1 Roots are capable of strong plastic responses to environmental signals, but the extent of such responses in the field is essentially unknown. Here, we aimed to identify interspecific root aggregations and segregations as indicators of plastic responses to the presence of other species. Further, we asked whether aggregations change with heterogeneity in overall root density along soil depth and whether root communities show any relationship to species' functional traits.

2 We used quantitative Real-Time PCR to estimate root quantities of nine grassland species in soil blocks at a centimetre scale. We combined these estimates with fine-scale recording of species' positions aboveground. We used deviations between measured root density and that expected from the relationship to aboveground species abundance to correct for different chances of individual species to meet due to the non-random distribution of their individuals. This approach allows us to identify whether species associations belowground are due to plastic response of their roots.

3 We showed that while there were signals of both interspecific aggregation and segregation, prevalence of aggregation over segregation strongly increased with increasing soil depth. In shallow layers, where root density is high and its heterogeneity low, roots are likely to respond to a number of signals which produce both interspecific aggregation and segregation. In contrast, in deeper layers, where root density is low and its heterogeneity higher, root distribution is likely to be driven primarily by foraging for resources, producing aggregations. Species composition of the root community did not show any relationship to functional traits (SLA, shoot lifespan, spreading distance and rooting depth) of the examined species.

4 Synthesis. The results imply that processes that determine interspecific root associations change at a scale of centimetres which is close to the scale of individual fine roots. They also imply that while root foraging might explain root co-occurrence under low root density, very different processes determine it when root density is high. Our findings also support the notion that roots, while extremely plastic, may not receive sufficient signals to elicit a response in the field if overall root density is high and homogeneous