Forested riparian area response to upland alternative forest management

Background/Question/Methods

Riparian forests provide unique ecosystem functions as a transition between upland and aquatic habitats. Reflecting their ecological importance, riparian buffers are implemented to protect these functions from management. However, many second-growth forests including riparian areas in western Oregon, USA, were regenerated as dense Douglas-fir plantations and may not be on an efficient successional trajectory towards complex old-growth forests. Management techniques to accelerate the development of old-growth characteristics are tested in uplands but buffer regulations prevent management in riparian zones. Leaving simplified riparian vegetation structures to develop on their own may negatively impact ecosystem function in the future. Our study utilizes an operational scale alternative management experiment, the Density Management and Riparian Buffer Study to investigate the impacts of upland forest management on the growth and physiological response of trees in riparian buffers. We collected increment cores from trees along a riparian upland-transect extended into three different thinning treatments; 99 trees per hectare, 0.4 ha gaps and an unthinned control. We compared annual growth and carbon (δ¹³C) to investigate: (1) the impact of upland management on riparian trees, (2) a possible decoupling of the climate-growth relationship and, (3) the water-use efficiency of trees along the riparian-upland transition.  

Results/Conclusions

Trees within 15 m of the treatment edge responded to an apparent edge effect downslope of thinned areas but not of gaps. The trees’ response were not affected by slope or topography. Initial δ¹³C values indicate that trees within the thinned uplands had higher discrimination (Δ¹³C) indicating lower water use efficiency. Trees close to the stream also showed an increase in Δ¹³C and diameter growth compared to trees farther away from the stream in upland areas. However, we did not find any significant relationships between drought severity and tree growth. Our results indicate that upland management can be used to influence the growth and water use efficiency of trees growing within riparian buffer zones. Trees in our study did not appear water-stressed even in drought years so testing whether management led to higher adaptability to drought was inconclusive. We also suggest that stream microclimate conditions, potentially including access to vadose zone water increase the growth of trees near the stream. Further study is needed, including the use of the stable isotopes of oxygen and local climate drivers to make robust conclusions on the physiological responses of trees in riparian zones to upland management.