The importance of functional diversity in the stability of Mediterranean shrubland communities after the impact of extreme climatic events

Aims Extreme climatic events may have important consequences for plant community structure and composition.in 2005,a severe drought together with a cold winter promoted extensive damage and mortal-ity in shrubland communities of southwest Spain(Doñana National Park).Here,we aim to identify the mechanisms underlying commu-nity stability(resistance and resilience)in response to this extreme climatic event,considering changes in the functional structure of these communities.Methods We used a trait-based approach,quantifying variations in 10 func-tional traits at the community level(community weighted means,CWM)and the functional diversity(functional richness,evenness and divergence)in 18 plots at three different times:predating the climatic event(estimated from the sum of the live and dead volume of each species in 2007),and 2 and 8 years after the 2005 episode.We also quantified the differences in functional traits and functional diversity between adult and recruit stages,which allowed us to bet-ter understand the contribution of the recruitment to the mainte-nance of the functional structure and diversity of the community.Important Findings Communities with higher functional divergence before the cli-matic event maintained nearly constant their levels of functional divergence 8 years after,but they were more prone to changes in species composition.Community resistance in terms of vegeta-tion cover was positively correlated with root dry matter content,whereas community resilience was positively correlated with leaf chlorophyll(LChl).We also found that some values(weighted means)of functional community traits(such as root dry matter con-tent and LChl)had increased 2 years after the event,returning to the pre-event conditions after 8 years.in addition,there was hardly any establishment of new species in the community and the recruits did not make substantial differences to the community functional structure.Only seed mass differed significantly between the adult and seedling stages.in summary,the extreme climatic event induced rapid vegetation changes,modifying several functional properties of the community,but,in spite of the occurrence of changes in species composition,a rapid convergence of these shrubland communities took place due to the replacement of species with functional redundancy,thus recovering the initial conditions and supporting the existence of strong mechanisms of functional resilience.

Climatic and fire determinants of early life-history stages in the Mediterranean shrub Cistus albidus

Aims Cistus albidus reproductive traits have been studied on typical Mediterranean shrublands along a water availability gradient in Northeastern Iberian Peninsula.Germination of this species is known to be highly favoured by fire.Moreover,Mediterranean species are particularly dependent on water availability.Therefore,we establish the hypothesis that in addition to fire disturbance,seedling recruitment in this Mediterranean seeder will be improved in drought-induced episodes resulting in generalized canopy die-off.Methods Individuals of several populations of C.albidus were collected and the size,weight and number of fruits and seeds were measured.Germination tests were also carried out on five pre-germination treatments:seeds’exposure to heat shock,imbibition,two cycles of imbibition/desiccation and the combination of heat shock and imbibition and imbibition/desiccation cycles.Moreover,the number of seedlings after a drought event was surveyed in the field and correlated with canopy die-off.Important findings Our study shows the variability of the C.albidus reproductive traits,such as germination rate or fruit production,along the water availability gradient.This variability resulted in a decrease in fruit production but an increase in successful germination under drier conditions.Cistus albidus seeds increased germination with heat,demonstrating their ability to successfully establish after fire.However,recruitment was not exclusively fire dependent since seedling establishment was higher under C.albidus canopies that had collapsed after the extreme drought.Finally,adult density increased C.albidus die-off and mortality,as well as seedling establishment.These results suggest that this species exhibits a trade-off between different reproductive outcomes(i.e.seed production vs.viability),which in turn is determined by climate.This study also provides evidence of how intra-specific competition,climate,particularly drought events and fire disturbance,can determine the success of key early stages of the life history of a common,representative Mediterranean fire-prone seeder shrub.

Plant-herbivore assemblages under natural conditions are driven by plant size,not chemical defenses

Aims Plant secondary metabolites have been traditionally recognized as key traits regulating plant-herbivore assemblages.However,the ecological relevance of secondary metabolites as resistance mecha-nisms in comparison to other plant attributes,including physical,morphological or ecological traits,has been recently questioned.We aim to evaluate the role of chemical defenses,plant size and the presence of insect competitors on driving the differences in her-bivory damage under natural conditions.Methods We performed a replicated field study on the herbivore commu-nity associated with four Senecio species(S.lividus,S.vulgaris,S.inaequidens and S.pterophorus)during a full-reproductive season in Montseny Natural Park(catalonia,NE Spain).Pyrrolizidine alka-loids(PAs),the most characteristic chemical defenses of Senecio due to their toxic effects on herbivores,were analyzed by gas chro-matography.Individual plant size was estimated by the number of flower heads produced over the entire reproductive season.We used linear mixed models to explore the relationships between total PA concentrations,plant size and herbivory levels.Important Findings PA concentrations were not related to the natural guild of herbivores within any plant species or insect type.Moreover,no significant interactions were found between insect species sharing the same host plants.In contrast,herbivore abundance was positively related to plant size in S.vulgaris,S.lividus and S.inaequidens.We found no evidence that PAs confer an increased plant resistance against herbivores in Senecio.Our study supports the hypothesis that plant chemical defenses have a secondary role in determining plant-her-bivore assemblages in comparison to other plant traits under the complexity of natural conditions.