Effects of Prescribed Fire Timing on Native Plant Composition, Forage Biomass Accumulation, and Root Carbohydrate Reserves in the Kansas Flint Hills: Year 2 of 6

Objective: The objective was to document the effects of prescribed fire timing on native plant composition, forage biomass, and root carbohydrate concentrations in key native tallgrass species in the Kansas Flint Hills. Study Description: This experiment was conducted at the Kansas State University Beef Stocker Unit. Eighteen pastures were grouped by watershed and assigned to one of three prescribed-burn treatments: spring (April 7 ± 2.1 days), summer (August 21 ± 5.7 days), or fall (October 2 ± 9.9 days). Botanical composition, forage biomass, and root carbohydrate concentrations in big bluestem, little bluestem, Indian grass, and purple prairie clover were evaluated annually. The Bottom Line: Data from the first two years of a six-year study were interpreted to indicate that basal cover of forage grasses and forbs, forage biomass accumulation, and root carbohydrate concentrations in key tallgrass species did not differ between prescribed-fire treatments.


Abstract
Sericea lespedeza (Lespedea cuneata) is a highly-invasive forb that has degraded more than 960 square miles of Kansas rangeland. Recent research has demonstrated that mid-summer or early-fall prescribed fire can achieve comprehensive control of sericea lespedeza; however, ranchers have voiced concerns that fire later in the year (i.e., August-October) may negatively impact native warm-season plant populations. In year two of a six-year study, 18 pastures were grouped by watershed and assigned to one of three burn treatments: early spring (April 7 ± 2.1 days), mid-summer (August 21 ± 5.7 days), or early fall (October 2 ± 9.9 days). All fire treatments were applied prior to grazing by yearling stocker calves. Soil cover, botanical composition, forage biomass, and root carbohydrate reserves were evaluated over a three-year period. Total grass and forb basal cover did not differ (P = 0.15) between treatments. In addition, no differences (P = 0.23) were observed in total cool-season grass cover or warm-season grass cover between fire regimes; however, native-grass species were greatest (P = 0.05) in the summer fire treatment, intermediate in the spring fire treatment, and least in the fall fire treatment. Forage biomass, root starch, and total water-soluble carbohydrate levels in three key warm-season forage grasses and one key native legume did not differ (P = 0.27) between treatments. We interpreted these data to suggest that prescribed fire timing caused small changes in range-plant composition but did not reduce forage biomass or root carbohydrate reserves of key native plant species.

Introduction
The Kansas Flint Hills represent the largest intact remnant of the original tallgrass prairie on earth. Traditionally, ranchers apply annual spring-season prescribed fire to these native rangelands to improve stocker cattle growth performance, increase warmseason grass production, and limit the encroachment of woody and invasive plant species. On average, 2.1 million acres throughout the Flint Hills are burned annually from mid-March to early May. Burning during this time of year presents certain challenges including low relative humidity, strong unpredictable winds, and elevated fuel loads. When combined, these factors increase the potential for uncontrolled fire. Another concern associated with spring prescribed fire is smoke management. Each spring, smoke produced from burning the Flint Hills travels to downwind municipalities, reduces air quality, and can negatively affect human health.
In addition, sericea lespedeza (Lespedea cuneata) likely proliferates with annual spring prescribed fire. Sericea lespedeza was introduced to Kansas in the 1930s and has since invaded more than 960 square miles of Kansas grasslands. Recent research demonstrated that shifting the timing of prescribed fire from spring to mid or late summer provided comprehensive control of sericea lespedeza. Although control of sericea lespedeza can be achieved through late-summer burning (i.e., August-September), ranchers have voiced concerns that native warm-season grass populations or forage biomass may be affected negatively. The objective of our experiment was to document the effects of prescribed-fire timing on plant composition, forage biomass accumulation, and root carbohydrate concentrations of key tallgrass plant species over a six-year period.

