Quantifying shortwave and longwave radiation inputs to headwater streams under differing canopy structures

The extent to which riparian understory vegetation in harvested areas can partially or fully negate changes in incoming shortwave and longwave radiation was investigated along 1st-order headwater streams. Despite similarities in shade provisioning, others have noted increased stream temperatures in post-harvest understory-only canopies, relative to mature forest canopies. To advance the process understanding of how management of riparian vegetation affects stream energetics, incoming shortwave and longwave radiation was measured directly at the stream surface for 6 stream reaches in a mid-latitude, temperate, montane, forested watershed with strongly contrasting riparian canopies. Radiometer arrays were installed during mid-summer, clear-sky conditions underneath combined tree and understory vegetation canopies within non-impacted forested reaches (references), a partial-cut reach (∼50% overstory canopy removal), and two recently (∼5 year.) clear-cut reaches with contrasting densities of understory vegetation. Results indicate that incoming all-wave radiation was 37-39% less in the three undisturbed reference reaches, compared to an open area reference. The radiative regimes in the partial-cut and dense-understory clear-cut reaches were similar to the intact forested reaches, with 37% and 39% less all-wave radiation, respectively, in comparison to the open reference location. The sparse-understory clear-cut exhibited a 16% reduction in all-wave radiation. Findings from this study indicate that some recently clear-cut and partial-cut headwater riparian ecosystems can rapidly recover (<5 years) to pre-harvest radiation levels if adequate regrowth of understory occurs. Findings clarify previous process understanding by demonstrating that a low-lying understory-only canopy does not enhance longwave radiation, relative to an intact full-canopy forest. Instead, enhanced stream heating post-harvest within an understory-only canopy is hypothesized to occur through potential increases in sensible heat input at the stream surface controlled by the lowering of the height of maximum radiative emittance after removal of the overstory canopy, possibly allowing for increased advective energy transfer from the above canopy and surrounding harvested area.