Increasing aboveground biomass and stable belowground biomass are controlled by greater species richness and dominant functional groups in Qinghai-Tibet Plateau

Ecological studies place great importance on understanding the profound significance of plant diversity in maintaining the functioning of grassland ecosystems. However, despite decades of research, ecologists have faced persistent challenges in fully comprehending the intricate relationships between biodiversity and ecosystem functioning, as well as the influence of dominant plant functional groups on overall ecosystem function. To investigate two key aspects, namely the association between species richness and above- and below-ground biomass (AGB and BGB), as well as the relative contributions of functional groups in maintaining ecosystem function, we investigated the grassland biomass productivity in meadow steppe and alpine meadow ecosystems in the Qinghai-Tibet Plateau (QTP) for the period 2015-2019. 

The pristine grassland sites belonging to the alpine meadow and meadow steppe were maintained for data collection over 5 years. Shoot biomass was harvested during the peak growing season (mid-late September) by clipping vegetation samples from 3 typical 0.25 × 0.25 m² quadrats within the central 4 m² (2 × 2 m²)of the plot (5 × 5 m²) at the respective site of the total 36 sites. Root biomass was sampled using soil cores at depths of 0-40 cm (0-10, 10-20, 20-30, and 30-40 cm) in 3 typical 0.25 × 0.25 m² quadrats over the 5 years. Before harvesting the biomass, the number of species and functional groups in the selected quadrat was counted. We used multiple tests including Mann-Kendall, Generalized Linear Model, Kruskal-Wallis, and Wilcoxon texts for analyzing the data.

The AGB of both grasslands exhibited an increasing trend over 5 years, while the BGB remained stable. This rise in AGB was attributed to the upward trajectory observed in AGB for forbs and grasses, which are the dominant functional groups in the QTP. These results underscore the crucial role of dominant species and functional groups in maintaining ecosystem functioning. Higher species richness plays a crucial role in ecosystem stability, as evidenced by the significantly positive relationships between biodiversity and AGB, and the stable relationships between biodiversity and BGB. In both grasslands, the top soil layer (0-10 cm) exhibited a dominant contribution to the observed BGB. This can be attributed to the abundance of nutrients present in the topsoil layer, which creates favorable conditions for root proliferation. Within the meadow steppe, there was an isometric allocation pattern observed in biomass, indicating that the BGB increased proportionally with the AGB.

The findings of this study demonstrate the influence of species richness on ecosystem functioning, with forbs and grasses playing a dominant role in biomass productivity. Notably, the top soil layer was responsible for three-quarters of the BGB. These empirical results provide valuable evidence that higher species richness enhances ecosystem functioning, serving as a scientific basis for informing policymaking regarding ecosystem stability.