Abstract
AbstractPacific salmon (Oncorhynchus spp.) carcasses can fertilize riparian forests with marine‐derived nutrients when populations make their annual return to natal streams to spawn; however, the strength of this cross‐system linkage likely varies substantially among years due to the interannual fluctuations in abundance that characterize most salmon populations. Here, we used a 36‐yr time series (1984–2019) of satellite imagery and salmon abundance estimates to assess spatiotemporal associations between forest greenness (a measure of plant productivity) and adult sockeye salmon (Oncorhynchus nerka) abundance in the lower Adams River, British Columbia, Canada. The Adams River sockeye population displays a quadrennial pattern of abundance, with a dominant cohort that spawns every four years in numbers that are typically two to three orders of magnitude larger than non‐dominant cohorts. We found that variation in forest greenness was consistently explained best by models including dominant cohort year, whereas models lacking an index of salmon abundance were the lowest‐ranked. Greenness of riparian vegetation increased by an average of 0.015 NDVI units (approximately 1%) in the summer after a dominant cohort return, and this effect on greenness persisted into the subsequent fall (11–13 months after spawning). The positive association between quadrennial pulses of salmon and riparian greenness occurred in plots both within 30 m of the stream and 95–125 m away from the stream, indicating that the spatial extent of fertilization may occur well beyond areas directly adjacent to the riverbank. These results suggest that forests respond to cyclical variation in salmon abundance and that overwinter storage of marine‐derived nutrients within catchments allows plants to capitalize on these nutrients in the following growing season. Continued advances in remote sensing technology will enhance our understanding of cross‐system resource linkages and can inform the ecosystem‐based management of Pacific salmon.
Highlights
The salmon spawning migrations of Pacific North America offer a unique example of mass resource transport across ecosystem boundaries, with hundreds of metric tons of marine-derived nitrogen and phosphorus transported into inland freshwater systems and surrounding forest ecosystems each year (Gresh et al 2000)
These nutrients can reach the riparian zone through the activity of terrestrial salmon consumers such as bears, wolves, and scavenging birds (e.g., Hilderbrand et al 1999, Reimchen 2000, Quinn et al 2001, Harding et al 2019), water movements that wash post-spawn carcasses out of streams (e.g., Ben-David et al 1998, Dunkle et al 2020), and subsurface water flow in the floodplain (e.g., O’Keefe and Edwards 2002). These pulses of nutrients influence the dynamics of riparian ecosystems, where salmon alter the composition of riparian plant assemblages (Hocking and Reynolds 2011, Morris and Stanford 2011), increase the growth rate of riparian trees (Helfield and Naiman 2001, Reimchen and Fox 2013, Quinn et al 2018), are heavily consumed by terrestrial insects (Hocking and Reimchen 2006), and increase the density of forest birds (Field and Reynolds 2013, Wagner and Reynolds 2019)
This study found a positive relationship between Normalized Difference Vegetation Index (NDVI) in summer months and the combined abundance of pink (Oncorhynchus gorbuscha) and chum (Oncorhynchus keta) salmon among streams in the central coast of British Columbia (BC), Canada, as well as biennial increases in summer NDVI that corresponded with the two-year cycle of pink salmon returns to the lower Fraser River over an 18-yr time series
Summary
The salmon spawning migrations of Pacific North America offer a unique example of mass resource transport across ecosystem boundaries, with hundreds of metric tons of marine-derived nitrogen and phosphorus transported into inland freshwater systems and surrounding forest ecosystems each year (Gresh et al 2000). 99% of their body mass in the ocean before they return to freshwater ecosystems to spawn and die (Quinn 2005), delivering marine-derived nutrients to aquatic and riparian ecosystems in the form of gametes, excretion, and carcasses These nutrients can reach the riparian zone through the activity of terrestrial salmon consumers such as bears, wolves, and scavenging birds (e.g., Hilderbrand et al 1999, Reimchen 2000, Quinn et al 2001, Harding et al 2019), water movements that wash post-spawn carcasses out of streams (e.g., Ben-David et al 1998, Dunkle et al 2020), and subsurface water flow in the floodplain (e.g., O’Keefe and Edwards 2002). Given that growing season varies with latitude and that the response of riparian vegetation to salmon is likely mediated by local factors such as geomorphology and historical salmon returns, the degree to which plants respond to salmon between geographic locations and within and among years remains generally unclear
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