Functional diversity buffers biomass production across variable rainfall conditions through different processes above- versus below-ground

Abstract

Understanding precipitation controls on functional diversity is important in predicting how change in rainfall patterns will alter plant productivity in the future. Trait-based approaches can provide predictive knowledge about how certain species will behave and interact with the community. However, how functional diversity relates to above- and below-ground biomass production in variable rainfall conditions remains unclear. Here, we tested the role of mass ratio and niche complementarity hypotheses in shaping above- and below-ground biomass–functional diversity relationships in seasonal drought. We implemented a fully crossed experiment that manipulated drought timing (fall dry, spring dry, consistent dry and ambient rainfall) and community composition (grass-dominated, forb-dominated and mixed grass–forb) in a California annual grassland. Plant communities with mixed functional groups showed higher above- and below-ground biomass than either the grass- or forb-dominant communities. We found divergent functional diversity–biomass relationships for above- and below-ground biomass. Above-ground biomass decreased with community weighted means (CWMs) of specific leaf area (SLA) and height, supporting the mass ratio hypothesis, which posits that dominant species with specific traits drive biomass production of the community. Below-ground biomass showed no evidence of either mass ratio hypothesis or niche complementarity. While biomass was largely unaffected by the timing of drought in one season, we found community-wide functional trait shifts in response to rainfall treatments. Above-ground traits shifted to higher SLA in consistent dry compared to ambient. Below-ground traits shifted to longer, finer and denser roots in fall and consistent dry, and shorter and coarser roots in spring dry. Functional diversity buffered biomass production by enabling shifts in above- and below-ground functional traits across variable rainfall conditions. Read the free Plain Language Summary for this article on the Journal blog.