Biomass fine-scale variation is predictive of functional composition and diversity in grazed grassland

Prepared by Nicolas Caram, Felipe Casalás, Marcelo O. Wallau, Pablo Soca, Valério D. Pillar, Monica Cadenazzi & Pablo Boggiano

Sampling at the Experimental Station “Bernardo Rosengurtt” of Faculty of Agronomy, Universidad de la República (32°21’59” S, 54°26’37” W). Photo credit: Felipe Casalás

The adjustment of livestock number per unit area affects the aboveground biomass and its variation in space and time, which is a tool in grassland management for biodiversity conservation and livestock production. Given the antecedents, we hypothesized that the aboveground biomass could be an effective tool to predict the species composition with similar or dissimilar functionality to the ecosystem, such as light interception, primary production, plant competition and resource use, response to herbivory and stability. Grouping species with similar functionality and predicting the functional composition and diversity can help design and improve sustainable management practices for complex native grasslands.

We developed the study in a native grassland, Campos, of north-eastern Uruguay, managed under high and low grazing pressure. We monitored the aboveground biomass and species composition during autumn, winter and spring 2017 using permanent quadrats of 20 × 20 cm. Thirteen of the most important species, in terms of contribution to aboveground biomass, were chosen for leaf traits measurements, which are highly correlated with the species’ functionality to the ecosystem. The traits selected were leaf dry matter content, specific leaf area, leaf tensile strength, and leaf width. Furthermore, we computed the community weighted mean of each trait at the 20 × 20 cm quadrat scale, and related it to the aboveground biomass. Species were grouped into four functional groups with similar traits and functionality (A, B, C and D).

We found that lower levels of biomass increased the dominance of species with lower leaf dry matter content and wider leaves, which corresponded to functional groups A and B. On the contrary, higher levels of biomass increased species with higher leaf dry matter content and narrower leaves, such as functional group D represented by a tussock-forming species, Nassella charruana [Arechav.] Barkworth. Moreover, higher levels of biomass were linked to a drop in the performance of species belonging to functional group A, classified as resource-acquisitive species, and determined an increase in functional diversity. Our results indicate that the fine-scale variation of aboveground biomass is predictive of functional composition and diversity in grazed grasslands. Grazing management practices that create and maintain spatiotemporal heterogeneity can be used to favour the conservation of biodiversity and the functionality of native grasslands.

This is a plain language summary for the paper of Caram et al. published in Applied Vegetation Science (