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The interactive effects of nutrients and fire on subtropical wet prairie ecosystems

PIs: Betsey Boughton and Patrick Bohlen

 

Rationale

The primary limits of productivity in grasslands are the availability of water, nutrients and light.  There have been many excellent long-term studies of how the major ecological drivers in grassland systems, such as fire and grazing, influence productivity by altering the relative availability of these different resources.  There have been few studies in subtropical grasslands to examine basic controls on productivity and nutrient cycling, inter-annual variability in productivity, and the interactive effects of fire and nutrients on productivity, community structure and soil processes.  This experiment, initiated in 2002, examines these interactions in a subtropical wet prairie at MAERC.  We are examining the effect of fire frequency, season of fire and nutrient addition on annual productivity, and species composition of dominant vegetation and over time will also examine the interactive effects of these treatments on belowground nutrient cycling processes and microbial functioning.  Season of fire is particularly applicable to these subtropical prairies because the management burns used by cattle ranches occur in the winter; whereas the "natural" fires in these systems occurred primarily in summer.  These systems are likely limited by the availability of N and we expect N additions to increase productivity and alter species composition of the dominant vegetation.  Phosphorus may also be limiting in these nutrient poor ecosystems and P addition is an additional component of this experiment.

 

Prescription Burn of a winter plot

Prescription burn of a winter burn plot that receives nitrogen + phosphorus fertilizer.

 

Experimental Design

The experiment is based on a completely randomized split-plot design in which fire is the main plot treatment and nutrient addition is the split-plot treatment.  The three fire treatments are: 1) annual winter fires 2) annual summer fires and  3) unburned control plots applied to 20 x 20 m plots.  Winter fires will set in January and summer fires in June of each year pending on burn conditions.  Unburned control plots may be burned at a longer interval (e.g. 5 years), which has yet to be determined.  The four nutrient treatments, applied to 10 x 10 m subplots are: 1) annual fertilization with N (50 kg/ha) 2) annual fertilization with P (20 kg/ha) 3) annual fertilization with N and P and 4) unfertilized control.  The nutrients are added to the plots in March of each year.  The entire experimental area is surrounded with a wire fence to keep out cattle.  To account for the effects of grazing, four set-aside plots with the winter burn treatment only have been set up just to the south of the fenced area to allow grazing in treatment plots.  These plots can be used for rigorous replicated comparison to ungrazed winter burned plots in the main experimental area.

 

A few interesting findings:

 

Current Work

Minimum annual sampling in the plots includes species composition.  Aboveground biomass is collected as needed (separated by live (by species), litter, standing dead). Additional measurements, such as soil nutrient cycling, will be made on a project-by-project basis, and the plots are available for student projects and cross site studies.  Meteorological conditions at the site are monitored by a nearby meteorological station. The plots were invaded by feral swine in 2011, providing an opportunity to investigate feral swine rooting impacts and plant community recovery.

 

Selected Recent Publications:

Boughton, E. H and R. K. Boughton. 2014. Environmental modification by feral swine shifts a diverse wet prairie to a monotypic stand. Biological Invasions.

 

Boughton, E.H. Bohlen, P., Steele, C. 2013. Season of fire and nutrient enrichment affect plant community dynamics in subtropical semi-natural grasslands released from agriculture Biological Conservation 158: 239-247.

 

For more information see poster presentations based on this research:

 

Fireplots poster

Grasslands poster

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