Physiological mechanism for the reduction in soil water in poplar (Populus deltoides) plantations in Dongting Lake wetlands

Youzhi Li,Canming Zhang,Xinsheng Chen,Yonghong Xie,Hongyan Qin,Wei Wang

doi:10.1007/s11273-013-9319-1

Abstract

The use of large-scale tree plantations has provoked increasing concern regarding the negative effects on local environments in different ecosystems. However, the physiological mechanism underlying the reduction in soil water by tree plantations in wetlands is not clear. The aims of this study were to investigate the effects of poplar (Populus deltoides) plantations on soil water content and to elucidate the underlying physiological mechanisms. To this end, we conducted a 1-year fixed-plot investigation of soil water content (SWC), plant photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and water-use efficiency (WUE) of individual leaves of 11- and 5-year-old poplars and of reed (Triarrherca sacchariflora, a native herbaceous plant) in the Dongting Lake wetlands, China. SWC was highest in reed, intermediate in 11-year-old poplar, and lowest in 5-year-old poplar, suggesting that poplar plantations produce a lower soil water content in wetlands. From May to July, Pn was significantly higher in reed than in the two poplar stands, but did not differ between the different-aged poplars. As a whole, Gs and Tr were higher, but WUE was lower, in the poplar stands than in reed during the growing season, indicating that Gs and Tr are the key physiological mechanisms associated with the lower soil water in poplar stands. Relationships among Pn, Gs, and Tr showed positive correlations (P < 0.01) for each type of vegetation. These data suggest that poplar plantations may cause the transformation of wetlands into dry land due to a lower WUE leading to a massive water loss from soil. This, in turn, would have an influence on community composition and ecosystem function after establishment of the plantations.

Highlights

The use of fast-growing tree plantations has been increasing throughout the world to satisfy demands for industrial timber and pulp, to mitigate climatic changes (Richards et al 2007), and to produce biofuels (IEA 2004)
It was clear that poplar plantations resulted in a lower soil water content (SWC)
Soil water is affected by flooding due to the high groundwater level (Xie et al 2011). This may be the main explanation for similar seasonal changes in SWC and precipitation from January to August, and for the relatively higher SWC during the flood period (April to August)

Summary

Introduction

The use of fast-growing tree plantations has been increasing throughout the world to satisfy demands for industrial timber and pulp, to mitigate climatic changes (Richards et al 2007), and to produce biofuels (IEA 2004). Fast-growing trees are generally planted in monocultures, and these species have obvious advantages over native plants in competing for light, nutrients, and water resources. These factors have led to increasing concerns regarding the negative effects of large-scale tree plantations on local environments, including nutrition depletion (Onyekwelu et al 2006), reduced biodiversity (Morris et al 2008), water-table decline (Almeida et al 2007), and outbreaks of pests and diseases (Kelty 2006). Compared to groundwater, soil moisture content can more closely reflect the water status of the rhizosphere of tree plantations, which is more important in terms of regulating species distribution, community composition, and biodiversity (Engelbrecht et al 2007; Gaitan et al 2011). The effects of tree plantations on soil water status have remained far from clear

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