The food webs of small streams and the adjacent riparian forests are inextricably linked. Riparian forests contribute substantial amounts of material to small streams, including leaf litter, conifer needles, and terrestrial invertebrates. These terrestrial inputs dramatically increase the productivity of stream food webs, and can make up the bulk of energy assimilated by local stream insect and fish populations (1,2). However, recent research has shown that headwater stream networks act as conduits for the downstream movement of terrestrially-derived nutrients (3). Such materials from upstream may thus ?subsidise? several levels of downstream stream food webs, including not only fish and benthic invertebrates, but also fungi, bacteria, and aquatic plants. Indicators of the degree to which riparian management strategies directly influence the ecology of small streams, and how those influences are transmitted downstream, are needed as tools that allow managers to determine whether stream systems are being protected by current or alternative practices.
Where channels effectively retain inputs of terrestrial materials and downstream transport is limited, local ecological effects will be more pronounced while the downstream effects of riparian perturbations may be minimal. In contrast, if streams act as highly efficient conduits for the transport of materials downstream, perturbations to riparian zones upstream will have ecological effects far downstream. However, while numerous studies have shown that key geomorphological features of stream channels such as bed roughness and average channel slope influence the retention of sediment and water, we know little about the specific features of stream channels that act to retain coarse, suspended organic materials of different types (4). If the geomorphological features of stream channels act as crucial elements in the retention of terrestrial inputs, the strength of linkages between riparian forests, headwater streams, and downstream reaches will depend on the physical characteristics of the channel itself.
In coastal British Columbia, riparian management strategies that encompass timber harvesting, use by livestock, and road building are known to effect changes in riparian forests and the rates of forest-to-stream inputs to adjacent streams. The goal of the proposed project is to determine if channel geomorphology controls (a) the sensitivity of small streams to the altered rates of terrestrial inputs that result from riparian management, and (b) the extent to which these changes are conveyed downstream. The proposed project will focus on the transport and retention dynamics of three different types of terrestrial organic matter (TOM) that function as important forest-to-stream subsidies in coastal watersheds: fallen terrestrial insects, red alder (Alnus rubra) leaf litter, and conifer (Douglas-fir Pseudotsuga menziesii) needles. The project will use a three-phase design to examine how channel geomorphology mediates the ecological linkages between riparian forests and stream food webs.
In the first phase (year one), the amount of terrestrial material that enters small coastal streams with different riparian management histories will be surveyed to establish a relationship between riparian management and terrestrial inputs. If possible, the streams currently used in the FSP-funded Riparian Management Project will be included, in order to take advantage of the infrastructure and background data associated with that project. Following this, a series of 18 coastal stream reaches will be selected for use in experiments; this stream series will include channels across a range of channel gradients and bed roughnesses. Drift nets and cross-channel block nets will be used to measure the amounts of TOM in transport, to provide estimates of the distances that TOM travels before being retained, and to clarify the role of geomorphology in TOM retention. Sampling will be conducted three ...
Richardson, John S., Hoover, Trent M.. 2009. Assessing the sensitivity of streams to riparian changes: Does channel geomorphology determine how tightly forests and small streams are linked to downstream reaches?. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report. FIA2009MR433
Topic: FLNRORD Research Program
Keywords: Forest, Investment, Account, (FIA), British, Columbia
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