Grassland ecosystems exhibit large annual variation in primary productivity. Our objectives in this study were to investigate the causes of annual variability in peak aboveground biomass production, net ecosystem productivity (NEP) and gross ecosystem productivity (GEP) during an eight-year period (1999-2006) in a northern Great Plains grassland near Lethbridge, Alberta, Canada. In particular, we were interested to test for a significant relationship between growing season precipitation and productivity and to determine whether soil moisture carry-over from the previous fall-winter could alter this relationship. We also investigated the interaction between moisture availability and temperature in controlling grassland productivity. Finally, we tested whether annual variation in grassland productivity was correlated with El Nino Southern Oscillation (ENSO) events because of associated variation in summer precipitation. The eddy covariance technique was used to measure NEP, and GEP values were calculated from NEP and estimates of total ecosystem respiration.
Results/Conclusions
There was a very strong correlation between total precipitation input and average soil moisture content during the May-October growing season. However, the growing season average soil moisture content in 2003 and 2006 were very similar to that recorded in 1999, despite the lower than normal precipitation occurring in these two years. This resulted from a positive difference between precipitation and evapo-transpiration that allowed significant soil moisture to be carried-over from the previous fall-winter during both 2003 and 2006. Strong logistic relationships were observed between soil moisture and annual productivity based on data from all years except 2003 and 2006, years which had higher productivity than was predicted from the logistic regression. Interaction between temperature and soil moisture explained this difference. Productivity values in 2003 and 2006 were high compared to 1999, a year with approximately the same soil moisture content, and this resulted from the higher average growing season temperatures that were apparent in 2003 and 2006. Analysis of weather records indicated that precipitation in the month of June was significantly higher during El Nino years than during La Nina years in Lethbridge. Aboveground biomass, NEP and GEP were generally higher in El Nino years and lower in La Nina years because of associated variation in summer precipitation.