Conclusion
Forested headwater streams drain much of the land area in North America (Colvin et al., 2019) and are a major contributor of C to the atmosphere (Cavallaro et al., 2018). Regeneration of pine stands following harvest of old-growth forests is a common land cover change in the western United States (Collins et al., 2011) that will have lasting consequences for the quantity and composition of DOM exported from forest hillslopes and watershed C cycling. We found that the DOM exported from second-growth hillslopes was five times more reactive than that from old-growth, though the total amount of C released was ten times lower. As such, export of bioavailable C from second-growth was roughly half of that released from old-growth forest, reducing the amount of DOM available to stream microbes. The slow recovery of these forests reinforces the persistent effects of land cover change on DOM composition and reactivity in headwater ecosystems.