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The evolution of disciplinary silos and increasingly narrow disciplinary boundaries have together resulted in one‐sided approaches to the study of land‐atmosphere interactions—a field that requires a bi‐directional approach to understand the complex feedbacks and interactions that occur. The integration of surface flux and atmospheric boundary layer measurements is therefore essential to advancing our understanding. The Land‐Atmosphere 2021 workshop (held virtually, June 10‐11, 2021) involved almost 300 participants from around the world and promoted cross‐discipline collaboration by way of talks from invited speakers, moderated discussions, breakout sessions, and a virtual poster session. The workshop focused on five main theme areas: "big picture" overview, instrumentation and remote sensing, modeling, water, and aerosols and clouds. In talks and breakout groups, there were frequent calls for more AmeriFlux sites to be instrumented for boundary layer height measurements, and for the development of some "super sites" where profiling instruments would be deployed. There was further agreement on the need for the standardization of various datasets. There was also a consensus that funding agencies need to be willing to support the sorts of large projects (including associated instrumentation) which can drive interdisciplinary work. Early‐career scientists, in particular, expressed enthusiasm for working across disciplinary boundaries but noted that there need to be more financial support and training opportunities so they would be better prepared for interdisciplinary work. Investment in these career development opportunities would enable today's cohort of early‐career scientists to advance the frontiers of interdisciplinary work over the next couple of decades. Plain Language Summary: The hypothesis we investigate is that a more collaborative, interdisciplinary approach is needed to advance the science of land‐atmosphere interactions. We provide historical perspective by discussing the evolution of the field of land‐atmosphere interactions but note that in recent decades, narrow disciplinary boundaries have emerged as a barrier to progress. However, synergies between newly developed observational capacities, and the power of a network‐based approach centered around AmeriFlux, have the potential to revolutionize the field over the coming decade. This background provides a summary of the keynote talks presented at the virtual Land‐Atmosphere 2021 workshop, in which 300 scientists from around the world participated this past June. The keynote talks offered a range of perspectives on key knowledge gaps and set up discussions on proposed new measurements and opportunities for future cross‐disciplinary collaboration. The main conclusions emerging from the final plenary discussion session at this workshop include the need to improve the availability and accessibility of key data sets; the need for federal agencies to provide funding support for large, interdisciplinary projects; and the need for training and other career development opportunities, particularly for early career scientists. Our review highlights the key challenges and opportunities facing the field of land‐atmosphere interactions. Key Points: The field of land‐atmosphere interactions can be advanced through emerging technologies to characterize the atmospheric boundary layerCross‐disciplinary research is needed but would require new funding mechanisms to tackle transformative research questionsTraining programs for early career scientists are essential so that they are prepared to work across disciplinary boundaries [ABSTRACT FROM AUTHOR]

In eastern North America, many deciduous forest ecosystems grow at the northernmost extent of their geographical ranges, where climate change could aid or impede their growth. This region experiences frequent extreme weather conditions, allowing us to study the response of these forests to environmental conditions, reflective of future climates. Here we determined the impact of seasonal and annual climate variations and extreme weather events on the carbon (C) uptake capacity of an oak‐dominated forest in southern Ontario, Canada, from 2012 to 2016. We found that changes in meteorology during late May to mid‐July were key in determining the C sink strength of the forest, impacting the seasonal and annual variability of net ecosystem productivity (NEP). Overall, higher temperatures and dry conditions reduced ecosystem respiration (RE) much more than gross ecosystem productivity (GEP), leading to higher NEP. Variability in NEP was primarily driven by changes in RE, rather than GEP. The mean annual GEP, RE, and NEP values at our site during the study were 1,343 ± 85, 1,171 ± 139, and 206 ± 92 g C m−2 yr−1, respectively. The forest was a C sink even in years that experienced heat and water stresses. Mean annual NEP at our site was within the range of NEP (69–459 g C m−2 yr−1) observed in similar North American forests from 2012 to 2016. The growth and C sequestration capabilities of our oak‐dominated forest were not adversely impacted by changes in environmental conditions and extreme weather events experienced over the study period. Plain Language Summary: Globally, forests are an important carbon sink, removing carbon dioxide from the atmosphere on an annual basis. Midlatitude temperate deciduous forests are an important component of the global forest carbon sink, representing ~11% of the global carbon stock. The strength of carbon uptake by these forests remains uncertain under future climate projections. With increasing temperatures and changes in precipitation patterns expected in many regions, tree species may advance north, changing the carbon uptake capacities of local forest ecosystems. In addition, extreme weather events such as heat waves and droughts may place these forests under added stress and reduce their carbon sink capacities. Our study found that despite frequently changing environmental conditions, an oak‐dominated forest, growing at the northernmost extent of temperate deciduous forests within the Great Lakes region of North America, was a strong sink of carbon with mean annual net ecosystem productivity of 2 t of C per hectare per year. Key Points: Seasonal and annual carbon fluxes were studied from 2012 to 2016 at a mature temperate deciduous forest in southern Ontario, CanadaMeteorological conditions over a short midsummer period greatly impacted annual NEPOn average, the forest was an annual carbon sink of 206 ± 92 g C m−2 yr−1 [ABSTRACT FROM AUTHOR]

