Purple threeawn (Aristida purpurea) is a native perennial grass classified as undesirable to cattle production because it is a low-quality forage. Fire is one of the management tools that has been used to improve grasslands invaded with purple threeawn. The objective of this study was to characterize the total nonstructural carbohydrates (TNC) dynamics of purple threeawn throughout the growing season and their response to clipping and fire treatments during three phenological stages. This research was conducted at the Texas Tech University Native Rangeland in Lubbock, TX. During the 2010 growing season, 90 purple threeawn plants were randomly selected regarding plant size and treated with clipping and burning. Treatments were applied to 30 plants during each phenological stage. Phenological stages were vegetative, reproductive and post-reproductive. In each stage ten plants were clipped, ten were burned and ten more were used as control plants. Clipping treatments simulated 90% utilization while the burning treatment consisted on prescribed fire to plants individually. Samples were collected from basal crowns 30 and 45 days after treatments. TNC concentration measurements were performed on basal crowns using the acid hydrolysis process, then TNC concentrations were measured spectrophotometrically at 612 nm. TNC concentrations of control plants showed a constant increase from the first sampled collection in mid-July to the last collection at the end of November. Sampling time, phenological stage and defoliations type interacted (P > 0.05) in their effects on TNC concentrations. TNC concentrations in the basal crowns were higher (P > 0.05) in plants treated during the post- reproductive stage among phenological stages, on control plants between defoliation treatments and in plants sampled 45 days after treatment applications. Our results showed that threeawn TNC concentration storage period extends as far as late November. Purple threeawn TNC concentration is dynamic across the growing season and it is easily affected by common defoliation events such as burning and clipping. The results of this study can be used to identify the best season to apply prescribed burning in order to control this species.
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