Fluxes all of the time? A primer on the temporal representativeness of FLUXNET

Abstract

FLUXNET, the global network of eddy covariance flux towers, provides the largest synthesized data set of CO2, H2O, and energy fluxes. To achieve the ultimate goal of providing flux information “everywhere and all of the time,” studies have attempted to address the representativeness issue, i.e., whether measurements taken in a set of given locations and measurement periods can be extrapolated to a space‐ and time‐explicit extent (e.g., terrestrial globe, 1982–2013 climatological baseline). This study focuses on the temporal representativeness of FLUXNET and tests whether site‐specific measurement periods are sufficient to capture the natural variability of climatological and biological conditions. FLUXNET is unevenly representative across sites in terms of the measurement lengths and potentials of extrapolation in time. Similarity of driver conditions among years generally enables the extrapolation of flux information beyond measurement periods. Yet such extrapolation potentials are further constrained by site‐specific variability of driver conditions. Several driver variables such as air temperature, diurnal temperature range, potential evapotranspiration, and normalized difference vegetation index had detectable trends and/or breakpoints within the baseline period, and flux measurements generally covered similar and biased conditions in those drivers. About 38% and 60% of FLUXNET sites adequately sampled the mean conditions and interannual variability of all driver conditions, respectively. For long‐record sites (≥15 years) the percentages increased to 59% and 69%, respectively. However, the justification of temporal representativeness should not rely solely on the lengths of measurements. Whenever possible, site‐specific consideration (e.g., trend, breakpoint, and interannual variability in drivers) should be taken into account.

Evaluations of Climate and Land Management Effects on Lake Carbon Cycling Need to Account for Temporal Variability in CO2 Concentrations

“…While it is well known that CO 2 concentration and budget estimates depend on the temporal coverage of sampling (Morales‐Pineda et al, ; Natchimuthu et al, ), we here show that this is also true for evaluations of environmental drivers of lake CO 2 . Our data confirms recent insights from terrestrial carbon flux studies (Chu et al, ; Montagnani et al, ) and highlights a general problem in empirical sciences: The scale of observed patterns and underlying mechanisms must match if observations are used to understand and predict patterns and processes (Levin, ).…”

Complementary‐Relationship‐Based Modeling of Terrestrial Evapotranspiration Across China During 1982–2012: Validations and Spatiotemporal Analyses

“…The emergence of shared eddy‐covariance (EC) ET a observations during the past few years (e.g., FLUXNET; Baldocchi et al, ) has substantially advanced our knowledge of terrestrial evapotranspiration in various ecosystems. Unfortunately, however, the observation period of most EC flux towers is shorter than a decade, thus impeding the depiction of long‐term variability in ET a (see Figure 1 in Chu et al, , for a detailed list of observational periods of the FLUXNET towers). Furthermore, the spatial distribution of the EC flux stations is uneven globally, with the majority of the stations operated in Europe and North America, having far fewer stations in Asia, South America, and Africa (Chu et al, ; Jung et al, ; Yu et al, ).…”

“…Unfortunately, however, the observation period of most EC flux towers is shorter than a decade, thus impeding the depiction of long‐term variability in ET a (see Figure 1 in Chu et al, , for a detailed list of observational periods of the FLUXNET towers). Furthermore, the spatial distribution of the EC flux stations is uneven globally, with the majority of the stations operated in Europe and North America, having far fewer stations in Asia, South America, and Africa (Chu et al, ; Jung et al, ; Yu et al, ). Due to these difficulties, especially in developing countries, a need arises to develop robust estimates of large‐scale terrestrial evapotranspiration rates for a better understanding of land‐atmosphere energy and water exchanges at the regional, continental, and/or global scales and also to detect variations in diverse components of the hydrological cycle due to the ongoing climate change.…”

Ecohydrological processes and ecosystem services in the Anthropocene: a review

“…Ecosystems do not generate water, but they modify the quantity, quality, and timing of water cycles. Global measurements of carbon and water fluxes using the eddy covariance method (Baldocchi et al 2001;Chu et al 2017) in recent decades clearly show that ecosystem primary productivity and ecosystem respiration are closely coupled with evapotranspiration processes (Sun et al 2011b). At the leaf level, plant stomata play a critical role in regulating both water and CO 2 fluxes in ecosystems.…”