Biomass and nitrogan dynamics of a herbicide converted sand shinnery oak community

Seasonal nitrogen and biomass dynamics were documented for a native sand shinnery oak community and areas treated with tebuthiuron three years and six years prior to sampling. Herbicidal treatment resulted in oak death and a change in plant community structure. Biomass, percent nitrogen, and organic carbon were measured for above-ground and below-ground compartments at three dates during the year which represented growth initiation, peak production, and winter dormancy.

Total biomass was not significantly changed by treatment. Above-ground biomass decreased significantly over time from the control to the 3-year and 6-year treatments. The absence of above-ground oak material and the significant decrease in litter on the two treated sites accounted for the change. Above-ground herbaceous material, however, increased 6 fold from the control to the two treated sites. Below-ground biomass was similar for all treatments and was only slightly lower on the 6-year treatment. Oak root biomass was reduced 1%, three years following oak death and 23% after six years. Significant (2- to 3-fold) increases in herbaceous roots on the treated sites offset the oak root decrease and stabilized the weights for below-ground biomass.

Available soil nitrogen increased from the control to the 3-year treatment and to the 6-year treatment. Soil nitrogen was significantly higher in the surface 15 cm and in the top of the B2t argillic subhorizon than in the two intermediate depths. Differences were primarily related to concentration of soil organic matter which increased as a result of oak root and litter decomposition.