Effect of Herbicides on Evapotranspiration of Willow Marshes in the Upper St. Johns River Basin, Florida

During the last 40years, Carolina willow (Salix caroliniana Michx.) expanded into areas previously dominated by herbaceous marshes in the Upper St. Johns River basin (USJRB) of east-central Florida, United States. This change in vegetation affects evapotranspiration (ET). To quantify changes in ET at the community level after willows were removed using herbicides, a two-year field experiment with a randomized complete block design was implemented. The design included an unsprayed control and two different aerially sprayed herbicide treatments and was replicated in four blocks along the Upper St. Johns River. Daily ET was estimated using the Penmen-Monteith equation from July 1, 2014, to August 31, 2016. Cumulative ET difference between the control and treated plots increased substantially during the growing season after herbicide application. Mean annual evapotranspiration was 1,368 +/- 51mmyear-1 on control plots, 1,096 +/- 137mmyear-1 on plots treated with Aquasweep or Ecomazapyr herbicide, and 968 +/- 117mmyear-1 on plots treated by Clearcast herbicide. A single-parameter annual ET model derived from a Budyko-type equation was applied to the study area, and the model parameter (E) strongly correlated with willow fractional coverage in April. The empirical equation obtained from this study can be potentially used for evaluating the impacts of willow treatment and climate on long-term evapotranspiration in the study area.