Abstract
Models predict that vertical gradients of foliar nitrogen (N) allocation, increasing from bottom to top of plant canopies, emerge as a plastic response to optimise N utilisation for carbon assimilation. While this mechanism has been well documented in monocultures, its relevance for mixed stands of varying species richness remains poorly understood. We used 21 naturally assembled grassland communities to analyse the gradients of N in the canopy using N allocation coefficients (KN) estimated from the distribution of N per foliar surface area (KN-F) and ground surface area (KN-G). We tested whether: 1) increasing plant species richness leads to more pronounced N gradients as indicated by higher KN-values, 2) KN is a good predictor of instantaneous net ecosystem CO2 exchange and 3) functional diversity of leaf N concentration as estimated by Rao’s Q quadratic diversity metric is a good proxy of KN. Our results show a negative (for KN-G) or no relationship (for KN-F) between species richness and canopy N distribution, but emphasize a link (positive relationship) between more foliar N per ground surface area in the upper layers of the canopy (i.e. under higher KN-G) and ecosystem CO2 uptake. Rao’s Q was not a good proxy for either KN.
Highlights
During the last two decades, substantial progress has been made in understanding the role of biodiversity for ecosystem functioning[1,2,3]
As we found a significant increase of net ecosystem CO2 exchange (NEE) in mixed stands concomitant with a tendency of lower KN-G values, we further analysed the relationship between the N allocation coefficients and NEE in all communities as well as in mixed stands only
This is the first study assessing the potential links between canopy N allocation coefficients (KN) and net ecosystem CO2 exchange (NEE) in stands of varying species richness under field conditions
Summary
During the last two decades, substantial progress has been made in understanding the role of biodiversity for ecosystem functioning[1,2,3]. We test the following hypotheses: 1) increasing plant species richness leads to more pronounced vertical N gradients estimated with the N allocation coefficients (KN) according to the method of Hirose and Werger (1987)23, 2) KN is a good predictor of instantaneous NEE and 3) functional diversity of leaf N concentration as estimated by Rao’s Q quadratic diversity metric is a good proxy of KN (see Table 1 for a list of abbreviations). To this end we tested two versions of KN, one based on the distribution of foliar N content per leaf surface area (KN-F), the other based on the distribution of foliar N content per ground surface area (KN-G)
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