Arnan X., Rodrigo A., Retana J. (2007) Uncoupling the effects of shade and food resources of vegetation on Mediterranean ants: An experimental approach at the community level. Ecography. 30: 161-172.EnlaceDoi: 10.1111/j.2006.0906-7590.04796.x
Vegetation is one of the main factors affecting the composition and structure of ant communities. The effect of vegetation is both by offering food resources and by modifying the proportion of ground cover exposed to shade conditions. However, it is difficult in field studies to uncouple the effects of food resources and shade on animal communities. The goal of this study was to analyze experimentally the effects of vegetation through food and shade supply on the structure and composition of a Mediterranean ant community. We have crossed these two factors in a factorial design where we have eliminated vegetation in experimental treatments and we have simulated the effects of vegetation by manipulating separately the supply of food resources and shade. The expected decrease in ant abundance and richness in plots without food resources but with shade was only partially confirmed by the results, because there was a decrease in the number of ant species but not a significant decrease in overall ant abundance in plots without food resources. We did not confirm the second hypothesis that the decrease in shade while maintaining food resources resulted in an increase of heat-tolerant, subordinate species and, consequently, ant richness. Species composition differed in the different experimental treatments. Stress-tolerant species were abundant in areas without shade and without food resources. Heat-tolerant species were mainly present in plots without shade but with food resources. Finally, species with high requirements of shade and/or food resources were associated to control plots with both shade and food resources from vegetation. Overall, this study provides an experiment that uncouples experimentally shade and food resources and dissects their effects on the whole ant community, and shows that the effects of the two factors are independent and affect different components of ant community structure and composition. Copyright © Ecography 2007.
Arnan X., Rodrigo A., Retana J. (2007) Post-fire regeneration of Mediterranean plant communities at a regional scale is dependent on vegetation type and dryness. Journal of Vegetation Science. 18: 111-122.EnlaceDoi: 10.1658/1100-9233(2007)18[111:PROMPC]2.0.CO;2
Question: We tested whether (1) the change in composition and structure of whole plant communities after fire is directly related to regeneration of the dominant tree species in the canopy; (2) the change in structure and composition of plant communities several years after fire decreases with the proportion of obligate seeders and (3) the proportion of obligate seeders in plant communities increases with the dryness gradient. Location: Catalonia (NE Spain). Methods: We measured floristic differences between burned and long-since burned sites in eight vegetation types across a climate gradient. We compared 22 sites burnt in 1994 in paired plots with 22 sites that had not been burnt since the 1940s. In each site we placed plots in burned and long-since burned areas, where we identified the presence and abundance of all plant species. Results: When the tree canopy recovers, structure and composition of the vegetation also return to the long-since burned community; when tree canopy does not recover, composition of the post-fire community varies compared to the long-since burned one. A higher proportion of obligate seeders in the pre-fire community promotes quicker regeneration of the original community. The proportion of obligate seeders increased along the dryness gradient. Conclusions: Regeneration of plant communities after fire depends on the vegetation type before the fire. Regeneration increases when the dominant tree or shrub species persists after fire and with a higher proportion of obligate seeders in the pre-fire community. The proportion of obligate seeders varies along the dryness gradient, which suggests that vegetation in drier areas (when seeders are more abundant) recovers earlier than in moister areas. © IAVS; Opulus Press.
Carpintero S., Retana J., Cerdá X., Reyes-López J., Arias De Reyna L. (2007) Exploitative strategies of the invasive Argentine ant (Linepithema humile) and native ant species in a southern Spanish pine forest. Environmental Entomology. 36: 1100-1111.EnlaceDoi: 10.1603/0046-225X(2007)36[1100:ESOTIA]2.0.CO;2
The Argentine ant, Linepithema humile (Mayr, 1868), is displacing native ant species in Doñana National Park (Spain). This paper discusses the results of experiments aimed at analyzing exploitation competition between the invading species and other ant species in a park community. The Argentine ant was found to implement several strategies favoring its success in exploitation competition: mass recruitment, use of various microhabitats (on the ground and in trees), and activity over a wide range of temperatures. Although these strategies were not exclusive to L. humile, their joint use, together with the large number of workers forming each "unicolony," conferred a clear advantage for resource exploitation. Some native species were more severely affected than others by the presence of L. humile in terms of both abundance and behavior. The worst affected species were those whose ecological characteristics were similar to those of the Argentine ant, e.g., Pheidole pallidula (Nylander, 1849); the species least affected was Cataglyphis floricola Tinaut, 1993, possibly because of its subordinate and thermophilous nature (little overlap of daily activity rhythms with the exotic species). © 2007 Entomological Society of America.
