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Abstract
Numerous experiments, mostly performed in particular environments, have shown positive diversity-productivity relationships. Although the complementary use of resources is discussed as an important mechanism explaining diversity effects, less is known about how resource availability controls the strength of diversity effects and how this response depends on the functional composition of plant communities. We studied aboveground biomass production in experimental monocultures, two- and four-species mixtures assembled from two independent pools of four perennial grassland species, each representing two functional groups (grasses, forbs) and two growth statures (small, tall), and exposed to different combinations of light and nutrient availability. On average, shade led to a decrease in aboveground biomass production of 24% while fertilization increased biomass production by 36%. Mixtures were on average more productive than expected from their monocultures (relative yield total, RYT>1) and showed positive net diversity effects (NE: +34% biomass increase; mixture minus mean monoculture biomass). Both trait-independent complementarity effects (TICE: +21%) and dominance effects (DE: +12%) positively contributed to net diversity effects, while trait-dependent complementarity effects were minor (TDCE: +1%). Shading did not alter diversity effects and overyielding. Fertilization decreased RYT and the proportion of biomass gain through TICE and TDCE, while DE increased. Diversity effects did not increase with species richness and were independent of functional group or growth stature composition. Trait-based analyses showed that the dominance of species with root and leaf traits related to resource conservation increased TICE. Traits indicating the tolerance of shade showed positive relationships with TDCE. Large DE were associated with the dominance of species with tall growth and low diversity in leaf nitrogen concentrations. Our field experiment shows that positive diversity effects are possible in grass-forb mixtures irrespective of differences in light availability, but that the chance for the complementary use of resources increases when nutrients are not available at excess.
Highlights
Experimental studies have repeatedly shown that higher species or functional group richness increases primary productivity in grassland ecosystems and influences ecosystem processes [1,2,3]
The selection effect hypothesis explains positive diversity-productivity relationships by the greater probability of more diverse communities to contain a dominant and productive species [6,7]. Both hypotheses propose distinct ecological mechanisms, which are controlled by the functional composition of the communities: complementarity effects depend on the interactions between functionally different species, while selection effects require the presence of a species with particular functional characteristics [8]
While aboveground biomass production did not change dependent on functional group composition, communities comprising only small-statured species produced less biomass than mixtures of only tall-statured species, while mixtures of both growth statures showed intermediate productivity
Summary
Experimental studies have repeatedly shown that higher species or functional group richness increases primary productivity in grassland ecosystems and influences ecosystem processes [1,2,3]. The selection effect hypothesis explains positive diversity-productivity relationships by the greater probability of more diverse communities to contain a dominant and productive species [6,7] Both hypotheses propose distinct ecological mechanisms, which are controlled by the functional composition of the communities: complementarity effects depend on the interactions between functionally different species, while selection effects require the presence of a species with particular functional characteristics [8]. A study across 29 grassland diversity experiments using trait diversity alone as predictor for community biomass production has shown that diversity in traits related to nitrogen acquisition and use (leaf nitrogen concentrations, N2 fixation) and light competition (plant height) have the greatest predictive power for biomass production [21] Using both communityweighted means of trait values and trait diversity, another study in the Jena Experiment has found that the dominance of particular trait values ( tall growth and high leaf nitrogen concentrations) is more important than trait diversity for high community biomass production [22]
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