An official website of the United States government
Headwater streams are reliant on riparian tree leaf litterfall to fuel brown food webs. Terrestrial agents like herbivores and contaminants can alter plant growth, litter production, litter quality, and the timing of litterfall into streams, influencing aspects of the brown food web. At Mount St. Helens (USA), early successional streams are developing willow (Salix sitchensis) riparian zones. The willows are attacked by stem-boring herbivores, altering litter quality and the timing of litterfall. Within a established experimental plots, willows (male and female plants) were protected from herbivores using insecticides and provided with experimental additions of nitrogen. This enabled us to test the interacting influences of herbivores, nitrogen deposition, and willow sex on leaf litter quality, aquatic litter decomposition, and microbial and invertebrate detritivores. We found weak litter quality effects (higher N and lower C:N) for the herbivore treatment, but no effect of nitrogen deposition. Although litter decomposition rates were not strongly affected by litter treatments, detritivore communities were altered by all treatments. Nitrogen deposition resulted in decreased bacterial richness and decreased fungal diversity in-stream. Aquatic macroinvertebrate communities were influenced by the interacting effects of herbivory and nitrogen addition, with abundances highest in herbivore litter with the greatest N addition. Shredders showed the highest abundance in male, herbivore-attacked litter. The establishment of riparian willows along early successional streams and their interacting effects with herbivores and nitrogen deposition may be influencing detritivore community assembly at Mount St. Helens. More broadly, global changes like increased wet and dry N deposition and expanded ranges of key herbivores might influence tree litter decomposition in many ecosystems.
more »
« less
Plant sex influences aquatic–terrestrial interactions
Abstract A growing body of research shows that plant genetic factors can influence ecosystem processes and structure communities, but one aspect that has received little study is sex differentiation in dioecious plants. Since headwater streams are reliant on riparian leaf litterfall, plant sex differences in leaf traits may influence in‐stream processes. Sitka willow (Salix sitchensis) at Mount St. Helens is dioecious and heavily infested with the stem‐boring weevil (Cryptorhynchus lapathi), which causes branch dieback and summer litterfall. We found that female willow shrubs tend to grow closer to the stream bank, are more likely to be infected by the weevil, and have 42% higher litter C:N than male willows. These factors may lead to increased litter inputs and slower litter mass loss for female willows. The combination of colonization location, herbivore attack, altered litter quality, and slower mass loss results in female shrubs providing more sustained carbon and nutrient resources to microbes and invertebrates in the early successional streams at Mount St. Helens. In addition, since dioecy is a relatively common trait in riparian habitats, it is possible that plant sex plays a far more interesting role in structuring linked terrestrial–aquatic communities and ecosystem processes than previously understood.
more »
« less
- Award ID(s):
- 1836387
- PAR ID:
- 10459341
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Ecosphere
- Volume:
- 11
- Issue:
- 1
- ISSN:
- 2150-8925
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Leaf litter inputs can influence the structure and function of both terrestrial and adjacent aquatic ecosystems. Dioecy and herbivory are two factors that together have received little attention, yet have the potential to affect the quantity, quality, and timing of riparian litterfall, litter chemistry, and litter decomposition processes. Here, we explore litter chemistry differences for the dioecious Sitka willow ( Salix sitchensis Sanson ex. Bong), which is establishing on primary successional habitats at Mount St. Helens (WA, USA) and is heavily infested with a stem‐boring weevil ( Cryptorhynchus lapathi ). Weevil‐attacked branches produced summer senesced litter that had significantly higher %N, lower C:N ratios, and lower condensed tannins than litter from branches that were unattacked by the weevil and senesced naturally in the autumn. Weevils more often attack female willows; however, these common litter chemicals did not significantly differ between males and females within the weevil‐attacked and ‐unattacked groups. High‐resolution mass spectrometry was used to isolate compounds in litter from 10 Sitka willow individuals with approximately 1500–1600 individual compounds isolated from each sample. There were differences between weevil‐attacked litter and green leaf samples, but at this level, there was no clustering of male and female samples. However, further exploration of the isolated compounds determined a suite of compounds present only in either males or females. These findings suggest some variation in more complex litter chemistry between the sexes, and that significant differences in weevil‐attacked litter chemistry, coupled with the shift in seasonality of litter inputs to streams, could significantly affect in‐stream ecological processes, such as decomposition and detritivore activity.more » « less
