Climate change and grazing interact to affect flower production in the Mongolian steppe

Background/Question/Methods

Societal and economic changes threaten traditional practices of nomadic pastoralism worldwide, sometimes altering grazing patterns that have been in place for millennia.  Climate change is occurring simultaneously.  While both are likely to affect ecosystem functioning, interactions between the two are poorly studied.  In Mongolia, a rise in mining, industry and urbanization is causing human migration from the countryside and altering grazing pressures by domestic herds.  In an experiment set in mountain steppe of northern Mongolia, we manipulated climate using passive open top warming chambers (OTCs) together with grazing (ambient levels versus cessation of grazing) to understand combined effects on numbers of flowers produced.  For two years, we measured soil moisture and temperature in the different treatment combinations and conducted weekly censuses to determine the number of species flowering and the number of flowers per species. We also used canonical analysis of principal components (CAP) to describe how the flower community composition at peak flowering responded to experimental treatments.

Results/Conclusions

Cessation of grazing, which normally occurs in autumn and winter, caused accumulation of plant litter and reduced soil temperature, while OTCs increased air temperature and reduced soil moisture.  We found for forbs, but not for graminoids, that grazing and OTCs interacted to affect the number of species flowering; cessation of grazing decreased the number of species flowering regardless of climate treatment while OTCs decreased the number of species flowering on grazed plots only.  At the community level, the cessation of grazing increased the average number of flowers produced at peak among graminoid species in one year, but otherwise grazing cessation and warming had no effect on overall community composition.  Of the four abundant graminoid species examined individually, three showed increased flower number with grazing removal, while one showed the reverse.  Four abundant forb species showed no significant response to either treatment.  Our results highlight that climate change impacts in the mountain steppe will be contingent on land use.  Studies designed to understand ecosystem responses to climate change should incorporate co-occurring drivers of change, such as altered grazing regimes.