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1. Cloud cover and delayed herbivory relative to timing of spring onset interact to dampen climate change impacts on net ecosystem exchange in a coastal Alaskan wetland

   https://doi.org/10.1088/1748-9326/ab1c91  

   Leffler, A. Joshua ; Beard, Karen H. ; Kelsey, Katharine C. ; Choi, Ryan T. ; Schmutz, Joel A. ; Welker, Jeffrey M. ( August 2019 , Environmental Research Letters)

   Rapid warming in northern ecosystems over the past four decades has resulted in earlier spring, increased precipitation, and altered timing of plant–animal interactions, such as herbivory. Advanced spring phenology can lead to longer growing seasons and increased carbon (C) uptake. Greater precipitation coincides with greater cloud cover possibly suppressing photosynthesis. Timing of herbivory relative to spring phenology influences plant biomass. None of these changes are mutually exclusive and their interactions could lead to unexpected consequences for Arctic ecosystem function. We examined the influence of advanced spring phenology, cloud cover, and timing of grazing on C exchange in the Yukon–Kuskokwim Delta of western Alaska for three years. We combined advancement of the growing season using passive-warming open-top chambers (OTC) with controlled timing of goose grazing (early, typical, and late season) and removal of grazing. We also monitored natural variation in incident sunlight to examine the C exchange consequences of these interacting forcings. We monitored net ecosystem exchange of C (NEE) hourly using an autochamber system. Data were used to construct daily light curves for each experimental plot and sunlight data coupled with a clear-sky model was used to quantify daily and seasonal NEE over a range of incident sunlight conditions.more »Cloudy days resulted in the largest suppression of NEE, reducing C uptake by approximately 2 g C m⁻²d⁻¹regardless of the timing of the season or timing of grazing. Delaying grazing enhanced C uptake by approximately 3 g C m⁻²d⁻¹. Advancing spring phenology reduced C uptake by approximately 1.5 g C m⁻²d⁻¹, but only when plots were directly warmed by the OTCs; spring advancement did not have a long-term influence on NEE. Consequently, the two strongest drivers of NEE, cloud cover and grazing, can have opposing effects and thus future growing season NEE will depend on the magnitude of change in timing of grazing and incident sunlight.

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2. Phenological mismatch between season advancement and migration timing alters Arctic plant traits

   https://doi.org/10.1111/1365-2745.13191  

   Choi, Ryan T. ; Beard, Karen H. ; Leffler, A. Joshua ; Kelsey, Katharine C. ; Schmutz, Joel A. ; Welker, Jeffrey M. ; Cornelissen, ed., Hans ( May 2019 , Journal of Ecology)

   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 hadmore »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.

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3. Dynamic global vegetation models underestimate net CO ₂ flux mean and inter-annual variability in dryland ecosystems

   https://doi.org/10.1088/1748-9326/ac1a38  

   MacBean, Natasha ; Scott, Russell L ; Biederman, Joel A ; Peylin, Philippe ; Kolb, Thomas ; Litvak, Marcy E ; Krishnan, Praveena ; Meyers, Tilden P ; Arora, Vivek K ; Bastrikov, Vladislav ; et al ( August 2021 , Environmental Research Letters)

   Abstract Despite their sparse vegetation, dryland regions exert a huge influence over global biogeochemical cycles because they cover more than 40% of the world surface (Schimel 2010 Science 327 418–9). It is thought that drylands dominate the inter-annual variability (IAV) and long-term trend in the global carbon (C) cycle (Poulter et al 2014 Nature 509 600–3, Ahlstrom et al 2015 Science 348 895–9, Zhang et al 2018 Glob. Change Biol . 24 3954–68). Projections of the global land C sink therefore rely on accurate representation of dryland C cycle processes; however, the dynamic global vegetation models (DGVMs) used in future projections have rarely been evaluated against dryland C flux data. Here, we carried out an evaluation of 14 DGVMs (TRENDY v7) against net ecosystem exchange (NEE) data from 12 dryland flux sites in the southwestern US encompassing a range of ecosystem types (forests, shrub- and grasslands). We find that all the models underestimate both mean annual C uptake/release as well as the magnitude of NEE IAV, suggesting that improvements in representing dryland regions may improve global C cycle projections. Across all models, the sensitivity and timing of ecosystem C uptake to plant available moisture was at fault. Spring biases inmore »gross primary production (GPP) dominate the underestimate of mean annual NEE, whereas models’ lack of GPP response to water availability in both spring and summer monsoon are responsible for inability to capture NEE IAV. Errors in GPP moisture sensitivity at high elevation forested sites were more prominent during the spring, while errors at the low elevation shrub and grass-dominated sites were more important during the monsoon. We propose a range of hypotheses for why model GPP does not respond sufficiently to changing water availability that can serve as a guide for future dryland DGVM developments. Our analysis suggests that improvements in modeling C cycle processes across more than a quarter of the Earth’s land surface could be achieved by addressing the moisture sensitivity of dryland C uptake.« less
4. ORCHIDEE-PEAT (revision 4596), a model for northern peatland CO₂, water, and energy fluxes on daily to annual scales

