Riparian Forest Conditions Influence Food Supplies for Stream Salmonids: Managing for Increased Aquatic Productivity

Timber harvesting throughout the coastal rainforests of western North America has led to dramatic shifts in riparian plant cover along both fish-bearing and upland fishless streams.  The post-harvest, regenerating riparian forests are typically characterized by canopies that contain varying mixtures of deciduous (commonly red alder, Alnus rubra) and coniferous species, ranging from nearly pure stands of one or the other to blends of the two. These regenerating riparian forest communities play a major role in regulating food (i.e., invertebrate) quality and quantity for fishes and other invertivores, along both fishless headwater channels and larger fish-rearing streams. Red alder and its understory associates have commonly been shown to increase invertebrate abundance 2-5X where studied in Alaska and the Pacific Northwest, relative to streams bordered by coniferous species. And these invertebrates (both terrestrial and aquatic) fall prey to stream fishes, directly through allochthonous infall from riparian habitats bordering fish-bearing tributaries, and indirectly via transport (invertebrate drift) from small, fishless, headwater streams. Maintaining an alder blend in regenerating riparian forests for food production while ensuring adequate supplies of large wood from conifers for stream habitat offers a strategy and opportunity for protecting or even augmenting aquatic and fish productivity in riverine systems impacted by past timber harvesting.