Soil water content effects on net ecosystem CO2 exchange and actual evapotranspiration in a Mediterranean semiarid savanna of Central Chile

Francisco J Meza,Felipe Bravo-Martínez,Carlo Montes,Andrew S Kowalski,Penélope Serrano-Ortiz

doi:10.1038/s41598-018-26934-z

Abstract

Biosphere-atmosphere water and carbon fluxes depend on ecosystem structure, and their magnitudes and seasonal behavior are driven by environmental and biological factors. We studied the seasonal behavior of net ecosystem CO2 exchange (NEE), Gross Primary Productivity (GPP), Ecosystem Respiration (RE), and actual evapotranspiration (ETa) obtained by eddy covariance measurements during two years in a Mediterranean Acacia savanna ecosystem (Acacia caven) in Central Chile. The annual carbon balance was −53 g C m−2 in 2011 and −111 g C m−2 in 2012, showing that the ecosystem acts as a net sink of CO2, notwithstanding water limitations on photosynthesis observed in this particularly dry period. Total annual ETa was of 128 mm in 2011 and 139 mm in 2012. Both NEE and ETa exhibited strong seasonality with peak values recorded in the winter season (July to September), as a result of ecosystem phenology, soil water content and rainfall occurrence. Consequently, the maximum carbon assimilation rate occurred in wintertime. Results show that soil water content is a major driver of GPP and RE, defining their seasonal patterns and the annual carbon assimilation capacity of the ecosystem, and also modulating the effect that solar radiation and air temperature have on NEE components at shorter time scales.

Highlights

Arid and semiarid regions of the world cover more than 40% of the total land surface, are characterized by low precipitation amounts and high evaporation rates, are exposed to high hydroclimatic variability and usually exhibit relatively low soil fertility, factors that condition biomass accumulation
net ecosystem CO2 exchange (NEE) is controlled by precipitation, and droughts are identified as major elements that reduce gross primary productivity (GPP) by limiting photosynthetic rates, and shortening the length of the growing season[3,4]
Other studies[12] indicate that aridity overrides the climatological sensitivity of NEE to temperature in dry areas. This could be the case of semiarid Chile, where increasing dryness in spring and summer has been reported during recent decades[13], which are consistent with climate change projections[14], suggesting high vulnerability of NEE as climate becomes drier and warmer[15]

Summary

Introduction

Arid and semiarid regions of the world cover more than 40% of the total land surface, are characterized by low precipitation amounts and high evaporation rates, are exposed to high hydroclimatic variability and usually exhibit relatively low soil fertility, factors that condition biomass accumulation. Recent studies suggest that current global trends in the carbon sink of the biosphere are dominated by semiarid ecosystems[2] In these regions, NEE is controlled by precipitation, and droughts are identified as major elements that reduce GPP by limiting photosynthetic rates, and shortening the length of the growing season[3,4]. NEE is controlled by precipitation, and droughts are identified as major elements that reduce GPP by limiting photosynthetic rates, and shortening the length of the growing season[3,4] This is the case of the exceptionally positive anomaly in global carbon uptake registered in 2011, which is mainly ascribed to increased productivity of semiarid vegetation in the Southern Hemisphere, and especially to the effect of wet conditions in Australia during a La Niña year[5,6]. A shift to net carbon source was reported in a grassland in California due to a shorter growing season[24]

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