Diversity-productivity relationships in ericaceous plants are influenced by interactions with ericoid mycorrhizal fungi

The positive effect of plant diversity on ecosystem productivity is among the most widespread trends in ecosystem ecology. Most studies suggest that niche differences (complementarity) play a significant role in shaping these effects. As most plant species require mutualistic mycorrhizal fungi to access limiting nutrients, recent evidence suggests that these mutualisms may mediate the complementarity detected in plant studies. Yet, until now, previous studies have focused primarily on the role of mycorrhizal presence in altering these relationships, but no studies have examined the combined effects of plant and mycorrhizal diversity in shaping productivity. Here, we use a mesocosm experimental system to manipulate ericoid mycorrhizal richness (ranging from one to seven fungal isolates) and plant richness (ranging from one to eight plant species) to assess how plant and fungal diversity interact to influence plant productivity. We find clear support for the positive effect of plant diversity on productivity, but this effect only emerged at higher levels of fungal diversity. Within plant monocultures, we found that increasing fungal diversity reduced plant productivity, consistent with antagonism between symbionts limiting plant productivity. Moreover, we found that although most species follow community-level patterns, plant species vary in their response to changing plant and fungal diversity. These results suggest that plant productivity within the ericoid mycorrhizal system is impacted by both plant and fungal diversity, with implications for the use of ericoid mycorrhizal fungi in both agriculture and restoration.