Spatial variations in terrestrial net ecosystem productivity and its local indicators

Erqian Cui,Shuli Niu,Jianyang Xia,Yingping Wang,Chenyu Bian,Yiqi Luo

doi:10.5194/bg-17-6237-2020

Abstract

Abstract. Multiple lines of evidence have demonstrated the persistence of global land carbon (C) sink during the past several decades. However, both annual net ecosystem productivity (NEP) and its inter-annual variation (IAVNEP) keep varying over space. Thus, identifying local indicators for the spatially varying NEP and IAVNEP is critical for locating the major and sustainable C sinks on land. Here, based on daily NEP observations from FLUXNET sites and large-scale estimates from an atmospheric-inversion product, we found a robust logarithmic correlation between annual NEP and seasonal carbon uptake–release ratio (i.e. U ∕ R). The cross-site variation in mean annual NEP could be logarithmically indicated by U ∕ R, while the spatial distribution of IAVNEP was associated with the slope (i.e. β) of the logarithmic correlation between annual NEP and U ∕ R. Among biomes, for example, forests and croplands had the largest U ∕ R ratio (1.06 ± 0.83) and β (473 ± 112 g C m−2 yr−1), indicating the highest NEP and IAVNEP in forests and croplands, respectively. We further showed that these two simple indicators could directly infer the spatial variations in NEP and IAVNEP in global gridded NEP products. Overall, this study provides two simple local indicators for the intricate spatial variations in the strength and stability of land C sinks. These indicators could be helpful for locating the persistent terrestrial C sinks and provide valuable constraints for improving the simulation of land–atmospheric C exchanges.

Highlights

Terrestrial ecosystems reabsorb about one-quarter of anthropogenic CO2 emission (Ciais et al, 2019) and are primarily responsible for the recent temporal fluctuations in the measured atmospheric-CO2 growth rate (Randerson, 2013; Le Quéré et al, 2018)
The mean annual net ecosystem productivity (NEP) is related to the strength of carbon exchange of a specific ecosystem (Randerson et al, 2002; Luo and Weng, 2011; Jung et al, 2017), while IAVNEP characterizes the stability of such a carbon exchange (Musavi et al, 2017)
To find local indicators for the spatially varying NEP in terrestrial ecosystems, we tested the relationship between NEP and its direct components (U and R) across the 72 flux tower sites

Summary

Introduction

Terrestrial ecosystems reabsorb about one-quarter of anthropogenic CO2 emission (Ciais et al, 2019) and are primarily responsible for the recent temporal fluctuations in the measured atmospheric-CO2 growth rate (Randerson, 2013; Le Quéré et al, 2018). Evidence based on eddyflux measurements (Baldocchi et al, 2018; Rödenbeck et al, 2018), aircraft atmospheric budgets (Peylin et al, 2013) and process-based model simulations (Poulter et al, 2014; Ahlström et al, 2015) has shown a large spatial variability in net ecosystem productivity (NEP) on land. The mean annual NEP is related to the strength of carbon exchange of a specific ecosystem (Randerson et al, 2002; Luo and Weng, 2011; Jung et al, 2017), while IAVNEP characterizes the stability of such a carbon exchange (Musavi et al, 2017). Whether and how NEP and IAVNEP change over space is important for predicting the future locations of carbon sinks on land (Yu et al, 2014; Niu et al, 2017).

Objectives

Methods

Results

Discussion

Conclusion