Bi-directional exchange of greenhouse gases and pollutants between a Mediterranean holm oak forest and the atmosphere

Abstract

Forests play a major role in regulating air quality and climate. However, plant exposure to atmospheric pollution may cause detrimental effects on vegetation. Among others, tropospheric O3 is probably the most damaging to forest ecosystems: entering the leaves thought stomata, it causes oxidative stress and damages photosynthetic apparatus, reducing carbon assimilation. This PhD work aimed to quantify fluxes of greenhouse gases (CO2, CH4, O3) in a Mediterranean holm oak forest located near Rome (Italy) exposed to high levels of tropospheric O3 concentration. A primary goal was also to evaluate the O3 damage on the forest carbon assimilation capacity. Four years of eddy covariance measurements demonstrated that the ecosystem is an active CO2 sink all year long (485 – 690 g C m-2 y-1). For the first time, long term monitoring of CH4 exchange above a Mediterranean forest showed that the ecosystem is a net CH4 sink during the cold season and a net source during the dry season, thus making the annual net CH4 exchange close to the neutrality. Precipitation events during Summer season have been found to be one of the most important factors controlling carbon exchanges at the forest, thus influencing the CO2 uptake and the magnitude of the CH4 emission. The dry condition in Summer period depressed CH4-oxidizing bacteria, so that the ecosystem acted as a net source of CH4 in this period, thanks to translocation of CH4 from the soil to above canopy through xylematic pathways. Furthermore, the Summer high UV radiation has been found to promote CH4 production from photochemical reactions on leaves waxes, thus contributing to the overall strength of the Summer CH4 emissions. Besides being a sink for carbon, the holm oak forest represents a net sink of O3. The annual budget has been estimated around 80 mmol m-2 y-1. The major sink were stomata, being O3 uptake strongly connected to the physiological plant activity and subject to water availability.The influence of O3 over NEE was tested using a novel approach, which combines non parametric time series decomposition and the explanatory capacity of Artificial Neural Networks. Results suggested that O3 has a detrimental effect over NEE during Spring and Summer seasons, although the magnitude of this reduction is low (rate of reduction of NEE for unit change of O3 stomatal uptake = 0.015). The results of this PhD work demonstrated that this Mediterranean forest is an active carbon sink and contributes to ameliorate air quality removing O3 from the atmosphere. However, O3 uptake through stomata reduces carbon assimilation, although it does not represent the main limiting factor to forest productivity as compared to drought. The latter showed to be the major limiting factor at the forest leading to strong reduction of CO2 assimilation, enhancement of CH4 emission and limitation to O3 uptake. These results help to foresee possible future effects of climate change and pollution on Mediterranean forest ecosystems.