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  1. abstract

    Long-term observations and experiments in diverse drylands reveal how ecosystems and services are responding to climate change. To develop generalities about climate change impacts at dryland sites, we compared broadscale patterns in climate and synthesized primary production responses among the eight terrestrial, nonforested sites of the United States Long-Term Ecological Research (US LTER) Network located in temperate (Southwest and Midwest) and polar (Arctic and Antarctic) regions. All sites experienced warming in recent decades, whereas drought varied regionally with multidecadal phases. Multiple years of wet or dry conditions had larger effects than single years on primary production. Droughts, floods, and wildfires altered resource availability and restructured plant communities, with greater impacts on primary production than warming alone. During severe regional droughts, air pollution from wildfire and dust events peaked. Studies at US LTER drylands over more than 40 years demonstrate reciprocal links and feedbacks among dryland ecosystems, climate-driven disturbance events, and climate change.

     
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  2. Whether the terrestrial biosphere will continue to act as a net carbon (C) sink in the face of multiple global changes is questionable. A key uncertainty is whether increases in plant C fixation under elevated carbon dioxide (CO2) will translate into decades-long C storage and whether this depends on other concurrently changing factors. We investigated how manipulations of CO2, soil nitrogen (N) supply, and plant species richness influenced total ecosystem (plant + soil to 60 cm) C storage over 19 y in a free-air CO2enrichment grassland experiment (BioCON) in Minnesota. On average, after 19 y of treatments, increasing species richness from 1 to 4, 9, or 16 enhanced total ecosystem C storage by 22 to 32%, whereas N addition of 4 g N m−2⋅ y−1and elevated CO2of +180 ppm had only modest effects (increasing C stores by less than 5%). While all treatments increased net primary productivity, only increasing species richness enhanced net primary productivity sufficiently to more than offset enhanced C losses and substantially increase ecosystem C pools. Effects of the three global change treatments were generally additive, and we did not observe any interactions between CO2and N. Overall, our results call into question whether elevated CO2will increase the soil C sink in grassland ecosystems, helping to slow climate change, and suggest that losses of biodiversity may influence C storage as much as or more than increasing CO2or high rates of N deposition in perennial grassland systems.

     
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  3. null (Ed.)
  4. Abstract

    Cold‐air pooling is a global phenomenon that frequently sustains low temperatures in sheltered, low‐lying depressions and valleys and drives other key environmental conditions, such as soil temperature, soil moisture, vapor pressure deficit, frost frequency, and winter dynamics. Local climate patterns in areas prone to cold‐air pooling are partly decoupled from regional climates and thus may be buffered from macroscale climate change. There is compelling evidence from studies across the globe that cold‐air pooling impacts plant communities and species distributions, making these decoupled microclimate areas potentially important microrefugia for species under climate warming. Despite interest in the potential for cold‐air pools to enable species persistence under warming, studies investigating the effects of cold‐air pooling on ecosystem processes are scarce. Because local temperatures and vegetation composition are critical drivers of ecosystem processes like carbon cycling and storage, cold‐air pooling may also act to preserve ecosystem functions. We review research exploring the ecological impacts of cold‐air pooling with a focus on vegetation, and then present a new conceptual framework in which cold‐air pooling creates feedbacks between species and ecosystem properties that generate unique hotspots for carbon accrual in some systems relative to areas more vulnerable to regional climate change impacts. Finally, we describe key steps to motivate future research investigating the potential for cold‐air pools to serve as microrefugia for ecosystem functions under climate change.

     
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  5. Nie and colleagues suggest a key role for interannual climate variation as an explanation for the temporal dynamics of an unexpected 20-year reversal of biomass responses of C 3 -C 4 grasses to elevated CO 2 . However, we had already identified some climate-dependent differences in C 3 and C 4 responses to eCO 2 and shown that these could not fully explain the temporal dynamics we observed. 
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  6. Abstract

    Global changes can interact to affect photosynthesis and thus ecosystem carbon capture, yet few multi‐factor field studies exist to examine such interactions. Here, we evaluate leaf gas exchange responses of five perennial grassland species from four functional groups to individual and interactive global changes in an open‐air experiment in Minnesota, USA, including elevated CO2(eCO2), warming, reduced rainfall and increased soil nitrogen supply. All four factors influenced leaf net photosynthesis and/or stomatal conductance, but almost all effects were context‐dependent, i.e. they differed among species, varied with levels of other treatments and/or depended on environmental conditions. Firstly, the response of photosynthesis to eCO2depended on species and nitrogen, became more positive as vapour pressure deficit increased and, for a C4grass and a legume, was more positive under reduced rainfall. Secondly, reduced rainfall increased photosynthesis in three functionally distinct species, potentially via acclimation to low soil moisture. Thirdly, warming had positive, neutral or negative effects on photosynthesis depending on species and rainfall. Overall, our results show that interactions among global changes and environmental conditions may complicate predictions based on simple theoretical expectations of main effects, and that the factors and interactions influencing photosynthesis vary among herbaceous species.

     
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  7. Abstract

    Uncertainty about long‐term leaf‐level responses to atmospheric CO2rise is a major knowledge gap that exists because of limited empirical data. Thus, it remains unclear how responses of leaf gas exchange to elevated CO2(eCO2) vary among plant species and functional groups, or across different levels of nutrient supply, and whether they persist over time for long‐lived perennials. Here, we report the effects of eCO2on rates of net photosynthesis and stomatal conductance in 14 perennial grassland species from four functional groups over two decades in a Minnesota Free‐Air CO2Enrichment experiment, BioCON. Monocultures of species belonging to C3grasses, C4grasses, forbs, and legumes were exposed to two levels of CO2and nitrogen supply in factorial combinations over 21 years. eCO2increased photosynthesis by 12.9% on average in C3species, substantially less than model predictions of instantaneous responses based on physiological theory and results of other studies, even those spanning multiple years. Acclimation of photosynthesis to eCO2was observed beginning in the first year and did not strengthen through time. Yet, contrary to expectations, the response of photosynthesis to eCO2was not enhanced by increased nitrogen supply. Differences in responses among herbaceous plant functional groups were modest, with legumes responding the most and C4grasses the least as expected, but did not further diverge over time. Leaf‐level water‐use efficiency increased by 50% under eCO2primarily because of reduced stomatal conductance. Our results imply that enhanced nitrogen supply will not necessarily diminish photosynthetic acclimation to eCO2in nitrogen‐limited systems, and that significant and consistent declines in stomatal conductance and increases in water‐use efficiency under eCO2may allow plants to better withstand drought.

     
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