Maximum carbon uptake rate dominates the interannual variability of global net ecosystem exchange

Terrestrial ecosystems contribute most of the interannual variability (IAV) in atmospheric carbon dioxide (CO₂) concentrations, but processes driving the IAV of net ecosystem CO₂ exchange (NEE) remain elusive. For a predictive understanding of the global C cycle, it is imperative to identify indicators associated with ecological processes that determine the IAV of NEE. Here, we decompose the annual NEE of global terrestrial ecosystems into their phenological and physiological components, namely maximum carbon uptake (MCU) and release (MCR), the carbon uptake period (CUP), and two parameters, α and β, that describe the ratio between actual versus hypothetical maximum C sink and source, respectively. Using long‐term observed NEE from 66 eddy covariance sites and global products derived from FLUXNET observations, we found that the IAV of NEE is determined predominately by MCU at the global scale, which explains 48% of the IAV of NEE on average while α, CUP, β, and MCR explain 14%, 25%, 2%, and 8%, respectively. These patterns differ in water‐limited ecosystems versus temperature‐ and radiation‐limited ecosystems; 31% of the IAV of NEE is determined by the IAV of CUP in water‐limited ecosystems, and 60% of the IAV of NEE is determined by the IAV of MCU in temperature‐ and radiation‐limited ecosystems. The Lund‐Potsdam‐Jena (LPJ) model and the Multi‐scale Synthesis and Terrestrial Model Inter‐comparison Project (MsTMIP) models underestimate the contribution of MCU to the IAV of NEE by about 18% on average, and overestimate the contribution of CUP by about 25%. This study provides a new perspective on the proximate causes of the IAV of NEE, which suggest that capturing the variability of MCU is critical for modeling the IAV of NEE across most of the global land surface.     

Endogenous circadian regulation of carbon dioxide exchange in terrestrial ecosystems

It is often assumed that daytime patterns of ecosystem carbon assimilation are mostly driven by direct physiological responses to exogenous environmental cues. Under limited environmental variability, little variation in carbon assimilation should thus be expected unless endogenous plant controls on carbon assimilation, which regulate photosynthesis in time, are active. We evaluated this assumption with eddy flux data, and we selected periods when net ecosystem exchange (NEE) was decoupled from environmental variability in seven sites from highly contrasting biomes across a 74° latitudinal gradient over a total of 36 site‐years. Under relatively constant conditions of light, temperature, and other environmental factors, significant diurnal NEE oscillations were observed at six sites, where daily NEE variation was between 20% and 90% of that under variable environmental conditions. These results are consistent with fluctuations driven by the circadian clock and other endogenous processes. Our results open a promising avenue of research for a more complete understanding of ecosystem fluxes that integrates from cellular to ecosystem processes.     

Carbon sequestration potential of tropical pasture compared with afforestation in Panama

Tropical forest ecosystems play an important role in regulating the global climate, yet deforestation and land‐use change mean that the tropical carbon sink is increasingly influenced by agroecosystems and pastures. Despite this, it is not yet fully understood how carbon cycling in the tropics responds to land‐use change, particularly for pasture and afforestation. Thus, the objectives of our study were: (1) to elucidate the environmental controls and the impact of management on gross primary production (GPP), total ecosystem respiration (TER) and net ecosystem CO₂ exchange (NEE); (2) to estimate the carbon sequestration potential of tropical pasture compared with afforestation; and (3) to compare eddy covariance‐derived carbon budgets with biomass and soil inventory data. We performed comparative measurements of NEE in a tropical C₄ pasture and an adjacent afforestation with native tree species in Sardinilla (Panama) from 2007 to 2009. Pronounced seasonal variation in GPP, TER and NEE were closely related to radiation, soil moisture, and C₃ vs. C₄ plant physiology. The shallow rooting depth of grasses compared with trees resulted in a higher sensitivity of the pasture ecosystem to water limitation and seasonal drought. During 2008, substantial amounts of carbon were sequestered by the afforestation (–442 g C m^–2, negative values denote ecosystem carbon uptake), which was in agreement with biometric observations (–450 g C m^–2). In contrast, the pasture ecosystem was a strong carbon source in 2008 and 2009 (261 g C m^–2), associated with seasonal drought and overgrazing. In addition, soil carbon isotope data indicated rapid carbon turnover after conversion from C₄ pasture to C₃ afforestation. Our results clearly show the potential for considerable carbon sequestration of tropical afforestation and highlight the risk of carbon losses from pasture ecosystems in a seasonal tropical climate.     

