Belowground fine root productivity, traits, and trees.

A long-standing debate in plant ecology is how plant diversity – including species richness, functional, and phylogenetic diversity – determines primary productivity aboveground and belowground (Tilman et al., 1997; Cadotte et al., 2009). A leading mechanism linking diversity and productivity posits that diverse assemblages of plants are more likely to complement each other in resource acquiring abilities, leading to greater biomass production than one would expect in low diversity communities, an outcome known as overyielding (Cardinale et al., 2007). Much of our understanding of the relationship between biodiversity and primary productivity comes from grassland communities and experiments. Forest trees are less tractable experimentally for biodiversity productivity studies than grasses and herbs and therefore require observational and correlative studies (Chisholm et al., 2013). Because of this, we know surprisingly little about how long-lived forest trees directly interact with each other belowground and compete for limiting soil resources, and in particular, if they partition space or nutrients to reduce interspecific competition – a classic explanation for species co-existence in diverse communities. Aboveground plant stems are easilymeasured and identifiedwhereas belowground plant roots are buried, making species interactions and basic patterns of occurrence and co-occurrence difficult to observe.While researchers have used a variety of DNA-basedmethods to identify roots and explore species interactions and rooting profiles for some time now (Jackson et al., 1999; Hiiesalu et al., 2012) there have been fewer studies done in closed canopy forests (Jones et al., 2011). In this issue ofNew Phytologist, Valverde-Barrantes et al. (2014, pp. 731– 742) use DNA identification of fine roots to dissect complex belowground interactions in a temperate hardwood forest and test several interrelated hypotheses on the relationship between: belowground species, functional, and phylogenetic diversity; soil resource availability; and productivity.