Fig. 1.

Different scales of interest typically found in the plant–plant interactions literature, relevant questions regarding their relationship with the environment and suitable tests to answer them. Species-specific (four species, Sp A–D) changes in performance across the same environmental gradient (left box; Level 1) affect the outcome of plant–plant interactions across such gradients (Level 3, pairwise). This affects the frequency of positive interactions found at the community level (Level 3, community). Plant–plant interactions become more positive with decreasing target plant performance in the open; however, for species A and D no changes in their performance are detected through the gradient, and therefore no changes in the outcome of the interactions between these latter species and their neighbours are expected. Discontinuous lines in the panels separate positive (above) and negative (below) plant–plant interaction net outcomes (represented by the lnRR [log Response Ratio] index). The grey quadrat surrounds high-priority areas for future research. SGH, stress-gradient hypothesis.