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Eddy covariance measurement systems provide direct observation of the exchange of greenhouse gases between ecosystems and the atmosphere, but have only occasionally been intentionally applied to quantify the carbon dynamics associated with specific climate mitigation strategies. Natural climate solutions (NCS) harness the photosynthetic power of ecosystems to avoid emissions and remove atmospheric carbon dioxide (CO2), sequestering it in biological carbon pools. In this perspective, we aim to determine whichkinds of NCS strategies are most suitable for ecosystem-scale flux measurements and howthese measurements should be deployed for diverse NCS scales and goals. We find that ecosystem-scale flux measurements bring unique value when assessing NCS strategies characterized by inaccessible and hard-to-observe carbon pool changes, important non-CO2greenhouse gas fluxes, the potential for biophysical impacts, or dynamic successional changes. We propose three deployment types for ecosystem-scale flux measurements at various NCS scales to constrain wide uncertainties and chart a workable path forward: “pilot”, “upscale”, and “monitor”. Together, the integration of ecosystem-scale flux measurements by the NCS community and the prioritization of NCS measurements by the flux community, have the potential to improve accounting in ways that capture the net impacts, unintended feedbacks, and on-the-ground specifics of a wide range of emerging NCS strategies.

One paradigm in biogeochemistry is that frequent disturbance tends to deplete carbon (C) in soil organic matter (SOM) by reducing biomass inputs and promoting losses. However, disturbance by fire has challenged this paradigm because soil C responses to frequent and/or intense fires are highly variable, despite observed declines in biomass inputs. Here, we review recent advances to illustrate that fire-driven changes in decomposition, mediated by altered SOM stability, are an important compensatory process offsetting declines in aboveground biomass pools. Fire alters the stability of SOM by affecting both the physicochemical properties of the SOM and the environmental drivers of decomposition, potentially offsetting C lost via combustion, but the mechanisms affecting the SOM stability differ across ecosystems. Thus, shifting our focus from a top-down view of fire impacting C cycling via changes in plant biomass to a bottom-up view of changes in decomposition may help to elucidate counterintuitive trends in the response of SOM to burning. Given that 70% of global topsoil C is in fire-prone regions, using fire to promote SOM stability may be an important nature-based climate solution to increase C storage.

Land use change and management affect climate by altering both the biogeochemical and biophysical interactions between the land and atmosphere. Whereas climate policy often emphasizes the biogeochemical impact of land use change, biophysical impacts, including changes in reflectance, energy partitioning among sensible and latent heat exchange, and surface roughness, can attenuate or enhance biogeochemical effects at local to regional scales. This study analyzes 3 years (2015–2017) of turbulent flux and meteorological data across three contrasting wetland restoration sites and one agricultural site, colocated in the Sacramento‐San Joaquin Delta, California, USA, to understand if the biophysical impacts of freshwater wetland restoration can be expected to attenuate or enhance the potential biogeochemical benefits. We show that despite absorbing more net radiation, restored wetlands have the potential to cool daytime surface temperature by up to 5.1 °C, as compared to a dominant drained agricultural land use. Wetland canopy structure largely determines the magnitude of surface temperature cooling, with wetlands that contain areas of open water leading to enhanced nighttime latent heat flux and reduced diurnal temperate range. Daytime surface cooling could be important in ameliorating physiological stress associated with hotter and drier conditions and could also promote boundary layer feedbacks at the local to regional scale. With a renewed focus on the mitigation and adaptation potential of natural and working lands, we must better understand the role of biophysical changes, especially in novel land use transitions like wetland restoration. Land use change and management affect climate by altering both the cycling of greenhouse gases and how energy and water are exchanged between the ecosystem and the atmosphere. These energy and water impacts can have local to regional implications on surface and air temperature. This study analyzes 3 years (2015–2017) of measured water and energy exchange at three contrasting wetland restoration sites and one agricultural site, all located in the Sacramento‐San Joaquin Delta, California, USA. Restoring drained agricultural fields to flooded wetlands causes a rougher canopy, with more exposed water, and alters the energy partitioning. This results in growing season and daytime surface cooling, which could enhance the other benefits of wetland restoration, like soil buildup, habitat creation, and carbon sequestration. Land use policies should consider both the greenhouse gas and the energy and water implications of the promoted land use change to fully account for the climatic impact. We found significant surface temperature differences over drained agricultural peatland and restored wetlandsRestored wetlands' tall, emergent canopy structure delays heat exchange with the atmosphere, causing a daytime and growing season cooling effectLand use policy must consider biophysical impacts in addition to biogeochemical benefits, especially in novel land use transitions

