Management and ecology of willow baccharis in the Texas Rolling Plains

Willow baccharis (Baccharis salicina T&G) has recently become a brush problem in the Texas Rolling Plains. Willow baccharis was historically confined to river banks and creek channels in the Rolling Plains, but the initiation of the Conservation Reserve Program provided an opportunity for it to expand to upland sites. Willow baccharis is a perennial shrub that can reach heights of 4 m, and has a life span of usually less than 15 years. It is a hardy plant often tolerating saline soils and exhibiting strong resprouting characteristics. It is a prolific seed producer with an affinity for disturbed sites, which makes it an aggressive invader that usually dominates areas with a previous mechanical disturbance or extended overgrazing. Willow baccharis will spread by adventious buds located on lateral roots, which often creates a tight colony resulting in a closed canopy.

Studies were conducted near Post, Texas to evaluate the ecology of willow baccharis, total nonstructural carbohydrate trends (TNC) of willow baccharis, and willow baccharis response to applications of glyphosate and 2,4-D. Ecology focused on the phenology of trees, flower and seed production, and seed germination rates. These studies were conducted from January 1996 through October 1998. Roots were collected from January 1996 through August 1997. The TNC concentrations were correlated with phenology as visual references for the TNC trends. Trees treated in summer of 1997 with glyphosate and 2,4-D, were evaluated at the end of the 1998 growing season. The objectives of this study were to: (1) determine the phenology of willow baccharis growing in the Texas Rolling Plains, (2) determine the seasonal trends in total nonstructural carbohydrate concentrations for willow baccharis in the Texas Rolling Plains, (3) evaluate the efficacy of glyphosate and 2,4-D applied at three concentrations (1,2, and 5%) and five phenological stages on willow baccharis mortality, and (4) develop economical, practical, and sound recommendations for willow baccharis management.

Observations of willow baccharis phenology showed nine major stages: leaf emergence, full leaf, bud initiation, flower opening, full flower, seed maturing, seed mature, seed dispersal, and plant dormancy. Flower production ranged from 1430 to 94540 flowers per plant with a mean of 15200 ± 3650 in females and 820 to 15760 flowers with a mean of 6050 ± 960 in males. Seed production ranged from 23700 to 2640930 seeds with a mean of 383700 ± 103570. An estimated 23.3 ± 4.8 seeds per flower was determined from all the flowers counted. Seed germination was 18.9 ± 0.53%. This information is important in implementing management strategies, and in assessing the potential for willow baccharis to become a serious problem in the Texas Rolling Plains.

The TNC concentrations were correlated to seven phenological stages of willow baccharis to show annual trends. Results indicated that TNC decreased from full leaf to bud initiation, and increased to flower opening. The TNC decreased to full flower, and then stabilized until the seeds were nearly mature. Finally, the TNC increased through seed maturity and dispersal until plant dormancy. The management implications are to use chemical control methods when the trend is increasing and mechanical methods when the TNC concentration is lowest.

Willow baccharis trees were treated with either glyphosate or 2,4-D at 1%, 2%, and 5% concentrations during the full leaf, bud initiation, flower opening, full flower, and mature seed phenological stages. Tree mortality was evaluated 12 months after the final treatment. Mortality expressed as a percentage, was the difference between pre-treatment and post-treatment stem counts. Analysis of variance indicated all treatments were significantly different from the control set (oc=0.05). There were no differences between treatments, with total stem mortality occurring in 298 of 300 treated trees. Foliar applications of 1% glyphosate or 2,4-D between the full leaf and seed dispersal stages is the recommended treatment.

The following thesis is arranged in four chapters as follows: Chapter I is a review of the literature. Chapter II discusses the phenology, flower and seed production, and seed germination rates for willow baccharis. Chapter III pertains to the evaluation of seasonal trend in TNC concentration for willow baccharis. Chapter IV describes the effectiveness of glyphosate and 2,4-D treatments on willow baccharis management and provides recommendations for management. Chapters II, III, and IV are written in format for publication as refereed journal articles. The following appendices provide analysis of variance tables, statistical analysis, data and forms used to record data, as well as the procedure for determining TNC.