Ozone uptake at the canopy level in Robinia pseudoacacia in beijing based on sap flow measurements

Abstract

The accelerating global urbanization caused severe air pollutions. In Beijing, the air pollution has been exacerbated due to the wide-spread construction activities and increasing number of vehicles. The increasing concentration of ozone (O3) in the troposphere has been recognized as a source of air pollution, due to its adverse effects on human health and plant and animal growth, and its contribution to global climate change. O3 is known to impact forest trees in many ways including morphological and histological injuries, decreasing photosynthesis, increasing respiration, and alteration of carbon allocation and water balance. Considerable researches have been conducted to investigate ozone uptake by urban forests at the regional scale. On the other hand, trees are able to improve urban air quality by taking up and removing gaseous pollutants, and the O3 uptake by urban forests at the regional scale has been extensively studied. However, research on O3 uptake by urban trees at the canopy scale is rare. The main objectives of this study are: (1) to quantify the whole-tree O3 uptake by Robinia pseudoacacia, one of urban greening tree species in Beijing; and (2) to examine how O3 flux in R. pseudoacacia trees is regulated by the stomata and environmental conditions. In this study, the whole-tree O3 uptake in R. pseudoacacia trees during spring and summer was estimated based on sap flow measurements and the data of micro-climate and ambient O3 concentration were also collected. The diurnal ozone uptake rate (FO3) by R. pseudoacacia showed a single peak pattern with the maximum rate occurring at around 15:00 pm. The diurnal FO3 showed a narrow peak during summer and a wide peak during autumn. The most obvious increase in accumulated stomatal ozone flux(AFst)occurred around noon time. FO3 showed a seasonal pattern with higher values found in summer than in autumn. The increase in AFst was most obvious in summer than in autumn. The diurnal and seasonal patterns of O3 uptake were related to the temporal variations of ambient air O3 concentrations and canopy conductance (GO3). Ambient air O3 concentration showed a similar diurnal and seasonal pattern to FO3. Under a given ambient air O3 concentrations, the whole-tree FO3 was dependent on canopy conductance, and hereby was further influenced by the vapour pressure deficit (D) and total radiation (Rs). GO3 decreased exponentially with increasing D. High D caused low GO3, and thus low FO3 in spite of relatively high ambient air O3 concentrations. On the contrary, GO3 was high under low D conditions, and thus FO3 was high in spite of relatively low ambient air O3 concentrations. However, FO3 was relatively low under very low D conditions, such as in early mornings, which may be attributed to the weak photosynthesis and small stomatal apertures in the early morning at this time. Moreover, GO3 decreased rapidly with increasing Rs when Rs was higher than 600 W/m2. Similarly, ambient air O3 concentration decreased with increasing Rs when Rs was higher than 800 W/m2. Therefore, FO3 exhibited an asymmetric single-peak pattern: FO3 slightly increased with increasing Rs when Rs was below 800 W/m2, however, it decreased rapidly with increasing Rs when Rs was higher than 800 W/m2. The annual O3 uptake by R. pseudoacacia trees estimated in our study was 0.16 g/m2, which was much lower than the values estimated from the Urban Forest Effects Model. This difference suggests the necessity to consider the O3 uptake flux on canopy level when evaluate the O3 risks on urban trees.