-
The eruption of Mount St. Helens in 1980 resulted in a cataclysmic restructuring of its surrounding landscapes. The Pumice Plain is one of these landscapes, where tree species such as Sitka willow (Salix sitchensis) and their dependent communities have been established along newly-formed streams. Thus, the study of these dependent communities provides a unique and rare opportunity to investigate factors influencing metacommunity assembly during true primary succession. We analyzed the influence of landscape connectivity on metacommunity assembly through a novel application of circuit theory, alongside the effects of other factors such as stream locations, willow leaf chemistry, and leaf area. We found that landscape connectivity structures community composition on willows across the Pumice Plain, where the least connected willows favored active flyers such as the western tent caterpillar (Malacosoma fragilis) or the Pacific willow leaf beetle (Pyrrhalta decora carbo). We also found that multiple levels of spatial habitat structure linked via landscape connectivity can predict the presence of organisms lacking high rates of dispersal, such as the invasive stem-boring poplar weevil (Cryptorhynchus lapathi). This is critical for management as we show that the maintenance of a heterogeneous mixture of landscape connectivity and resource locations can facilitate metacommunity dynamics to promote ecosystem function and mitigate the influences of invasive species.more » « less
-
Abstract BackgroundWhile a considerable amount of research has explored plant community composition in primary successional systems, little is known about the microbial communities inhabiting these pioneer plant species. Fungal endophytes are ubiquitous within plants, and may play major roles in early successional ecosystems. Specifically, endophytes have been shown to affect successional processes, as well as alter host stress tolerance and litter decomposition dynamics—both of which are important components in harsh environments where soil organic matter is still scarce. ResultsTo determine possible contributions of fungal endophytes to plant colonization patterns, we surveyed six of the most common woody species on the Pumice Plain of Mount St. Helens (WA, USA; Lawetlat'la in the Cowlitz language; created during the 1980 eruption)—a model primary successional ecosystem—and found low colonization rates (< 15%), low species richness, and low diversity. Furthermore, while endophyte community composition did differ among woody species, we found only marginal evidence of temporal changes in community composition over a single field season (July–September). ConclusionsOur results indicate that even after a post-eruption period of 40 years, foliar endophyte communities still seem to be in the early stages of community development, and that the dominant pioneer riparian species Sitka alder (Alnus viridisssp.sinuata) and Sitka willow (Salix sitchensis) may be serving as important microbial reservoirs for incoming plant colonizers.more » « less
-
Abstract Large carnivores were persecuted in Yellowstone National Park, WY, USA, during the late 1800s and early 1900s, leading to extirpation of grey wolves (Canis lupus) and cougars (Puma concolor). Soon thereafter increased herbivory of riparian plant communities by Rocky Mountain Elk (Cervus elaphus) became widespread in the park's northern ungulate winter range or “northern range.” Wolves were reintroduced in 1995–1996, again completing the park's large carnivore guild. In 2004 and 2017, we sampled Geyer willow (Salix geyeriana), a commonly occurring tall willow, along the West and East Forks of Blacktail Deer Creek in the central portion of the northern range. Results indicated high levels of elk herbivory in the 1990s, as in previous decades, not only continued to keep willows short, generally ≤52 cm in height, but also resulted in stream widening and incision, leading to “oversized” channel cross‐sections and a drastically reduced frequency of overbank flows. However, by 2017, willow heights ≥200 cm ( = 310 cm) were prevalent, and canopy cover over the stream, essentially absent in 1995, had increased to 43% and 93% along the West Fork and East Fork, respectively. These recent increases in tall willow heights, greater canopy cover, well‐vegetated streambanks, and the recent development of an inset floodplain all pointed towards a riparian/aquatic ecosystem beginning to recover. Overall, results were consistent with a landscape‐scale trophic cascade, whereby reintroduced wolves, operating in concert with other large carnivores, appear to have sufficiently reduced elk herbivory in riparian areas to initiate the recovery of Blacktail Deer Creek's riparian plant communities and stream channels.more » « less
