Characterizing short-term light dynamics in forested headwater streams

• Published in: Freshwater science Vol. 36; no. 2; pp. 259 - 271
• Main Authors: Heaston, Emily D., Kaylor, Matthew J., Warren, Dana R.
• Format: Journal Article
• Language: English
• Published: Lawrence Society for Freshwater Science (SFS) 01.06.2017; University of Chicago Press
• Subjects: Autotrophy; Canopies; Canopy; Canopy gaps; Computer simulation; Dynamics; Forest ecosystems; Forests; Headwaters; Light; Locations (working); Mountains; Primary production; Rivers; Sensors; Short term; Streams; Temperature; Terrestrial ecosystems; Terrestrial environments; Time of use; Understory; transient offset light; riparian forest; forest age; sunfleck; HJ Andrews Experimental Forest; stream light
• ISSN: 2161-9549; 2161-9565
• DOI: 10.1086/691540

Light controls local- and reach-scale primary production in forested headwater streams. In terrestrial ecosystems, short periods of elevated light beneath canopies (sunflecks) are instrumental in maintaining understory autotrophic production, but sunflecks are unexplored and uncharacterized in stream systems. We described short-term light dynamics in 4 forested headwater streams in the western Cascade Mountains of Oregon. We quantified the prevalence and characteristics of sunflecks, compared the timing of light exposure at stream sites to a sensor in full sun, and evaluated whether sunfleck characteristics differed between stream sections in old-growth vs second-growth forest. Sunflecks were common in streams. A minimum of 3 individual sunflecks/d occurred at each of 47 sensor locations. Short sunflecks were more common than long sunflecks, a result consistent with findings in terrestrial studies. However, streams had longer sunflecks than upland forests in this region, probably because canopy gaps are larger over stream channels than in the forest. In >½ of the 47 sensor locations, daily light distributions differed between in-stream and full-sun locations, revealing that canopies affect both the total light and the timing of light delivery to understory locations. The timing of peak stream temperature coincided better with light delivery by sunflecks than in full sun. Peak stream temperature was offset by ≥3 h from midday peak light in full sun, but beneath the forest canopy at the 4 surveyed streams, 26 to 60% of sunfleck events occurred within 2 h of peak stream temperature. These findings highlight the presence and potential importance to in-stream autotrophy of short-term light events (sunflecks) in forested streams. In a model simulation, a 1-h sunfleck increased stream primary production 5 to 17%, depending on the time of day that sunfleck occurred.