Increasing productivity through time in an Illinois tallgrass prairie restoration

Background/Question/Methods

College class projects offer an ideal opportunity to monitor long term ecological processes associated with plant community development.   Studies documenting compositional change are quite abundant, but efforts focused on ecosystem properties are much rarer.  Starting in 2002, ecology students at Southern Illinois University Edwardsville have sampled peak above-ground biomass of a ca. 20 year-old  tallgrass prairie restoration on Campus, ca. 30 km northeast of St. Louis, MO.  The original focus of the exercise was to evaluate the effect of a steep slope gradient (10%) on peak biomass production, but the fairly long time series also permits comparison of productivity through time.  The decade 2002-2011 is particularly notable for several years of low rainfall and high temperature.  Sampling typically occurred during October, employing 22-46, 0.25 m² quadrats distributed over a 100-130 m downslope transect.  Vegetation was cut as close to the ground as possible, dried thoroughly, and weighed to the nearest 0.5 g.  Slope positions were determined using GPS or estimations from maps. 

Results/Conclusions

Peak biomass production has varied widely through time, ranging 6460 to 18,700 kg / ha.  The lower value is far more typical of Midwestern tallgrass prairies.  Despite fluctuating dormant season rainfall accumulation, standing crop has increased fairly smoothly over time (r = 0.82, p < 0.004).  Standing crop is strongly correlated with increasing average June-August temperatures (r = 0.64, p < 0.05).  Cumulative rainfall calculated for the immediate growing season (June-August), October-December of the preceding year, the dormant season October-March , and January- March of the current year were not correlated with standing crop.  Standing crop decreased with higher elevation, consistent with the existence of a moisture gradient (partial r = -0.34 p < 0.0001 after correcting for temperature).  The high proportion of C4 grasses in the prairie (ca. 90%) is concordant with increasing productivity in response to increasing summer temperature through the decade.  In addition, fertilization occurring during a slow process of colonization by the N-fixing legume Robinia pseudoacacia may have played a role.