Canopy photosynthesis modeling

Beyschlag W, Ryel R. Canopy photosynthesis modeling. In: Pugnaire FI, Valladares F, eds. Functional Plant Ecology. 2nd. Boca Raton: CRC Press; 2007

Gender effects on the post-facilitation performance of two dioecious Juniperus species

7 páginas, 2 figuras, 2 tablas.1. Plant facilitation usually changes to competition as plants age. In dioecious plants, females should be affected more negatively than males by stressful conditions because of the greater costs of female reproduction.2. We investigated the gender effects on the post-facilitation performance of adult plants of two dioecious Juniperus species from the high mountains of eastern Spain: J. sabina L. acts as a nurse plant for J. communis L. We compared physiological (water potential, carbon isotope discrimination and nitrogen concentration), vegetative (shoot growth) and reproductive (number of male flowers, and number of fruits and seeds) characters of associated and non-associated plants of both species, to test the hypothesis that this association represents a more stressful condition for females than for males because of the greater costs of female reproduction.3. Despite their close phylogenetic relatedness, both species showed a distinct performance pattern after the facilitation phase. Association with the nurse plant reduced the growth and reproductive capacity of both genders in J. communis, the facilitated species. In contrast, the association with J. communis did not affect the fitness of the nurse plant, J. sabina , although in accordance with our hypothesis a gender effect was found on several hysiological parameters. Thus J.sabina associated females had a more negative water potential and carbon isotope discrimination than the associated males, but there were no differences between genders when growing in isolation.4. The consequences of the post-facilitation interaction between the two long-lived woody Juniperus species are asymmetrical: harmful for the facilitated species, but harmless for the nurse.5. Gender had also asymmetrical consequences on some functional traits of the nurse – but not the facilitated – species.We sincerely thank all the people of Puebla de San Miguel who helped us with the field work. Carbon isotope analyses were run in the Laboratorio de Isótopos Estables(Universidad Autónoma de Madrid, Spain) and analyses of leaf nitrogen content in the Centro de Investigaciones sobre Desertificación (Valencia, Spain). This research was supported by projects FEDER 1FD97-0551, AGL2001-1061 and REN 2000-0163- P4-05 from the Ministerio de Ciencia y Tecnología. M.V. received grants from contracts of the Reincorporación de Doctores y Tecnólogos and Programa Ramón y Cajal del Ministerio de Ciencia y Tecnología during this study.Peer reviewe

Do biotic interactions shape both sides of the humped-back model of species richness in plant communities?

A humped-back relationship between species richness and community biomass has frequently been observed in plant communities, at both local and regional scales, although often improperly called a productivity–diversity relationship. Explanations for this relationship have emphasized the role of competitive exclusion, probably because at the time when the relationship was first examined, competition was considered to be the significant biotic filter structuring plant communities. However, over the last 15 years there has been a renewed interest in facilitation and this research has shown a clear link between the role of facilitation in structuring communities and both community biomass and the severity of the environment. Although facilitation may enlarge the realized niche of species and increase community richness in stressful environments, there has only been one previous attempt to revisit the humped-back model of species richness and to include facilitative processes. However, to date, no model has explored whether biotic interactions can potentially shape both sides of the humped-back model for species richness commonly detected in plant communities. Here, we propose a revision of Grime's original model that incorporates a new understanding of the role of facilitative interactions in plant communities. In this revised model, facilitation promotes diversity at medium to high environmental severity levels, by expanding the realized niche of stress-intolerant competitive species into harsh physical conditions. However, when environmental conditions become extremely severe the positive effects of the benefactors wane (as supported by recent research on facilitative interactions in extremely severe environments) and diversity is reduced. Conversely, with decreasing stress along the biomass gradient, facilitation decreases because stress-intolerant species become able to exist away from the canopy of the stress-tolerant species (as proposed by facilitation theory). At the same time competition increases for stress-tolerant species, reducing diversity in the most benign conditions (as proposed by models of competition theory). In this way our inclusion of facilitation into the classic model of plant species diversity and community biomass generates a more powerful and richer predictive framework for understanding the role of plant interactions in changing diversity. We then use our revised model to explain both the observed discrepancies between natural patterns of species richness and community biomass and the results of experimental studies of the impact of biodiversity on the productivity of herbaceous communities. It is clear that explicit consideration of concurrent changes in stress-tolerant and competitive species enhances our capacity to explain and interpret patterns in plant community diversity with respect to environmental severit