Integrating aquatic and terrestrial components to construct a complete carbon budget for a north temperate lake district

The development of complete regional carbon (C) budgets for different biomes is an integral step in the effort to predict global response and potential feedbacks to a changing climate regime. Wetland and lake contributions to regional C cycling remain relatively uncertain despite recent research highlighting their importance. Using a combination of field surveys and tower‐based carbon dioxide (CO₂) flux measurements, modeling, and published literature, we constructed a complete C budget for the Northern Highlands Lake District in northern Wisconsin/Michigan, a ～6400 km² region rich in lakes and wetlands. This is one of the first regional C budgets to incorporate aquatic and terrestrial C cycling under the same framework. We divided the landscape into three major compartments (forests, wetlands, and surface waters) and quantified all major C fluxes into and out of those compartments, with a particular focus on atmospheric exchange but also including sedimentation in lakes and hydrologic fluxes. Landscape C storage was dominated by peat‐containing wetlands and lake sediments, which make up only 20% and 13% of the landscape area, respectively, but contain >80% of the total fixed C pool (ca. 400 Tg). We estimated a current regional C accumulation of 1.1±0.1 Tg yr^?1, and the largest regional flux was forest net ecosystem exchange (NEE) into aggrading forests for a total of 1.0±0.1 Tg yr^?1. Mean wetland NEE (0.12±0.06 Tg yr^?1 into wetlands), lake CO₂ emissions and riverine efflux (each ca. 0.03±0.01 Tg yr^?1) were smaller but of consequence to the overall budget. Hydrologic transport from uplands/wetlands to surface waters within the region was an important vector of terrestrial C. Regional C fluxes and pools would be misrepresented without inclusion of surface waters and wetlands, and C budgets in heterogeneous landscapes open opportunities to examine the sensitivities of important fluxes to changes in climate and land use/land cover.     

How eddy covariance flux measurements have contributed to our understanding of Global Change Biology

A global network of long‐term carbon and water flux measurements has existed since the late 1990s. With its representative sampling of the terrestrial biosphere's climate and ecological spaces, this network is providing background information and direct measurements on how ecosystem metabolism responds to environmental and biological forcings and how they may be changing in a warmer world with more carbon dioxide. In this review, I explore how carbon and water fluxes of the world's ecosystem are responding to a suite of covarying environmental factors, like sunlight, temperature, soil moisture, and carbon dioxide. I also report on how coupled carbon and water fluxes are modulated by biological and ecological factors such as phenology and a suite of structural and functional properties. And, I investigate whether long‐term trends in carbon and water fluxes are emerging in various ecological and climate spaces and the degree to which they may be driven by physical and biological forcings. As a growing number of time series extend up to 20 years in duration, we are at the verge of capturing ecosystem scale trends in the breathing of a changing biosphere. Consequently, flux measurements need to continue to report on future conditions and responses and assess the efficacy of natural climate solutions.     

Seasonal hysteresis of net ecosystem exchange in response to temperature change: patterns and causes

Understanding how net ecosystem exchange (NEE) changes with temperature is central to the debate on climate change‐carbon cycle feedbacks, but still remains unclear. Here, we used eddy covariance measurements of NEE from 20 FLUXNET sites (203 site‐years of data) in mid‐ and high‐latitude forests to investigate the temperature response of NEE. Years were divided into two half thermal years (increasing temperature in spring and decreasing temperature in autumn) using the maximum daily mean temperature. We observed a parabolic‐like pattern of NEE in response to temperature change in both the spring and autumn half thermal years. However, at similar temperatures, NEE was considerably depressed during the decreasing temperature season as compared with the increasing temperature season, inducing a counter‐clockwise hysteresis pattern in the NEE–temperature relation at most sites. The magnitude of this hysteresis was attributable mostly (68%) to gross primary production (GPP) differences but little (8%) to ecosystem respiration (ER) differences between the two half thermal years. The main environmental factors contributing to the hysteresis responses of NEE and GPP were daily accumulated radiation. Soil water content (SWC) also contributed to the hysteresis response of GPP but only at some sites. Shorter day length, lower light intensity, lower SWC and reduced photosynthetic capacity may all have contributed to the depressed GPP and net carbon uptake during the decreasing temperature seasons. The resultant hysteresis loop is an important indicator of the existence of limiting factors. As such, the role of radiation, LAI and SWC should be considered when modeling the dynamics of carbon cycling in response to temperature change.     

