CREAF

[No abstract available]

Disturbances favour plant invasions because they provide a pulse of resources for seedling establishment and growth. However, little attention has been paid to the importance of the different disturbance regimes in the extent of invasion. In this paper we asked to what extent invasion depends on spatial and temporal patterns of disturbance. By using the spatially explicit model Fateland, we simulated how Cortaderia selloana, a large-wind dispersed perennial grass invading Mediterranean grasslands, responds to different medium-term (up to 55 years) disturbance regimes. The different scenarios combine disturbance frequency, extent of disturbance and temporal and spatial disturbance variability, and they roughly mimic real situations of habitat management and disturbances in cultural landscapes. The model predicted that the proportion of invaded area would increase with disturbance frequency. The extent of the disturbance overcame the importance of frequency. Aggregated disturbances randomly distributed in space had a more positive effect on invasion than scattered disturbances at repeatedly fixed locations. Whether the disturbances regularly occurred in time or were irregularly distributed did not influence the percentage of invaded landscape. Our simulation analysis unambiguously supports the hypothesis that disturbances are essential for the persistence and expansion of Cortaderia sellona. The high amount of seed rain from a few individuals (e.g., gardens) make this species very sensitive to landscape changes produced by different disturbance patterns (e.g., land abandonment, habitat management or rubbish dumping). Our results can contribute to redirect land management options. © 2005 Elsevier Ltd. All rights reserved.

Presence and cover of alien plants were analysed in relation to recent naturalness changes (1956-1999) in the Llobregat delta by means of GIS techniques and field surveys. Two land cover maps of 1956 and 1999 were generated by photo-interpretation of orthoimages and they were then reclassified into naturalness classes, defined as the degree of preservation of the pristine state. The resulting naturalness maps were combined in order to obtain a naturalness change map, which was used to design field sampling in four pristine communities: reedbeds, rushbeds, halophilous scrubs and fixed dune communities. Two study areas were selected for each community and three stability regimes (stable, semi-stable and non-stable) obtained from the naturalness change map. Five vegetation inventories were performed on average in each of these areas using the classical sigmatist method. Results showed a negative relationship between stability and invasibility, with several variations between communities. No alien species were found in stable areas of all communities. Alien species number, species percentage and relative cover increased from semi-stable to non-stable regimes in reedbeds and dune communities, indicating that reversion towards the climax reduces opportunities for alien establishment in these communities. In contrast, halophilous habitats such as rushbeds and scrubs did not exhibit significant differences between semi-stable and non-stable plots, probably because saline stress makes their invasion by alien plants difficult, even under disturbance. © Springer 2006.

This study analyzes the variations in the structure and composition of ant communities in burned Pinus nigra forests in central Catalonia (NE Spain). Pinus nigra forests do not recover after fire, changing to shrublands and oak coppices. For this reason, we suggest that ant communities of burned P. nigra forests will change after fire, because the post-fire scenario, in particular with the increase of open areas, is different to the unburned one, and more favourable for some species than for others. In four locations previously occupied by P. nigra forests where different fires occurred 1, 5, 13 and 19 yr before the sampling, we sampled the structure and composition of ant communities with pitfall traps, tree traps and net sweeping in unburned plots and in plots affected by canopy and understory fire. The results obtained suggest that canopy and understory fire had little effect on the structure of ant communities. Thus, many variables concerning ant communities were not modified either by fire type (understory or canopy fire) or by time since fire. However, a number of particular species were affected, either positively or negatively, by canopy fire: three species characteristic of forest habitats decreased after fire, while eight species characteristic of open habitats increased in areas affected by canopy fire, especially in the first few years after fire. These differences in ant community composition between burned and unburned plots imply that the maximum richness is achieved when there is a mixture of unburned forests and areas burned with canopy fire. Moreover, as canopy cover in P. nigra forests burned with canopy fire is not completed in the period of time studied, the presence of the species that are characteristic of burned areas remains along the chronosequence studied, while the species that disappear after fire do not recover in the period of time considered. Overall, the results obtained indicate that there is a persistent replacement of ant species in burned P. nigra forests, as is also the case with vegetation. Copyright © ECOGRAPHY 2006.