   https://doi.org/10.5194/gmd-11-497-2018  

   Qiu, Chunjing ; Zhu, Dan ; Ciais, Philippe ; Guenet, Bertrand ; Krinner, Gerhard ; Peng, Shushi ; Aurela, Mika ; Bernhofer, Christian ; Brümmer, Christian ; Bret-Harte, Syndonia ; et al ( January 2018 , Geoscientific Model Development)

   Peatlands store substantial amounts of carbon and are vulnerable to climate change. We present a modified version of the Organising Carbon and Hydrology In Dynamic Ecosystems (ORCHIDEE) land surface model for simulating the hydrology, surface energy, and CO₂ fluxes of peatlands on daily to annual timescales. The model includes a separate soil tile in each 0.5° grid cell, defined from a global peatland map and identified with peat-specific soil hydraulic properties. Runoff from non-peat vegetation within a grid cell containing a fraction of peat is routed to this peat soil tile, which maintains shallow water tables. The water table position separates oxic from anoxic decomposition. The model was evaluated against eddy-covariance (EC) observations from 30 northern peatland sites, with the maximum rate of carboxylation (V_cmax) being optimized at each site. Regarding short-term day-to-day variations, the model performance was good for gross primary production (GPP) (r² =  0.76; Nash–Sutcliffe modeling efficiency, MEF  =  0.76) and ecosystem respiration (ER, r² =  0.78, MEF  =  0.75), with lesser accuracy for latent heat fluxes (LE, r² =  0.42, MEF  =  0.14) and and net ecosystem CO₂ exchange (NEE, r² =  0.38, MEF  =  0.26). Seasonal variations in GPP, ER, NEE, and energy fluxes on monthly scales showed moderate to high r² values (0.57–0.86). For spatial across-site gradients of annual meanmore »GPP, ER, NEE, and LE, r² values of 0.93, 0.89, 0.27, and 0.71 were achieved, respectively. Water table (WT) variation was not well predicted (r² < 0.1), likely due to the uncertain water input to the peat from surrounding areas. However, the poor performance of WT simulation did not greatly affect predictions of ER and NEE. We found a significant relationship between optimized V_cmax and latitude (temperature), which better reflects the spatial gradients of annual NEE than using an average V_cmax value.« less
5. Seasonal Water Mass Evolution and Non‐Redfield Dynamics Enhance CO ₂ Uptake in the Chukchi Sea

   https://doi.org/10.1029/2021JC018326  

   Ouyang, Zhangxian ; Collins, Andrew ; Li, Yun ; Qi, Di ; Arrigo, Kevin R. ; Zhuang, Yanpei ; Nishino, Shigeto ; Humphreys, Matthew P. ; Kosugi, Naohiro ; Murata, Akihiko ; et al ( August 2022 , Journal of Geophysical Research: Oceans)

   The Chukchi Sea is an increasing CO₂sink driven by rapid climate changes. Understanding the seasonal variation of air‐sea CO₂exchange and the underlying mechanisms of biogeochemical dynamics is important for predicting impacts of climate change on and feedbacks by the ocean. Here, we present a unique data set of underway sea surface partial pressure of CO₂(pCO₂) and discrete samples of biogeochemical properties collected in five consecutive cruises in 2014 and examine the seasonal variations in air‐sea CO₂flux and net community production (NCP). We found that thermal and non‐thermal effects have different impacts on sea surfacepCO₂and thus the air‐sea CO₂flux in different water masses. The Bering summer water combined with meltwater has a significantly greater atmospheric CO₂uptake potential than that of the Alaskan Coastal Water in the southern Chukchi Sea in summer, due to stronger biological CO₂removal and a weaker thermal effect. By analyzing the seasonal drawdown of dissolved inorganic carbon (DIC) and nutrients, we found that DIC‐based NCP was higher than nitrate‐based NCP by 66%–84% and attributable to partially decoupled C and N uptake because of a variable phytoplankton stoichiometry. A box model with a non‐Redfield C:N uptake ratio can adequately reproduce observedpCO₂and DIC, which reveals that, during the intensivemore »growing season (late spring to early summer), 30%–46% CO₂uptake in the Chukchi Sea was supported by a flexible stoichiometry of phytoplankton. These findings have important ramification for forecasting the responses of CO₂uptake of the Chukchi ecosystem to climate change.

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