Land‐use strategies to balance livestock production,biodiversity conservation and carbon storage in Yucatán,Mexico

Balancing the production of food, particularly meat, with preserving biodiversity and maintaining ecosystem services is a major societal challenge. Research into the contrasting strategies of land sparing and land sharing has suggested that land sparing—combining high‐yield agriculture with the protection or restoration of natural habitats on nonfarmed land—will have lower environmental impacts than other strategies. Ecosystems with long histories of habitat disturbance, however, could be resilient to low‐yield agriculture and thus fare better under land sharing. Using a wider suite of species (birds, dung beetles and trees) and a wider range of livestock‐production systems than previous studies, we investigated the probable impacts of different land‐use strategies on biodiversity and aboveground carbon stocks in the Yucatán Peninsula, Mexico—a region with a long history of habitat disturbance. By modelling the production of multiple products from interdependent land uses, we found that land sparing would allow larger estimated populations of most species and larger carbon stocks to persist than would land sharing or any intermediate strategy. This result held across all agricultural production targets despite the history of disturbance and despite species richness in low‐ and medium‐yielding agriculture being not much lower than that in natural habitats. This highlights the importance, in evaluating the biodiversity impacts of land use, of measuring population densities of individual species, rather than simple species richness. The benefits of land sparing for both biodiversity and carbon storage suggest that safeguarding natural habitats for biodiversity protection and carbon storage alongside promoting areas of high‐yield cattle production would be desirable. However, delivering such landscapes will probably require the explicit linkage of livestock yield increases with habitat protection or restoration, as well as a deeper understanding of the long‐term sustainability of yields, and research into how other societal outcomes vary across land‐use strategies.     

Scaling-up knowledge of growing-season net ecosystem exchange for long-term assessment of North Dakota grasslands under the Conservation Reserve Program

Scaling‐up knowledge of land‐atmosphere net ecosystem exchange (NEE) from a single experimental site to numerous perennial grass fields in the Northern Great Plains (NGP) requires appropriate scaling protocols. We addressed this problem using synoptic data available from the Landsat sensor for 10 growing seasons (April 15 to September 30) over a North Dakota field‐site, where we continuously measured CO₂ exchange using a Bowen Ratio Energy Balance (BREB) system. NEE observed during the growing season at our field‐site from 1997 to 2006 vacillated with drought and deluge, with net carbon (C) losses to the atmosphere in 2006. We used stepwise linear regression with 10 years of Landsat and NEE data to construct and validate a model for estimating grassland growing‐season NEE from field to landscape scales. Eighty‐nine percent of the variability in NEE was explained by year, live biomass, carbon : nitrogen ratio, day of image acquisition, and annual precipitation. We then applied this model on 20 620 ha of North Dakota perennial grass fields enrolled in the Conservation Reserve Program (CRP), including 1272 fields east of the Missouri River and 165 fields west‐river. Growing‐season NEE for individual CRP fields was highly variable from 1997 to 2006, ranging from ?366 to 692 g C m^?2 growing season^?1. Mean annual growing‐season fluxes over 10 years for CRP fields located east‐river and west‐river were 317 g C m^?2 growing season^?1 and 239 g C m^?2 growing season^?1, respectively. Average cumulative growing‐season NEE modeled for fields east‐ and west‐river diverged from one another in 2002–2006, when west‐river fields received < 70% of the long‐term annual average precipitation during these years. Results indicate assessment of conservation practices on grassland CO₂ exchange during the growing season can be remotely estimated at field and landscape scales under variable environmental conditions and should be followed up with similar, spatially explicit investigations of NEE during the dormant season.     

