Root plasticity as a potential driver of niche complementarity

The current loss of biodiversity has negatively affected ecosystem functioning, as shown by decreased plant production of grasslands with declining species richness. The mechanisms underlying this diversity-productivity relationship have been hotly debated, but among the explanations put forward belowground niche complementarity is assumed to be the most important mechanism. Belowground, plant species differ inherently in rooting architecture and distribution, but roots are also incredibly plastic in these traits. We hypothesise that root plasticity may be an important driver of belowground niche complementarity, since plastic root responses may change the inherent differences in rooting pattern between species.  To investigate this hypothesis we designed a long-term experiment with monocultures and mixtures of four grassland species in the Nijmegen Phytotron (the Netherlands), a state-of-the-art ecological root research facility (http://www.eco.science.ru.nl/expploec/phytotron/). We monitor the species-specific short-term root responses under near-outdoor conditions using minirhizotron tubes over two growing seasons in order to elucidate the effect of belowground plant-plant interactions for plant productivity aboveground. Half of the experimental containers have a nutrient-poor sandy top soil layer to speed up the process of nutrient depletion due to root competition. We also take root samples per soil layer of 5 cm every growing season. Since the determination of the species-specific distribution of roots in species mixtures is currently almost impossible, we have developed a molecular method to quantify the species composition of root samples (based on RT-PCR on ISSR markers), to reveal the exact root distribution patterns of each of the four species. The first results will be presented during the ESA 2007 symposium.