Understory Invasion by Acacia longifolia Alters the Water Balance and Carbon Gain of a Mediterranean Pine Forest
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In water-limited ecosystems, where potential evapotranspiration exceeds precipitation, it is often assumed that plant invasions will not increase total ecosystem water use, because all available water is evaporated or transpired regardless of vegetation type. However, invasion by exotic species, with high water use rates, may potentially alter ecosystem water balance by reducing water available to native species, which may in turn impact carbon assimilation and productivity of co-occurring species. Here, we document the impact of invasion by an understory exotic woody species (Acacia longifolia) in a semi-arid Mediterranean dune pine forest. To quantify the effects of this understory leguminous tree on the water use and carbon fixation rates of Pinus pinaster we compare an invaded and a non-invaded stand. A. longifolia significantly altered forest structure by increasing plant density and leaf area index in the mid-stratum of the invaded forest. A. longifolia contributed significantly to transpiration in the invaded forest (up to 42%) resulting in a slight increase in stand transpiration in the invaded relative to non-invaded forest. More importantly, both water use and carbon assimilation rates of P. pinaster were significantly reduced in the invaded relative to non-invaded stand. Therefore, this study shows that exotic plant invasions can have significant impacts on hydrological and carbon cycling even in water-limited semi-arid ecosystems through a repartitioning of water resources between the native and the invasive species.
Keywordscompetition delta 13-C ecohydrology invasion ecology Mediterranean nitrogen Pinus pinaster sap flux transpiration water cycle
Funding for this project was provided by the Deutsche Forschungsgemeinschaft, (TRANSDUNE Project: # WE 2681/3-1). KGR gratefully acknowledges additional funding from the PEO Scholar Award. We also thank the Estabelecimento Prisional de Pinheiro da Cruz for logistical support and allowing the establishment of our field site. We are also grateful to Ana Julia Pereira, Tine Hellmann and Rabea Sutter for their assistance in the field and Babsi Teichner for isotope analyses.
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