Gracia M., Montané F., Piqué J., Retana J. (2007) Overstory structure and topographic gradients determining diversity and abundance of understory shrub species in temperate forests in central Pyrenees (NE Spain). Forest Ecology and Management. 242: 391-397.EnlaceDoi: 10.1016/j.foreco.2007.01.056
Understanding the factors that determine the distributions of understory plants is important because they represent the largest component of temperate forest diversity. In the present study, we investigate the influence of topographic gradients and overstory structure on composition and abundance of understory shrub species in Massís de l'Orri (central Pyrenees, Lleida, Spain). We established 329 plots in two opposing hills in the massif at elevations ranging from 1500 to 2200 m, where we measured understory composition, overstory structure and physiographic features. Overall richness and diversity did not vary with aspect and showed only a small decrease with elevation. However, understory composition showed drastic changes related to elevation and aspect. Most species had differences between slopes, some of them showing higher cover in south-facing plots and the others in north-facing plots. Elevation also influenced percent cover of many understory shrub species, which showed either a decrease (Arctostaphylos uva-ursi, Rosa sp.), a maximum at intermediate values (Cytisus purgans) or an increase (Rhododendron ferrugineum) with elevation. The effect of the two variables was not independent, and percent cover of the different species depended on the overall effect of aspect and elevation. The species with highest occurrence in plots, Vaccinium myrtillus, increased with elevation in south-facing slopes but showed similar values in north-facing slopes. Compared to topographic variables, the effect of overstory variables on understory was considerably smaller. Understory species richness and diversity were inversely but weakly correlated with overstory cover but not with basal area. These overstory variables only affected negatively the presence of C. purgans and Juniperus communis, two shade-intolerant species that decrease their abundance when light decreases. Overall, the results obtained suggest that topographic variables have a stronger influence on understory composition than overstory structure in the study area, which is located in the southern distribution limit of temperate forests in Europe. © 2007 Elsevier B.V. All rights reserved.
Rodrigo A., Quintana V., Retana J. (2007) Fire reduces Pinus pinea distribution in the northeastern Iberian Peninsula. Ecoscience. 14: 23-30.EnlaceDoi: 10.2980/1195-6860(2007)14[23:FRPPDI]2.0.CO;2
This study analyzes the recovery of Pinus pinea in forested areas that have burned in the last 30 years. The main objectives of the study were (i) to analyze the success of the regeneration of P. pinea in areas with the presence or absence of surviving adult trees and (ii) to investigate whether or not the post-fire response of P. pinea may affect its present distribution. We analyzed the survival of adult P. pinea trees and the regeneration of P. pinea seedlings in 8 areas burned by wildfires in Catalonia (northeast Spain) between 1979 and 2001. This study shows that the inability of seedlings to establish after fire, together with the short seed dispersal distance of this species and the low level of seedling survival, renders the natural regeneration of P. pinea in burned areas difficult. A comparison of the presence of P. pinea in plots located in burned and unburned areas throughout its distribution area in Catalonia indicates that its presence at the regional level is clearly reduced after fire. Moreover, in the areas where it persists, tree density and cover are smaller than in unburned areas. This decrease of P. pinea forests is exacerbated by the increase in surface area burned by forest fires during recent decades in the Mediterranean basin and by decreasing profits associated with this species as a consequence of the falling value of its pine kernel.
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