Michael R. Morris and Jack A. Stanford. Flathead Lake Biological Station and Division of Biological Sciences, The University of Montana, 32125 Bio Station Lane, Polson, MT 59860-9659
Some five million salmon return annually to the Kol River, contributing 270,000 kg N yr-1. We quantified N dynamics in soils and vegetation across a 100-year riparian chronosequence. Soil of early succession alluvial bars (0-5 yrs) contained 200 kg total N ha-1 (to 10 cm depth) and net mineralization was 0.05 kg N ha-1 day-1 before the salmon migration. In 100 year-old stands, total N and mineralization were an order of magnitude higher. Thus, as is in most terrestrial ecosystems, early succession should have been N-limited. However, flood-deposited carcasses contributed up to 150 kg N ha-1 yr-1 to these otherwise N-poor alluvial bars. Early successional willows actively assimilated N and doubled in mass during the decomposition of salmon carcasses, even though this N-pulse was late in the growing season. Early succession foliage was highly enriched with the 15N isotope (δ15N: 2 to 6, depending on species) relative to terrestrially fixed N (δ15N: -1), confirming salmon were a major N-source in early succession. Unlike other Pacific-rim succession studies, vegetation across this chronosequence was composed of fast growing, N-rich (foliar C:N = 8 to 14 versus avg. for temperate forests: 35) species and, thus, we concluded vegetation growth was not N-limited.