|Abstract or Summary
- This study quantified the magnitude and timing of summer streamflow deficits in paired-watershed experiments in the Cascade Range of Oregon where mature and old-growth conifer forests were subjected to clearcutting, patch cutting, and overstory thinning treatments in the 1960s and 1970s. Hydrologic effects of clearcutting, small-patch cutting, and overstory thinning in the mixed conifer/brush zone were studied (1 watershed (WS) each) in the Coyote Creek WS of the South Umpqua Experimental Forest at 42° 1' 15"N and 122° 43' 30"W. Hydrologic effects of clear cutting (3 WS), shelterwood cutting (1 WS), patch cutting (1 WS), and young forest thinning (1 WS) were examined in the Tsuga heterophylla zone at the H. J. Andrews experimental forest at 44° 14' 0"N and 122° 11' 0" W. Climate of both sites is marine west coast with winter precipitation and dry summers, producing minimum streamflows in August and September. Changes in flow frequency distributions were detected by counting days below streamflow thresholds where the thresholds were established using percentiles from pre-cutting streamflow records. Changes in relative streamflow were established by the station pair method. Summer streamflow deficits were largest and most persistent in 35 to 50-year-old forest plantations created from clearcutting and shelterwood cutting in the 1960s and 1970s. Summer streamflow deficits were smallest and most ephemeral in a stand that experienced 50% overstory thinning in 1971. Summer streamflow deficits of intermediate size and persistence developed in watersheds in which 25 to 30% of the area had been patchcut in the 1960s or 1970s. A sparse (12%) precommercial thin of a 27-year-old stand exhibiting summer streamflow deficits had comparatively little effect on streamflow deficits. Streamflow deficits emerged as early as March or April and persisted into October and November in the warmer, drier site in southern Oregon (Coyote Creek), whereas summer streamflow deficits were restricted to July through September in the cooler, wetter Andrews Forest. These findings are consistent with previous studies demonstrating (1) increases in water use in certain conifer species relative to others (e.g. Douglas-fir versus pine); (2) higher water use in young (i.e., 10 to 50-yr-old) compared to old (100 to 250-yr-old) stands of many tree species; and (3) decreased interception capacity of young relative to old forest stands associated with loss of canopy epiphytes. Results appear to be robust, despite gaps in data availability, uncertainties associated with changes in stream gauging, streamflow trends over time in control watersheds, and multi-decadal fluctuations in regional climate over the study period. These findings support the notion that variable-intensity logging prescriptions over small areas to approximate natural forest structure may have the least effect on summer streamflows. However, more research, preferably new paired watershed experiments, is needed to quantify the magnitude and duration of summer streamflow effects from various levels of overstory and understory thinning treatments.