Suchergebnisse

11 pages, 4 figures, 20 references. We thank the "Consejería de Medio Ambiente" (Andalusian Government), and Felipe Oliveros, the Director of "Los Alcornocales" Natural Park for the facilities and support to carry out our field work. We thank Ana Murillo, Juan Rubio, Francisco Conde, Miguel Ángel Calero, Loles Bejarano and Carlos Casimiro for their help in the greenhouse experiment. El libro referenciado ha sido publicado con motivo de Conference Proceedings: Conference held at the University of Huelva on 20-22 october, 2005. Organised bay: Departamento de Ciencias Agroforestales, Universidad de Huelva, Spain. Centro de Estudos Florestais, Universidade Técnica de Lisboa, Portugal. ; The objective of this study is to understand the response of Quercus suber seedlings to light and water, two limiting factors of their growth and survival in Mediterranean woody plants. Greenhouse and field experiments were conducted to contrast the results from both types of experiments. At the greenhouse experiment, Quercus suber seedlings were grown at three treatments of light (3%, 27% and 100% of incident light) and daily irrigation. When plants had four months old, half of the individuals did not receive any water, so we had then two water treatments (water and drought) within each light treatment. After two months, different variables were measured. At the field experiment, Quercus suber acorns were sown in three microhabitats with different light conditions (open, under tree and deep shade) in "Los Alcornocales" Natural Park (Cádiz). These microhabitats have similar light radiation than the treatments used at the greenhouse experiment. The greenhouse experiment results showed that the acorn mass was important for determining the seedling mass on the first stages, and that importance increased with light limitation. Thus, a greater seedling biomass was strongly associated with a bigger acorn mass under deep shade, but with increasing light levels the association between seedling and acorn mass became lower and non-significant. Quercus suber seedlings responded to light restriction with a decrease in the relative growth rate (RGR) and then the seedlings had lower biomass. This decrease in RGR was mainly caused by a decrease of the net assimilation rate (NAR). However, specific leaf area (SLA) and stem mass ratio (SMR) increased under shade as a way to enhance light capture. The response to water limitation was a general decrease of RGR, but interestingly, the effect of drought was negligible under deep shade conditions. The results of the field experiment were similar to those in the greenhouse experiment, which reinforce the conclusions of this study. Seedlings from open microhabitats (with more light) had a greater biomass than seedlings under tree or deep shade microhabitats. Also, the response to light limitation was an increase of specific leaf area (SLA), an increase in stem mass ratio and stem length. In contrast with the growth data, the survival rate of seedlings after the summer drought was greater under trees than in the open habitats. Then, it seems to exist an uncoupling between growth and survival of Quercus suber seedlings under different conditions. In this way, the best microhabitat for the growth (open microhabitat) is not the best for seedling survival, while medium shade microhabitat seems to be the best suited for Quercus suber regeneration. ; This study was supported by grants PFPU-MEC to IMPR and PFPI-MEC to JLQ (BES- 2003-1716), and by the coordinated Spanish CICYT project HETEROMED (REN2002- 04041-C02). This research is part of the REDBOME and GLOBIMED (www.globimed. net) network on forest ecology. ; Peer reviewed

17 pages, 3 figures, 5 tables, 63 references. We thank the Consejería de Medio Ambiente (Andalusian Government) and the directors of the Sierra Nevada National Park and Alcornocales Natural Park for facilities and support to carry out the field work. We also thank Rafael Villar for his advice in the experimental design of seedling growth measurements and for providing Q. pyrenaica acorns; and Luis V. García, Eduardo Gutiérrez, Ana Polo, Rocío Sánchez, Maite Domínguez, Carmen Navarro, Sergio de Haro, Asier Herrero, Ángel Navarro and Joaquín Sánchez for invaluable lab and field assistance. Many thanks to Lora Murphy and Charlie Canham for developing and providing the likelihood package for statistical analyses. Eugene W. Schupp provided helpful comments on an earlier version of the manuscript. ; Understanding seedling performance across resource gradients is crucial for defining the regeneration niche of plant species under current environmental conditions and for predicting potential changes under a global change scenario. A 2-year field experiment was conducted to determine how seedling survival and growth of two evergreen and two deciduous Quercus species vary along gradients of light and soil properties in two Mediterranean forests with contrasting soils and climatic conditions. Half the seedlings were subjected to an irrigation treatment during the first year to quantify the effects on performance of an alteration in the summer drought intensity. Linear and non-linear models were parameterized and compared to identify major resources controlling seedling performance. We found both site-specific and general patterns of regeneration. Strong site-specificity was found in the identity of the best predictors of seedling survival: survival decreased linearly with increasing light (i.e. increasing desiccation risk) in the drier site, whereas it decreased logistically with increasing spring soil water content (i.e. increasing waterlogging risk) in the wetter site. We found strong empirical support for multiple resource limitation at the drier site, the response to light being modulated by the availability of soil resources (water and P). Evidence for regeneration niche partitioning among Quercus species was only found at the wetter site. However, at both sites Quercus species shared the same response to summer drought alleviation through water addition: increased first-year survival but not final survival (i.e. after two years). This suggests that extremely dry summers (i.e. the second summer in the experiment) can cancel out the positive effects of previous wetter summers. Therefore, an increase in the intensity and frequency of summer drought with climate change might cause a double negative impact on Quercus regeneration, due to a general reduction in survival probability and the annulment of the positive effects of (infrequent) 'wet' years. Overall, results presented in this study are a major step towards the development of a mechanistic model of Mediterranean forest dynamics that incorporates the idiosyncrasies and generalities of tree regeneration in these systems, and that allow simulation and prediction of the ecological consequences of resource level alterations due to global change. ; This study was supported by a Juan de la Cierva contract to LGA, grants FPU-MEC to IMPR and IM, grants FPIMEC to J.L.Q. and L.M. and by the coordinated Spanish MEC projects HETEROMED (REN2002-04041-C02) and DINAMED (CGL2005-05830-C03). This research is part of REDBOME (www.ugr.es/ redbome/ ) and GLOBIMED (www.globimed.net) networks on forest ecology. ; Peer reviewed