How afforestation affects the water cycle in drylands: A process‐based comparative analysis

The world's largest afforestation programs implemented by China made a great contribution to the global “greening up.” These programs have received worldwide attention due to its contribution toward achieving the United Nations Sustainable Development Goals. However, emerging studies have suggested that these campaigns, when not properly implemented, resulted in unintended ecological and water security concerns at the regional scale. While mounting evidence shows that afforestation causes substantial reduction in water yield at the watershed scale, process‐based studies on how forest plantations alter the partitioning of rainwater and affect water balance components in natural vegetation are still lacking at the plot scale. This lack of science‐based data prevents a comprehensive understanding of forest‐related ecosystem services such as soil conservation and water supply under climate change. The present study represents the first “Paired Plot” study of the water balance of afforestation on the Loess Plateau. We investigate the effects of forest structure and environmental factors on the full water cycle in a typical multilayer plantation forest composed of black locust, one of the most popular tree species for plantations worldwide. We measure the ecohydrological components of a black locust versus natural grassland on adjacent sites. The startling finding of this study is that, contrary to the general belief, the understory—instead of the overstory—was the main water consumer in this plantation. Moreover, there is a strict physiological regulation of forest transpiration. In contrast to grassland, annual seepage under the forest was minor in years with an average rainfall. We conclude that global long‐term greening efforts in drylands require careful ecohydrologic evaluation so that green and blue water trade‐offs are properly addressed. This is especially important for reforestation‐based watershed land management, that aims at carbon sequestration in mitigating climate change while maintaining regional water security, to be effective on a large scale.     

Carbon dioxide exchange over multiple temporal scales in an arid shrub ecosystem near La Paz,Baja California Sur,Mexico

Arid environments represent 30% of the global terrestrial surface, but are largely under‐represented in studies of ecosystem carbon flux. Less than 2% of all FLUXNET eddy covariance sites exist in a hot desert climate. Long‐term datasets of these regions are vital for capturing the seasonal and interannual variability that occur due to episodic precipitation events and climate change, which drive fluctuations in soil moisture and temperature patterns. The objectives of this study were to determine the meteorological variables that drive carbon flux on diel, seasonal, and annual scales and to determine how precipitation events control annual net ecosystem exchange (NEE). Patterns of NEE from 2002 to 2008 were investigated, providing a record with multiple replicates of seasons and conditions. Precipitation was extremely variable (55–339 mm) during the study period, and reduced precipitation in later years (2004–2008) appears to have resulted in annual moderate to large carbon sources (62–258 g C m^?2 yr^?1) in contrast to the previously reported sink (2002–2003). Variations in photosynthetically active radiation were found to principally drive variations in carbon uptake during the wet growing season while increased soil temperatures at a 5 cm depth stimulated carbon loss during the dry dormant season. Monthly NEE was primarily driven by soil moisture at a 5 cm depth, and years with a higher magnitude of precipitation events showed a longer growing season with annual net carbon uptake, whereas years with lower magnitude had drier soils and displayed short growing seasons with annual net carbon loss. Increased precipitation frequency was associated with increased annual NEE, which may be a function of increased microbial respiration to more small precipitation events. Annual precipitation frequency and magnitude were found to have effects on the interannual variability of NEE for up to 2 years.     

Regime shifts of Mediterranean forest carbon uptake and reduced resilience driven by multidecadal ocean surface temperatures

The mechanisms translating global circulation changes into rapid abrupt shifts in forest carbon capture in semi‐arid biomes remain poorly understood. Here, we report unprecedented multidecadal shifts in forest carbon uptake in semi‐arid Mediterranean pine forests in Spain over 1950–2012. The averaged carbon sink reduction varies between 31% and 37%, and reaches values in the range of 50% in the most affected forest stands. Regime shifts in forest carbon uptake are associated with climatic early warning signals, decreased forest regional synchrony and reduced long‐term carbon sink resilience. We identify the mechanisms linked to ocean multidecadal variability that shape regime shifts in carbon capture. First, we show that low‐frequency variations of the surface temperature of the Atlantic Ocean induce shifts in the non‐stationary effects of El Niño Southern Oscillation (ENSO) on regional forest carbon capture. Modelling evidence supports that the non‐stationary effects of ENSO can be propagated from tropical areas to semi‐arid Mediterranean biomes through atmospheric wave trains. Second, decadal changes in the Atlantic Multidecadal Oscillation (AMO) significantly alter sea–air heat exchanges, modifying in turn ocean vapour transport over land and land surface temperatures, and promoting sustained drought conditions in spring and summer that reduce forest carbon uptake. Third, we show that lagged effects of AMO on the winter North Atlantic Oscillation also contribute to the maintenance of long‐term droughts. Finally, we show that the reported strong, negative effects of ocean surface temperature (AMO) on forest carbon uptake in the last decades are unprecedented over the last 150 years. Our results provide new, unreported explanations for carbon uptake shifts in these drought‐prone forests and review the expected impacts of global warming on the profiled mechanisms.     

