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Abstract Given the current rates of climate change, with associated shifts in herbivore population densities, understanding the role of different herbivores in ecosystem functioning is critical for predicting ecosystem responses. Here, we examined how migratory geese and resident, non‐migratory reindeer—two dominating yet functionally contrasting herbivores—control vegetation and ecosystem processes in rapidly warming Arctic tundra.We collected vegetation and ecosystem carbon (C) flux data at peak plant growing season in the two longest running, fully replicated herbivore removal experiments found in high‐Arctic Svalbard. Experiments had been set up independently in wet habitat utilised by barnacle geeseBranta leucopsisin summer and in moist‐to‐dry habitat utilised by wild reindeerRangifer tarandus platyrhynchusyear‐round.Excluding geese induced vegetation state transitions from heavily grazed, moss‐dominated (only 4 g m−2of live above‐ground vascular plant biomass) to ungrazed, graminoid‐dominated (60 g m−2after 4‐year exclusion) and horsetail‐dominated (150 g m−2after 15‐year exclusion) tundra. This caused large increases in vegetation C and nitrogen (N) pools, dead biomass and moss‐layer depth. Alterations in plant N concentration and CN ratio suggest overall slower plant community nutrient dynamics in the short‐term (4‐year) absence of geese. Long‐term (15‐year) goose removal quadrupled net ecosystem C sequestration (NEE) by increasing ecosystem photosynthesis more than ecosystem respiration (ER).Excluding reindeer for 21 years also produced detectable increases in live above‐ground vascular plant biomass (from 50 to 80 g m−2; without promoting vegetation state shifts), as well as in vegetation C and N pools, dead biomass, moss‐layer depth and ER. Yet, reindeer removal did not alter the chemistry of plants and soil or NEE.Synthesis. Although both herbivores were key drivers of ecosystem structure and function, the control exerted by geese in their main habitat (wet tundra) was much more pronounced than that exerted by reindeer in their main habitat (moist‐to‐dry tundra). Importantly, these herbivore effects are scale dependent, because geese are more spatially concentrated and thereby affect a smaller portion of the tundra landscape compared to reindeer. Our results highlight the substantial heterogeneity in how herbivores shape tundra vegetation and ecosystem processes, with implications for ongoing environmental change.
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Plant community response to warming and herbivory on sub-arctic coastal terraces in Western Alaska, 2015 - 2016
To predict future changes in high latitude biomes, it is important to understand how plant communities will respond to increased temperature. Across sub-arctic systems, warming generally increases aboveground biomass in plant communities. Specifically, in arctic graminoid systems, experimental warming has been shown to increase productivity, aboveground biomass and leaf litter production, and stimulate early-season growth. Warming can also decrease species richness, and reduce foliar nitrogen (N) in aboveground biomass over the growing season. Migrating geese are important grazers in arctic and subarctic ecosystems during summer breeding months. Avian herbivores depend on high quality forage (high N) and are often found at high enough densities to impact vegetation communities. Exclosure experiments show that goose herbivory reduces biomass of herbaceous species but increases net above-ground primary production and N concentrations of grazing-tolerant sedges, and sometimes even increases species richness. Goose herbivory also alters plant physiological processes as evidenced by increased N uptake by plants, as well as the biophysical processes that affect N cycling through trampling and fecal deposition. Thus, high-density populations of avian herbivores can have top-down control on their vegetation communities. While increasing global temperatures may increase aboveground biomass and decrease species richness in plant communities, herbivory could potentially mediate, or even reverse, these responses. For example, Post and Pedersen (2008) suggest that herbivory may exacerbate plant response to warming because both effects increase rates of productivity, while simultaneously reducing the effects of warming on aboveground biomass. If the interaction between warming and herbivory causes a shift in plant abundance and community functional groups, this could cascade through the system resulting in changes in nutrient cycling and plant-animal feedbacks. The Yukon-Kuskokwim (Y-K) Delta is one of the largest river deltas in the world and is a globally important breeding area for millions of long-distance migratory waterfowl and shorebird species. The majority of these species nest in high densities close to the ocean among lowland coastal habitat. Geese populations utilize overlapping habitats and shift from more coastal to more interior habitats over the growing season. The expectations for how vegetation responds to increasing temperature and changes in herbivory with climate change will vary for different plant communities. We propose to conduct an experiment that investigates the impact of warming and herbivory on three coastal sub-arctic vegetation communities in the Y-K Delta addressing the following questions: 1) How does warming impact vegetation biomass and community composition; 2) How does herbivory impact species composition and plant functional groups; and 3) How do the different responses to warming and herbivory interact?
