditions,
they were unable to do so under field conditions. In the field the biomass on
the branch could not predict the maximum branch water storage because the
branches: 1) were partially saturated for most of the measurement period; and
2) required greater than 30 mm of rain to saturate due to preferential flow
routing water through the epiphyte mats. The frequent storms and the slow
saturation of the canopy resulted an underestimation of S and an
overestimation of E/R by standard regression based techniques for estimating
canopy variables. Lastly, the water stored on epiphyte-laden branches after
exposure to natural rainfall was positively associated with rainfall intensity.
The absorption of atmospheric water vapor by epiphytes in old-growth
Douglas-fir forests in the PNW may have facilitated carbon uptake by green lichens and altered the energy budget of the forest during the seasonal
summer drought. During the summer months the green lichens absorb
sufficient quantities of atmospheric water to reactivate their photosystems.
The diurnal absorption/evaporation of atmospheric water will result in
significant uptake of water at night and may account for 5 to 21 % of the
canopy latent heat flux in the early morning (600 to 1000 h).
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