Carbon dioxide exchange above a Mediterranean C3/C4 grassland during two climatologically contrasting years

Eddy‐covariance measurements of net ecosystem carbon exchange (NEE) were carried out above a grazed Mediterranean C3/C4 grassland in southern Portugal, during two hydrological years, 2004–2005 and 2005–2006, of contrasting rainfall. Here, we examine the seasonal and interannual variation in NEE and its major components, gross primary production (GPP) and ecosystem respiration (R_eco), in terms of the relevant biophysical controls. The first hydrological year was dry, with total precipitation 45% below the long‐term mean (669 mm) and the second was normal, with total precipitation only 12% above the long‐term mean. The drought conditions during the winter and early spring of the dry year limited grass production and the leaf area index (LAI) was very low. Hence, during the peak of the growth period, the maximum daily rate of NEE and the light‐use and water‐use efficiencies were approximately half of those observed in the normal year. In the summer of 2006, the warm‐season C4 grass, Cynodon dactylon L., exerted an evident positive effect on NEE by converting the ecosystem into a carbon sink after strong rain events and extending the carbon sequestration for several days, after the end of senescence of the C3 grasses. On an annual basis, the GPP and NEE were 524 and 49 g C m^?2, respectively, for the dry year, and 1261 and ?190 g C m^?2 for the normal year. Therefore, the grassland was a moderate net source of carbon to the atmosphere, in the dry year, and a considerable net carbon sink, in the normal year. In these 2 years of experiment the total amount of precipitation was the main factor determining the interannual variation in NEE. In terms of relevant controls, GPP and NEE were strongly related to incident photosynthetic photon flux density on short‐term time scales. Changes in LAI explained 84% and 77% of the variation found in GPP and NEE, respectively. Variations in R_eco were mainly controlled by canopy photosynthesis. After each grazing event, the reduction in LAI affected negatively the NEE.     

Modelling the interannual variability of net ecosystem CO2 exchange at a subarctic sedge fen

This paper presents an empirical model of net ecosystem CO₂ exchange (NEE) developed for a subarctic fen near Churchill, Manitoba. The model with observed data helps explain the interannual variability in growing season NEE. Five years of tower‐flux data are used to test and examine the seasonal behaviour of the model simulations. Processes controlling the observed interannual variability of CO₂ exchange at the fen are examined by exploring the sensitivity of the model to changes in air temperature, precipitation and leaf area index. Results indicate that the sensitivity of NEE to changing environmental controls is complex and varies interannually depending on the initial conditions of the wetland. Changes in air temperature and the timing of precipitation events have a strong influence on NEE, which is largely manifest in gross ecosystem photosynthesis (GEP). Climate change scenarios indicate that warmer air temperatures will increase carbon acquisition during wet years but may act to reduce wetland carbon storage in years that experience a large water deficit early in the growing season. Model simulations for this subarctic sedge fen indicate that carbon acquisition is greatest during wet and warm conditions. This suggests therefore that carbon accumulation was greatest at this subarctic fen during its early developmental stages when hydroclimatic conditions were relatively wet and warm at approximately 2500 years before present.     

