Causes and consequences of exotic tree invasion in the Iberian Peninsula

Abstract

This Thesis is focused in the study of exotic and native tree species co-occurring in the riparian forests of central Spain and in the mesic forests of the Northwest. The main aims are: 1) determining the environmental conditions that favour the establishment success of native and exotic tree species; 2) evaluating the impacts on the nitrogen cycling of the exotic species spreading in riparian forests of central Spain. To achieve aim 1, a two year experiment was conducted to compare differences in the establishment success between native and exotic seedlings under different treatments of light and soil moisture (Chapters 2 and 3). To achieve aim 2, we evaluated the effects of exotic and native litter on invaded soils (Chapter 5) and compared differences in nitrogen use strategies between native and exotic tree species (Chapter 4). The studied species can be grouped regarding their responses to the light and soil moisture treatments, but independently of their native or exotic origin, due to the heterogeneity found in the responses of the species included in each group. The exotic species may provoke changes in the nitrogen cycle of the riparian forests of central Spain, due to differences in litter quality, litter decomposition and nitrogen use strategies between native and exotic tree species. The differences found in the nitrogen use strategies among studied exotic species highlight the difficulties of predicting the impacts of exotic species in the invaded ecosystems.