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- PAR ID:
- 10647394
- Publisher / Repository:
- NSF Arctic Data Center
- Date Published:
- Subject(s) / Keyword(s):
- Climate change forbs grasses International tundra experiment (ITEX) plant-herbivore interactions plant functional groups sedges shrubs tundra Yukon-Kuskokwim Delta
- Format(s):
- Medium: X Other: text/xml
- Sponsoring Org:
- National Science Foundation
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Abstract QuestionUnderstanding the sensitivity and magnitude of plant community responses in tundra wetlands to herbivory and warming is pressing as these ecosystems are increasingly threatened by changes in grazing pressure and higher temperatures. Here, we ask to what extent different low‐Arctic coastal wetland plant communities are affected by short‐term goose grazing and warming, and whether these communities differ in their responses. LocationYukon–Kuskokwim Delta, Alaska. MethodsWe conducted an experiment where we simulated goose grazing by clipping the vegetation and summer warming by using open‐top chambers in three plant communities along a 6‐km coastal–inland gradient. We assessed plant community compositional changes following two years of treatments. ResultsGrazing had stronger effects than warming on both plant functional group and species composition. Overall, grazing decreased the abundance of grasses and sedges and increased the abundance of forbs, whereas warming only caused a decrease in forb abundance. However, plant communities and functional groups, both within and across communities, varied widely in their responses to treatments. Grazing decreased grass abundance (−25%) and increased forb abundance (+44%) in the two more coastal communities, and reduced sedge abundance (−22%) only in the most inland community. Warming only decreased forb abundance (−18%) in the most coastal community, which overall was the most responsive to treatments. ConclusionsWe show that short‐term goose grazing predominates over short‐term summer warming in eliciting compositional changes in three different low‐Arctic coastal wetland plant communities. Yet, responses varied among communities and the same functional groups could respond differently across them, highlighting the importance of investigating the effects of biotic and abiotic drivers in different contexts. By showing that tundra wetland plant communities can differ in their immediate sensitivity to goose grazing and, though to a lesser extent, warming, our findings have implications for the functioning of these rapidly changing high‐latitude ecosystems.more » « less
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The large carbon (C) stock of wetlands is vulnerable to climate change, especially in high latitudes that are warming at a disproportional rate. Likewise, low-lying Arctic areas will experience increased coastal flooding under climate change and sea-level rise, which may alter goose herbivory and fecal deposition patterns if geese are pushed inland. While temperature, flooding, and feces impact soil C emissions, their interactive effects have been rarely studied. Here, we explore the impact of these interactions on carbon dioxide (CO2) and methane (CH4) emissions and nitrogen (N) mineralization (ammonification) in soils collected from four plant communities in the Yukon-Kuskokwim (Y-K) Delta, a high latitude coastal wetland in western Alaska. Communities included a Grazing Lawn, which is intensely grazed and susceptible to flooding, a Lowland Wetland and an Upland Wetland that experience moderate grazing and frequent (Lowland) and less frequent (Upland) flooding, and a rarely grazed and flooded Tundra community, located at the highest elevation. Soils were incubated for 16 weeks at 8 degrees Celsius (°C) or 18°C in microcosms and subjected to flooding and feces addition treatments with no-flood and no-feces controls. We quantified C emissions weekly and ammonification over the course of the experiment. While warming increased ammonification and C demand in the Lowland Wetland and always increased CO2 and CH4 emissions, interactions with flooding complicated warming impacts on C emissions in the Grazing Lawn and Tundra. In the Grazing Lawn, flooding increased CH4 emissions at 8°C and 18 °C, but in the Tundra, flooding suppressed CH4 emissions at 18°C. Flooding alone reduced CO2 emissions in the Upland Wetland. Feces addition increased CO2 emissions in all communities, but feces impacts on CH4 emissions and ammonification were minimal. When feces and flooding occurred together in the Lowland Wetland, CH4 emissions decreased compared to when feces was added without concomitant flood. Feces decreased the immobilization of ammonium and N demand in the Tundra only. Our results suggest that flooding could partially offset C loss from warming in less frequently flooded, higher elevation communities, but this offset could be negligible if flooding and warming drastically increase C loss in more flooded lowland areas.more » « less
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This dataset was created to understand plant trait responses to warming, flooding, and herbivory in the Yukon-Kuskokwim (Y-K) Delta (western Alaska, USA). We conducted a one-year field mesocosm experiment in which we passively increased temperatures, simulated periodic tidal flooding at two intensity levels (low and high), and applied three components of goose herbivory (grazing, feces addition, and trampling) during the summer growing season. Our treatments reflect changes expected in the Y-K Delta in the next 10-20 years. We conducted the experiment in three community types: a wet sedge-shrub meadow, a tundra, and a transitional wet community between the meadow and tundra, and only sampled the dominant species in these communities. At the end of the season, we harvested height, leaf area, specific leaf area, and leaf dry matter content from randomly selected individuals.more » « less
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Abstract Climate change is creating phenological mismatches between herbivores and their plant resources throughout the Arctic. While advancing growing seasons and changing arrival times of migratory herbivores can have consequences for herbivores and forage quality, developing mismatches could also influence other traits of plants, such as above‐ and below‐ground biomass and the type of reproduction, that are often not investigated.In coastal western Alaska, we conducted a 3‐year factorial experiment that simulated scenarios of phenological mismatch by manipulating the start of the growing season (3 weeks early and ambient) and grazing times (3 weeks early, typical, 3 weeks late, or no‐grazing) of Pacific black brant (Branta bernicla nigricans), to examine how the timing of these events influence a primary goose forage species,Carex subspathacea.After 3 years, an advanced growing season compared to a typical growing season increased stem heights, standing dead biomass, and the number of inflorescences. Early season grazing compared to typical season grazing reduced above‐ and below‐ground biomass, stem height, and the number of tillers; while late season grazing increased the number of inflorescences and standing dead biomass. Therefore, an advanced growing season and late grazing had similar directional effects on most plant traits, but a 3‐week delay in grazing had an impact on traits 3–5 times greater than a similarly timed shift in the advancement of spring. In addition, changes in response to treatments for some variables, such as the number of inflorescences, were not measurable until the second year of the experiment, while other variables, such as root productivity and number of tillers, changed the direction of their responses to treatments over time.Synthesis. Factors affecting the timing of migration have a larger influence than earlier springs on an important forage species in the breeding and rearing habitats of Pacific black brant. The phenological mismatch prediction for this site of earlier springs and later goose arrival will likely increase above‐ and below‐ground biomass and sexual reproduction of the often‐clonally reproducingC. subspathacea. Finally, the implications of mismatch may be difficult to predict because some variables required successive years of mismatch to respond.more » « less
