Fungal pairwise interactions shift from positive to negative under warming stress

The impact of climate change on soil processes depends on how abiotic stressors (drought, flood, heatwave…) affect soil species physiology directly, but also indirectly through their interactions. A number of different theoretical frameworks have been designed to conceptualize the latter. The most widely used is the stress gradient hypothesis, which states that facilitation should be more common in stressful environments. However, these interactions are notoriously difficult to investigate in soils, because of the difficulty to isolate low competitor species and of the sheer number of species in soil, and therefore of interactions to test experimentally. Consequently, this topic has been traditionally approached through network analyses, that are based on frequency of co-occurrences, but has its own flaws.

In this paper, we used a combination of isolation methods to recover the widest possible range of soil fungi with randomized co-cultivation tests to cover as many interactions as possible. This way, we investigated how multiple pairwise competitive interactions were affected by abiotic stress (high temperature, low water availability).

In absence of abiotic stress, the presence of another species mostly affected growth positively, demonstrating facilitation among soil fungal species under benign conditions. In presence of temperature stress, either alone or combined with water stress, these positive effects became negative, contradicting the stress gradient hypothesis. We did not find the interaction outcomes to be predictable by some of the trait data we investigated, such as abiotic stress tolerance or intrinsic growth rate.

Overall, our results demonstrate that in a very simplified but controlled system, climate change shifts the interaction types from mostly facilitative to mostly negative. We discuss further potential implications in the presentation.