Antagonistic interactions between above- and belowground biota reduce their negative effects on a tree species

Aims: Plants in nature are confronted by a variety of beneficial and antagonistic above- and belowground organisms, including leaf herbivores, soil fungi, and soil nematodes. While their individual effects are usually well studied, their joint effects on plant performance are less well known. Synergistic or antagonistic interactions between these organisms would mean that their joint effects on plant performance are more or less detrimental or beneficial than expected from their individual effects. Methods: We conducted a factorial greenhouse experiment in which we manipulated the presence of aboveground herbivores (weevils), soil nematodes, and soil fungi using addition (weevil) or removal (fungicide, nematicide) treatments to test how these groups of organisms alone and in combination affect Triadica sebifera biomass production, when grown individually or under intraspecific competition. Results: Soil fungi and aboveground weevils alone each strongly decreased plant root and total biomass. Interestingly, soil nematodes alone slightly reduced plant biomass but they mitigated the negative impacts of aboveground weevils, indicating antagonism in their effects on plant biomass. However, in the presence of soil fungi this antagonism was less pronounced, illustrating the complexity of interactive effects of aboveground and belowground biota on plant biomass. Aboveground herbivory increased nematode infections, but only in the absence of soil fungi. Intraspecific competition strongly enhanced nematode infection loads and slightly decreased T. sebifera root biomass but did not modulate the direction or the strength of interactions among these aboveground and belowground biota. Conclusions: Our findings indicate that joint effects of antagonistic above- and belowground biota on plant performance can be less detrimental than expected from their individual effects. These results highlight the importance of considering the roles of plant aboveground and belowground interactions from a systems perspective