Question: What characteristics of local biotic neighbourhood is the best proxy of competitive effects experienced by plants in a herbaceous community: (1) total above-ground biomass, (2) root mass or (3) relative above-ground abundance of selected species?

Location: Grassland at ca. 1100 m a.s.l. in the Krkonoše Mts., northern Czech Republic.

Methods: We implanted two phytometer species, Anthoxanthum alpinum and Festuca rubra, into a mountain grassland, and examined their response to local variation in (1) total above-ground biomass, (2) root mass at three soil depths, and (3) relative abundance of individual species above-ground.

Results: Performance of both phytometer species was determined much more consistently by the mass of neighbouring roots and by species composition of neighbours than by the total above-ground biomass. The two phytometer species showed different responses to these parameters. The most important relationships were (1) negative relationship between performance of Anthoxanthum and mass of neighbouring roots at 0–3 cm, (2) positive relationship between performance of Festuca and mass of neighbouring roots at 3–6 cm, and (3) negative relationship between performance of Festuca and relative abundance of Festuca in the neighbourhood.

Conclusions: Neighbouring root mass and above-ground species composition are better determinants of biotic interactions than total above-ground biomass of neighbours in the studied mountain grassland. However, the relationships found are not necessarily due to variation in competitive intensity but can be due to other hidden factors as well, e.g. local availability of resources.