Peatland drainage is an important driver of global soil carbon loss and carbon dioxide (CO2) emissions. Restoration of peatlands by reflooding reverses CO2losses at the cost of increased methane (CH4) emissions, presenting a biogeochemical compromise. While restoring peatlands is a potentially effective method for sequestering carbon, the terms of this compromise are not well constrained. Here we present 14 site years of continuous CH4and CO2ecosystem‐scale gas exchange over a network of restored freshwater wetlands in California, where long growing seasons, warm weather, and managed water tables result in some of the largest wetland ecosystem CH4emissions recorded. These large CH4emissions cause the wetlands to be strong greenhouse gas sources while sequestering carbon and building peat soil. The terms of this biogeochemical compromise, dictated by the ratio between carbon sequestration and CH4emission, vary considerably across small spatial scales, despite nearly identical wetland climate, hydrology, and plant community compositions. Wetlands play an important role in the climate system, with restoration commonly undertaken for the benefit of atmospheric carbon dioxide removal and carbon storage in the soil. While flooded conditions suppress carbon dioxide emissions from decomposition and sequester carbon, they also generate methane, another potent heat‐trapping greenhouse gas. Understanding the balance between these exchanges is important to our understanding of how restored or created wetlands will contribute to mitigating climate change. Here we present a long‐term record of continuous carbon dioxide and methane exchange from restored wetlands in Northern California to understand the ultimate climate impact of wetland restoration. We report 14 site years of in situ ecosystem‐scale carbon dioxide and methane measurements over a range of restored wetlands in Northern CaliforniaLarge methane emissions caused the wetlands to be neutral to strong greenhouse gas sources while sequestering carbon and building peat soilDespite experiencing nearly identical meteorological conditions and similar species composition, methane and carbon sequestration scaled within, but not across, wetland sites

