Environmental and biological regulations of interannual variability of net ecosystem exchange across regional scales

Background/Question/Methods Interannual variability (IAV) in net ecosystem exchange of carbon (NEE) is a common phenomenon observed at almost all eddy covariance flux sites worldwide. However, our understanding of spatial patterns of IAV is extremely limited. As a result of different impact patterns of photosynthesis (GPP) and ecosystem respiration on climate change, there can be considerable IAV in NEE. Based on the eddy covariance data from AmeriFlux and EuroFlux sites, we investigated the spatial pattern of IAV in NEE, and concluded the environmental and biological (GPP and ecosystem respiration) regulations. 43 sites consisting of 151 years of data were included for analyzing, covering three major terrestrial biomes: deciduous broadleaf forests, evergreen needleleaf forests and grasslands. Standard deviation was used to represent the absolute IAV and coefficient of variation to express the magnitude of relative IAV in GPP, Re, NEE and environmental variables.

Results/Conclusions In deciduous broadleaf forests, there was significant negative correlation between NEE and ecosystem respiration. However, spatial pattern of NEE appears more related to GPP than to ecosystem respiration in evergreen needleleaf forests. The absolute IAV of NEE had a significant positive correlation with both absolute IAV of GPP and ecosystem respiration in deciduous broadleaf forests, and the relative IAV of NEE correlated significantly with relative IAV of GPP and ecosystem respiration.

Correlation analyses showed mean annual ecosystem respiration in deciduous broadleaf forests correlated negatively and significantly with the mean annual air temperature, while gross primary production tends to be constant, inducing to positive correlation of NEE along the temperature gradient. In evergreen needleleaf forests, GPP, ecosystem respiration and NEE increased significantly with air temperature, and GPP and ecosystem respiration also showed significantly negative correlation with annual precipitation. Annual precipitation was major cause for spatial patterns of GPP and ecosystem respiration in grasslands, however, no significant correlation was found about NEE.

In deciduous broadleaf forests, relative IAV of air temperature had significant correlation with relative IAV in GPP, Re and NEE, except for the two sites in Europe. Changes of precipitation were the reason for interannual variability in GPP and ecosystem respiration in grasslands. Absolute IAV of Precipitation explained 40% and 63% variables of absolute IAV in GPP and ecosystem respiration respectively in grasslands, and also relative IAV was a reason for 41% and 56% variables of relative IAV in GPP and ecosystem respiration.