Do the invasive trees, Ailanthus altissima and Robinia pseudoacacia, alter litterfall dynamics and soil properties of riparian ecosystems in Central Spain?

To assess the impacts of Ailanthus altissima and Robinia pseudoacacia on the whole litterfall process and on soil properties of riparian ecosystems of Central Spain by comparing invaded and non-invaded forests. We selected 3-4 plots of four different types of forests: invaded by A. altissima or R. pseudoacacia, or dominated by the native Fraxinus angustifolia or Populus alba. In each plot we collected the litter monthly over 2 years and separated the litter from different plant organs. We calculated annual inputs of litter, nitrogen (N) and phosphorus (P) to soil. We also analyzed soil pH, organic matter (OM), N and P. Most litter fractions from the two invasive trees contained over twice as much N as those from the native P. alba. Although not significantly different, the annual inputs of N in invaded forests were 1.7-2.2 times higher than in native forests. In the invaded forests, extra litterfall peaks were found in summer and/or late-spring (June), corresponding to reproductive and/or leaf litter fractions. The percentage of annual litter represented by these peaks was over twice those in native forests. Soil OM, N and P were higher in invaded than in P. alba forests. In addition, forests invaded by R. pseudoacacia had higher soil OM and N than F. angustifolia forests. This study showed the capability of A. altissima and R. pseudoacacia to alter soil properties and litterfall dynamics in riparian ecosystems.