In this study, we evaluated the effect of land cover, atmospheric subsidence, and advection on the annual dynamics of atmospheric boundary layer (ABL) height from two contrasting sites. The first site is the Walker Branch forest, a deciduous forest of temperate climate, complex topography, and cloudy summers. The second site is the Sacramento‐San Joaquin River Delta, a site of Mediterranean climate, flat terrain on a local scale, and clear summers. After testing a new algorithm to calculate ABL heights from 915 MHz radar wind profilers, we evaluated a hierarchy of three slab models to recreate the diurnal and annual patterns of ABL growth. We found that the lower ABL heights in the Delta, particularly during late summer, are driven by the combined effects of increased atmospheric subsidence and marine air advection. In both sites, the annual pattern of ABL height was strongly correlated to total daily incoming radiation, and in the Delta, the annual pattern of ABL height closely followed the seasonal patterns of sensible heat flux from a mosaic of different land covers. A land composite of latent and sensible heat fluxes obtained through a meso‐network of eddy covariance measurements and the ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) mission resulted in higher model skill, thus showing that land cover heterogeneity is an important driver of ABL growth. Model simulations show that in the Delta, restoring agricultural land to wetlands with large open water areas could result in a reduction of ABL height during those months with low subsidence and advection. The height of the atmospheric boundary layer drives the dilution of contaminants and other trace gases; therefore, it is of vital importance for the creation of accurate atmospheric and weather models. In this study, we report on comparisons of boundary layer height from radar wind profilers against three models with different levels of complexity in two sites with contrasting land covers. The first site is the Walker Branch forest, a forest of temperate climate, complex topography, and cloudy summers. The second site is the Sacramento‐San Joaquin River Delta in California, a site of Mediterranean climate, flat terrain, and clear summers with high rates of atmospheric subsidence and cold‐air advection from the adjacent ocean. We demonstrate that the annual cycle in boundary layer height is mostly driven by daily solar radiation in both sites; however, the presence of different land covers also has an important effect on driving the boundary layer height annual pattern, especially for those times where there is low atmospheric subsidence and advection. These results can help to understand the effects of changes in land cover on the boundary layer height and the air quality within it. Atmospheric boundary layer (ABL) height is studied using radar wind profilers and slab models in two contrasting ecosystems over annual time scalesAtmospheric subsidence and cold‐air advection limit the growth of the boundary layer during the summer in Northern CaliforniaLand‐cover‐mediated changes in ABL height are important but muted by strong subsidence and advection Atmospheric boundary layer (ABL) height is studied using radar wind profilers and slab models in two contrasting ecosystems over annual time scales Atmospheric subsidence and cold‐air advection limit the growth of the boundary layer during the summer in Northern California Land‐cover‐mediated changes in ABL height are important but muted by strong subsidence and advection

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 C4corn crop, intermediate for the C3tules 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. 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

There are few observational studies measuring the ecosystem‐scale productivity effects of changes in incident diffuse photosynthetically active radiation (PARdiffuse), especially related to wildfire smoke. Climate change‐induced increases to the duration and intensity of fire conditions have made smoke a common occurrence across western North America, with largely unquantified ecosystem feedbacks. Under equivalent amounts of radiation, increased atmospheric particulate matter could lead to a boost in productivity as scattering redistributes photons throughout multilayer canopies. In this work, we leverage a meso‐network of eddy covariance measurement sites across a unique array of managed and restored C3and C4canopy types to understand how recent wildfire smoke affected ecosystem productivity during the summer of 2018, an especially smoky year in the agriculturally productive Central Valley. We find that diffuse PARdiffuseincreased by more than a third compared to the previous growing season, while total PAR was only slightly diminished. These conditions caused nearly a doubling of light use efficiency over the range of diffuse fraction observed, with the highest sensitivity to diffuse fraction exhibited by corn and alfalfa crops. We utilized an empirical model to assess the trade‐off between enhanced diffuse fraction and reduced total PAR. Under mean radiation conditions, daily integrated gross ecosystem productivity increased by 1.2–4.2% compared to the previous growing season. Finally, we explore the potential negative effect of heightened ozone, a copollutant often associated with wildfire. In addition to the effects of wildfire smoke, the results of this natural experiment can help validate future predictions of aerosol‐productivity feedbacks. Exacerbated by climate change, wildfire smoke is becoming a more common occurrence, especially across western North America. While we know that wildfire smoke has negative health impacts for humans, there is evidence that it could increase plant productivity. This occurs due to the way that smoke scatters incoming sunlight, allowing the Sun's energy to reach further into dense plant canopies. Here, we measure the impacts of wildfire smoke on vegetation productivity across a range of vegetation types, all with different canopy structures and physiological function, during the summer of 2018, an especially smoky year in the agriculturally productive Central Valley. We find that smoky conditions increased the efficiency by which these plant canopies photosynthesized, leading to productivity increases, depending on trade‐offs with total light and other pollutants. Historically large wildfires caused heightened smoke particulate that increased the fraction of diffuse radiation during the growing seasonLight use efficiency, the ratio of productivity to absorbed radiation, was nearly doubled during smoke events at crop and wetland land usesProductivity enhancements depend on the trade‐off between reduced total radiation and increased diffuse radiation