Floristic homogenization by native ruderal and alien plants in north-east Spain: The effect of environmental differences on a regional scale

Pino J., Font X., de Cáceres M., Molowny-Horas R. (2009) Floristic homogenization by native ruderal and alien plants in north-east Spain: The effect of environmental differences on a regional scale. Global Ecology and Biogeography. 18: 563-574.
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Doi: 10.1111/j.1466-8238.2009.00458.x

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Aim: To evaluate the relative potential contribution of native ruderals and aliens to plant homogenization at a regional scale, after taking into account the effect of diverse environmental distances. Location: Catalonia (north-east Spain) Methods: We have used the flora module of the BDBC project (Catalonian Database of Biodiversity), which provides information on plant species distribution per 10 × 10 km Universal Transverse Mercator (UTM) cell. Pairwise floristic similarities of: (1) total, (2) native non-ruderal, (3) native ruderal, and (4) alien vascular plant species have been calculated for a particularly well-sampled subset of UTM cells, using a modified version of the Simpson index. These similarities have been compared per UTM pair using Mantel tests, before and after considering their relative association with geographical, climatic and landscape distances from linear regression models. The floristic similarity of both total and native non-ruderal species was also correlated with the proportion of alien and native ruderal species after discounting the effects of environmental distances. Results: The proportion of variance explained by environmental correlates was highest for the floristic similarity of native non-ruderal plants and lowest for that of aliens. In all plant groups, climatic distance was the main significant variable of species similarity. Geographical distance was only significant for total and native non-ruderal species and was of secondary importance in both cases. Landscape distance was not significant in any case. Similarities among both aliens and native ruderals were significantly higher than among native non-ruderals, but these differences disappeared after removing the effect of environmental distances. Main conclusions: Species similarity between sites may depend on differences in environmental factors other than geographical distance. This has to be taken into account when exploring the implications for biotic homogenization. In the case of Catalonian flora, the potentially homogenizing effect of native ruderal and alien species seems to be associated with their lower dependence on geographical distance and climatic factors compared with those of native, non-ruderal species. © 2009 Blackwell Publishing Ltd.

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Masting mediated by summer drought reduces acorn predation in mediterranean oak forests

Espelta J.M., Cortés P., Molowny-Horas R., Sánchez-Humanes B., Retana J. (2008) Masting mediated by summer drought reduces acorn predation in mediterranean oak forests. Ecology. 89: 805-817.
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Doi: 10.1890/07-0217.1

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Temporally variable production of seed crops by perennial plants (masting) has been hypothesized to be a valuable mechanism in the reduction of seed predation by satiating and starving seed consumers. To achieve these benefits, coexisting species subjected to the same predator would benefit from a similar pattern of seeding fluctuation over time that could lead to a reduction in predation at the within-species level. We tested for the existence of an environmental factor enforcing synchrony in acorn production in two sympatric Mediterranean oaks (Quercus ilex and Q. humilis) and the consequences on within-species and between-species acorn predation, by monitoring 15 mixed forests (450 trees) over seven years. Acorn production in Q. ilex and Q. humilis was highly variable among years, with high population variability (CVp) values. The two species exhibited a very different pattern across years in their initial acorn crop size (sum of aborted, depredated, and sound acorns). Nevertheless, interannual differences in summer water stress modified the likelihood of abortion during acorn ripening and enforced within- and, particularly, between-species synchrony and population variability in acorn production. The increase in CVp from initial to mature acorn crop (after summer) accounted for 33% in Q. ilex, 59% in Q. humilis, and 60% in the two species together. Mean yearly acorn pre-dispersal predation by invertebrates was considerably higher in Q. humilis than in Q. ilex. Satiation and starvation of predators was recorded for the two oaks, and this effect was increased by the year-to-year variability in the size of the acorn crop of the two species combined. Moreover, at a longer time scale (over seven years), we observed a significant reduction in the mean proportion of acorns depredated for each oak and the variability in both species' acorn production combined. Therefore, our results demonstrate that similar patterns of seeding fluctuation over time in coexisting species mediated by an environmental cue (summer drought) may contribute to the reduction of the impact of seed predation at a within-species level. Future research should be aimed at addressing whether this process could be a factor assisting in the coexistence of Q. ilex and Q. humilis. © 2008 by the Ecological Society of America.

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A model of the recruitment of Pinus nigra from unburned edges after large wildfires

Ordóñez J.L., Molowny-Horas R., Retana J. (2006) A model of the recruitment of Pinus nigra from unburned edges after large wildfires. Ecological Modelling. 197: 405-417.
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Doi: 10.1016/j.ecolmodel.2006.03.027

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We have developed a simulation model for predicting the recruitment response of Pinus nigra from unburned edges in areas affected by large wildfires, where the regeneration of burned forests of this tree species is very low. We have simulated the distribution of P. nigra seedlings at different distances from the unburned edges by integrating empirical field data for the different processes affecting seed and seedling success. The simulation model determines the final number of seedlings established in any subplot, within the burned area, as the multiplication of a number of statistically independent processes, from cone and seed production and predation, to seed germination and seedling establishment. Probabilities are drawn at random at each time step from the experimental distributions that describe those processes. The model runs for a given number of years, thus simulating the accumulative effects on the total number of established seedlings of the different processes involved. That represents one model run, which is then repeated a sufficient number of times to calculate simulated distributions of established seedlings as a function of distance from the forest edge. The validation of those results with data from field measurements of seedling establishment in old-burned areas has demonstrated the ability of the simulation algorithm to reproduce observed results. Furthermore, the simulations carried out with the model for actual plots, located near the margins of the burned areas affected by the largest fires affecting P. nigra forests in Catalonia in recent years, have all shown striking similarities. The simulated values of established seedlings follow a Gaussian-like distribution in the first 100 m, with a wide range of 2000-25,000 seedlings/ha. There is also a clear trend for plots with medium and large trees to show an increment in increasing seedling establishment with tree density, whereas plots dominated by small trees give very low regeneration values. © 2006 Elsevier B.V. All rights reserved.

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