At each watershed, all species showed an increase in foliar δ15N values across the waterfall barrier (Figure 1). At Clatse River, foliar 15N
among species was enriched from 1.4‰ to 5.6‰ below the falls relative
to those above the falls while at Neekas River, enrichment ranged from
7‰ to 9‰ across the barrier. Analyses of barrier and vegetation
demonstrated that both are highly significant sources of variation in
each watershed (barrier: Clatse, F1 = 70.14; Neekas, F1 = 409.63, p < 0.001 for both, two-way ANOVA; vegetation species Clatse, F5 = 6.31; Neekas F5 = 7.05, p < 0.001, for both). A greater proportion of the variance in 15N was attributed to barrier (R2: Clatse= 0.58, Neekas= 0.88) than to vegetation species (R2: Clatse= 0.38, Neekas= 0.38).
Figure 1. δ15N
values in riparian vegetation collected immediately below and above
waterfall barriers to salmon at Clatse and Neekas rivers, B.C. T-test
results: * denotes p < 0.05; ** denotes p < 0.01.
Total contribution of marine-derived nitrogen (MDN) in these riparian habitats varied among watershed and among species (Figure 2). MDN was higher at Neekas River (range 45–65%) than at Clatse River (range 12–48%) (t10 = 4.13, p = 0.002). At Clatse, lowest values occurred in T. heterophylla and highest values in R. spectabilis while at Neekas River, T. heterophylla had the lowest while V. alaskaense had the highest values.
%MDN in riparian vegetation collected below waterfalls at Clatse and
Neekas rivers, B.C. Overall, %MDN values at Neekas are higher than at
Clatse (t-test p < 0.001).
Total tissue nitrogen was examined as a potential proxy for primary
productivity. There were no overall differences between watersheds
(Clatse = 2.16%, Neekas = 2.00%, t22 = 0.70, p = 0.49). At each river, there were significant differences above and below waterfalls (Clatse: F1 = 13.04, p = 0.001; Neekas: F1 = 6.63, p = 0.013) with a marginal tendency towards higher mean values of %N in foliage collected below the falls (Figure 3). However, among individual species, these comparisons were significant (p < 0.05) only for M. ferruginea at Clatse River and for R. spectabilis at Neekas River. Within each habitat, there were also consistent differences in %N among species (Clatse above barrier: F5 = 38.14, p < 0.001; Clatse below barrier: F5 = 48.22, p < 0.001; Neekas above barrier: F5 = 19.50, p < 0.001; Neekas below barrier: F5 = 37.18, p < 0.001). In both watersheds, above and below the falls, T. heterophylla, and to a lesser extent B. spicant exhibited the lowest tissue nitrogen while R. spectabilis had the highest levels.
Total %N values in riparian vegetation collected immediately below and
above waterfall barriers to salmon at Clatse and Neekas rivers, B.C. R. spectabilis and O. horridus represent nitrogen-rich soil indicator species, T. heterophylla is
unclassified, and all others are indicators of nitrogen-poor soil.
T-test results: * denotes p < 0.05; ** denotes p < 0.01.
Soil-nitrogen indicator species
We examined the relative cover of vegetation corresponding to the indicator groups (Table 1).
Nitrogen-rich soil indicator species had a higher cover below than
above the falls in all plots on both watersheds (Clatse: above barrier:
2.5%, below barrier: 31.5%, p = 0.06, Mann-Whitney; Neekas: above
barrier: 25.5%, below barrier: 60%, p = 0.06, MW; Figure 4).
Nitrogen-poor soil indicator species demonstrated the reverse trend
with marginally higher percent cover above than below the falls
(Clatse: above barrier: 70.2%, below barrier: 8.75%, p = 0.06, MW;
Neekas: above barrier: 16%, below barrier: 6.5%, p = 0.23, MW).
Total understory vegetation cover by soil nitrogen indicator category
in 10 m × 10 m plots on the Clatse and Neekas rivers, B.C. Species are
separated into nitrogen-rich soil and nitrogen-poor soil indicator
categories based on data by Klinka et al. .
Understory vegetation soil indicator species from research plots on the
Clatse and Neekas rivers, B.C. Species are separated into nitrogen-rich
soil and nitrogen-poor soil indicator categories based on data by
Klinka et al. .