The annual carbon and water dynamics of two eastern North American temperate forests were compared over a 6-year period from 2012 to 2017. The geographic location, forest age, soil, and climate were similar between the two stands; however, stand composition varied in terms of tree leaf-retention and shape strategy: one stand was a deciduous broadleaf forest, while the other was an evergreen needleleaf forest. The 6-year mean annual net ecosystem productivity (NEP) of the coniferous forest was slightly higher and more variable (218±109 g C m -2 yr -1) compared to that of the deciduous forest NEP (200±83 g C m -2 yr -1). Similarly, the 6-year mean annual evapotranspiration (ET) of the coniferous forest was higher (442±33 mm yr -1) than that of the deciduous forest (388±34 mm yr -1), but with similar interannual variability. Summer meteorology greatly impacted the carbon and water fluxes in both stands; however, the degree of response varied among the two stands. In general, warm temperatures caused higher ecosystem respiration (RE), resulting in reduced annual NEP values – an impact that was more pronounced at the deciduous broadleaf forest compared to the evergreen needleleaf forest. However, during warm and dry years, the evergreen forest had largely reduced annual NEP values compared to the deciduous forest. Variability in annual ET at both forests was related most to the variability in annual air temperature (Ta), with the largest annual ET observed in the warmest years in the deciduous forest. Additionally, ET was sensitive to prolonged dry periods that reduced ET at both stands, although the reduction at the coniferous forest was relatively larger than that of the deciduous forest. If prolonged periods (weeks to months) of increased Ta and reduced precipitation are to be expected under future climates during summer months in the study region, our findings suggest that the deciduous broadleaf forest will likely remain an annual carbon sink, while the carbon sink–source status of the coniferous forest remains uncertain. [ABSTRACT FROM AUTHOR]

The annual carbon and water dynamics of two Eastern North American forests were compared over a six year period from 2012 to 2017. The geographic location, forest age, soil, and climate were similar between the sites, however, the species composition varied: one was a deciduous broadleaf forest, while the other an evergreen needleleaf forest. During the 6-year study period, the mean annual net ecosystem productivity (NEP) of the coniferous forest was slightly higher and more variable (218±109 g C m -2 yr -1) compared to that of the deciduous broadleaf forest NEP of 200±83g C m-2 yr -1. Similarly, the mean annual evapotranspiration (ET) of the conifer forest over the 6-year study period was higher (442±33 mm yr-1) compared to that of the broadleaf forest (388±34 mm yr-1), but with similar interannual variability. Significant abnormalities in fluxes were measured between sites during drought years. Summer meteorology greatly impacted fluxes at both sites, but to varying degrees and with varying responses. In general, warm temperatures caused higher ecosystem respiration (RE), resulting in reduced mean annual NEP values -- an impact that was more pronounced at the deciduous broadleaf forest compared to the evergreen needle-leaf forest. However, during drought years, the evergreen forest saw greater annual reduction in carbon sequestration compared to the deciduous forest. In the evergreen conifer forest, variability of summer meteorology greatly controlled the forest's annual carbon sink-source strength. Annual ET at both forests was driven by changes in air temperature (Ta), with the largest annual ET measured in the warmest years in the deciduous forest. Additionally, prolonged dry periods with increased Ta, greatly reduced ET. During drought years, the carbon and water fluxes of the deciduous forest were less sensitive to changes in temperature or water availability compared to the evergreen forest. If longer periods of increased temperatures and larger precipitation variability during summer months are to be expected under future climates, our findings suggest the carbon sink capacity of the deciduous forest will continue, while that of the conifer forest remains uncertain in the study region. [ABSTRACT FROM AUTHOR]

