Effects of experimentally added salmon subsidies on resident fishes via direct and indirect pathways

Artificial additions of nutrients of differing forms such as salmon carcasses and analog pellets (i.e. pasteurized fishmeal) have been proposed as a means of stimulating aquatic productivity and enhancing populations of anadromous and resident fishes. Nutrient mitigation to enhance fish production in stream ecosystems assumes that the central pathway by which effects occur is bottom-up, through aquatic primary and secondary production, with little consideration of reciprocal aquatic-terrestrial pathways. The net outcome (i.e. bottom-up vs. top-down) of adding salmon-derived materials to streams depend on whether or not these subsidies indirectly intensify predation on in situ prey via increases in a shared predator or alleviate such predation pressure. We conducted a 3-year experiment across nine tributaries of the N. Fork Boise River, Idaho, USA, consisting of 500-m stream reaches treated with salmon carcasses (n = 3), salmon carcass analog (n = 3), and untreated control reaches (n = 3). We observed 2–8 fold increases in streambed biofilms in the 2–6 weeks following additions of both salmon subsidy treatments in years 1 and 2 and a 1.5-fold increase in standing crop biomass of aquatic invertebrates to carcass additions in the second year of our experiment. The consumption of benthic invertebrates by stream fishes increased 110–140% and 44–66% in carcass and analog streams in the same time frame, which may have masked invertebrate standing crop responses in years 3 and 4. Resident trout directly consumed 10.0–24.0 g·m-2·yr-1 of salmon carcass and \u3c1–11.0 g·m-2·yr-1 of analog material, which resulted in 1.2–2.9 g·m-2·yr-1 and 0.03–1.4 g·m-2·yr-1 of tissue produced. In addition, a feedback flux of terrestrial maggots to streams contributed 0.0–2.0 g·m-2·yr-1 to trout production. Overall, treatments increased annual trout production by 2–3 fold, though density and biomass were unaffected. Our results indicate the strength of bottom-up and top-down responses to subsidy additions was asymmetrical, with top-down forces masking bottom-up effects that required multiple years to manifest. The findings also highlight the need for nutrient mitigation programs to consider multiple pathways of energy and nutrient flow to account for the complex effects of salmon subsidies in stream-riparian ecosystems

Resilience and Vulnerability of Northern Regions to Social and Environmental Change

The arctic tundra and boreal forest were once considered the last frontiers on earlh because of their vast expanses remote from agriculturaland-use change and industrial development. These regions are now, however, experiencing environmental and social changes that are as rapid as those occurring anywhere on eadh. This paper summarizes the role of northern regions in the global system and provides a blueprint for assessing the factors that govern their sensitivity to social and environmental chanqe

Increased temperature sensitivity and divergent growth trends in circumpolar boreal forests

[1] Tree rings have been used to both reconstruct past climate, and to estimate and project carbon uptake of forest ecosystems. Here we show that large groups of trees of the dominant tree species within widely-distributed circumpolar forest sites show opposing growth trends during recent warming. These opposing growth trends are present at a sub-chronology level and, if averaged into chronologies, may have contributed to the widely reported overall decreased temperature sensitivity of high-latitude chronologies over recent decades. Unlike previous studies, we find that temperature sensitivity has actually increased for most individual trees at these sites. This recent, widespread divergence in growth response seems unique over the past three centuries, and may relate to different microsite responses of individual trees to temperatureinduced drought stress or other factors. This divergence needs to be taken into account in dendroclimatic reconstructions, estimations of global warming impacts, and carbon uptake projections. Citation: Wilmking, M., R. D’Arrigo, G. C. Jacoby, and G. P. Juday (2005), Increased temperature sensitivity and divergent growth trends in circumpolar boreal forests, Geophys. Res. Lett., 32, L15715, doi:10.1029/2005GL023331. 1