Carbon Sequestration and Storage in Mountain Meadow Ecosystems and Implications for Climate Resiliency

Healthy mountain meadows have been shown to provide a number of important ecosystem services, including their ability to sequester large amounts of atmospheric CO₂ on an annual basis and store it in a semi-stable form in meadow soils. More than a century of anthropogenic activities have greatly impacted the hydrological and biological functioning of mountain meadows in the Sierra Nevada and elsewhere. In the past few decades, meadow restoration projects have been designed to reverse this decline through various hydrological interventions. Observations of ecosystem CO₂ exchange are compared from five growing seasons in three mountain meadows in the northern Sierra Nevada, California, ranging in elevation from 1,400 to 1,900 masl. We have found that wet mountain meadows can be strong net carbon sinks on an annual basis and can sequester impressive daily magnitudes of carbon during the peak of the growing season. They showed a strong seasonal pattern with weak CO₂ losses during the winter, including under snowpack, followed by a rapid transition to growth onset as soil temperatures climbed above 0 °C in late March - April. Net CO₂ uptake rates peaked in June/July, followed by a long senescence phase, triggered by declining soil water levels, switching from a net sink to source around August. The duration of peak uptake rates was governed by the retention of high soil moisture levels into the dry summer. Subtle shifts in the timing of growth onset, peak uptake, and speed of senescence produced significant differences in the annual sequestration rate. These findings suggest that future sequestration rates of mountain meadows will be sensitive to both changes in winter snowpack and spring melt rates as well as further degradation or restoration of meadow hydrology.