SalmonSalmon nutrients,nutrients, nitrogennitrogen isotopesisotopes andand coastalcoastal forestsforests

Photo: Don Vipond Photo:

The yearly return of salmon from the vidually captured about 700 largely spawned- open Pacific Ocean to coastal waters of West- out salmon over the six-week spawning pe- ern North America is one of nature’s grand riod and carried the majority of these into displays and recent investigations by research- the where they could feed relatively ers in Washington, British Columbia and undisturbed. At Bag Harbour, where most of Tom Reimchen Alaska indicate that the signature of salmon the data were collected, eight bears trans- finds its way into both aquatic and terrestrial ferred 3,000 salmon into the forest over the , as far inland as the Rockies. one kilometre of stream where salmon The most widespread species associated spawned. On average, about one-half of each with these formerly immense schools of salmon carcass was consumed by the bears salmon are black and grizzly bears, which and the remnants were scavenged by eagles, migrate from alpine and distant habitats to marten and flocks of crows, ravens and gulls. congregate along streams and rivers during A diversity of insects including flies and bee- the spawning migration. Recent studies show tles were found with the carcasses and typi- that these predators play a much more sig- cally within five days, all carcasses were a nificant ecological role in coastal than seething mass of maggots which consumed previously recognized. all remaining soft tissues, leaving the bone. During an investigation on the foraging The cumulative effect of decomposing car- behaviour of Queen Charlotte Island black casses combined with the faecal and urine bear, begun in 1992, I found that bears indi- discharge from bears and other animals pro-

Ecoforestry Fall, 2001 13 duces a highly odiforous . Another student in my lab, Morgan Hock- Expansion of these studies on the Queen ing, is examining isotope signatures in insects Charlottes indicates that transfer of salmon and other invertebrates and has shown ma- carcasses into riparian zones is widespread jor amplification of 15N at multiple trophic throughout the British Columbia coast wher- levels, including herbivores, omnivores, car- ever bears and salmon are common and these nivores and detritivores. This amplification nutrients represent a significant part of the does not extend from direct consumption of nitrogen budget of and soil inver- salmon carcasses but rather from indirect tebrates. The use of stable nitrogen isotopes food web effects. Dr. Jonathan Moran, an- allows us to identify the relative contribution other member of our group, has examined of salmon to the . Researchers have nitrogen isotopes in soil at increased distance noted that 15N, the heavy but from the stream at each of six watersheds dif- Our research over the rare isotope of nitrogen, is more fering in the numbers of salmon and finds a abundant in marine algae than direct relationship between soil 15N and last decade, combined in terrestrial vegetation. Ras- salmon density. with those of other mussen at McGill University, That riparian plants or insects are using and others, have shown that the salmon-derived nutrients does not itself pro- investigators in the isotope is further enriched with vide evidence that this source of nutrients is Pacific Northwest, has each successive trophic level. required or essential for the plants. One line Salmon, occurring at the 4th of evidence that plants directly benefit from yielded previously trophic level in marine waters, salmon nutrients would be evidence for im- are very enriched in 15N. Conse- proved growth rate in . Nitrogen is usu- unrecognized linkages quently, comparisons of 15N ally limiting in coastal forests, as can be read- ‘‘ levels in vegetation beside a ily seen by the positive effect of adding ferti- between the open salmon stream with control lizers. At Bag Harbour, these carcasses con- ocean and forests. plants nearby without access to tribute up to 120 kg nitrogen per hectare into salmon provides a direct meas- the forest, comparable to applied fertilization ure of the contribution of salmon-derived rates by industry in coastal forests. nitrogen to the plants. Researchers in Alaska As a preliminary test of this, I examined and Washington, such as Kline, Bilby and yearly growth rings of Western Hemlock at Ben-David, have shown evidence for 15N en- Bag Harbour of 13 trees of similar size from richment in aquatic and streamside vegeta- sites differing in carcass density. Average tion. My student, Deanna Mathewson, has growth rate over the last 50 years was 2.5mm looked at needles or leaves from 10 riparian per year within 10m of the stream where car- plant species, including Western Hemlock, casses were most abundant and less than 1mm devil’s club, false azalea, red huckleberry, per year where carcasses were not present. salmon berry, buckbean and false lily of the Furthermore, individual trees grow more valley from some 20 watersheds throughout rapidly following years with high salmon the British Columbia Coast that differ in abundance, while control trees nearby with- abundance of salmon. These data demon- out access to salmon show no changes in strate that up to 40 percent of the nitrogen growth for the equivalent period. These data used by the riparian plants is derived from are at best ambiguous, as multiple factors salmon nutrients, with values dependent on influence plant growth, including light, mois- the salmon density in the stream, abundance ture and nutrients. I am currently examin- of bears, plant species and distance from the ing yearly growth in Western Hemlock and stream. Sitka Spruce from 80 watersheds through-

