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Assessing spatio-temporal rates, patterns and determinants of biological invasions in forest ecosystems. The case of Acacia species in NW Spain

Hernández, Laura, Martínez- Fernández, Jesús, Cañellas, Isabel, de la Cueva, Antonio Vázquez
Forest ecology and management 2014 v.329 pp. 206-213
Acacia dealbata, Acacia melanoxylon, basal area, biotic factors, control methods, ecological invasion, environmental factors, forest ecosystems, forest inventory, forest types, introduced plants, invasive species, landscapes, linear models, monitoring, stand structure, trees, Spain
Invasive species currently pose a major environmental challenge. Understanding their development and the factors associated with their expansion is the first step towards developing effective control measures. This work proposes the use of detailed spatio-temporal information from forest monitoring systems to assess the demographic rates, spatio-temporal patterns and spread determinants of invasive plants in forest ecosystems. For this purpose, we selected two of the most widespread non-native plants in Europe: Acacia dealbata and Acacia melanoxylon. Focusing on the forested area of northwest Spain and based on the comparison of two cycles of the Spanish National Forest Inventory, this study analyzes the dynamics of Acacia species between 1998 and 2008 in regards to changes in their spatial distribution, dominance, abundance, diametrical (dbh) structure and regeneration. In addition, the forested area was classified into forest types to identify the forests which are more susceptible to invasion. Finally, through general linear models, this study analyzes the relative importance of abiotic and biotic factors determining the spread of Acacia species over the studied period. The results confirm a rapid expansion in the presence of Acacia species in the forests of NW Spain, with annual spread rates around 0.1%. These two species are increasing their dominance across most forest types in the study area, where they are becoming the dominant species in the regeneration of some of them. Environmental factors and connectivity between Acacia populations are identified as the main factors associated with their spread into new areas. Additionally, the combination of disturbances and biotic factors associated with stand structure (total basal area, richness and tree cover) appear to determine the vulnerability or resistance of some forest to their spread. The early stage of invasion detected highlights the potential of Acacia species to continue spreading. This aspect, in conjunction with the high degree of disturbances (mainly fires) in this region, could be critical in determining the configuration of future forest landscapes in NW Spain. This study demonstrates the value of considering broad-scale periodic forest surveys to monitor biological invasions in forests ecosystems. The spatially-explicit data obtained from these surveys can contribute not only to furthering our knowledge with regard to invasion biology but also to developing more efficient conservation and management strategies.