Lack of superiority of invasive over co-occurring native riparian tree seedling species

The invasive tree species Acer negundo, Ailanthus altissima, Elaeagnus angustifolia and Robinia pseudoacacia are nowadays spreading in inner Spain riparian forests, where they co-occur with the natives Fraxinus angustifolia, Populus alba and Ulmus minor. In these forests, the natural light and soil moisture conditions are being altered by different human activities, as river channelization and regulation. These new environmental conditions may favor invasive species to the detriment of natives. In order to predict potential shifts of species composition in inner Spain riparian forests, we experimentally compared seedling and sapling performance across the four invasive and the three native mentioned species. Seeds were sown along an experimental gradient with four levels of light (100, 65, 35, 7 % of full irradiance) factorially combined with two levels of soil moisture (61 and 40 % of soil gravimetric water content). We compared plant biomass at the end of the first and second growing seasons, relative growth rates, biomass allocation to roots (RWR), stems (SWR) and leaves (LWR), time to emergence (T-emerg) and net assimilation rate per unit of leaf mass (NAR(m)) between origins and across species. Biomass accumulation greatly varied across species. However, invaders did not grow on average more than natives. Under high resource conditions, all species tended to grow more and similarly. Only the native U. minor and the invader A. negundo were not hampered by low moisture and/or low light availabilities. The absence of superiority of invaders over natives suggests that the former will not displace the latter in inner Spain riparian forests. However, human activities promoting shade and drought stress in floodplains may benefit the invasive A. negundo in the long term, as the growth of its seedlings is less declined by these conditions than the growth of the rest of the studied species. Thus, management effort should be focused in monitoring and preventing this species spread.