Can frequent mowing counteract the negative effect of multiple nutrients on plant diversity?

Cecilia Molina discusses her recently published Journal of Ecology article: Mowing does not redress the negative effect of nutrient addition on alpha and beta diversity in a temperate grassland. Find out more about the mechanisms driving diversity loss in a temperate grassland.

Temperate grasslands are one of the major and most modified vegetation types in the world. They occupy ~8% of the earth’s terrestrial surface and provide a range of essential ecosystem services to human societies, such as crop and livestock production, habitats for wildlife, etc. Unfortunately, grasslands biodiversity has been affected by the combination of multiple global drivers, such as climate change, species invasions, altered disturbance regimes and nutrient inputs. These drivers affect grassland diversity at both local (e.g. species richness) and larger spatial scales (e.g. beta diversity estimate as differences in species composition). Understanding the mechanisms driving diversity loss at multiples scales is central not only for theoretical community ecology, but also for conservation and management of this unique biome.

Pampa grasslands support a unique biodiversity, and nutrient addition can threaten it. In grazing exclosures, we asked if frequent mowing disturbances may redress the negative effect of increasing number of soil nutrients on diversity. Photo: Cecilia Molina.

Increasingly limiting soil nutrients, through fertilizers or atmospheric deposition, can drive species loss, affecting mainly less abundant species (including rare species). This is well known, and it is a huge problem for aquatic and terrestrial biomes. Moreover, this negative effect increases with the number of added nutrients. For plant communities, there are two main hypotheses that have been proposed to understand the mechanisms underpinning species loss with nutrients addition. On the one hand, the ‘light asymmetry hypothesis’ postulates that nutrient addition increases aboveground biomass of dominant species and generates strong light competition reducing species. On the other, the ‘niche dimension hypothesis’ argues that, apart from light effects, belowground competition is important because species have specific trade-offs for different resources, and a decreasing number of resources could lead to species loss. However, it remains unknown whether canopy disturbance, by reducing biomass and increasing light availability, can reverse the negative effect of nutrient accumulation on diversity. This was the main question of our research.

Grazing exclosures. Left image focus in mown plot, next to applied mowing treatment and right image shows intact plot dominate by tall fescue with reduce light availability. Photo: Pedro Tognetti.

Our results showed that, in a tall fescue dominated grassland in the Pampas (Argentina), frequent mowing increased light availability and thus allowed species richness recovery (Fig. 1). However, mowing did not completely counteract the negative nutrient effect. Increasing the number of added nutrients still reduced local and spatial plant diversity in mown plots, and this effect was intensified along time increasing species dominance and promoting rare species loss (Fig. 1a). Importantly, we found that the relative importance of the number of added nutrients on diversity depended on initial species richness and on disturbance regime. We demonstrate that under conditions of strong dominance, in intact plots, species loss can be better explained by light limitation; whereas in mown plots, where the light was less limiting, species loss can be explained by belowground competition and niche dimension.

Fig. 1: Changes in species richness (a) inverse Simpson index (b) and beta diversity (c) with the increasing number in added nutrients (i.e. control, 1, 2, 3) and mowing over four experimental years.

In summary, our study highlights that removing plant biomass through mowing positively affected local and spatial diversity, but it could not completely counteract the negative effect of nutrient addition. Furthermore, our experiment supported both the light limitation and niche dimension hypotheses, as both helped to explain nutrient addition effects. Nutrient addition generated biological homogenization, threatening the ability of grasslands to provide ecosystem services at different spatial scales. In a global context, as temperate grasslands are one of the most endangered ecosystems worldwide, a focus on intervention and management of eutrophized systems is necessary to aid their conservation.

Cecilia Molina Universidad de Buenos Aires and CONICET, Argentina

You can read the full paper online: Mowing does not redress the negative effect of nutrient addition on alpha and beta diversity in a temperate grassland

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