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1. Goose grubbing and warming suppress summer net ecosystem CO₂ uptake differentially across high‐Arctic tundra habitats

   https://doi.org/10.1002/ecy.4498  

   Petit_Bon, Matteo; Beard, Karen H; Bråthen, Kari Anne; Lee, Hanna; Jónsdóttir, Ingibjörg S (January 2025, Ecology)

   Abstract Environmental changes, such as climate warming and higher herbivory pressure, are altering the carbon balance of Arctic ecosystems; yet, how these drivers modify the carbon balance among different habitats remains uncertain. This hampers our ability to predict changes in the carbon sink strength of tundra ecosystems. We investigated how spring goose grubbing and summer warming—two key environmental‐change drivers in the Arctic—alter CO₂fluxes in three tundra habitats varying in soil moisture and plant‐community composition. In a full‐factorial experiment in high‐Arctic Svalbard, we simulated grubbing and warming over two years and determined summer net ecosystem exchange (NEE) alongside its components: gross ecosystem productivity (GEP) and ecosystem respiration (ER). After two years, we found net CO₂uptake to be suppressed by both drivers depending on habitat. CO₂uptake was reduced by warming in mesic habitats, by warming and grubbing in moist habitats, and by grubbing in wet habitats. In mesic habitats, warming stimulated ER (+75%) more than GEP (+30%), leading to a 7.5‐fold increase in their CO₂source strength. In moist habitats, grubbing decreased GEP and ER by ~55%, while warming increased them by ~35%, with no changes in summer‐long NEE. Nevertheless, grubbing offset peak summer CO₂uptake and warming led to a twofold increase in late summer CO₂source strength. In wet habitats, grubbing reduced GEP (−40%) more than ER (−30%), weakening their CO₂sink strength by 70%. One‐year CO₂‐flux responses were similar to two‐year responses, and the effect of simulated grubbing was consistent with that of natural grubbing. CO₂‐flux rates were positively related to aboveground net primary productivity and temperature. Net ecosystem CO₂uptake started occurring above ~70% soil moisture content, primarily due to a decline in ER. Herein, we reveal that key environmental‐change drivers—goose grubbing by decreasing GEP more than ER and warming by enhancing ER more than GEP—consistently suppress net tundra CO₂uptake, although their relative strength differs among habitats. By identifying how and where grubbing and higher temperatures alter CO₂fluxes across the heterogeneous Arctic landscape, our results have implications for predicting the tundra carbon balance under increasing numbers of geese in a warmer Arctic. 

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2. Plant community response to warming and herbivory on sub-arctic coastal terraces in Western Alaska, 2015 - 2016

   https://doi.org/10.18739/a2hm52m2s  

   Beard, Karen; Choi, Ryan (January 2017, NSF Arctic Data Center)

   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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3. Long-term herbivore removal experiments reveal how geese and reindeer shape vegetation and ecosystem CO2-fluxes in high-Arctic tundra

   https://doi.org/10.1111/1365-2745.14200  

   Petit Bon, Matteo; Hansen, Brage B.; Loonen, Maarten J.; Petraglia, Alessandro; Bråthen, Kari Anne; Böhner, Hanna; Layton‐Matthews, Kate; Beard, Karen H.; Le Moullec, Mathilde; Jónsdóttir, Ingibjörg S.; et al (December 2023, Journal of Ecology)

   1. 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. 2. 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 geese Branta leucopsis in summer and in moist-to-dry habitat utilised by wild reindeer Rangifer tarandus platyrhynchus year-round. 3. Excluding geese induced vegetation state transitions from heavily grazed, moss-dominated (only 4 g m−2 of live above-ground vascular plant biomass) to ungrazed, graminoid-dominated (60 g m−2 after 4-year exclusion) and horsetail-dominated (150 g m−2 after 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 nu- trient 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). 4. Excluding reindeer for 21 years also produced detectable increases in live aboveground 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. 5. 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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4. Projected near‐future flooding and warming increase graminoid biomass in a high‐latitude coastal wetland

   https://doi.org/10.1111/1365-2745.14418  

   Petit_Bon, Matteo; Leffler, A Joshua; Kelsey, Katharine C; Williams, Tyler J; Beard, Karen H (December 2024, Journal of Ecology)

   Abstract: With rapid climate warming, some coastal high‐latitude ecosystems are experiencing more frequent tidal floods. Yet little is known about tundra plant‐community responses to flooding, and whether Arctic warming may modulate such responses.In a 2‐year, full‐factorial field experiment in coastal tundra wetlands of the Yukon‐Kuskokwim (Y‐K) Delta (western Alaska), we simulated periodic tidal flood events at two severities under both ambient and warmed summer conditions and measured above‐ground plant‐community responses. Low‐severity flooding represented overbank flooding 1 day per month, which is consistent with projections in the next 5 years. High‐severity flooding represented a more impactful flooding regime (three consecutive days per month) that is projected to occur in the next 10 years. Our warming treatment (+1°C) also represented a change projected in the next 10 years.Regardless of temperature, high‐severity flooding increased graminoid biomass by >45%, in turn increasing live plant‐community biomass by >18%. Low‐severity flooding had similar, though weaker, effects. Flooding had overall negative effects on both forb and shrub biomass, though shrub responses were weaker. Only during the second summer, warming increased graminoid biomass by 20% and tended to increase shrub biomass, regardless of flooding. Concurrently, warming enhanced standing‐dead graminoid biomass by 20%, while high‐severity flooding decreased it by 15%. Therefore, wet tundra that was both flooded and warmed had the greatest proportion of graminoids and total live biomass, but standing‐dead biomass comparable to that of unmanipulated wet tundra. While our manipulations simulated flooding and warming regimes expected in the wetlands of the Y‐K Delta over the same, near‐future (5‐to‐10 years) time frame, flooding had stronger effects than warming. What is striking is the rate at which graminoid increases occurred, becoming apparent after only two monthly flood events in the first experimental year. Flooding‐induced decreases in standing‐dead biomass suggests that the incorporation of dead plant material into the litter layer might be facilitated by tidal floods. These rapid increases in plant biomass and potentially biomass turnover, especially of graminoids, which are characterized by high‐quality litter, may have major implications for carbon and nutrient cycling of more frequently flooded coastal ecosystems in a warmer Arctic. 

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5. Wetland plant functional trait responses to experimental warming, flooding, and herbivory, Yukon-Kuskokwim Delta (Western Alaska, USA) (2022-2023)

   https://doi.org/10.18739/a2m03z02f  

   Chirvasă, Cristina; Petit_Bon, Matteo; Beard, Karen (January 2025, NSF Arctic Data Center)

   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. 

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