Positive interactions by cushion plants in high mountains:fact or artifact?

Positive interactions are defined as non-trophic interactions where at least one of the interacting species is benefited in terms of fitness and the other remains unaffected.Nevertheless,the bidirectional feedbacks between species may be positive,neutral or negative.Thus,if facilitated species induce negative effects on their‘nurses’,the assumed definition of positive interactions could be reconsidered.Methods We assessed if ecological interactions between cushions of Azorella madreporica and their facilitated species are positive.Specifically,we tested if cover of facilitated species has any costs for cushion plants from an ecophysiological perspective,and if these costs increase with the amount of cover of facilitated species.In addition,through pathway analysis and correlations,we assessed if cover and richness of facilitated species have a direct and/or indirect effect on the fitness of cushion plants.Important Findings We found that facilitated plant species induced a significant cost for their nurses(cushion plants),and this cost increases with cover of the facilitated species.Additionally,the facilitated species exert a strong direct negative effect on the cushion’s fitness and a moderate indirect negative cost evident through the nutrient status and physiological performance of cushion plants.We thus contribute evidence that positive interactions between high mountain cushion plants of central Chile and their‘facilitated’species may be an artifact more than a fact,especially when bidirectional effects are considered;contrasting with the majority of studies that document only one side of the interaction.

Ecophysiological basis of the Jack-and-Master strategy:Taraxacum officinale(dandelion)as an example of a successful invader

Aims Successful invasive plants are often assumed to display significant levels of phenotypic plasticity.Three possible strategies by which phenotypic plasticity may allow invasive plant species to thrive in changing environments have been suggested:(i)via plasticity in morphological or physiological traits,invasive plants are able to maintain a higher fitness than native plants in a range of environ-ments,including stressful or low-resource habitats:a‘Jack-of-all-trades’strategy;(ii)phenotypic plasticity allows the invader to better exploit resources available in low stress or favorable habitats,show-ing higher fitness than native ones:a‘Master-of-some’strategy and(iii)a combination of these abilities,the‘Jack-and-Master’strategy.Methods We evaluated these strategies in the successful invader Taraxacum officinale in a controlled experiment mimicking natural environmen-tal gradients.We set up three environmental gradients consisting of factorial arrays of two levels of temperature/light,temperature/water and light/water,respectively.We compared several ecophysiologi-cal traits,as well as the reaction norm in fitness-related traits,in both T.officinale and the closely related native Hypochaeris thrin-cioides subjected to these environmental scenarios.Important Findings Overall,T.officinale showed significantly greater accumulation of biomass and higher survival than the native H.thrincioides,with this difference being more pronounced toward both ends of each gradient.T.officinale also showed significantly higher plasticity than its native counterpart in several ecophysiological traits.Therefore,T.officinale exhibits a Jack-and-Master strategy as it is able to main-tain higher biomass and survival in unfavorable conditions,as well as to increase fitness when conditions are favorable.We suggest that this strategy is partly based on ecophysiological responses to the environment,and that it may contribute to explaining the successful invasion of T.officinale across different habitats.