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 ((E) over bar/(R) over bar) and interception loss (I-n), of a Douglas-fir forest are influenced by short (seasonal) and long-term (decades to centuries) changes in the forest canopy. Gross precipitation (P-G) and net precipitation (P-n) 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 (E) over bar/(R) over bar for individual storms using the relationship between P-G and P-n. The individual storm estimates of S, p and (E) over bar/(R) over bar 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 I-n (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 I-n that was only slightly larger in the old-growth forest because the (E) over bar/(R) over bar for the two forests were similar (young = 0.18 +/- 0.06; old-growth = 0.17 +/- 0.08). (E) over bar/(R) over bar in the young and old-growth forests may have been similar because developmental changes associated with old-growth forest may alter the external resistance (r(a)) and the effective area for evaporation. The Gash model successfully predicted I-n 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, I-n and (E) over bar/(R) over bar of Douglas-fir forests, (c) 2005 Elsevier B.V. All rights reserved.