A multidecade experiment shows that fertilization by salmon carcasses enhanced tree growth in the riparian zone

Abstract

As they return to spawn and die in their natal streams, anadromous, semelparous fishes such as Pacific salmon import marine‐derived nutrients to otherwise nutrient‐poor freshwater and riparian ecosystems. Diverse organisms exploit this resource, and previous studies have indicated that riparian tree growth may be enhanced by such marine‐derived nutrients. However, these studies were largely inferential and did not account for all factors affecting tree growth. As an experimental test of the contribution of carcasses to tree growth, for 20 yr, we systematically deposited all sockeye salmon (Oncorhynchus nerka) carcasses (217,055 individual salmon) in the riparian zone on one bank of a 2‐km‐long stream in southwestern Alaska, reducing carcass accumulation on one bank and enhancing it on the other. After accounting for partial consumption and movement of carcasses by brown bears (Ursus arctos) and variation in salmon abundance and body size, we estimated that 267,620 kg of salmon were deposited on the enhanced bank and 45,200 kg on the depleted bank over the 20 yr, for a 5.9‐fold difference in total mass. In 2016, we sampled needles of 84 white spruce trees (Picea glauca) the dominant riparian tree species, for foliar nitrogen (N) content and stable isotope ratios (δ15N), and took core samples for annual growth increments. Stable isotope analysis indicated that marine‐derived N was incorporated into the new growth of the trees on the enhanced bank. Analysis of tree cores indicated that in the two decades prior to our enhancement experiment, trees on the south‐facing (subsequently the depleted) bank grew faster than those on the north‐facing (later enhanced) bank. This difference was reduced significantly during the two decades of fertilization, indicating an effect of the carcass transfer experiment against the background of other factors affecting tree growth.

Riparian soil nitrogen cycling and isotopic enrichment in response to a long‐term salmon carcass manipulation experiment

“…Briefly, from 1997 to 2016 an average of 10,853 sockeye salmon returned to the stream annually. To avoid doublecounting carcasses on the river left bank, carcasses naturally occurring on the river left bank were also relocated to a distance of about 5 m; thus, all carcasses (with the exception of those moved by wildlife; see Quinn et al 2018) were located between 3 and 6 m on the river left bank. Helfield and Naiman 2002).…”

“…Overstory vegetation is dominated by white spruce and paper birch (Betula papyrifera), and unlike many other watersheds in the region, it has a low density of symbiotic N 2 -fixing alder (Alnus spp. Quinn et al (2018) calculated that prior to manipulation, the both banks averaged 4545.6 kg of salmon annually and that after manipulation, the river left bank averaged 13,381 kg of salmon and the river right bank averaged 2,260 kg of salmon annually, a 9.6-fold difference. From 1997 to 2016, the stream was surveyed daily during the annual sockeye salmon (Oncorhynchus nerka) run and all dead salmon were removed from the creek and the river right bank to a distance of about 5 m and tossed onto the river left bank.…”

“…Helfield and Naiman 2002). Approximately 108,530 individual fish (in many cases partially consumed by bears) were translocated over the 20-yr period representing a total of 267,620 kg of salmon, 8,028 kg of N, and 1,356 kg of phosphorus (P; Quinn et al 2018). To avoid doublecounting carcasses on the river left bank, carcasses naturally occurring on the river left bank were also relocated to a distance of about 5 m; thus, all carcasses (with the exception of those moved by wildlife; see Quinn et al 2018) were located between 3 and 6 m on the river left bank.…”

“…Helfield and Naiman (2001) measured tree growth increments in areas with and without salmon and found higher growth in one species (Sitka spruce) in areas where salmon nutrients were present, although these findings were later contested on statistical grounds (Kirchhoff 2003). Most recently, Quinn et al (2018) examined tree growth increments in the riparian zone of a small Alaskan stream before and after a 20-yr, >200,000 kg, salmon carcass manipulation. Reimchen and Fox (2013) suggested that salmon abundance increased tree growth, but tree ring 15 N/ 14 N values were not related to salmon abundance; other growth limiting factors such as temperature and location were important covariates.…”

“…While Quinn et al focused on tree growth before and after the manipulation, the objective of this work was to determine whether prolonged enhancement and reduction of salmon subsidies altered long-term soil N cycling, similar to that documented in forests receiving N fertilizer additions (Prescott et al 1992, Lu et al 2010). If not, then the addition of nutrients as a management response to low salmon returns may have unintended negative consequences (sensu Compton et al 2006 Quinn et al 2018 at the same site. If not, then the addition of nutrients as a management response to low salmon returns may have unintended negative consequences (sensu Compton et al 2006 Quinn et al 2018 at the same site.…”