Weathering the storms
One of the consequences of current climate warming is that the amount and intensity of extreme weather events is increasing. A rapidly growing number of studies show that these extreme events may have complex and often indirect ecological consequences, however, little is known about the role of soil biodiversity in these processes.
In a recent paper in PNAS (2013; 110:9835-9838) Annelein Meisner and colleagues show that extreme weather events may tip the balance between native and exotic plant species in vegetation. The researchers dried and wetted soils before planting a mixed vegetation of native and genetically related exotic plant species. Although all soils were re-set to standard moisture conditions before planting, the differences in plant biomass distribution between natives and exotics were striking. In soils that had been dried, exotic plant species became dominant, whereas in control soils, exotics and natives were more in balance. The role of different nutrient availabilities leading to the shifts in plant dominance could be largely ruled out.
In a second experiment, soil samples were inoculated into sterilized soils and planted again with the same plant mixtures as before. Interestingly, most effects could be recalled, suggesting that soil microbes were responsible for the plant responses to the extreme weather events. This study revealed that soil biodiversity might memorize effects of extreme weather events, which may influence plant community composition that was established after the abiotic stress had been taken away.
The study by Meisner and colleagues shows that effects of extreme weather events - due to climate warming - may work out on ecosystems in quite unpredicted ways.
The study also gives rise to some questions.
First, what is the mechanism leading to dominance of the exotic plant species? It is increasingly recognized that feedback interactions between plants and soil properties influence plant community composition. The question now is how these feedback effects are organized. In most studies, plant-soil feedback effects are measured as net effects of all decomposers, pathogens, and symbiotic mutualists. Each of these major groups of soil biota can include hundreds to even thousands of species. The challenge will be to tease apart the contribution of the various groups, as well as the species within these groups in explaining how plant-soil feedback effects may depend on extreme weather events and other environmental conditions. This is a major issue for soil ecologists to study in the near future.
Second, it needs to be analyzed how the effects reported by Meisner and colleagues might work out in the field, where many other influences can affect plant performance, such as aboveground and belowground invertebrates, aboveground vertebrates, resource limitation, as well as other abiotic environmental conditions. Although these issues need more work, a fact is that extreme weather events have been reported to enhance plant invasions. A memorizing effect of the abiotic stress conditions in the soil biodiversity might provide a new explanation for plant invasiveness.
This study on effects of abiotic stress that can be memorized in the soil community is a demonstration of the complex involvement of soil biodiversity in plant community composition and ecosystem functioning.
Still, Pandora´s box does not seem to be fully opened yet. Plant-soil interactions are net effects of myriad interactions of plant roots with the diverse soil community. Some of these interactions can be positive, whereas others are negative. Studies during the past years have elucidated that soil biota can be much more specific in their interactions with plants than has been assumed for long. It is a major challenge in order to further explore soil biodiversity, its specificity, and effects on plant community dynamics.