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Earth System Models (ESMs) are increasingly representing agriculture due to its impact on biogeochemical cycles, local and regional climate, and fundamental importance for human society. Realistic large scale simulations may require spatially varying crop parameters that capture crop growth at various scales and among different cultivars, as well as common crop management practices, but their importance is uncertain, and they are often not represented in ESMs. In this study, we examine the impact of using constant versus spatially varying crop parameters using a novel, realistic crop rotation scenario in the Energy Exascale Earth System Model (E3SM) Land Model version 2 (ELMv2). We implemented crop rotation by using ELMv2's dynamic land unit capability, and then calibrated and validated the model against observations collected at three AmeriFlux sites in the US Midwest with corn soybean rotation. The calibrated model closely captured the magnitude and observed seasonality of carbon and energy fluxes across crops and sites. We performed regional simulations for the US Midwest using the calibrated model and found that spatially varying only a few crop parameters across the region, as opposed to using constant parameters, had a large impact, with the carbon fluxes and energy fluxes both varying by up to 40%. These results imply that large scale ESM simulations using spatially invariant crop parameters may result in biased energy and carbon fluxes estimation from agricultural land, and underline the importance of improving human‐earth systems interactions in ESMs. Plain Language Summary: Crops are increasingly being characterized in global land models because of their impact on local and regional climate. However, there is limited understanding of the impact of crop rotation and of different crop cultivars on carbon and energy fluxes from the land surface. Our study implements crop rotation and spatially varying crop parameters in the Energy Exascale Earth System Model Land Model and finds that doing so improves carbon and energy flux estimation from cropland area. These findings emphasize the importance of capturing agricultural management practices and variability in growth characteristics across different crop cultivars in global land models. Key Points: This study implements corn soybean rotation and spatially varying crop parameters in the Energy Exascale Earth System Land ModelThe model is calibrated and validated against observations collected at three AmeriFlux sites in the US MidwestWe find that spatially varying crop parameters resulted in improved flux estimation from cropland areas [ABSTRACT FROM AUTHOR]

Long‐running eddy covariance flux towers provide insights into how the terrestrial carbon cycle operates over multiple timescales. Here, we evaluated variation in net ecosystem exchange (NEE) of carbon dioxide (CO2) across the Chequamegon Ecosystem‐Atmosphere Study AmeriFlux core site cluster in the upper Great Lakes region of the USA from 1997 to 2020. The tower network included two mature hardwood forests with differing management regimes (US‐WCr and US‐Syv), two fen wetlands with varying levels of canopy sheltering and vegetation (US‐Los and US‐ALQ), and a very tall (400 m) landscape‐level tower (US‐PFa). Together, they provided over 70 site‐years of observations. The 19‐tower Chequamegon Heterogenous Ecosystem Energy‐balance Study Enabled by a High‐density Extensive Array of Detectors 2019 campaign centered around US‐PFa provided additional information on the spatial variation of NEE. Decadal variability was present in all long‐term sites, but cross‐site coherence in interannual NEE in the earlier part of the record became weaker with time as non‐climatic factors such as local disturbances likely dominated flux time series. Average decadal NEE at the tall tower transitioned from carbon source to sink to near neutral over 24 years. Respiration had a greater effect than photosynthesis on driving variations in NEE at all sites. Declining snowfall offset potential increases in assimilation from warmer springs, as less‐insulated soils delayed start of spring green‐up. Higher CO2 increased maximum net assimilation parameters but not total gross primary productivity. Stand‐scale sites were larger net sinks than the landscape tower. Clustered, long‐term carbon flux observations provide value for understanding the diverse links between carbon and climate and the challenges of upscaling these responses across space. Plain Language Summary: The terrestrial biosphere features the largest global sources and sinks of atmospheric carbon. Changes in growing season length, disturbance frequency, human management, increasing atmospheric carbon dioxide (CO2) concentrations, amount and timing of precipitation, and warmer air temperature all influence the carbon cycle. Observations from the global eddy covariance flux tower network have been key for diagnosing these changes. However, data from most sites are limited in length. Here, we explore how multi‐decadal carbon flux measurements from a cluster of flux towers in forests and wetlands in the upper Midwest USA respond to environmental