Functional traits of a broad-niched metallophyte along a toxicity gradient: disentangling intra and inter-population variation

Studies on the relative contribution of inter- and intra-population trait variations in broad-niched species along edaphic gradients are still scarce. Here, we explore the variation of eight traits in five populations of Cyanotis longifolia, a broad-niched metallophyte thriving on very broad gradients of soil copper concentration in Central Africa. Variation at species level was decomposed into covariation with copper, variation among sites, and residual variation. The proportion of the variance explained by the site and by the local copper gradient in each site ranged from 10 to 32% and between 5 and 51% respectively, depending on trait. At the species level, specific leaf area decreased and leaf Cu and Co concentrations increased with increasing soil Cu concentration. Surprisingly, traits related to plant size show a quadratic response with higher values on both high and low soil Cu concentrations. This could be accounted for by population-specific patterns of covariation with soil copper. Population specific traits responses were found, with some populations exhibiting no significant response while the others sometimes show opposing variation patterns. The reasons for such idiosyncratic patterns are discussed in terms of interactions with other soil factors influencing Cu availability and toxicity. In further studies on trait variation along toxicity gradients, we recommend to investigate the small scale edaphic variability and to measure traits at the population level to capture the diversity of functional responses.