Experimental Procedures
Our experiment was conducted at the Kansas State University Beef Stocker Unit. The Beef Stocker Unit is comprised of approximately 1,100 acres of native tallgrass prairie and is fenced into 18 pastures. Pastures were grouped by watershed and each watershed was assigned to one of three prescribed-fire treatments (n = 6 pastures per treatment): spring (April 7 ± 2.1 days), summer (August 21 ± 5.7 days), or fall (October 2 ± 9.9 days). A single, permanent 328-ft transect was established in each pasture. Pre-treatment botanical composition, basal cover, standing forage biomass, and root carbohydrate concentrations were determined in June 2018 and re-evaluated after fire application in 2019 and 2020. Prescribed fire treatments were applied prior to grazing in 2019 and 2020.
Botanical composition and soil cover were evaluated along each permanent 328-ft transect using a modified step-point method. Standing forage biomass was determined by clipping the vegetation within ten 0.82-ft 2 frames randomly placed at 33-ft intervals along each transect. Plant material was clipped at a height of 0.39-in above the soil and dried in a forced-air oven (122ºF; 96 hours). Root-carbohydrate concentrations of three native C4 grasses (i.e., big bluestem, little bluestem, and Indiangrass), and one leguminous, native forb (i.e., purple prairie clover) were also evaluated. Individual roots and rhizomes were collected from each pasture, washed with tap water, dried in a forced-air oven (122ºF; 96 hours), and analyzed for both total starch and total water-soluble carbohydrate concentrations.

Results and Discussion
Following the second full cycle of prescribed fire application, bare soil cover was greater (P ≤ 0.01; Table 1) in the spring burn treatment compared with the summer and fall burn treatments. Conversely, litter cover on the soil surface was greater (P ≤ 0.01; Table 1) in pastures burned in the summer or fall compared with pastures burned in the spring. These trends can likely be attributed to the length of time since prescribed fire application. Soil cover was evaluated annually between late June and early July. As the time since fire application increased, bare soil cover was reduced while litter cover on the soil surface increased. In contrast, basal vegetation cover did not differ (P = 0.22; Table 1) between prescribed fire treatments.
When botanical composition was evaluated, no differences (P = 0.15; Table 2) in total grass cover, cool-season grass cover, or warm-season grass cover were detected; however, differences within warm-season grass growth forms were apparent. Warm-season perennial tallgrass cover tended to be greater (P = 0.07; Table 2) in pastures burned in the summer or fall compared with those pastures burned in the spring. Warm-season perennial mid-grass cover was greater (P = 0.05; Table 2) in the spring-burn treatment compared with the fall-burn treatment, whereas summer-burned pastures were intermediate. Spring-season prescribed fire increased (P = 0.01; Table 2) warm-season perennial short grass cover compared with summer or fall prescribed fire. In addition, native grass species cover was greatest (P = 0.05; Table 2) in the summer-fire treatment, least in the fall-fire treatment, and intermediate in the spring-fire treatment. No differences (P = 0.17; Table 2) were observed between treatments when total forb cover and native forb cover were evaluated; however, nectar-producing forb cover was greater (P = 0.02; Table 2) in fall-burned pastures compared to spring-and summer-burned pastures. Similarly, annual forb cover was greater (P = 0.03; Table 2) in the fall treatment compared to the spring treatment, whereas the summer treatment was intermediate. Shrub cover was minimal (i.e., ≤ 1.5%) and not different (P = 0.08) between treatments.
No differences (P = 0.91; Table 1) in forage biomass were observed between prescribedfire treatments. Furthermore, root starch and total water-soluble carbohydrate concentrations in big bluestem, little bluestem, Indiangrass, and purple prairie clover did not differ (P = 0.27; Table 3, Table 4) between the spring-, summer-, or fall-fire treatments. The lack of differences in forage biomass accumulation and root-carbohydrate concentrations were interpreted to suggest that prescribed fire timing may not affect the growth potential of key native tallgrass species.

Implications
We interpreted our data to suggest that prescribed fire timing is associated with small changes in range-plant composition; however, fire timing did not affect forage biomass accumulation or root carbohydrate reserves of key native tallgrass species. We will continue to evaluate these trends and modify our conclusions over the next five years. Within rows, means with unlike superscripts differ (P ≤ 0.05).