Intra and inter-annual climatic conditions have stronger effect than grazing intensity on root growth of permanent grasslands

  1. Catherine Picon-Cochard
  2. Nathalie Vassal
  3. Raphaël Martin
  4. Damien Herfurth
  5. Priscilla Note
  6. Frédérique Louault

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Abstract

Understanding how direct and indirect changes in climatic conditions, management, and species composition affect root production and root traits is of prime importance for the delivery of carbon sequestration services of grasslands. This study considers the effects of climatic variability and gradients of herbage utilisation by grazing on root production over the course of two years. The root and leaf traits of the plant communities were determined to detect their capacity to predict above- and below-ground net primary production, ANPP and BNPP, respectively. A long-term field experiment was used to compare the effects of abandonment and low (Ca-) and high (Ca+) grazing intensities (resulting in mean residual plant heights of 15.2 cm and 7.7 cm, respectively) induced by grazing rotations on upland fertile grasslands after 10 years of treatment application. Ingrowth cores and exclusion cages were used to measure, respectively, the root and shoot mass production several times each year and at an annual scale. The root and leaf traits of the communities were measured near the vegetations peak growing season. We observed strong seasonal root production across treatments in both a wet and a dry year, but the response to grazing intensity was hardly observable within growing seasons. In the abandonment treatment, the spring and autumn root growth peaks were delayed by approximately one month compared to the two cattle treatments, possibly due to a late plant canopy green-up induced by lower soil temperatures and an accumulation of litter. The BNPP was slightly lower in the abandonment treatment compared to the cattle treatments only during the dry year, whereas a decline of the ANPP in the abandonment treatment compared to the Ca+ treatment was observed during the wet year. In response to drought, which occurred during the second year, the root-to-shoot biomass ratio was stable in the cattle treatments but declined in the abandonment treatment. The higher allocation to root mass could benefit plant communities under drier conditions. Rotational grazing pressures and climatic condition variabilities had limited effects on root growth seasonality, although drought had stronger effects on the BNPP than on the ANPP. The stability of the root-to-shoot biomass ratio during the dry year evidenced a higher resistance to drought by grazed versus abandoned grassland communities.

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  1. Version published to 10.24072/pcjournal.54
  2. Peer Community in Ecology

    We know that herbivory can have profound influences on plant communities with respect to their distribution and productivity (recently reviewed by Jia et al. 2018). However, the degree to which these effects are realized belowground in the rhizosphere is far less understood. Indeed, many independent studies and synthesis find that the environmental context can be more important than the direct effects of herbivore activity and its removal of plant biomass (Andriuzzi and Wall 2017, Schrama et al. 2013). In spite of dedicated attention, generalizable conclusions remain a bit elusive (Sitters and Venterink 2015). Picon-Cochard and colleagues (2021) help address this research conundrum in an elegant analysis that demonstrates the interaction between long-term cattle grazing and climatic variability on primary production aboveground and belowground. 

    Over the course of two years, Picon-Cochard et al. (2021) measured above and belowground net primary productivity in French grasslands that had been subject to ten years of managed cattle grazing. When they compared these data with climatic trends, they find an interesting interaction among grazing intensity and climatic factors influencing plant growth.  In short, and as expected, plants allocate more resources to root growth in dry years and more to above ground biomass in wet and cooler years. However, this study reveals the degree to which this is affected by cattle grazing. Grazed grasslands support warmer and dryer soils creating feedback that further and significantly promotes root growth over green biomass production.  

    The implications of this work to understanding the capacity of grassland soils to store carbon is profound. This study addresses one brief moment in time of the long trajectory of this grazed ecosystem. The legacy of grazing does not appear to influence soil ecosystem functioning with respect to root growth except within the environmental context, in this case, climate. This supports the notion that long-term research in animal husbandry and grazing effects on landscapes is deeded. It is my hope that this study is one of many that can be used to synthesize many different data sets and build a deeper understanding of the long-term effects of grazing and herd management within the context of a changing climate.  Herbivory has a profound influence upon ecosystem health and the distribution of plant communities (Speed and Austrheim 2017), global carbon storage (Chen and Frank 2020) and nutrient cycling (Sitters et al. 2020). The analysis and results presented by Picon-Cochard (2021) help to resolve the mechanisms that underly these complex effects and ultimately make projections for the future.

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  3. Version published to 10.1101/2020.08.23.263137 on bioRxiv