Effects of Photosynthetically Active Radiation on Vegetative Growth of Texas Wild Rice and Consequences for Population Augmentation

Abstract

As urbanization and urban sprawl increases, habitat for native flora and fauna often becomes threatened. Reestablishing wildlife habitats within designed landscapes has become increasingly popular with horticultural consumers, who are becoming more aware of the benefits of using native plants and the threats of invasive species. Texas wild rice (Zizania texana Hitchc.) is a federally endangered aquatic plant known to occur only in the San Marcos River, Hays County, TX. The objective of this study was to experimentally test the impact of light availability on the vegetative growth of Texas wild rice (TWR) ex situ. The effect of photosynthetically active radiation (PAR) was tested by establishing treatment and control groups of plants in a river raceway located on the campus of Texas State University, San Marcos, TX. At the onset of the experiment, baseline growth data were collected on a random sample of 15 plants to determine starting conditions. The 75 plants within the control and treatment groups were also randomly selected. Two sequential experimental trials were designed involving the same treatment of PAR reductions with 15 TWR plants in the control group (100% of ambient PAR conditions) and 15 within each of four experimental treatment units. Treatments included a reduction in ambient light values at each of the following rates: PAR reduced by 10% (90% ambient light), 20% (80% ambient light), 40% (60% ambient light), and 80% (20% ambient light). Results of the study indicated high shade areas contained reduced areal coverage or complete lack of TWR. There was a significant decrease in both above and below ground biomass, with an 80% reduction in available PAR (20% available ambient light), and other growth parameters of TWR were negatively impacted by reductions in PAR greater than 40% (60% ambient light availability) during the short-term early establishment growth period. Therefore, light availability is a critical environmental factor that must be given consideration when deciding areas of the river to plant TWR for population augmentation.