Forest Investment Account (FIA) - Forest Science Program
FIA Project Y081241

    Assessing the sensitivity of streams to riparian changes: Does channel geomorphology determine how tightly forests and small streams are linked to downstream reaches?
Project lead: Hoover, Trent (University of British Columbia)
Contributing Authors: Richardson, John S.; Hoover, Trent M.
Subject: Forest Investment Account (FIA), British Columbia
Series: Forest Investment Account (FIA) - Forest Science Program
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 twelve coastal streams 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 times over the course of the year (April, July, October) to obtain seasonal estimates of terrestrial inputs and transport dynamics.
In the second phase (year two), detailed experimental work will examine how the mechanisms of retention differ in headwater reaches and downstream reaches, and how the patterns of retention and breakdown are associated with specific geomorphological features of the stream bed. Releases of dyed particles of each of the three different TOM types will allow for the calculation of channel-specific retention curves. Experimental placements of artificially-created patches of different TOM types in stream channels will allow us to calculate the rates at which terrestrially-derived organic matter is assimilated into stream food webs. In the third phase (year three), physical and computer modelling (5) will be used to determine how TOM derived from riparian zones is incorporated into stream food webs across several spatial scales (from microhabitat- to watershed-scales). The experimental stream channel at Blaney Creek in the Malcolm Knapp Research Forest will be used to examine the relationship between channel bedforms and type-specific organic matter retention and breakdown. Software and expertise provided by Aquatic Informatics, Inc. of Vancouver will be used to model high-resolution discharge records for select stream channels used in the study. These modelled discharge records will be then be combined with rating curves to estimate annual variation in the transport capacity of small coastal streams.
The results of this project will provide practitioners with additional tools when prescribing appropriate levels of riparian retention. For example, many headwater streams are logged to the bank during timber harvesting. However, in-stream productivity is low in many coastal streams, and fish populations are dependent on both drifting terrestrial insects and aquatic detritivorous invertebrates as sources of food (6). The guidelines will enable managers to identify the types of channels that retain little TOM, and prescribe increased levels of riparian retention in headwater reaches in order to maintain rates of organic matter export. Alternatively, managers could implement riparian management schemes that create successional canopy types (e.g., mixed deciduous and coniferous trees) that act to maximise inputs of the TOM types that are retained at higher rates.
References (1) Webster, J.R. & Meyer J.L. (1997) J. N. Am. Benthol. Soc. 16: 141-161. (2) Nakano, S., Miyasaka, H., & Kuhara N. (1999) Ecology 7: 2435-2441. (3) Wipfli, M.S. (2005) Landsc. Urban Plan. 72: 205-213. (4) Hoover, T.M., Richardson, J.S., & Yonemitsu, N. (2006) Freshwater Biol. 51: 435-447. (5) Karlsson, O.M., Richardson, J.S., and P.M. Kiffney. (2005) Ecol. Model. 183: 463-476. (6) Wipfli, M.S. (1997) Can. J. Fish. Aquat. Sci. 54: 1259-1269.
Related projects:  FSP_Y092241FSP_Y103241


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Updated September 19, 2011 

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