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Abiotic conditions can increase the costs of services and/or the benefits of rewards provided by mutualistic partners. Consequently, in some situations, the outcome of mutualisms can move from beneficial to detrimental for at least one partner. In the case of protective mutualisms between ant bodyguards and plants bearing extrafloral nectaries (EFNs), plants from arid environments face a trade-off between EFN production and maintenance and water and carbon economy. This trade-off may increase EFN costs and decrease their value as a defensive strategy to plants in such environments. Despite this, the presence of EFNs is an ubiquitous trait in plants from arid environments, suggesting that they provide greater benefits to plants in these environments to compensate for their higher costs. We used a meta-analysis to investigate if such benefits do increase with decreasing water availability and the possible underlying causes (such as ant behaviour or ant diversity). As predicted, ant effect on EFN plants performance increased as mean annual precipitation decreased. We also found that the frequency of dominant ants on EFN plants increased in drier areas. Due to the more aggressive behaviour of dominant ants, we suggest that they represent an important factor shaping the adaptive value of EFNs to plants in arid environments., (© 2016 Cambridge Philosophical Society.)

Plant traits have become popular as predictors of interspecific variation in important ecosystem properties and processes. Here we introduce foliar pH as a possible new plant trait, and tested whether (1) green leaf pH or leaf litter pH correlates with biochemical and structural foliar traits that are linked to biogeochemical cycling; (2) there is consistent variation in green leaf pH or leaf litter pH among plant types as defined by nutrient uptake mode and higher taxonomy; (3) green leaf pH can predict a significant proportion of variation in leaf digestibility among plant species and types; (4) leaf litter pH can predict a significant proportion of variation in leaf litter decomposability among plant species and types. We found some evidence in support of all four hypotheses for a wide range of species in a subarctic flora, although cryptogams (fern allies and a moss) tended to weaken the patterns by showing relatively poor leaf digestibility or litter decomposability at a given pH. Among seed plant species, green leaf pH itself explained only up to a third of the interspecific variation in leaf digestibility and leaf litter up to a quarter of the interspecific variation in leaf litter decomposability. However, foliar pH substantially improved the power of foliar lignin and/or cellulose concentrations as predictors of these processes when added to regression models as a second variable. When species were aggregated into plant types as defined by higher taxonomy and nutrient uptake mode, green-specific leaf area was a more powerful predictor of digestibility or decomposability than any of the biochemical traits including pH. The usefulness of foliar pH as a new predictive trait, whether or not in combination with other traits, remains to be tested across more plant species, types and biomes, and also in relation to other plant or ecosystem traits and processes. [ABSTRACT FROM AUTHOR]