Seasonal and interannual variations in carbon dioxide exchange over a cropland in the North China Plain

In China, croplands account for a relatively large form of vegetation cover. Quantifying carbon dioxide exchange and understanding the environmental controls on carbon fluxes over croplands are critical in understanding regional carbon budgets and ecosystem behaviors. In this study, the net ecosystem exchange (NEE) at a winter wheat/summer maize rotation cropping site, representative of the main cropping system in the North China Plain, was continuously measured using the eddy covariance technique from 2005 to 2009. In order to interpret the abiotic factors regulating NEE, NEE was partitioned into gross primary production (GPP) and ecosystem respiration (R_eco). Daytime R_eco was extrapolated from the relationship between nighttime NEE and soil temperature under high turbulent conditions. GPP was then estimated by subtracting daytime NEE from the daytime estimates of R_eco. Results show that the seasonal patterns of the temperature responses of R_eco and light‐response parameters are closely related to the crop phenology. Daily R_eco was highly dependent on both daily GPP and air temperature. Interannual variability showed that GPP and R_eco were mainly controlled by temperature. Water availability also exerted a limit on R_eco. The annual NEE was ?585 and ?533 g C m^?2 for two seasons of 2006–2007 and 2007–2008, respectively, and the wheat field absorbed more carbon than the maize field. Thus, we concluded that this cropland was a strong carbon sink. However, when the grain harvest was taken into account, the wheat field was diminished into a weak carbon sink, whereas the maize field was converted into a weak carbon source. The observations showed that severe drought occurring during winter did not reduce wheat yield (or integrated NEE) when sufficient irrigation was carried out during spring.     

The effect of charcoal production on carbon cycling in African biomes

Using biomass for charcoal production in sub-Saharan Africa (SSA) may change carbon stock dynamics and lead to irreversible changes in the carbon balance, yet we have little understanding of whether these dynamics vary by biome in this region. Currently, charcoal production contributes up to 7% of yearly deforestation in tropical regions, with carbon emissions corresponding to 71.2 million tonnes of CO₂ and 1.3 million tonnes of CH₄. With a projected increased demand for charcoal in the coming decades, even low harvest rates may throw the carbon budget off-balance due to legacy effects. Here, we parameterized the dynamic global vegetation model LPJ-GUESS for six SSA biomes and examined the effect of charcoal production on net ecosystem exchange (NEE), carbon stock sizes and recovery time for tropical rain forest, montane forest, moist savanna, dry savanna, temperate grassland and semi-desert. Under historical charcoal regimes, tropical rain forests and montane forests transitioned from net carbon sinks to net sources, that is, mean cumulative NEE from −3.56 ± 2.59 kg C/m² to 2.46 ± 3.43 kg C/m² and −2.73 ± 2.80 kg C/m² to 1.87 ± 4.94 kg C/m² respectively. Varying charcoal production intensities resulted in tropical rain forests showing at least two times higher carbon losses than the other biomes. Biome recovery time varied by carbon stock, with tropical and montane forests taking about 10 times longer than the fast recovery observed for semi-desert and temperate grasslands. Our findings show that high biomass biomes are disproportionately affected by biomass harvesting for charcoal, and even low harvesting rates strongly affect vegetation and litter carbon and their contribution to the carbon budget. Therefore, the prolonged biome recoveries imply that current charcoal production practices in SSA are not sustainable, especially in tropical rain forests and montane forests, where we observe longer recovery for vegetation and litter carbon stocks.     

Mammals in South American drylands: faunal similarity and trophic structure

We compared the fauna of small mammals (less than 500 g body weight) among five major South American drylands (Atacama, Altiplano, Monte, Patagonia and Caatinga) and found considerable heterogeneity and distinctiveness in species richness and composition between these biomes. From a total of 89 recorded species, 76 of them are restricted to only one of these drylands. The highland desert, or Altiplano, is the biome with the highest number of species. Despite the marked differences in the composition of the mammalian fauna, the trophic structure shows a rather consistent pattern: herbivores are the most important trophic group in all drylands. This consistency seems to be more the result of phylogenetic inertia than of similar ecological processes. Our results are compared with recent studies on desert small mammals across continents.     

