Drought and phosphorus affect productivity of a mesic grassland via shifts in root traits of dominant species

Aims: Precipitation and soil nutrients play an important role in regulating grassland productivity. However, little is known regarding the sensitivity of grasslands to changes in water and nutrient availability and the mechanisms driving productivity responses. Methods: We examined the effects of extreme drought (65% rainfall reduction) and phosphorus fertilization on aboveground net primary production (ANPP) and plant functional traits (PFTs) of four dominant mesic grassland species in a semi-natural grassland in southeast Australia. We used piecewise structural equation modelling to determine which PFTs contribute to the treatment effects on ANPP. Results: Reduced rainfall decreased ANPP by 29% while P addition increased ANPP by 62% at the community-level. Significant drought-related reductions in ANPP were apparent for Setaria parviflora, while Cynodon dactylon was the only species exhibiting increased ANPP under P addition. There was no interaction between rainfall and P addition. Structural equation modelling indicated specific root length was a key trait underpinning community-level ANPP responses to P; this was, however, primarily driven by a single dominant (~61% of community biomass) species (Cynodon). Conclusions: Our results indicate the negative impacts of drought on ANPP – driven primarily by Setaria– were not ameliorated by P addition. The positive effect of P addition on community-level ANPP was attributed to the response of the most dominant species, Cynodon, and mediated by changes in specific root length. This study highlights the importance of understanding the link between belowground traits and ANPP for predicting dominant species’ response to global change drivers.