What drives changes in plant group proportions over time? An analysis from an international multi-site grassland experiment

Abstract

Traditionally, productive agricultural grassland systems have been sown with a single grass species and managed using a high input of synthetic nitrogen fertiliser, which can be expensive and ecologically harmful. However, recent research has shown that increasing the number of sown species and including legumes can promote yield production and reduce the reliance on fertiliser, thus improving the sustainability of the system. Most research into the relationship between species diversity and responses (such as yield production) in ecosystems has focused on the number of species as the primary driving component of diversity in the ecosystem. However, the proportions in which species are sown can be a strong driver of responses, in addition to the number. In multi-species mixtures, selecting species from varying plant groups with complementary symbiotic roles within the ecosystem can maximise resource use and increase outputs. While it is possible to control the proportions of different plant groups that are sown, these proportions may change over time as plant species compete against each other and some species or groups may not persist well. It is of interest to know how sown plant diversity and site-specific conditions, such as climate, impact on changes of plant proportions over time. Here, we report on the international multi-site LegacyNet experiment. At each of multiple sites, a common field experiment was conducted where the initial sown proportions of up to six grassland species were manipulated across plots. The six species were categorised into three plant groupings, grasses, legumes, and herbs. One aim of the study was to evaluate the dynamics of these plant grouping proportions in yield production over time. Using a hierarchical Bayesian baseline logit-category model, we assess how the initial sown species diversity affects the observed functional group proportions in grassland mixtures. This approach captures the complex nature of this experimental dataset, considers the spatial dependencies between sites, and provides an understanding of the drivers of plant groups proportions over time. We found that plant groups tended to remain relatively constant over time (e.g., 18 to 24 months), with estimates from single plant groups having larger associated uncertainty than multi-species mixtures proportions. Evaluating the dynamics of plant group proportions over a gradient of initial sown proportions, we discuss potential trends between different climate types. Lastly, we translate the results of this work into recommendations for future agricultural practices.