Does red alder (Alnus rubra) in upland riparian forests elevate macroinvertebrate and detritus export from headwater streams to downstream habitats in southeastern Alaska?

Abstract

We assessed the influence of riparian forest canopy type on macroinvertebrate and detritus export from headwater streams to downstream habitats in the Tongass National Forest, southeastern Alaska. Twenty-four fishless headwater streams were sampled monthly, from April to August 1998, across four riparian canopy types: old growth, clearcut, young-growth alder, and young-growth conifer. Young-growth alder sites exported significantly greater count (mean = 9.4 individuals·m–3 water, standard error (SE) = 3.7) and biomass (mean = 3.1 mg dry mass·m–3 water, SE = 1.2) densities of macroinvertebrates than did young-growth conifer sites (mean = 2.7 individuals·m–3 water, SE = 0.4, and mean = 1.0 mg dry mass·m–3 water, SE = 0.2), enough prey to support up to four times more fish biomass if downstream habitat is suitable. We detected no significant differences in macroinvertebrate export between other canopy types or in detritus export among different canopy types. Roughly 70% of the invertebrates were aquatic; the rest were terrestrial or could not be identified. Although we do not recommend clearcutting as a means of generating red alder, maintaining an alder component in previously harvested stands may offset other potentially negative effects of timber harvest (such as sedimentation and loss of coarse woody debris) on downstream, salmonid-bearing food webs.

Getting quantitative about consequences of cross-ecosystem resource subsidies on recipient consumers

“…For instance, Swain et al (2014) showed a positive relation of invertebrate, algae, and sculpin densities along a gradient of long-term escapement numbers of spawning sockeye salmon (Oncorhynchus nerka) in central British Columbia. Wipfli and Musslewhite (2004) and Piccolo and Wipfli (2002) demonstrated higher rates of invertebrate subsidies from upland forested streams to downstream habitats as riparian alder increased in coastal Alaska. Similarly, reductions in subsidy inputs can have negative consequences for recipient ecosystems.…”

Red Alder-Conifer Stands in Alaska: An Example of Mixed Species Management to Enhance Structural and Biological Complexity

“…These effects could lead to changes in aquatic productivity and terrestrial invertebrate inputs to streams [117]. Some riparian tree species contribute more terrestrial invertebrate mass to streams than others [118] and red alder appears to support relatively high levels of prey for fish [117,119,120]. This is significant because over half of the prey biomass ingested by juvenile salmonids in southeastern Alaska is terrestrial and originates from adjacent riparian vegetation [117].…”

“…While conifer-derived LWD plays an important structural role in aquatic systems (pool formation, sediment capture, cover for fish), smaller alder-derived woody debris and leaf litter increased invertebrate densities and diversity important as instream or downstream sources of food for fish [89]. An alder component in previously harvested fishless headwater stands may offset negative effects of timber harvest (such as sedimentation and loss of coarse woody debris) on downstream, salmonid-bearing food webs [119]. They found that young-growth alder sites exported significantly greater count and biomass of macroinvertebrates (70% were aquatic) than did young-growth conifer sites, translating up to four times more fish biomass in suitable downstream habitats [119].…”

“…An alder component in previously harvested fishless headwater stands may offset negative effects of timber harvest (such as sedimentation and loss of coarse woody debris) on downstream, salmonid-bearing food webs [119]. They found that young-growth alder sites exported significantly greater count and biomass of macroinvertebrates (70% were aquatic) than did young-growth conifer sites, translating up to four times more fish biomass in suitable downstream habitats [119]. Riparian forests with red alder included in the overstory, along with denser shrub understory may also increase abundance of terrestrial-derived invertebrates that are important prey for juvenile salmonid and other predators [117].…”

“…Riparian forests with red alder included in the overstory, along with denser shrub understory may also increase abundance of terrestrial-derived invertebrates that are important prey for juvenile salmonid and other predators [117]. Similarly, headwater streams with a greater component of alder in riparian zones exported significantly more detritus than streams with less alder [115,119]. This particulate organic matter is the driving energy source for ecosystem functioning in low-order streams [120].…”