The importance of canopy structure in controlling the interception loss of rainfall: Examples from a young and an old-growth Douglas-fir forest

The canopy water storage capacity (S), direct throughfall fraction (p), the ratio of evaporation to rainfall intensity (View the MathML source) and interception loss (In), of a Douglas-fir forest are influenced by short (seasonal) and long-term (decades to centuries) changes in the forest canopy. Gross precipitation (PG) and net precipitation (Pn) were measured in a young (25-year-old) Douglas-fir forest and the results compared with measurements previously made in a nearby old-growth (>450-year-old) Douglas-fir forest [Link, T.E., Unsworth, M.H., Marks, D., 2004. The dynamics of rainfall interception by a seasonal temperate rainforest. Agric. Forest Meteorol. 124, 171–191.]. Canopy rainfall variables were estimated using a regression-based method that estimates S, p and View the MathML source for individual storms using the relationship between PG and Pn. The individual storm estimates of S, p and View the MathML source for the young forest were applied to a rainfall interception model (Gash model [Gash, J.H.C., 1979. An analytical model of rainfall interception by forest. Q. J. R. Meteorol. Soc. 105, 43–55.]) to determine the effect of seasonal changes in canopy hydrologic variables have on estimates of In (young forest only). The Gash model was previously applied to the old-growth forest [Link, T.E., Unsworth, M.H., Marks, D., 2004. The dynamics of rainfall interception by a seasonal temperate rainforest. Agric. Forest Meteorol. 124, 171–191.]. The young forest had significantly different S (1.40 mm ± 0.27) and p (0.12 ± 0.07) relative to the old-growth forest (S = 3.32 ± 0.35; p = 0.42 ± 0.07). Seasonal variation in canopy structure, such as deciduous leaf senescence and coniferous needle drop, were correlated with decreases in S. The differences in S and p between the two forests resulted in an In that was only slightly larger in the old-growth forest because the View the MathML source for the two forests were similar (young = 0.18 ± 0.06; old-growth = 0.17 ± 0.08). View the MathML source in the young and old-growth forests may have been similar because developmental changes associated with old-growth forest may alter the external resistance (ra) and the effective area for evaporation. The Gash model successfully predicted In for the young forest on a seasonal basis (3.29% error), but experienced larger errors (range = −91 to 36% error) for individual storms. The seasonal error and the error for individual storms improved when seasonal variations in canopy characteristics were incorporated in the model (seasonal error = 2.37%; individual storm error range = −12.0 to 21.7%). Therefore, short-term (seasonal) changes in phenology and long-term (decades to centuries) horizontal and vertical development of the forest canopy influence S, p, In and View the MathML source of Douglas-fir forests.