Effects of temperature, precipitation regime, and clipping on root and shoot litter decomposition in three Northern grasslands

Background/Question/Methods

Globally, grasslands comprise approximately 40% of terrestrial land mass and store more carbon (C) than forest ecosystems. In grasslands, C storage is primarily belowground, relatively protected from fire, and thus an important aspect of global C sequestration. Though there are numerous studies investigating climate-mediated effects on litter decomposition, surprisingly few include in-situ root samples.  Further, nearly nothing is known about climate-mediated decomposition at the northern edge of the Great Plains – an area projected to be strongly impacted in future climate scenarios. Grasslands in North America are also typically grazed by native and domestic animals, causing potential shifts in litter quality and microclimatic conditions. Thus, here we conducted a factorial experiment testing the interactive effects of increased temperature, reduced precipitation, and varied levels of clipping on root and shoot decomposition in three Northern grasslands. We used the litterbag method to study decomposition rates over a 2-year period in Alberta, Saskatchewan, and Manitoba; the three Prairie Provinces in Canada. We established plots in which we manipulated precipitation regime using rain-out shelters (2 levels=ambient, drought), manipulated temperature using open-top chambers (OTCs) (2 levels=warming, control), and manually clipped vegetation to simulate grazing (3 levels=none, low, high). Air and soil temperature was measured in two replicates of each treatment combination at each site every 30 minutes using data loggers.

Results/Conclusions

Across all sites, clipping intensity had no effect on decomposition for either roots or shoots. This suggests that in these systems, clipping intensity itself may not be a major contributor to litter turnover. In contrast, we found strong effects of precipitation on both root and shoot litter decomposition; whereby drought decreased litter mass loss across all sites. Increased temperature decreased shoot, but had no effect on root litter decomposition. Additionally, decomposition varied as function of site, and % mass loss was higher for root than shoot litter. In 2009, the average growing season (May – August) daily daytime air temperature increased by 1.5 ⁰C on average in warmed plots than in control plots compared to 0.5 ⁰C increase for soil temperature across sites. Reduced precipitation decreased soil moisture by an average of 6.4 %VWC across sites. These findings suggest that the in the short-term, temperature and precipitation may have direct consequence on carbon storage in these grassland systems, and shoot and root decomposition may impact the function of these systems differently.