Old-growth forests are different from younger forests in the eastern US in regard to forest structure, and the amount of coarse wood. As a result, streams bordered by old-growth forests can be profoundly different from those in young regenerating forests. As the forest canopy develops and changes over time due to stand success dynamics, responses to climate, natural disturbance events, or planned harvests, stream structure and function will also change. The differences in canopy structure between dense young forests and late succession forests correspond to differences in availability of light, amount of organic matter, woody debris, and nutrients. Several studies of northeastern streams draining forests ranging in age from 20 to 400 years old found differences in light availability, stream functional processes (nutrient uptake), open channel metabolism, and distribution and amount of algal biomass (chl a). Results from these studies suggest a conceptual framework where forest structural characteristics influence stream function such that riparian forest that has undergone small-moderate disturbances (e.g., frequent gaps and multiple canopy layers) have similar structure to old-growth forests and may have streams with greater rates of primary production and nutrient uptake than streams flowing through forests with fewer gaps typical of middle-aged forests.
Results/Conclusions
Riparian forests with more complex canopy architecture appear to have concurrent shifts in stream ecosystem function. A general trend from multiple studies has emerged, which suggests that streams draining older forests are more productive and retentive than streams draining middle-aged forests. This trend is most likely attributed to increased light availability and large wood found in the stream channel. Studies have found light irradiance greatest in both young open forests and older growth forests compared to secondary growth middle-aged forests. Studies also suggest that streams with open or old-growth riparian forest were more similar to each (greater chl a and elevated metabolism) compared to streams with middle-aged riparian forests. Thus, as riparian forests mature, their counterpart streams may also alter their ability to retain or process nutrients, support biota, or bounce back from disturbances. Our findings suggest that the sensitivity of stream ecosystem function to altering canopy structure will depend on the function of interest but follow projections that align with our conceptual framework. Combined, the response of northeastern streams to changes in forest structure will be multi-faceted and depend on the many drivers of these ecosystems.