Drivers of future alien species impacts: An expert‐based assessment

Understanding the likely future impacts of biological invasions is crucial yet highly challenging given the multiple relevant environmental, socio‐economic and societal contexts and drivers. In the absence of quantitative models, methods based on expert knowledge are the best option for assessing future invasion trajectories. Here, we present an expert assessment of the drivers of potential alien species impacts under contrasting scenarios and socioecological contexts through the mid‐21st century. Based on responses from 36 experts in biological invasions, moderate (20%–30%) increases in invasions, compared to the current conditions, are expected to cause major impacts on biodiversity in most socioecological contexts. Three main drivers of biological invasions—transport, climate change and socio‐economic change—were predicted to significantly affect future impacts of alien species on biodiversity even under a best‐case scenario. Other drivers (e.g. human demography and migration in tropical and subtropical regions) were also of high importance in specific global contexts (e.g. for individual taxonomic groups or biomes). We show that some best‐case scenarios can substantially reduce potential future impacts of biological invasions. However, rapid and comprehensive actions are necessary to use this potential and achieve the goals of the Post‐2020 Framework of the Convention on Biological Diversity.     

Using historical ecology to understand patterns of biodiversity in fragmented agricultural landscapes

Aim To enhance current attempts to understand biodiversity patterns by using an historical ecology approach to highlight the over‐riding influence of land‐use history in creating past, current and future patterns of biodiversity in fragmented agricultural landscapes. Methods We develop an integrative conceptual framework for understanding spatial and temporal variations in landscape patterns in fragmented agricultural landscapes by presenting five postulates (hypotheses) which highlight the important role of historical, anthropogenic disturbance regimes. We then illustrate each of these postulates with examples drawn from fragmented woodlands in agricultural areas of south‐eastern Australia, and discuss these findings in an international context. Location examples are drawn from agricultural areas in south‐eastern Australia. Results We conclude that there is limited potential to refine our understanding of patterns of biodiversity in human‐modified landscapes based on traditional concepts of island biogeography, or simple assumptions of ongoing destruction and degradation. Instead, we propose that in agricultural landscapes that were largely cleared over a century ago: (1) present‐day remnant vegetation patterns are not accidental, but are logically arrayed due to historic land‐use decisions, (2) historic anthropogenic disturbances have a major influence on current ecosystem conditions and diversity patterns, and (3) the condition of remnant ecosystems is not necessarily deteriorating rapidly. Main conclusions An historical ecology approach can enhance our understanding of why different species and ecosystem states occur where they do, and can explain internal variations in ecological conditions within remnant ecosystems, too often casually attributed to the ‘mess of history’. This framework emphasizes temporal changes (both past and future) in biotic patterns and processes in fragmented agricultural landscapes. Integration of spatially and temporally explicit historical land‐use information into ecological studies can prove extremely useful to test hypotheses of the effects of changes in landscape processes, and to enhance future research, restoration and conservation management activities.     

Model-data synthesis of diurnal and seasonal CO2 fluxes at Niwot Ridge, Colorado

Eddy covariance records hold great promise for understanding the processes controlling the net ecosystem exchange of CO₂ (NEE). However, NEE is the small difference between two large fluxes: photosynthesis and ecosystem respiration. Consequently, separating NEE into its component fluxes, and determining the process‐level controls over these fluxes, is a difficult problem. In this study, we used a model‐data synthesis approach with the Simplified PnET (SIPNET) flux model to extract process‐level information from 5 years of eddy covariance data at an evergreen forest in the Colorado Rocky Mountains. SIPNET runs at a twice‐daily time step, and has two vegetation carbon pools, a single aggregated soil carbon pool, and a soil moisture submodel that models both evaporation and transpiration. By optimizing the model parameters before evaluating model‐data mismatches, we were able to probe the model structure independent of any arbitrary parameter set. In doing so, we were able to learn about the primary controls over NEE in this ecosystem, and in particular the respiration component of NEE. We also used this parameter optimization, coupled with a formal model selection criterion, to investigate the effects of making hypothesis‐driven changes to the model structure. These experiments lent support to the hypotheses that (1) photosynthesis, and possibly foliar respiration, are down‐regulated when the soil is frozen and (2) the metabolic processes of soil microbes vary in the summer and winter, possibly because of the existence of distinct microbial communities at these two times. Finally, we found that including water vapor fluxes, in addition to carbon fluxes, in the parameter optimization did not yield significantly more information about the partitioning of NEE into gross photosynthesis and ecosystem respiration.