Variation in riparian and stream assemblages across the primary succession landscape of Mount St. Helens, U.S.A.

Abstract

Although most lotic ecosystems experience frequent and sometimes large disturbances, opportunities are uncommon to study primary succession in streams. Exceptions include new stream channels arising from events such as glacial retreat, volcanism, and catastrophic landslides. In 1980, the eruption and massive landslide at Mount St. Helens (WA, U.S.A.) created an entire landscape with five new catchments undergoing primary succession. We asked if riparian and lotic assemblages at early successional stages (36 years after the eruption) showed predictable change along longitudinal gradients within catchments, and whether assemblages were similar among five replicate catchments. In July 2016, we collected environmental data and characterised riparian, algal, and benthic macroinvertebrate assemblages at 21 stream reaches distributed within and among five neighbouring catchments. We evaluated patterns of richness, abundance, biomass, multivariate taxonomic community structure, and functional traits both longitudinally and among catchments. We found minimal evidence that longitudinal gradients had developed within catchments at 36 years post‐eruption. Increases in diatom and macroinvertebrate richness with downstream distance were the only biological responses with longitudinal trends. Conversely, we documented substantial variation in community structure of riparian plants, soft‐bodied algae, diatoms, and macroinvertebrates at the among‐catchment scale. Among‐catchment differences consistently separated two eastern catchments from three western catchments, and these two groups also differed in stream water chemistry, water temperature, and geomorphology. Overall, we documented greater diversity in the young catchments than predicted by ecologists in the years immediately following the eruption, yet functional traits indicate that these catchments are still in relatively early stages of succession. Variation at the among‐catchment scale is likely to be driven in part by hydrological source variation, with the two eastern catchments showing environmental signatures associated with glacial ice‐melt and the three western catchments probably fed primarily by springs from groundwater aquifers. Contemporary flow disturbance regimes also varied among catchments and successional trajectories were probably reset repeatedly in streams experiencing more frequent disturbance. Similar to new stream channels formed following glacial retreat, our results support a tolerance model of succession in streams. However, contrasting abiotic templates among Mount St. Helens catchments appear to be driving different successional trajectories of riparian plant, algal, and macroinvertebrate assemblages among neighbouring small catchments sharing the same catastrophic disturbance history.

Culturable fungal endophyte communities of primary successional plants on Mount St. Helens, WA, USA

“…Between July and September 2017, we sampled six common woody species found on the Pumice Plain of Mount St. Helens, WA [ 65 ]: Sitka alder, Sitka willow, cottonwood, Douglas fir, noble fir, and western hemlock (Table 1 ). Sites were identified by traversing previously-established horizontal transects [ 66 , 67 ] and collecting GPS coordinates for over 300 trees.…”

Spatial Habitat Structure Assembles Willow-Dependent Communities across the Primary Successional Watersheds of Mount St. Helens, USA

“…Dominant riparian species include green alder (Alnus viridis) and Sitka willow (Salix sitchensis; hereafter simply willow), the latter of which has been regularly attacked by a stem-boring weevil (Cryptorhynchus lapathi, Curculionidae, Coleoptera) since its introduction in 1989 [83]. [92]. Blue lines are stream features on the Pumice Plain, near which willow shrubs were sampled.…”

“…Between May-June of 2018, we haphazardly tagged 348 individual and relatively isolated willows across the Pumice Plain and identified each as either male or female [92]. Blue lines are stream features on the Pumice Plain, near which willow shrubs were sampled.…”

Canopy development influences early successional stream ecosystem function but not biotic assemblages

“…However, efforts to do so are hampered by the infrequent formation of new rivers on the landscape. This information gap can be filled by studying new stream reaches formed following glacial retreat (Milner et al 2008;Brown and Milner 2012) or new watersheds created following volcanic eruptions (Claeson et al 2021). For example, the dynamics of stream primary succession have been informed by prior research in Glacier Bay, Alaska, USA (Sidle and Milner 1989;Flory and Milner 1999;Milner and Gloyne-Phillips 2005;Milner et al 2011) and following volcanic eruptions in South America (Fuentes et al 2020;Carrillo and Díaz-Villanueva 2021) and the Pacific Northwest (WA, USA; Claeson et al 2021).…”

“…This information gap can be filled by studying new stream reaches formed following glacial retreat (Milner et al 2008;Brown and Milner 2012) or new watersheds created following volcanic eruptions (Claeson et al 2021). For example, the dynamics of stream primary succession have been informed by prior research in Glacier Bay, Alaska, USA (Sidle and Milner 1989;Flory and Milner 1999;Milner and Gloyne-Phillips 2005;Milner et al 2011) and following volcanic eruptions in South America (Fuentes et al 2020;Carrillo and Díaz-Villanueva 2021) and the Pacific Northwest (WA, USA; Claeson et al 2021). As research continues at these primary succession locations, we have unanswered questions about how newly established riparian forests along streams may alter the developmental trajectories of in-stream communities.…”