Comparing streambed light availability and canopy cover in streams with old-growth versus early-mature riparian forests in western Oregon

Abstract

Light availability strongly influences stream primary production, water temperatures and resource availability at the base of stream food webs. In headwater streams, light is regulated primarily by the riparian forest, but few studies have evaluated the influence of riparian forest stand age and associated structural differences on light availability. In this study, we evaluated canopy cover and streambed light exposure in four second-order streams within paired reaches of primary old-growth versus second- growth mature riparian forests. Stand age class was used as a proxy here for canopy complexity. We estimated stream canopy cover using a spherical densiometer. Local streambed light exposure was quantified and compared within and between reaches using fluorescein dye photo- degradation. Reaches with complex old-growth riparian forests had frequent canopy gaps which lead to greater stream light availability compared to adjacent reaches with simpler second-growth riparian forests. We quantified light exposure at relatively high resolution (every 5 m) and also found greater variability in stream light along the reaches with old-growth riparian forests in three of the four streams. Canopy gaps were particularly important in cre- ating variable light within and between reaches. This work demonstrates the importance of the age, developmental stage, and structure of riparian forests in controlling stream light. The highly variable nature of light on the stream benthos also highlights the value of multiple measurements of light or canopy structure when quantifying stream light.