Plant Responses to the GCE-LTER Seawater Addition Long Term Experiment (SALTEx) from 2013 to 2016

This study was conducted in a tidal freshwater marsh (31°20’16” N, 81°27’52” W) on the Altamaha River, GA. Each plot was defined by a 2.5 m x 2.5 m polycon frame, inserted 12 cm into the soil. The frames had drain holes in the sides above the soil surface that were plugged when plots were watered but open otherwise to allow tidal exchange. The plots were randomly assigned to five treatments. Plots were monitored without imposing any treatments in 2013. Treatments started on April 14, 2014. Plots receive diluted saltwater (press and pulse of saline water treatments, n=6), freshwater (freshwater addition treatment, n=6), or no water additions (control with frame and control treatments, n=6 and 7, respectively). The press plots received additions of a mixture of seawater and fresh river water four times each week. Pulse plots received the same mixture but only for 8 weeks in September and October. The freshwater treatment plots received additions of fresh river water four times each week. To document treatment effects on the plant community, we measured stem height, photosynthesis, and percent cover of plants, and blocking of light by the vegetation. We used the same 0.75 m x 0.75 m subplot inside each plot for plant measurements, and took the measurements 3 – 6 times during each growing season from 2013 to 2016. We measured height of all stems of Zizaniopsis miliacea and Persicaria hydropiperoides, and all leaves of Pontederia cordata. In each plot, we chose the tallest individual of those three species and measured their photosynthetic rates using LCi photosynthesis System (ADC BioScientific Ltd, Hoddesdon, UK). Around the same dates, we also visually estimated the percent cover of Ludwigia repens, Persicaria hydropiperoides, Pontederia cordata, and Zizaniopsis miliacea in the entire 2.5 m x 2.5 m plot. We summed the stem heights by species as a first approximation of species biomass. We measured the photosynthetically active radiation (PAR) above top and underneath (10 cm above the soil) the plant canopy in each plot using a Sunscan Canopy Analysis System (AT Delta-T Devices Cambridge England) 4-6 times during each growing season from 2014 to 2016. We report the proportion of light penetrating the canopy.