change. Despite the proximity of the sites, year‐to‐year variation in carbon fluxes was rarely similar between sites. Surprisingly, warmer winters promoting earlier snowmelt led to later spring green‐up because soil temperature was colder. Impacts of higher CO2 and warmer temperature on annual carbon fluxes were limited but did influence factors linking carbon flux sensitivity to climate. Differences in flux magnitudes from a very tall tower flux to the network show that the whole does not seem to be simply a sum of its measured parts. More elaborate approaches may be needed to understand the processes that control carbon fluxes across large landscapes. Key Points: Multi‐decadal eddy covariance flux tower site cluster provides insight into variation of regional carbon cyclingVariation of carbon exchange in two forests, two wetlands, and a tall tower responded differently to weather, phenology, and disturbanceChallenges in upscaling fluxes indicate need for advances in aquatic observations, disturbance mapping, and flux footprint decomposition [ABSTRACT FROM AUTHOR]

Linking biometric measurements of stand‐level biomass growth to tower‐based measurements of carbon uptake—gross primary productivity and net ecosystem productivity—has been the focus of numerous ecosystem‐level studies aimed to better understand the factors regulating carbon allocation to slow‐turnover wood biomass pools. However, few of these studies have investigated the importance of previous year uptake to growth. We tested the relationship between wood biomass increment (WBI) and different temporal periods of carbon uptake from the current and previous years to investigate the potential lagged allocation of fixed carbon to growth among six mature, temperate forests. We found WBI was strongly correlated to carbon uptake across space (i.e., long‐term averages at the different sites) but on annual timescales, WBI was much less related to carbon uptake, suggesting a temporal mismatch between C fixation and allocation to biomass. We detected lags in allocation of the previous year's carbon uptake to WBI at three of the six sites. Sites with higher annual WBI had overall stronger correlations to carbon uptake, with the strongest correlations to carbon uptake from the previous year. Only one site had WBI with strong positive relationships to current year uptake and not the previous year. Forests with low rates of WBI demonstrated weak correlations to carbon uptake from the previous year and stronger relationships to current year climate conditions. Our work shows an important, but not universal, role of lagged allocation of the previous year's carbon uptake to growth in temperate forests. Plain Language Summary: We compared the interannual variability of stand‐level biomass growth (estimated from annual tree‐level measurements) to carbon uptake (measured from towers monitoring gas exchange over forest canopies) to identify temporal mismatches between the two processes. We used data from multiple forested sites with long‐term measurements of carbon uptake to ask the question: is there a consistent temporal offset between the uptake of carbon and the allocation to plant biomass? We found that the relationship between growth and carbon uptake varies among sites, and there was no consistent temporal offset between the uptake of carbon and allocation to biomass growth. Sites with higher growth rates had higher interannual variability, and more apparent coupling between biomass growth and carbon uptake from the previous year. Forests with lower growth rates had weaker relationships with carbon uptake and stronger coupling with current year environmental conditions. We demonstrate that temporal lags between carbon uptake and allocation to growth are not universal among these temperate forests, and the carry‐over of uptake stored from the previous year is not as critical in slow‐growing forests, compared to fast‐growing forests, likely due to lower demands for growth. This work helps to clarify the limitations on the growth of slow‐turnover wood biomass. Key Points: We found evidence for lags in allocation of previous year's carbon uptake to wood biomass increment (WBI) at three of six sites studiedSites with high annual WBI demonstrated strong correlations to carbon uptake, with the strongest correlations including previous year uptakeWBI was less tightly coupled to carbon uptake in less productive forests that also exhibited low interannual variability in WBI [ABSTRACT FROM AUTHOR]

