Maintenance of hydrologic function and fish habitat

Highlights

  • Old growth forests contribute to salmon and trout survival because they provide a continuous source of energy to streams from leaves which supports stream food webs; they contribute large, fallen trees to streams and rivers that create channel structure (e.g., pools) and instream cover; they ameliorate temperature extremes and flood peaks in streams and rivers; and they hold together soil and rocks, controlling sediment input and stabilizing stream channels.
  • Logging and road building disturbances, whether within the riparian zone or in the broader watershed area, can result in detrimental impacts to stream hydrology, stream ecology, and fish habitat via increased erosion, fine sediment inputs and more landslides.
Many Vancouver Island rivers and streams provide important spawning and rearing habitat for salmonids including chinook, pink, chum, sockeye, coho, and steelhead trout. Old growth forests in the San Juan and Gordon River watersheds near Port Renfrew contribute to these species’ survival (Morton et al., 2021, and references within).

Logging and road building disturbances, whether within the riparian zone or in the broader watershed area, can result in detrimental impacts to stream hydrology, stream ecology, and fish habitat, as summarized below based on Hicks (2002) and, where noted, Franklin et al. (1981).

Stream Ecology

Streams flowing through old growth forests are distinctive from those in younger stands. The old growth forest landscape plays several ecologically crucial functions:
  1. it provides a continuous source of energy to streams from leaves which supports stream food webs;
  2. it contributes large, fallen trees to streams and rivers that create channel structure and instream cover;
  3. it ameliorates temperature extremes and flood peaks in streams and rivers; and
  4. it holds together the soil and rocks, controlling sediment input and stabilizing stream channels.
When riparian forests are cut down, the local stream ecology changes dramatically, and effects are generally detrimental to the stream and the fish that inhabit them.

Forest litter is the main energy base for small to medium-size streams in old growth forests, with large streams depending more on algae than litter for their energy base (Franklin et al., 1981). Riparian vegetation provides leaf inputs to the stream, which begins a food chain. Debris may be fully utilized by the biotic community within a stream reach or may be exported downstream to feed larger streams with a partially prepared food resource. In old growth forest streams, 60-70% of the annual organic inputs are retained long enough to be biologically used by stream organisms (Franklin et al., 1981). Large dams of woody debris resulting from falling trees sieve and deposit fine organic matter, allowing time for microbial colonization and eventual consumption of the material by aquatic insects and other invertebrates. These aquatic invertebrates form the food source for juvenile salmon and other stream vertebrates, including trout, sticklebacks, sculpins, tailed frogs, and salamanders. Terrestrial invertebrates falling from the riparian vegetation are also an important food source for fish. Mammals and birds then feed on the fish. This food chain is disrupted if riparian vegetation is removed.

Stream temperature, which impacts fish health and survival, is also changed when riparian forest cover is removed. This may be of less concern in cool environments where pre-logging stream temperatures are far from detrimental thresholds for fish.

Fish Habitat

Large wood in streams creates pools and beds of trapped gravel and fine sediments that are important habitat for numerous stream and stream-margin organisms (Franklin et al., 1981) including habitat for overwinter survival of fish such as Coho salmon. The amount of large wood in streams declines rapidly with the removal of adjacent old-growth forest, and even the largest wood from second growth forest is smaller, more mobile, and decays faster than wood from old forests. Without large wood, sediments and beds are less stable and less organic matter is stored. Logs are an important source of energy and woody debris is the major source of the nitrogen supply of a stream.

Stream in an old growth forest (Photo by Morten Rand Hendriksen / Flickr)

Stream Hydrology

Logging of riparian old growth affects stream structure and functions, but upstream logging and road building also affects stream hydrology. Numerous studies show that forest disturbance in the watershed as a whole changes the volume and timing of water flow to the stream. In coastal temperate rainforests, where rainfall can exceed 3000 mm yr-1, changes in stream hydrology following disturbance can be particularly large. The immediate response to tree removal is increased runoff, both in high and low flow conditions, due to less water loss through evapotranspiration. As well, snowpacks are greater in clearcuts than in forested areas resulting in greater streamflow during snowmelt. Flood flows can be increased by logging, especially in areas where rain-on-snow generates peak flows.
Rain falling on exposed soil surfaces where trees have been removed can result in fine sediment (sands and silts) inputs to streams. Fine sediment clogs the spaces between the gravels that incubating eggs rely on for transport of oxygen-carrying water, thereby smothering the eggs. Fry can be entombed in the gravels, and fine sediment also consolidates spawning material, making excavation impossible for the adult salmon. Studies in British Columbia, Oregon, and Idaho have shown declining emergence from eggs and declining salmonid abundance associated with sediment increases after logging.
Floods, debris avalanches, debris torrents, and windthrow are part of the natural environment yet intact forests generally have much less erosion and mass wasting compared to following timber harvesting. Once a forest is harvested, root systems decay and weaken. In coastal BC, Douglas-fir roots have been found to lose 50% of their tensile strength within five years after cutting. It may take 15 years of regrowth after clearcutting to regain 50% of the soil stabilization supplied by an old growth forest, and 26 years until soil strength is fully regained. When intense rainstorms occur in the critical period between root decay and re-establishment of root strength in the regenerating forest, the landscape is particularly vulnerable to mass wasting. In extreme cases, mass wasting can yield so much gravel and cobble substrate that the transport capacity of the stream is overwhelmed, and the stream ceases to flow above ground. A study in Oregon found that landslides occurred up to 27 times more frequently in clearcuts or near roads than in forested areas. Similarly, on Haida Gwaii, logging increased the frequency of landslides by a factor of 34. Hillslope steepness is one of the simplest indicators of areas prone to debris slides and flows following logging.
Road building associated with timber harvest not only acts as a primary source of fine sediments through mass wasting and erosion but logging roads can also increase fine sediment deposition by disrupting subsurface groundwater flows, leading to more frequent extreme flow events. Additionally, logging roads can alter streamflow patterns and create barriers to fish migration. Culverts that are poorly designed, installed, or maintained can prevent upstream migration.