Summary• This paper presents the results of 2 yr of CO 2 flux measurements on grassland communities of varying species richness, exposed to either the current or a warmer climate.• We grew experimental plant communities containing one, three or nine grassland species in 12 sunlit, climate-controlled chambers. Half of these chambers were exposed to ambient air temperatures, while the other half were warmed by 3 ° C. Equal amounts of water were added to heated and unheated communities, implying drier soils if warming increased evapotranspiration. Three main CO 2 fluxes (gross photosynthesis, above-ground and below-ground respiration) were measured multiple times per year and reconstructed hourly or half-hourly by relating them to their most important environmental driver.• While CO 2 outputs through respiration were largely unchanged under warming, CO 2 inputs through photosynthesis were lowered, especially in summer, when heat and drought stress were higher. Above-ground CO 2 fluxes were significantly increased in multispecies communities, as more complementary resource use stimulated productivity. Finally, effects of warming appeared to be smallest in monocultures.• This study shows that in a future warmer climate the CO 2 sink capacity of temperate grasslands could decline, and that such adverse effects are not likely to be mitigated by efforts to maintain or increase species richness.
Nijs, I., Lemmens, C. M. H. M. and Ceulemans, R. 2006. Underlying effects of spatial aggregation (clumping) in relationships between plant diversity and resource uptake. Á/ Oikos 113: 269 Á/278.In the debate on the influence of declining plant diversity on ecosystem functioning, spatial aggregation has received little attention, in spite of the fact that the local distribution of species determines interaction. Using a simple conceptual model, we visualize the effect that clumping has on below ground resource uptake (RU). Two general conclusions can be drawn from the model: (1) clumping results in a loss of RU, and does so to a larger extent when species differ more in rooting depth and when plant density increases;(2) the current view on complementarity may be flawed, as we find that neighborhood species richness only promotes spatial complementarity directly at the transition from monocultures to higher species richness levels, but not at transitions between higher richness levels. The number of species in a community does, however, affect the probability of the occurrence of clumping. In randomly assembled communities, clumps are statistically larger when the number of species is lower, leading to reduced overall below ground complementarity. Complementarity, and thus also RU, will therefore increase indirectly with increasing species richness. As a result of our findings, we propose that clumping could have a number of 'hidden' consequences, for example on invasibility of a community and on competition between species. Clumping could also prove to be a new and important explanatory factor in diversity studies, as it extends the definition of complementarity effects to include the effect of spatial heterogeneity of plant positions within the community.
Model ecosystems were grown in 12 sunlit, climate‐controlled chambers to gain insight into the effects of elevated (+3°C) air temperature (Tair) on temperate grasslands. In this study, the hypothesis of delayed senescence in response to elevated Tair was tested for Rumex acetosa L. and Plantago lanceolata L. During the autumn of the first treatment year, frequent measurements were made of leaf chlorophyll a (Chla) fluorescence transients. Chl fluorescence images of individual leaves as well as digital colour images of these ecosystems were captured. Chl fluorescence variables, such as the maximum quantum yield of primary photochemistry (Fv/Fm), indicated a decreasing efficiency with time. Despite no treatment effect on Fv/Fm, other variables derived from the Chl fluorescence transients showed a strong trend towards a positive effect of a 3°C temperature increase on the photosynthetic performance of R. acetosa and P. lanceolata in the first year. After mid‐September, the initial positive treatment effect disappeared for R. acetosa, strongly suggesting that leaf lifespan of this species was shortened by higher Tair. One possible explanation is more intense drought stress in the elevated compared to the ambient temperature treatments. Second‐year measurements were possibly too limited in time to confirm this trend. These results show that temperate grassland species may take advantage of a future increase in Tair during autumn. This will ultimately depend on the species' degree of acclimation to a temperature change and on the resistance to drought stress.
In view of the projected climatic changes and the global decrease in plant species diversity, it is critical to understand the effects of elevated air temperature (T(air)) and species richness (S) on physiological processes in plant communities. Therefore, an experiment of artificially assembled grassland ecosystems, with different S (one, three or nine species), growing in sunlit climate-controlled chambers at ambient T(air) and ambient T(air) + 3 degrees C was established. We investigated whether grassland species would be more affected by midday high-temperature stress during summer in a warmer climate scenario. The effect of elevated T(air) was expected to differ with S. This was tested in the second and third experimental years by means of chlorophyll a fluorescence. Because acclimation to elevated T(air) would affect the plant's stress response, the hypothesis of photosynthetic acclimation to elevated T(air) was tested in the third year by gas exchange measurements in the monocultures. Plants in the elevated T(air) chambers suffered more from midday stress on warm summer days than those in ambient chambers. In absence of severe drought, the quantum yield of PSII was not affected by elevated T(air). Our results further indicate that species had not photosynthetically acclimated to a temperature increase of 3 degrees C after 3 years exposure to a warmer climate. Although effects of S and T(air) x S interactions were mostly not significant in our study, we expect that combined effects of T(air) and S would be important in conditions of severe drought events.
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