The stress-gradient hypothesis does not fit all relationships between plant-plant interactions and abiotic stress: Further insights from arid environments

1 Our earlier meta-analysis of the effects of abiotic stress on the outcome of plant-plant interactions, suggested that the magnitude of the net effect provided by neighbours, whether positive or negative, was not higher under high abiotic stress conditions. This result, which does not support predictions of the stress-gradient hypothesis, has been questioned on the basis of limitations in our analytical approach, on the inappropriateness of some of the studies included in our data bases, and on the criteria used to select the levels of abiotic stress within each study. Here we provide additional arguments in defence of our approach and selection of studies, and perform further analyses of our data base that show that these criticisms are not well founded. 2 The inclusion of studies with contrasting abiotic stress conditions does not invalidate per se tests of predictions derived from the stress-gradient hypothesis because the hypothesis does not specify that predictions should hold for a given difference, or range of differences, in abiotic stress. 3 Our re-analyses show that differences in the length of stress gradient between the low and high stress levels across studies do not reduce the ability of meta-analysis to test predictions of the stress-gradient hypothesis, and that our approach does not suffer from 'hypothesis bias'. 4 Species interactions across abiotic stress gradients do not follow a simple pattern, and there are specific circumstances under which the predictions arising from the stress-gradient hypothesis do not hold. This hypothesis requires profound revision if all situations that emerge when evaluating the relationship between plant interactions and abiotic stress are to be 'fit' by a single conceptual paradigm.