Krista Bartz, UW Fisheries; Scott Bechtold, UW Forestry; Rick Edwards, UW Forestry; James Helfield, UW Forestry; Robert Naiman, UW Fisheries; Thomas O'Keefe, UW Fisheries; Gilles Pinay, Centre National de la Recherche Scientifique, Rennes, France

Introduction

Interactions between Pacific salmon (Oncorhynchus spp.) and bear (Ursus spp.) may play an important role in influencing the stature and dynamics of riparian vegetation. It has been known for some time that anadromous, semelparous salmon returning to their natal streams make significant contributions of marine-derived nutrients to the aquatic ecosystem (e.g., Mathisen et al. 1988, Kline et al. 1990, Bilby et al. 1996). These allochthonous inputs enhance productivity at various trophic levels within aquatic food webs, but they may also fertilize riparian vegetation. Bears may act as an important vector for moving nutrients from the stream to the riparian zone through the killing and consumption of salmon and subsequent deposition of carcasses and feces within the riparian forest. Uptake of enriched hyporheic waters by riparian root systems represents another potentially important pathway for this nutrient transfer. Annual inputs of marine-derived nutrients (MDN) may result in enhanced growth rates and altered community composition in riparian forests. Given that larger riparian trees tend to yield larger, more stable woody debris, resulting in comparatively deeper pools, more extensive cover and correspondingly enhanced aquatic habitat (Harmon et al. 1986, Sedell et al. 1988), enrichment of riparian vegetation may represent an important feedback mechanism by which interactions of salmon and bear populations help to maintain structural habitat and productivity within the stream corridor. The significance of this research lies in the discovery of how synergistic interactions between salmon and bear may act to influence riparian forests, and how materials from those forests act to influence the strength of salmon populations.

There were three specific research objectives in the initial project (1998-2002): (1) We determined the extent to which riparian vegetation acquired MD-nitrogen from spawning salmon. (2) We measured the impacts of MD-nitrogen on riparian growth and community composition. (3) We assessed the relative importance of bear activity (e.g., carcass and feces distribution) and hyporheic flow as pathways for MD-nitrogen transfer from stream to riparian systems.

Research Results This is what we have learned so far:

Objective 1:

Spatial Pattern of MD-nitrogen in Riparian Plants. We met this objective at two spatially extensive research sites in Alaska (Bartz 2002, Helfield 2001, Helfield and Naiman 2001, 2002) and by a synthesis of published information (Naiman et al. 2002). We learned that salmon make measurable contributions to the nutrient capital of riparian ecosystems. Riparian plant foliage contains ~15-30% MD-nitrogen. The exception is alder (Alnus spp.), a nitrogen-fixing genus that has little need for MD-nitrogen. Additionally, N concentrations in the foliage of salmonberry (Rubus spectabilis), Devil’s Club (Oplopanax horridum) and ferns (Dryopteris dilatata and Athyrium felixfemina) are significantly higher along salmon bearing reaches than streams without salmon. However, these data are from basically N-limited systems. Curiously, in the apparently P-limited systems, there are few significant differences in N and P concentrations in riparian foliage between streams with and without salmon (Bartz 2002).

MD-nitrogen can be distributed a considerable distance from the channel. The extent of lateral distribution of MD-nitrogen varies by site and plant species (Ben-David et al. 1998), and can be found in some plant species up to 200 m from the channel – with the proportion declining with distance (Helfield & Naiman 2001, Bartz 2002, Naiman et al. 2002).

Research Results Objective 2:

Assessment of MD-nitrogen on Riparian Composition and Growth. We met this objective at the spatially extensive research sites in Alaska (Bartz 2002, Helfield 2001, Helfield and Naiman 2001, 2002) and by the use of regional climate and salmon population data (Drake et al. 2002). We learned that MD-nitrogen subsidies decrease the competitive advantage of N-fixing plants such as alder (Alnus spp.), resulting in decreased abundance near spawning streams (Helfield and Naiman 2002). We suspect that spatial distributions of MD-nitrogen may influence patterns of browsing, which in turn affects nutrient cycling, successional processes and plant species composition (e.g., Kielland and Bryant 1998). These processes are realized through interactions between salmon and their consumers, so that neither can fulfill the keystone role without the other.

We examined soil, foliage, and riparian community composition on 10 rivers where barrier waterfalls limited salmon access (Bartz 2002). With the exception of 15N abundance, soil nutrients did not differ significantly based on the presence or absence of salmon. Both mineral and organic soil d15N values were higher near spawning reaches, yet this did not translate into significantly greater total N or P. Foliar nutrients parallel those of soils with median d15N values consistently higher at spawning sites for white spruce, willow (Salix alaxensis), and polar grass (Arctagrostis latifolia). Additionally, overstory stem density and basal area were generally greater near spawning sites, while understory stem density and basal area and ground layer vascular species richness and evenness were greater at non-spawning sites. While salmon appeared to drive upstream-downstream patterns in the understory, other variables were more strongly correlated with overstory and ground layer patterns (e.g., soil N and soil P). These results are being prepared for journal submission by autumn 2002.

