Warming & Grazing

Research: warming and grazing effects

Impacts of increased atmospheric temperature and livestock grazing on plant communities and soil processes
Researchers: Dr. Brenda Casper, Dr. Brent Helliker, Dr. Pierre Liancourt, Dr. Laura A. Spence, Dr. Alain Plante

The impacts of overgrazing on soils and plant communities are obvious in the Hövsgöl Basin. Soil humus, exchangeable Ca and NO₃ have declined in heavily grazed pastures. Grazers reduce plant biomass, cause soil compaction, and increase the surface area of bare soil, all of which elevate daytime soil temperatures. Heavy grazing also increases forbs abundance, apparently because grasses, sedges and mosses are selectively removed and because forbs respond more positively to increased soil temperatures. For these reasons, we hypothesize that atmospheric temperature increases associated with global climate change will affect heavily grazed pastures more strongly than lightly grazed pastures.

Our experiment examines how further increases in atmospheric temperature and livestock grazing will affect plant communities and soil processes. Passively heated greenhouses are being placed in heavily and lightly grazed areas in order to raise soil and air temperatures. The greenhouses are open topped chambers (OTC's) constructed of a fiberglass material that transmits visible but not infrared radiation. The 35 cm tall hexagonal chambers have sides angled at 60 degrees so that the top opening is smaller than the bottom, which measure 1.1 m across. Similar chambers have been used in the Arctic and alpine and depending on the design, OTC's raise air temperatures between 1.2 and 2.0°C. At several heavily and lightly grazed sites, we are installing warming OTC's and control OTC's, which transmit both visible and infrared radiation. Some chambers will be fenced to exclude large vertebrate grazers. The chambers will be left in place for four growing seasons and repaired as needed. Every 30 min. during the growing season, air temperature and humidity are measured in all OTC's about 13 cm above ground, and soil temperature is measured at 5 cm below the surface using Hobo® sensors and data loggers. In the summer of 2012, we will take destructive measurements to examine changes in plant communities and soil carbon. This design will enable us to examine the interaction of grazing and climate change.

At the beginning of the experiment and at the end of each growing season we estimate percentage cover of plant functional groups (e.g. grasses and sedges, forbs, and non-vascular plants). We also record the timing of new plant growth and flowering weekly. This effort builds upon the previous GEF plant studies.

Permafrost results in a large amount of persistent soil organic matter (SOM). It is expected that an increase in warming or growing season length will increase decomposition of SOM and rates of soil respiration, which can lead to further increases in atmospheric CO₂. Thus, we measure soil respiration monthly during the growing season in a closed gas exchange system. Each year, we measure the effect of elevated temperature on litter decomposition by placing a weighed amount of dead leaf material, representative of vegetation in the chamber, in nylon mesh bags attached to the soil surface and then reweighing at appropriate intervals. Similar decomposition studies were done as part of the GEF project, and our studies expand upon the GEF experiments.