Vertical Brush Sculpting for Heterogeneity in Mesquite Savanna: Implementation and Effect on Grass Production

Managing woody species that have encroached into grasslands and can resprout following top-kill has become an increasing problem. Treatments that yield complete mortality (i.e., root-kill) such as mechanical grubbing are often too costly at landscape scale. Less expensive treatments that only top-kill are problematic if treatments are not applied frequently to maintain regrowth suppression. More sustainable strategies are needed. Previous research found that a low rate (0.28 kg.ha(-1)) of clopyralid-only herbicide root-killed some honey mesquite (Prosopis glandulosa) but importantly caused partial top-kill (PTK) in surviving trees that maintained apical dominance and prevented basal sprouting. This management approach, referred to as vertical brush sculpting, increased herbaceous productivity compared with top-killed mesquite with dense resprouting. Here we determined if higher rates of clopyralid could increase mesquite root-kill and maintain the PTK physiognomy in surviving plants. We evaluated effects of clopyralid at 0.28, 0.42, and 0.56 kg.ha(-1) (C28, C42, and C56, respectively) and a mixture of clopyralid (0.28 kg.ha(-1)) + triclopyr (0.28 kg.ha(-1)) (C + T) on mesquite physiognomy. Averaged over two sites at 6 yr post treatment (YPT), root-kill was 0%, 20.0%, 42.8%, 58.9%, and 65.6% in untreated, C28, C42, C56, and C + T, respectively. Among surviving trees, percent PTK with no basal sprouts was greatest in clopyralid-only treatments; percent top-killed with basal sprouting was greatest in C + T. Grass production, measured in untreated, C28, and C + T, ranged from 110% to 215% greater in treated than untreated at 2 YPT. We developed a model to project stand-level mesquite competitive impact on grasses in all treatments over 30 yr. Mesquite competitive impact values were lowest in C42 and C56 due to high rootkill, and most survivors had noncompetitive PTK physiognomy. We viewed C42 as the best option due to lower cost. Vertical brush sculpting offers a method to reduce woody competition with grasses yet maintain landscape heterogeneity. (c) 2023 The Authors. Published by Elsevier nc. on behalf of The Society for Range Management.

Automated summary

Managing woody species that encroach into grasslands and can resprout following top-kill is a growing problem. Costly treatments that yield complete mortality are not feasible at a large scale, while less expensive top-kill treatments need frequent application to maintain regrowth suppression. A low rate of clopyralid-only herbicide root-killed some honey mesquite but caused partial top-kill, leading to increased herbaceous productivity. Higher rates of clopyralid increased mesquite root-kill and maintained partial top-kill in surviving plants. The best option was clopyralid at 0.42 kg.ha(-1) due to lower cost, offering a method to reduce woody competition with grasses while maintaining landscape heterogeneity.