A thirty-year assessment of the endangered aquatic macrophyte, Zizania texana, endemic to the upper reach of the San Marcos River in Central Texas, USA

Texas wild rice (Zizania texana Hitchc.) is an endemic, federally endangered aquatic macrophyte known only from the upper San Marcos River in central Texas. Annual surveys of Texas wild rice coverage have occurred for 30 years from 1989 to 2018, documenting the known coverage within its limited range. Texas wild rice exhibited a progressive increase in coverage from 1989 to 2018, but coverage increased following 2013 when plantings began. Based on an autoregressive model, the overall maximum coverage growth rate of Texas wild rice was estimated to be r(max) = 0.152 indicating a 15% increase in Texas wild coverage annually if discharge is maintained at an optimum of 3.99 m(3) s(-1). Significant (P < 0.05) increases in Texas wild rice coverage occurred in six of the seven upper segments of the river, while significant (P < 0.05) decreases occurred in five of the seven lower segments. The maximum discharge recorded during the survey was 175 m(3) s(-1) in 1998, which resulted in a 15.6% decrease in coverage during the 1999 survey, but Texas wild rice coverage increased during 2000. This endangered macmphyte exhibits resilience and resistance to high discharge events due to its fast growth rates, reproductive plasticity, high root-to-shoot ratios, and a perennial life cycle that contributes to its fast recovery following scouring. Decreased spring flows and associated declining water levels due to increased groundwater water extraction represents the greatest threat to the only known population of Texas wild rice as exposed plants become desiccated and suffer high mortality if the upper reach of the river becomes ephemeral.