The Editor’s Choice for this issue of Journal of Ecology, the paper ‘Unveiling below-ground species abundance in a biodiversity experiment: a test of vertical niche differentiation among grassland species’ by Liesje Mommer and colleagues, uses a novel molecular technique to answer a longstanding question about the mechanisms underlying the positive effect of plant species diversity on primary production.
The effects of biodiversity on ecosystem functioning has become a topical area of research in ecology. Greater biomass production in species mixtures, compared with that predicted using growth in monocultures, has been repeatedly reported for grasslands, but the underlying mechanisms are hotly debated. Enhanced biomass production in species-rich communities has been attributed to 1) increased nitrogen inputs caused by the presence of legumes, 2) the increased chance of a highly productive species occurring in diverse communities , or 3) complementarity in resource use—niche differences between species allow resources to be more efficiently exploited. Curiously, most studies have only examined above-ground productivity, while it has also been suggested that the mechanism of niche complementarity is below ground. Specifically, overyielding in species-rich communities can be achieved through vertical niche differentiation: a combination of shallow-rooting and deep-rooting species may allow more extensive exploration of soil volume and hence exploitation of the available nutrients, which in turn should lead to increased productivity both below and above ground. However, testing the vertical niche differentiation hypothesis is difficult because the species identity of roots in soil samples from diverse communities must be determined and this is not easy. This methodological limitation is a major obstacle to research on plant root interactions in general.
A novel molecular tool
Mommer and her colleagues have developed a molecular technique that enables accurate assessment of the relative abundance of different species in root samples. The technique involves extracting genomic DNA from root samples and running real-time PCR with primers for species-specific markers. This molecular tool was employed in a biodiversity experiment in which four grassland species—two dicots and two grasses—were each grown in a monoculture and a four-species mixture. The study was specifically designed to test the vertical niche differentiation hypothesis and so advance our understanding of the mechanisms underlying the positive effect of diversity on primary production.
The results showed that species mixtures produced considerably more root mass than predicted on the basis of below-ground production in the four monocultures. The over-yielding in species mixtures was not caused by a dominant, fast-growing species accruing a competitive advantage, but was a result of root growth stimulation in a subdominant grass with little change in root growth of other species. This result would normally be attributed to complementarity in resource use. However, molecular analysis of root samples found little support for the vertical niche differentiation hypothesis: the vertical root distributions of the different species overlapped widely, and most of the roots were aggregated in the upper soil layer.
The results were also hard to explain on the basis of resource-driven processes. If plants were merely responding to changes in nutrient availability, the fastest-growing species should have benefited most from growing in a multispecies mixture, but this was not the case - it was a subdominant grass species that responded most to changes in diversity. One possible explanation for these counterintuitive results can be found in the growing body of literature on root communication and microbe-mediated root interactions. The positive diversity–productivity relationship may have been caused by the suppression of root growth in the subdominant’s monocultures due to the accumulation of autotoxic exudates or species-specific pathogens. Alternatively, root growth in the mixture may have been stimulated by the root exudates of other species or by the presence of growth-promoting bacteria.
The study by Mommer et al. did not reveal the exact mechanism behind the positive diversity–productivity relationship, but it elegantly challenges explanations and fundamental assumptions about how plants interact and ecosystems function at different levels of species diversity. This first glimpse of the way roots respond to diversity also creates an important link between studies on ecosystem functioning and research on root communication.
Associate Editor, Journal of Ecology
- Mommer, L., van Ruijven, J., de Caluwe, H., Smit-Tiekstra, A.E., Wagemaker, C.A.M., Ouborg, N.J.,Bögemann, G.M., van der Weerden, G.M., Berendse, F., de Kroon, H. (2010) Unveiling below-ground species abundance in a biodiversity experiment: a test of vertical niche differentiation among grassland species. Journal of Ecology, 98, 1117–1127.
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