Can root trait diversity explain complementarity effects in a grassland biodiversity experiment?

Journal of Plant Ecology VOLUME 11, NUMBER 1, PAGES 73–84 February 2018 doi: 10.1093/jpe/rtw111 Advance Access publication 21 December 2016 available online at academic.oup.com/jpe Can root trait diversity explain complementarity effects in a grassland biodiversity experiment? Lisette M. Bakker*, Liesje Mommer and Jasper van Ruijven Plant Ecology and Nature Conservation Group, Wageningen University & Research, Droevendaalsesteeg 3a, 6708 PB Wageningen, The Netherlands *Correspondence address. Plant Ecology and Nature Conservation Group, Wageningen University & Research, P.O. Box 47, Droevendaalse steeg 3A, 6700 AA Wageningen, The Netherlands. Tel: +31-0-317-48-55-49; E-mail: Abstract Aims The positive relationship between plant biodiversity and community productivity is well established. However, our knowledge about the mechanisms underlying these positive biodiversity effects is still limited. One of the main hypotheses is that complementarity in resource uptake is responsible for the positive biodiversity effects: plant species differ in resource uptake strategy, which results in a more complete exploitation of the available resources in space and time when plant species are growing together. Recent studies suggest that functional diversity of the community, i.e. the diversity in functional characteristics (‘traits’) among species, rather than species richness per se, is important for positive biodiversity effects. However, experimental evidence for specific trait combinations underlying resource complementarity is scarce. As the root system is responsible for the uptake of nutrients and water, we hypothesize that diversity in root traits may underlie complementary resource use and contribute to the biodiversity effects. Methods In a common garden experiment, 16 grassland species were grown in monoculture, 4-species mixtures differing in root trait diversity and 16-species mixtures. The 4-species mixtures were designed to cover a gradient in average rooting depth. Above-ground biomass INTRODUCTION Many biodiversity experiments have shown a positive relationship between plant species richness (SR) and productivity (Balvanera et al. 2006; Cardinale et al. 2012; Hooper et al. 2005). However, the mechanisms underlying this positive biodiversity effect are still debated (Atwater and Callaway 2015; Cardinale et al. 2011; de Kroon et al. 2012; Kuebbing et al. 2015; Schnitzer et al. 2011). One of the main hypotheses was cut after one growing season and used as a proxy for plant productivity to calculate biodiversity effects. Important Findings Overall, plant mixtures showed a significant increase in biomass and complementarity effects, but this varied greatly between communities. However, diversity in root traits (measured in a separate greenhouse experiment and based on literature) could not explain this variation in complementarity effects. Instead, complementarity effects were strongly affected by the presence and competitive interactions of two particular species. The large variation in complementarity effects and significant effect of two species emphasizes the importance of community composition for positive biodiversity effects. Future research should focus on identifying the traits associated with the key role of particular species for complementarity effects. This may increase our understanding of the links between functional trait composition and biodiversity effects as well as the relative importance of resource complementarity and other underlying mechanisms for the positive biodiversity effects. Keywords: biodiversity effects, resource complementarity, trait diversity, roots, functional diversity, grassland Received: 11 March 2016, Revised: 15 September 2016, Accepted: 13 October 2016 is that positive biodiversity effects on productivity are the result of resource complementarity. Different plant species differ in resource uptake strategy, which results in complementarity in resource uptake when plant species are growing together. In species-rich plant communities, resources will thus be more completely exploited in space and time than in species-poor plant communities (e.g. Berendse 1982; Cardinale et al. 2007, 2011; Roscher et al. 2012; Tilman et al. 1997b). © The Author(s) 2016. Published by Oxford University Press on behalf of the Institute of Botany, Chinese Academy of Sciences and the Botanical Society of China. All rights reserved. For permissions, please email: 74 It has been suggested that resource complementarity occurs mainly belowground (Bardgett et al. 2014; Fischer et al. 2014; de Kroon et al. 2012; van Ruijven and Berendse 2005; Yang et al. 2015). However, empirical studies that have investigated resource complementarity belowground are scarce and the results are mixed. The most classical example of resource complementarity is in differential root distribution patterns (i.e. vertical niche differentiation) among individual plant species (Berendse 1982; Fitter 1986; Parrish and Bazzaz 1976; Silvertown et al. 2015). Differentiation in rooting depth (RD) could imply that species are able to acquire water and nutrients from separate parts of the soil (e.g. shallow and deep soil layers), thereby decreasing resource competition and increasing resource exploitation. However, experimental tests of vertical niche differentiation in terms of nutrient uptake (Bachmann et al. 2015; Hoekstra et al. 2015; Schultz et al. 2012; von Felten et al. 2009) or root biomass distribution (Mommer et al. 2010; Ravenek et al. 2014) have yielded little evidence supporting resource complementarity in grassland biodiversity experiments. An alternative approach to reveal resource complementarity is to focus on the functional traits of the species involved. Several studies have shown that species composition (Avolio et al. 2014; Hector et al. 2011) and functional group richness (Hooper and Dukes 2004; Marquard et al. 2009; Tilman et al. 1997a) greatly influence the biodiversity effects. The consensus is that differences between species in functional traits, rather than SR per se, can enhance total resource capture (Cardinale et al. 2012), but experimental evidence that links trait differences to biodiversity effects is limited (but see Flynn et al. 2011). One of the outstanding questions is whether the average trait value of the community (community weighted mean [CWM]) or the diversity in traits is more important for the complementarity effects. Given the fact that resource complementarity is based on differences among species, one would expect that trait diversity is more important than the mean trait value. However, few studies that linked the traits of the species to community performance found that CWMs explained more variation in biomass (Finegan et al. 2015) and biodiversity effects (Roscher et al. 2012) than functional diversity. However, these studies mainly focused on aboveground traits, whereas resource complementarity is predominantly expected to occur (...truncated)