Seed mass equalizes the strength of positive and negative plant-plant interactions in a semi-arid grassland

Verónica Zepeda, Facultad de Ciencias, Universidad Nacional Autónoma de México, Mexico City, DF, Mexico and Carlos Martorell, Departamento de Ecología y Recursos Naturales, Facultad de Ciencias, Universidad Nacional Autónoma de México, Mexico City, DF, Mexico

Background/Question/Methods
The interplay and balance between positive and negative interactions are important and recurrent topics in plant ecology. If facilitation occurs because benefactors ameliorate stress, such balance may be driven by seed size because stress tolerance and competitive ability of plants, particularly seedlings, are positively correlated with seed size. Thus, small-seeded, stress-intolerant species may require facilitation, but not large-seeded ones. This would equalise the magnitudes of opposite-signed interactions because in small-seeded species, positive effects of facilitators and negative effects of competitors should be strong, while both effects should be weak in large-seeded species. To test this hypothesis, we assessed the effects of interactions on different components of the performance of ten focal species. Interactions were estimated from experimental data in a semiarid grassland by analysing the effects of four associated species on the germination, survival, growth and fecundity. Also, the integrated effects of the interaction over the life cycle were estimated. To predict the relationship between the magnitude of the interactions and seed mass, we used a multi-model inference approach to compare a set of models that differ in terms of the properties of their mean and standard deviation.
Results/Conclusions
As expected, the largest facilitative and competitive effects were recorded in small-seeded species, and positive interactions had similar magnitudes to negative ones for any given seed size, especially when performance was integrated into lifelong fitness. This pattern was stronger when all the demographic processes were integrated over the life cycle, which might have a number of implications for coexistence in stressful environments. Furthermore, the fact that small-seeded species seem to be strongly facilitated may explain why they are not outcompeted by large-seeded species. This is an alternative to other hypotheses explaining the coexistence of plants with different-sized seeds. The close balance between opposite-signed interactions in the presence of stress may also explain why interactions have strong effects on individuals (that interact with only a few species), but seemingly weaker effects on populations, where interactions between many species would cancel out.