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Species composition and diversity were compared among twenty Mediterranean annual grasslands in northern and central California and central and southern Spain, encompassing climatic gradients and local site variation in topography and soils Geographic proximity was more important than environmental factors such as climate, topography and parent material in predicting the species composition of these grasslands, with Californian and Spanish grasslands sharing only 9% of the species and geographically separated regions within each country sharing only 20‐ 32% of the species This importance of geographic separation in predicting species composition suggests a strong role of dispersal limitation in determining current community composition Mean species diversity was lower in Californian than in Spanish grasslands and was negatively correlated with cover of annual grasses that tended to be greater in California than in Spain However, there were few differences in species diversity among sites within either country and patterns of species diversity were unrelated to soil fertility or productivity We suggest that current grazing regimes contribute to the greater abundance of grasses and lower species diversity of Californian than of Spanish grasslands The apparent importance of dispersal limitation and grazing m explaining differences in species composition and diversity between Californian and Spanish grasslands and within each country suggest that the structure of these grasslands has been and will continue to be sensitive to human influence

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Citation: Fernandez-Conradi, P., H. Jactel, A. Hampe, M. J. Leiva, and B. Castagneyrol. 2017. The effect of tree genetic diversity on insect herbivory varies with insect abundance. Ecosphere 8(1):e01637. 10.1002/ecs2.1637Abstract. Variance in edibility among plant genotypes is expected to be a key driver of plant genetic diversity (PGD) effects on abundance of insect herbivores and resulting herbivory. Yet, herbivore foraging behavior and leaf consumption may be also context-dependent and, in particular, influenced by herbivore density, which remains unexplored. We used a combination of field and laboratory experiments with saplings from four half-sib families (henceforth, families) of pedunculate oak (Quercus robur) to test how PGD and herbivore density interactively affect herbivory. Insect herbivory was assessed in a common garden experiment with plots containing all possible combinations of individuals from one to four oak families. Herbivore density was manipulated by spraying insecticide in a factorial design. Complementary feeding trials with gypsy moth larvae (Lymantria dispar) were used to further explore the mechanisms underlying observed patterns in the field. Herbivory decreased with increasing PGD under normal herbivore density, but not under reduced herbivore abundance. The most damaged oak family in the field was also the most consumed in non-choice tests and was consistently preferred over other families in choice tests. Trials showed that the presence of less edible families in the diet reduced overall consumption by gypsy moth larvae. Under field conditions, the most edible family consistently benefited most from being associated with less edible, neighboring genotypes. Our results demonstrate that small-scale PGD can provide associational resistance to insect herbivory, probably through change in herbivore foraging activity. Importantly, they also reveal that the magnitude of genetic diversity effect depends on herbivore density.

Iberian holm oak meadows are savannah-like ecosystems that result from traditional silvo-pastoral practices. However, such traditional uses are declining, driving changes in the typical tree—open grassland structure of these systems. Yet, there are no studies integrating the whole ecosystem—including the arboreal and the herbaceous layer—as drivers of greenhouse gas (GHG: CO2, CH4 and N2O) dynamics. Here, we aimed at integrating the influence of tree canopies and interactions among plant functional types (PFT: grasses, forbs, and legumes) of the herbaceous layer as GHG exchange drivers. For that purpose, we performed chamber-based GHG surveys in plots dominated by representative canopy types of Iberian holm oak meadows, including Quercus species and Pinus pinea stands, the last a common tree plantation replacing traditional stands, and unraveled GHG drivers through a diversity-interaction model approach. Our results show the tree–open grassland structure, especially drove CO2 and N2O fluxes, with higher emissions under the canopy than in the open grassland. Emissions under P. pinea canopies are higher than those under Quercus species. In addition, the inclusion of diversity and compositional terms of the herbaceous layer improve the explained variability, with legumes enhancing CO2 uptake and N2O emissions. Changes in the tree cover and tree species composition, in combination with changes in the structure and composition of the herbaceous layer, will imply deep changes in the GHG exchange of Iberian holm oak meadows. These results may provide some guidelines to perform better management strategies of this vast but vulnerable ecosystem.

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