Journal Article

Upside-down fluxes Down Under: CO2 net sink in winter and net source in summer in a temperate evergreen broadleaf forest

Isaac, Peter, Griebel, Anne, Duursma, Remko A, Pendall, Elise, Renchon, Alexandre A, Boer, Matthias M, Maier, Chelsea, Metzen, Daniel, Tissue, David T, Medlyn, Belinda E, Williams, Christopher A, Barton, Craig V. M, Resco de Dios, Víctor

2018

Overview

Predicting the seasonal dynamics of ecosystem carbon fluxes is challenging in broadleaved evergreen forests because of their moderate climates and subtle changes in canopy phenology. We assessed the climatic and biotic drivers of the seasonality of net ecosystem–atmosphere CO2 exchange (NEE) of a eucalyptus-dominated forest near Sydney, Australia, using the eddy covariance method. The climate is characterised by a mean annual precipitation of 800mm and a mean annual temperature of 18°C, hot summers and mild winters, with highly variable precipitation. In the 4-year study, the ecosystem was a sink each year (−225gCm−2yr−1 on average, with a standard deviation of 108gCm−2yr−1); inter-annual variations were not related to meteorological conditions. Daily net C uptake was always detected during the cooler, drier winter months (June through August), while net C loss occurred during the warmer, wetter summer months (December through February). Gross primary productivity (GPP) seasonality was low, despite longer days with higher light intensity in summer, because vapour pressure deficit (D) and air temperature (Ta) restricted surface conductance during summer while winter temperatures were still high enough to support photosynthesis. Maximum GPP during ideal environmental conditions was significantly correlated with remotely sensed enhanced vegetation index (EVI; r2 = 0.46) and with canopy leaf area index (LAI; r2= 0.29), which increased rapidly after mid-summer rainfall events. Ecosystem respiration (ER) was highest during summer in wet soils and lowest during winter months. ER had larger seasonal amplitude compared to GPP, and therefore drove the seasonal variation of NEE. Because summer carbon uptake may become increasingly limited by atmospheric demand and high temperature, and because ecosystem respiration could be enhanced by rising temperatures, our results suggest the potential for large-scale seasonal shifts in NEE in sclerophyll vegetation under climate change.

• Publisher: Copernicus Publications on behalf of the European Geosciences Union,Copernicus GmbH,Copernicus Publications
• Subject: Air temperature / Annual precipitation / Annual temperatures / Annual variations / Atmospheric precipitations / Canopies / Canopy / Carbon / Carbon dioxide / Carbon dioxide atmospheric concentrations / Carbon dioxide exchange / Carbon uptake / Climate and vegetation / Climate change / Conductance / Coniferous forests / Covariance / Drought / Dynamics / Ecosystems / Eddy covariance / Environmental conditions / Eucalyptus / Fluxes / Forest ecosystems / Forests / Heat / Herbivores / High temperature / Hot summers / Interannual variability / Leaf area / Leaf area index / Light intensity / Luminous intensity / Mean annual precipitation / Meteorological conditions / Mild winters / Phenology / Photosynthesis / Precipitation / Primary production / Rain / Rainfall / Remote sensing / Resistance / Respiration / Seasonal variation / Seasonal variations / Seasonality / Senescence / Soil / Summer / Summer rainfall / Surface temperature / Temperature effects / Temperature rise / Uptake / Vapor pressure / Vapour pressure / Vegetation / Vegetation index / Winter / Winter temperatures