A simplified method for characterising agronomic services provided by species-rich grasslands
Résumé
In order to encourage farmers and their advisors to promote species-rich grasslands, the agronomic services they can provide should be demonstrated. Methods based on functional ecology are promising, but difficult to use in an applied context. Thus, we aimed to construct a simplified method, tailored for non-researchers, for assessing grassland agronomic services. We assess them by focusing on the grass species that was coupled to a leaf dry matter content (LDMC) database. These simplifications are supported by several hypotheses tested in our paper: (i) trade-offs between agronomic properties; (ii) mass ratio hypothesis; and (iii) functional convergence and divergence between grass and dicotyledonous species. The results were based on 37 vegetation records taken in the south of France (central Pyrenees, from 650 to 1250 m. a.s.l.) for calculating the proportion of grass species and the aggregated LDMC of grass species (LDMCgw), weighting species values by their abundance. LDMC of grass species was taken from field measurements and from a database. The two methods were compared. The latter avoids tedious field measurements, and we found that it was at least as good as the former for assessing agronomic productivity and quality, and the same was true for a LDMC diversity index. Reducing the list of grass species to the two dominant ones did not significantly alter the LDMCgw. There were significant differences between the grass and dicotyledonous components coexisting within a plant community for the digestibility of the plant components (higher for dicots) and in the date at which ceiling yield occurred (earlier for dicots), but no significant effect for productivity. For assessing agronomic services, we proposed corrections based on the grass : dicotyledon ratio. The hypotheses used for designing the method were verified. Finally, agronomic services such as herbage productivity and quality provided by species-rich grasslands can be evaluated by recording dominant grass species and the proportion of grass in the herbage. However, taking account of the difference in sensitivity to the environment for agronomic properties and plant species composition, the main use of the method is for ranking plant communities within a given area and a given year, rather than for providing absolute values of these agronomic properties.