Quantification of dominant riparian tree growth greatly increased our understanding of this complex system: (1) Not all trees respond (in terms of xylem growth) to subsidies of MD-nitrogen even though they have a significant percentage in their tissues. Western hemlock (Tsuga heterophylla), for example, assimilates MD-nitrogen but it does not result in increased xylem growth. This has been observed in forest fertilization studies (Bix 1993) but the reason remains unknown. (2) Those species responding to MD-nitrogen exhibit surprising increases in growth rates. Sitka spruce grows three times faster adjacent to salmon rivers than along nearby sites without salmon (Helfield and Naiman 2001). At salmon-spawning sites it takes 86 yr for Sitka spruce to reach 50 cm dia, a size where woody debris contributes to the creation and maintenance of salmon habitat in the river, as opposed to 307 yr at sites without salmon. (3) Annual salmon escapement was significantly related to tree-ring growth at two sites in the Pacific coastal rainforest (PCRF) but not at two sites in the boreal forest. We used salmon and climate relationships (Pacific Decadal Oscillation) established at PCRF sites to reconstruct preliminary salmon spawning abundances to 1820 A.D. – and they compared favorably to SE Alaska fisheries catch data from 1924 – 1994 (Drake et al. 2002).

From the beginning we recognized the possibility that the MD-nitrogen ‘signal’ in the vegetation could be an artifact of metabolic transformations by microbes. We investigated this by measuring denitrification rates at a variety of contrasting sites (Pinay et al. In Press, Ecosystems). Indeed, there is significant denitrification occurring in the riparian soils along salmon-spawning streams but there are also differences in the vegetative communities and in tree growth rates that can only be explained by addition of MD-nutrients. We continue to assert that the ecological responses seen are due to MD-nutrients, however, we intend to submit a separate proposal to an appropriate agency later this year to focus on N transformations in riparian soils and plants in cooperation with Gilles Pinay (CNRS, France) and his research team.

Research Results Objective 3:

Original Objective 3: Relative Importance of MD-Nitrogen Pathways. This was the most difficult of the original objectives but we have made substantial progress. We examined the relative importance of pathways by an analysis of the literature and unpublished data sets (Helfield and Naiman submitted Am. Nat.) and intensive investigations of subsurface fluxes in boreal Alaska (O’Keefe and Edwards 2002). There are five dominant pathways for the lateral movement of MD-nitrogen from streams to riparian vegetation: direct atmospheric inputs, animal consumption followed by waste deposition in the riparian forest, fluxes into parafluvial hyporheic zones and uptake by plant roots, sequential year-to-year litterfall and sequestration into soils, and direct root transfer via mycorrhizal associations.

Some nutrients released from spawning fish and from carcasses decomposing in the stream are carried into the riparian root zone by hyporheic flow (O’Keefe and Edwards 2002). However, the data suggest that this may be only important during peak spawning runs when concentrations of MD-nutrients in the stream are high or in physical situations promoting strong downwelling of stream water. Quantification of actual flux has been difficult due to the technical complexities of measuring hyporheic in-flows and extrapolation to other physical settings.

We constructed a preliminary comparison of pathways for Lynx Creek, Alaska (our primary hyporheic research site; Helfield and Naiman, submitted Am. Nat.). We estimated inputs of MD-nitrogen via bear activity and hyporheic flows for comparison with inputs from other N sources such as precipitation, leaching from upland soils and symbiotic N fixation by alder. Precipitation could account for 18-37% of the annual N-flux to the riparian forest, leaching from upslope soils 23-44%, N-fixation by alder ~1% (but as high as 85% where alder is abundant), hyporheic flows 1-14% (but as high as 57% dependent on assumptions of rooting and water table depth), and bear activity 17-42% (depending on the annual abundance of spawning sockeye salmon). The variability in the range of different vectors is highly dependent on escapement and landscape features in the watershed and can thus vary considerably among different watersheds.

Two potential pathways that we have not been able to quantify are sequential year-to year lateral movement by litterfall and by roots and their associated mycorrhizal communities. We know that MD-nitrogen is quickly sequestered after entering the hyporheic zone (O’Keefe and Edwards 2002) and we know that soil nitrogen accumulated over the dry summer season is rapidly leached into the root zone during autumn rainstorms (Bechtold et al. in press). If either incremental lateral movement via litterfall or if mycorrhizal fungi are active they could explain why MD-nitrogen is found hundreds of meters from spawning sites. Quantifying these potential pathways will be a main research objective of the next phase.

Developing a System-scale Model. Our conceptual model of the environmental basis for salmon production along the Pacific Coast of North America has evolved as new information is generated. We now envision that oceanic climate conditions, operating on the scale of decades, have strong influences on salmon escapement to rivers. Once in the rivers, salmon-borne nutrients have differential affects depending whether the sites are in the N-limited coastal rain forest or in what appears to be a generally P-limited boreal forest. Where bear and other salmon predators are abundant they appear to be primary vectors moving MD-nutrients into the riparian zone, either through the deposition of carcasses or urine (Hildebrand et al. 1999). The hyporheic pathway appears to be important only during large runs relative to discharge or in systems that have limited nitrogen input from other sources. Lateral transfers of MD-nutrients by litterfall and roots remain unexplored. The MD-nutrients affect tree species differentially. Sitka spruce benefits greatly, while other species (e.g., western hemlock and red alder) do not show strong xylem responses. Nevertheless, when the large trees die they contribute LWD to the river, closing the feedback loop by enhancing overall habitat and survival of young fish.

Our research is further divided into three primary projects:

• Influence of spawning salmon on riparian vegetation productivity (Helfield, Naiman)
• The role of the hyporheic zone in nutrient processing and storage (Edwards, O'Keefe, Bechtold, Pinay)
• The vegetation community composition and geomorphology of salmon vs. non-salmon streams (Bartz, Naiman)

last update Nov 2002