Temporal variation in plant diversity and carbon dynamics of a mountain meadow in response to warming and species removal

Background/Question/Methods

Climate change is causing warmer temperatures globally, especially in cold, high elevation areas. Prior work using a combination of a 30-year observational study, active warming experiment, and synthesis of warming studies on the Tibetan Plateau found that although the climate was warmer and drier, net primary productivity (NPP) remained stable. These results indicate that warming and drying lead to changes in plant functional types from shallow-rooted forbs to deep-rooted grasses, suggesting ecosystem changes beyond NPP. In an observational gradient study in Colorado, net ecosystem exchange (NEE) and plant phylogenetic diversity were tightly coupled independent of large differences in soil moisture and temperature across elevations. While plant diversity influences NEE, how the relationship between biodiversity and ecosystem function is impacted by warming is uncertain. Here, we explored the temporal responses of NEE and plant species richness to experimental warming and the removal of dominant plant species at low and high elevations in the Rocky Mountains of Colorado. Starting in 2013, we warmed using open-top chambers and altered species composition by removing the dominant species in a full factorial design at a high and low elevation site. Each summer we measured, plant diversity (species richness), and NEE in each of the plots.

Results/Conclusions

We found differences in NEE (µmol/m2/sec) and plant diversity between years. However, there was no consistent pattern of response due to experimental warming, dominant species removal, or elevation. At the high site, NEE decreased by the largest magnitude in control plots (90% from 2017 to 2018), and the smallest magnitude of change in the removal-only plots (74% from 2017 to 2018). At the low site, the difference in NEE between 2017 and 2018 was even more extreme. In 2017, the low site had greater carbon fluxes to the ecosystem than the atmosphere (negative NEE values), while in 2018 it had greater carbon fluxes to the atmosphere than the ecosystem (positive NEE values). Plant diversity was more consistent between years. At the high site, in 2017 and 2018, removal-only plots had the greatest plant diversity values. At the low site in 2018, removal-only plots had the highest diversity, in 2017, the removal-warming plots were the greatest. There could be a relationship between NEE and plant diversity, at the high control site, Simpson’s Diversity Index explained 29.9% (R2= 0.299) of variation in NEE. The differences in NEE and plant diversity between years might be attributed to yearly precipitation and temperature differences.