Long-term patterns in soil moisture and revegetation after a clearcut of a Douglas-fir forest in Oregon

Adams, P.W., Flint, A.L. and Fredriksen, R.L., 1991. Long-term patterns in soil moisture and revegetation after a clearcut of a Douglas-fir forest in Oregon. For. Ecol..~lanage., 41: 249-263. Soil moisture levels during 1960-1980 were compared for two areas within a 101 ha watershed in the Oregon Cascade Range. In winter 1962-1963, the old-growth Douglas-fir (Pseudotsuga rnenziesii (Mirb.) Franco) forest in one area was clearcut. The site was then lightly broadcast-burned in September 1963. An adjacent forested area was left undisturbed as a control. In summer 1963, the upper 120 cm of soil in the clearcut averaged over 10 cm more moisture than that in the forested control. By 1967 these surpluses in the clearcut had declined to become deficits of at least 2 cm less moisture than in the control. These deficits, which were presumably caused by a rapid increase in plant cover after the light slash burn, persisted in the upper 30 cm of soil throughout the rest of the study. The fluctuations in soil moisture in the treated area are extensive enough to influence forest regeneration and watershed hydrology. I N T R O D U C T I O N In the Douglas-fir forests of the Pacific Northwest, clearcut logging and slash burning can markedly alter soil moisture. Logging temporarily reduces the plant cover, thus decreasing interception losses and moisture uptake and increasing soil moisture levels (Bethlahmy, 1963). Burning further increases moisture retention by suppressing plant survival and regrowth (Gaweda, 1983 ). In this region, slash burning is used to reduce hazardous fuels, but also to control shrubs and other vegetation that would otherwise compete with conifer seedlings for available moisture, light and nutrients (Cleary et al., ~Present address: Water Resources Division, U.S. Geological Survey, Mercury, NV 89023, USA. 0 3 7 8 1 1 2 7 / 9 l / $ 0 3 . 5 0 © 1991 Elsevier Science Publ ishers B.V. 2 5 0 P.W. ADAMS ET AL. 1978 ). A few years after a burn, however, invading vegetation may deplete soil moisture to levels comparable with those for forested areas (Hallin, 1967, 1968). Such observations indicate the value of long-term studies of how soil moisture and plant cover respond to logging and burning. In this paper we describe a comparison of soil moisture and vegetation in two adjacent areas of an oldgrowth, Douglas-fir forest in the Oregon Cascade Range. One area was clearcut and then slash-burned less than a year later. The other area was left undisturbed. Even though direct extrapolation of our results is limited by the lack of replication over a broad range of site conditions, analysis of our long-term sampling (1960-1980) has offered unique insights into nearly two decades of post-treatment response.