Wildfire-Grazing Impact on Forage Quality Assessed with Near-Infrared Spectroscopy and Generalized Partial Least Squares Regression

Abstract

Fire and grazing are key drivers of plant community structure in the Great Plains ecoregion, with critical impacts on the quantity and quality of forage. While the impact of fire on forage quality is well understood in many ecosystems, few studies have examined this question in the northern mixed-grass prairie. We investigated the postburn nutrient content of forbs and graminoids over a 4-yr timeframe, asking 1) how forage quality changed over the 4 yr post fire and 2) whether the effect of fire on quality was impacted by the resumption of grazing. We used near-infrared spectroscopy and generalized partial least squares regression (plsRglm) methods to assess crude protein. We show that the use of generalized models provides improved estimates of percent nitrogen than the standard partial least squares approach. Both crude protein and fiber varied annually after the wildfires, with no effect of grazing on forage quality. In the first and second years, burned sites displayed higher protein than unburned locations. Fire did not affect crude protein between plant functional groups; however, forbs showed higher nutrient values than grasses throughout the 4 yr. Fire affected fiber content with grasses showing higher fiber than forbs. There was a positive relationship between neutral and acid detergent fiber and litter in burned sites. These results indicate no long-term effect of wildfire on protein content, which stabilizes in the third year post burn. The improvement of forb digestibility and retention of protein under dry conditions could modify the grazing capacity of the prairies at large scales. This may also change the foraging behavior of large ungulate grazers through mechanisms such as increasing patch reselection (i.e., intensified grazing cycles) and broadening feeding preferences to forbs when grasses are limited or have high fiber content.