A long‐standing goal of ecology has been to understand the cycling of carbon in forests. This has taken on new urgency with the need to address a rapidly changing climate. Forests serve as long‐term stores for atmospheric CO2, but their continued ability to take up new carbon is dependent on future changes in climate and other factors such as age. We have been measuring many aspects of carbon cycling at an unmanaged evergreen forest in central Maine, USA, for over 25 years. Here we use these data to address questions about the magnitude and control of carbon fluxes and quantify flows and uncertainties between the different pools. A key issue was to assess whether recent climate change and an aging tree population were reducing annual C storage. Total ecosystem C stocks determined from inventory and quantitative soil pits were about 23,300 g C m−2 with 46% in live trees, and 48% in the soil. Annual biomass increment in trees at Howland Forest averaged 161 ± 23 g C m−2 yr−1, not significantly different from annual net ecosystem production (NEP = −NEE) of 211 ± 40 g C m−2 y−1 measured by eddy covariance. Unexpectedly, there was a small but significant trend of increasing C uptake through time in the eddy flux data. This was despite the period of record including some of the most climate‐extreme years in the last 125. We find a surprising lack of influence of climate variability on annual carbon storage in this mature forest. Plain Language Summary: Trees remove carbon dioxide (CO2) from the atmosphere by photosynthesis and store it in chemical form in wood and other plant tissues. Much of the stored carbon ends up in the soil. Plants and soil organisms use some of the stored carbon to provide energy for growth and plant maintenance processes which releases CO2 back to the atmosphere. The movement of carbon atoms into and out of the forest and through the plants and soils is termed the "carbon cycle". We have been studying the carbon cycle of the Howland Forest in central Maine, an unmanaged evergreen forest with most trees between 100 and 200 years old. Over the last 25 years the forest has stored almost 3.5 tons of CO2 per acre each year, even though that timespan has included the warmest, wettest, and driest years in the last 125. Although the forest is maturing, it is storing on average a bit more carbon each year and there is as yet no clear reason why. Forest growth is a natural solution to the problem of too much CO2 in the atmosphere. This research helps show the longer‐term stability and viability of forests as natural climate solutions. Key Points: An unmanaged evergreen forest has been a net carbon sink for over 25 years and the sink strength has increased significantly with timeAnnual average carbon increment measured by eddy flux or inventory data are not significantly differentThe forest remained a robust C sink in the warmest, wettest, and driest years since 1895, indicating resistance to climate variability [ABSTRACT FROM AUTHOR]

Environmental observatory networks (EONs) provide information to understand and forecast the spatial and temporal dynamics of Earth's biophysical processes. Consequently, representativeness analyses are important to provide insights for improving EONs' management, design, and interpretation of their value‐added products. We assessed the representativeness of registered FLUXNET sites (n = 41, revised on September 2018) across Latin America (LA), a region of great importance for the global carbon and water cycles, which represents 13% of the world's land surface. Nearly 46% of registered FLUXNET sites are located in evergreen broad‐leaf forests followed by sites in woody savannas (∼20%). Representativeness analyses were performed using a 0.05° spatial grid for multiple environmental variables, gross primary productivity (GPP), and evapotranspiration (ET). Our results showed a potential representativeness of 34% of the surface area for climate properties, 36% for terrain parameters, 34% for soil resources, and 45% when all aforementioned environmental variables were summarized into a principal component analysis. Furthermore, there was a 48% potential representativeness for GPP and 34% for ET. Unfortunately, data from these 41 sites are not all readily available for the scientific community, limiting synthesis studies and model benchmarking/parametrization. The implication is that global/regional data‐driven products are forced to use information from FLUXNET sites outside LA to predict patterns in LA. Representativeness could increase to 86% (for GPP) and 80% (for ET) if 200 sites are optimally distributed. We discussed ongoing challenges, the need to enhance interoperability and data sharing, and promote monitoring efforts across LA to increase the accuracy of regional‐to‐global data‐driven products. Plain Language Summary: Environmental observatory networks (EONs) provide information to better understand, model, and forecast the spatial and temporal dynamics of Earth's biophysical processes. Consequently, representativeness analyses of EON's are important to provide insights for improving EONs' management, design, and interpretation of their value‐added products (e.g., datasets, model predictions). We assessed the representativeness of registered FLUXNET sites (n = 41, revised on September 2018) across Latin America (LA), a region of great importance for the global carbon and water cycles, which represents nearly 13% of the world's land surface area. Representativeness analyses were performed using multiple environmental variables, gross primary productivity (GPP), and evapotranspiration (ET) across LA. Our results showed potential spatial representativeness of 34% of the surface area for climate properties, 36% for terrain parameters, 34% for soil resources, and 45% when all aforementioned environmental variables were summarized. Furthermore, there was a 48% potential representativeness for GPP and 34% for ET. Our results represent a best‐case scenario because data from these 41 sites is not all readily available for the scientific community. We discussed the need to enhance collaboration, promote the participation of active/inactive sites to share information with local, regional and international networks, and promote monitoring efforts across this region of the world. Key Points: There is a need to promote data sharing and improve the representativeness of environmental networks across Latin AmericaCurrently, 48% of gross primary productivity and 34% of evapotranspiration patterns are represented by FLUXNET sitesData‐driven products are forced to use information outside Latin America to predict patterns within this region [ABSTRACT FROM AUTHOR]

