Climate Change and Feedbacks from Vegetation Succession: Evidence from Long-Term Ecological Research Sites

Abstract

Long-term ecological research (LTER) sites, many of which are U.S. Forest Service Experimental Forests, have unique, long-term records of climate, streamflow and vegetation that illustrate many aspects of vegetation effects on hydrology and suggest implications for climate change. Because vegetation, and vegetation water use, are changing over time, changes in streamflow and the climate:streamflow relationships are not sufficient to demonstrate climate change. This study investigates the hypothesis that vegetation change effects on streamflow can be quantified and separated from pure climate change effects on streamflow. Long-term paired watershed experiments involving forest removal and regrowth have been conducted at sites in the eastern and northwestern United States. These experiments show that streamflow increases after forest removal, and then decreases as forests regrow. However, the rates and timing of these changes depend upon the type of vegetation (deciduous or evergreen forests), the climate (seasonally concentrated v. year-round rainfall), and the history of disturbances (fire, hurricanes, pests and diseases, prior forest harvest or agriculture) at the sites. Detailed examination of vegetation water use at these sites indicates that transpiration rates of trees also vary by the type of vegetation (broadleaf or conifer), the age of the tree (young plantation v. old-growth), and climate (wet/dry seasons, wet/dry years). Detailed study of hillslope hydrology at these sites shows that the transport of water through vegetation to soils and thence to streams depends upon preferential flow pathways that are in part related to soil depth, slope angle, and changes in permeability with depth. These findings imply that as vegetation undergoes succession, water fluxes to the atmosphere and streams will change. This study utilizes streamflow and climate records from multiple LTER sites to examine the changing relationship between streamflow and climate over periods of up to 6 decades in the course of forest vegetation succession. Results indicate that in early succession (0 to 50 yr-old stands) vegetation water use increases rapidly and is decoupled from water availability, whereas in old-growth stands (450 to 500 yr-old stands) vegetation water use may decline and is coupled to water availability. Understanding how vegetation responds dynamically to climate fluctuations over decadal time scales improves predictions of climate change effects on streamflow over very long time scales.