Annual root productivity and chemical composition responses to shrub encroachment and prescribed fire

Background/Question/Methods

Reduced fire frequency and climatic changes within tallgrass prairie have been attributed as the main factors contributing to increased shrub density.  A vegetative shift from a grass to shrub dominated ecosystem may change belowground productivity and soil carbon (C) storage.  Although 60 to 90% of net primary production occurs belowground, little research has focused on belowground productivity.  Fine root contributions from shrubs may contain higher lignin concentrations that could reduce root decomposition rates; however, belowground productivity of shrubs tends to be lower than grasses.  The objectives of my study were to determine fire and vegetative change effects on annual belowground productivity and soil C contributions in patches dominated by Indian grass and rough-leaf dogwood.  Soil and fine root samples were collected from Konza Prairie Biological Station (KPBS) and Ross Natural History Reservation (RNHR).  Eight grass and eight shrub dominated plots were sampled at each site.  Half the plots of each vegetation type were annually burned and the remaining plots were unburned.  Annual belowground productivity was measured using 2 mm mesh root in-growth bags (5 cm diameter, 20 cm depth), and lignin concentrations were determined using the acid detergent fiber by refluxing method.

Results/Conclusions

The results of annual belowground productivity indicate that prescribed fire stimulates fine root (< 1 mm diameter) production and total belowground net primary production (BNPP) in grass plots at the RNHR when compared to unburned patches.  Unburned shrub plots at KPBS had significantly more belowground productivity in fine root production (< 1 mm diameter) and total BNPP than burned shrub patches.  A reduction in belowground productivity in burned shrub patches could potentially be due to less aboveground biomass from long-term prescribed fire treatments, resulting in less overall long-term allocation to root productivity.  Cellulose and lignin concentrations did not differ between vegetation types or sites.  Since lignin concentrations did not differ between grass and shrub fine roots, fine root decomposition may occur at similar rates for both vegetative types, and therefore should not change soil organic C inputs from fine roots due to chemical composition.  However, soil C storage may change in response to a vegetative shift from a grass to shrub dominated ecosystem from changes in belowground fine root productivity because of the large contribution of roots to the soil C pool.