In October 2020, under COVID‐19 quarantine, AmeriFlux held its largest and one of its most successful annual meetings. Historically, ∼100 scientists attend; this meeting had over 400 registrants and participants. Participants expressed that this was among the best virtual meetings that they had ever attended, and 100% of post‐meeting survey respondents stated that they would attend again. Feedback revealed the meeting fostered a strong sense of connection to the AmeriFlux community, especially among early career and international scientists. How did a feeling of strong connection to the community arise from the seemingly cold and isolated structures of virtual meetings? The meeting emphasized Diversity, Equity, and Inclusion (DEI), which resulted in an unexpected enhancement of science communication and community connections. Additionally, the meeting experimented with an online virtual gaming‐like world (Gather.town), where users controlled video/audio‐enabled avatars in a conference center and poster hall environment to create spontaneous conversations and discussions. In lieu of a social‐bonding field trip, participants showed videos of their field sites accompanied by informal banter, which were watched in group settings in Gather. Social mixers were structured over Zoom breakout rooms that were limited in size to promote participation with accessible games. Science talks were selected based on appeal to a demographically diverse organizing committee, which enhanced appeal to a broad meeting audience. Finally, breakout reports were given not in the format of bullet point slides, but instead as creatively improvized fairytales, which dramatically enhanced engagement. Here, we describe some of the process that went into the AmeriFlux 2020 meeting. Keeping with the theme of experimenting and fairytales, we present this narrative in the form of a fairytale; and, without further ado... Key Points: Diversity, Equity, and Inclusion are keys to enhanced science communication and community connectionsLean‐in to experimenting with novel meeting tools and options for improved virtual meetingsStory telling as a means of effective science communication and engagement [ABSTRACT FROM AUTHOR]

We test the relationship between canopy photosynthesis and reflected near‐infrared radiation from vegetation across a range of functional (photosynthetic pathway and capacity) and structural conditions (leaf area index, fraction of green and dead leaves, canopy height, reproductive stage, and leaf angle inclination), weather conditions, and years using a network of field sites from across central California. We based our analysis on direct measurements of canopy photosynthesis, with eddy covariance, and measurements of reflected near‐infrared and red radiation from vegetation, with light‐emitting diode sensors. And we interpreted the observed relationships between photosynthesis and reflected near‐infrared radiation using simulations based on the multilayer, biophysical model, CanVeg. Measurements of reflected near‐infrared radiation were highly correlated with measurements of canopy photosynthesis on half‐hourly, daily, seasonal, annual, and decadal time scales across the wide range of function and structure and weather conditions. Slopes of the regression between canopy photosynthesis and reflected near‐infrared radiation were greatest for the fertilized and irrigated C4 corn crop, intermediate for the C3 tules on nutrient‐rich organic soil and nitrogen fixing alfalfa, and least for the native annual grasslands and oak savanna on nutrient‐poor, mineral soils. Reflected near‐infrared radiation from vegetation has several advantages over other remotely sensed vegetation indices that are used to infer canopy photosynthesis; it does not saturate at high leaf area indices, it is insensitive to the presence of dead legacy vegetation, the sensors are inexpensive, and the reflectance signal is strong. Hence, information on reflected near‐infrared radiation from vegetation may have utility in monitoring carbon assimilation in carbon sequestration projects or on microsatellites orbiting Earth for precision agriculture applications. Key Points: Reflected near‐infrared radiation is a strong proxy for canopy photosynthesisThis proxy works for a wide range of ecosystem structure and functionThe method has promise to be used for a variety of applications relating to carbon and water use [ABSTRACT FROM AUTHOR]