A wide range of globally significant biomass fuels were burned during the fourth Fire Lab at Missoula Experiment (FLAME-4). A multi-channel photoacoustic absorption spectrometer (PAS) measured dry absorption at 405, 532, and 660 nm and thermally denuded (250 °C) absorption at 405 and 660 nm. Absorption coefficients were broken into contributions from black carbon (BC), brown carbon (BrC), and lensing following three different methodologies, with one extreme being a method that assumes the thermal denuder effectively removes organics and the other extreme being a method based on the assumption that black carbon (BC) has an Ångström exponent of unity. The methodologies employed provide ranges of potential importance of BrC to absorption but, on average, there was a difference of a factor of 2 in the ratio of the fraction of absorption attributable to BrC estimated by the two methods. BrC absorption at shorter visible wavelengths is of equal or greater importance to that of BC, with maximum contributions of up to 92% of total aerosol absorption at 405 nm and up to 58% of total absorption at 532 nm. Lensing is estimated to contribute a maximum of 30% of total absorption, but typically contributes much less than this. Absorption enhancements and the estimated fraction of absorption from BrC show good correlation with the elemental-carbon-to-organic-carbon ratio (EC/OC) of emitted aerosols and weaker correlation with the modified combustion efficiency (MCE). Previous studies have shown that BrC grows darker (larger imaginary refractive index) as the ratio of black to organic aerosol (OA) mass increases. This study is consistent with those findings but also demonstrates that the fraction of total absorption attributable to BrC shows the opposite trend: increasing as the organic fraction of aerosol emissions increases and the EC/OC ratio decreases. [ABSTRACT FROM AUTHOR]

A wide range of globally significant biomass fuels were burned during the fourth Fire Lab at Missoula Experiment (FLAME-4). A multi-channel photoacoustic absorption spectrometer (PAS) measured dry absorption at 405, 532, and 660 nm and thermally denuded (250 �C) absorption at 405 and 660 nm. Absorption coefficients were broken into contributions from black carbon (BC), brown carbon (BrC) and lensing following three different methodologies, with one extreme being a method that assumes the thermal denuder effectively removes organics and the other extreme being a method based on the assumption that black carbon (BC) has an angstrom exponent of unity. The methodologies employed provide ranges of potential importance of BrC to absorption but, on average, there was a factor of 2 difference in the ratio of the fraction of absorption attributable to brown carbon estimated by the two methods. BrC absorption at shorter visible wavelengths is of equal or greater importance to that of BC with maximum contributions of up to 92 % of total aerosol absorption at 405 nm and up to 58 % of total absorption at 532 nm. Lensing is estimated to contribute a maximum of 30 % of total absorption, but typically contributes much less than this. Absorption enhancements and the estimated fraction of absorption from BrC show good correlation with the elemental to organic carbon ratio (EC/OC) of emitted aerosols and weaker correlation with the modified combustion efficiency (MCE). Previous studies have shown that brown carbon grows darker (larger imaginary refractive index) as the ratio of black to organic aerosol (OA) mass increases. This study is consistent with those findings but also demonstrates that the fraction of total absorption attributable to BrC shows the opposite trend: increasing as the organic fraction of aerosol emissions increases and the EC/OC ratio decreases. [ABSTRACT FROM AUTHOR]