14 Ecoforestry Fall, 2001 out the British Columbia coast differing in has proved, however, logistically challenging abundance of salmon and a variety of physi- due to the difficulty in detecting 15N in . cal parameters in an effort to partition the Standard mass spectrometers, the instru- influence of salmon from other factors. Some ments used to measure isotopic ratios, work of our study sites are particularly useful as well with leaves or needles in which the car- they include comparisons of trees immedi- bon to nitrogen ratio (C:N) is about 40:1. Yet, ately above and below waterfalls that are im- the C:N ratio in wood is about 1000:1 and as passable to salmon. such, the signature of 15N is masked by the One of the empirical observations emerg- large amount of carbon. Over a three-year ing from the Bag Harbour studies was that period and in collaboration with colleagues the amount of salmon transfer into the for- at McGill University, wood samples were sent est each year varied directly with the yearly out to four different mass spectrometer labo- numbers of spawning salmon returning to ratories but none could produce a repeatable the stream. As such, it seemed plausible to 15N signature. In 1998, I sent some wood sam- me that yearly differences in this nutrient ples from Bag Harbour to a mass spectro- pulse might be reflected in the 15N levels in meter researcher from California and he was yearly growth rings of the conifers. If so, the able to gradually solve technical constraints rings might retain evidence for past fluctua- and measure 15N on yearly growth rings of tions in salmon nutrients and potentially al- Western Hemlock and Sitka Spruce. The ex- low a reconstruction of movement of salmon cess carbon in the samples remains a con- into the watershed into past centuries. This straint and progress is slow. To the present, I

Photo: Don Vipond Photo:

Ecoforestry Fall, 2001 15 have sent wood samples from 25 trees and the The research is ongoing and we have taken results can be briefly summarized . cores from 750 trees at 80 watersheds along Comparisons among watersheds shows the coast that will be analyzed for dendro- that the 15N levels in the wood of trees adja- chronological and isotopic data. The results cent to streams is directly proportional to will provide a detailed historical assessment salmon numbers. The highest values, near 10 of nutrient cycling in watersheds throughout parts per thousand, occur in ancient Sitka the coast. Spruce at a mid-coast stream near Bella Bella Our research over the last decade, com- that has the highest salmon spawning density bined with those of other investigators in the identified in our studies (60,000 salmon /km). Pacific Northwest, has yielded previously Up to 80 percent of the yearly nitrogen budget unrecognized linkages between the open in some years in these spruce appears to have ocean and forests and these may be impor- been derived from salmon nutrients. tant to our understanding of forest ecosys- Comparisons within watersheds show that tems. The available evidence currently sug- the 15N levels are highest in trees near the gests that these linkages occur from the es- stream and decline with increased distance tuaries and small streams that fringe the into the forest, concordant with the decline North Pacific through to the headwaters of in salmon carcasses and bear activity. Even in the major rivers that penetrate far into the small watersheds, vegetation 150 m from the continents. The estimated 80-90 percent re- stream still has the signature of salmon. This duction in salmon returning to streams over suggests a much broader riparian zone than the last 100 years, largely the result of defor- the 10-30 m zone suggested in government estation and overfishing, will have ecosystem- policy on fish streams. Recent studies by level consequences for the remaining forests. Hilderbrand in Alaska suggest that the salmon What these are remain largely unknown but signature in vegetation occurs some 800 m into more of these effects will emerge from the the forest where grizzly bears are common. ongoing research programs. M Comparisons within trees demonstrate a correlation between 15N signatures among Acknowledgments yearly growth rings and DFO records of This research is financially supported by the salmon escapement over the last 50 years. The David Suzuki Foundation, Friends of Eco- Dr. T. E. Reimchen peaks in salmon can take from one to three logical Reserves and the Natural Sciences and Department of years to show up in the rings. Engineering Research Council of Canada. Biology, University of Older or larger trees exhibit higher 15N lev- I also thank my graduate students D. Victoria, BC. E-mail els in growth rings than younger or smaller Mathewson, M. Hocking, D. Klinka, my col- will reach him at trees. This could reflect either increased trans- laborators D. Harris, Bristol Foster and J. [email protected] fer of salmon nutrients to larger trees or in- Moran, volunteers including D. Coopland creased isotopic fractionation (reduced up- and I. Jacobs and, for sharing their lab space, take of 15N ) of young trees when the static B. Hawkins, N. Livingston and D. Smith. nitrogen supply in the soil may exceed the requirements.

(This article appeared in the Fall 2001 issue of "Ecoforestry," the quarterly journal of the Ecoforestry Institute, based in Victoria, British Columbia. The institute's website: www.ecoforestry.ca)a

16 Ecoforestry Fall, 2001