Southern exposure research project: A study evaluating the effectiveness of riparian buffers in minimizing impacts of clearcut timber harvest operations on shade-producing canopy cover, microclimate, and water temperature along a headwater stream in northern California.

摘要

This study examines the effects of two riparian buffer widths on microclimate variables, shade-producing canopy cover, and the temperature of stream water in multiple clearcut harvest units adjacent to a fish bearing (Class I) stream in northern California. Data collected before and after timber harvest operations in years 2000, 2001, and 2002 was analyzed to determine changes in response variables to wider (175 ft.) or narrower (100 ft.) riparian buffers. Angular canopy cover was measured to be 85% at mid-stream and no less than 80% within the riparian buffer regardless of buffer width. Vertical canopy cover was measured to be 50% within the riparian buffer for each harvest unit following the first phase of timber operations. Microclimate results show that edge effects from the adjacent upslope clearcut harvest units had no discernible impact within 40 ft. of the stream bank. No difference in the extent of microclimate edge effects within the riparian zone was found for either the 175 ft. buffer or the 100 ft. buffer under very warm summer conditions. This study found that two separate timber harvest operations, conducted in summer 2000 and summer 2001, resulted in only minor (±1.5°C) changes in the water temperature pattern along the experimental reach. The monthly maximum water temperature never exceeded 21.1°C before or after harvest throughout the study area. In general, the trend of the data over the three-year study showed upstream and downstream average, minimum, and maximum water temperature response values becoming more homogenous over time. Water temperature results from this study are consistent with previous research that concluded forested buffer strips either maintain stable water temperature patterns or sustain only minimal increases in daily maximum water temperatures (2°C) after upslope timber harvesting. In this experiment, no practical difference in the canopy cover, near-stream microclimate, or water temperature patterns were found between the wider 175-ft. and the narrower 100-ft. buffers. The lack of change in response variables was likely due to the very small measurable reduction in shade-producing canopy cover mid-stream and within the riparian buffer. Only minimal changes in the near stream microclimate and water temperature occurred despite the fact that 35% of the merchantable tree volume within the riparian buffer was removed during summer 2000 timber harvest operations. Results from this study show that 100-ft. vegetative buffers that maintain at least 50% vertical or 80% angular canopy cover minimize potential negative impacts to the temperature of stream water and the near-stream microclimate from adjacent upslope clearcut harvest operations.