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.本研究于佐治亚州阿尔塔马哈河沿岸的潮汐淡水沼泽（31°20’16” N，81°27’52” W）开展。每个样地以2.5 m×2.5 m的聚圆锥框架（polycon frame）划定，框架插入土壤12 cm深度。框架在土壤表面以上的侧壁开设排水孔，样地浇水时封堵该排水孔，其余时段保持敞开以保障潮汐水体交换。所有样地被随机分配至5组处理。2013年未施加任何处理，仅开展本底监测。处理实验于2014年4月14日正式启动。样地分别接受三类处理：稀释盐水组（包含压式与脉冲式盐水处理，重复数n=6）、淡水添加组（n=6），以及无水处理对照组（含框架对照组与空白对照组，重复数分别为n=6和n=7）。压式处理组每周四次施加海水与新鲜河水的混合液；脉冲式处理组同样施加该混合液，但仅在9月至10月的8周周期内开展；淡水添加组每周四次施加新鲜河水。为量化处理对植物群落的影响，我们测定了植物株高、光合速率、植被盖度及植被对光照的截留能力。每个样地内均设置固定的0.75 m×0.75 m小样方用于植物相关测定，2013年至2016年每个生长季开展3~6次测定。我们测定了小米状野稷（Zizaniopsis miliacea）与沼生蓼（Persicaria hydropiperoides）所有茎秆的高度，以及梭鱼草（Pontederia cordata）所有叶片的高度。在每个样地中，我们选取上述三个物种的最高个体，使用LCi光合测定系统（LCi photosynthesis System，ADC BioScientific有限公司，英国霍德德斯敦）测定其光合速率。在相近测定时段，我们还对整个2.5 m×2.5 m样地内的匍匐丁香蓼（Ludwigia repens）、沼生蓼（Persicaria hydropiperoides）、梭鱼草（Pontederia cordata）与小米状野稷（Zizaniopsis miliacea）的盖度进行目视估测。我们以各物种的总株高作为该物种生物量的初步估算值。2014年至2016年每个生长季，我们使用Sunscan冠层分析系统（Sunscan Canopy Analysis System，Delta-T Devices有限公司，英国剑桥），在每个样地的植被冠层顶部及冠层下方（距土壤10 cm处）测定光合有效辐射（Photosynthetically Active Radiation, PAR），每个生长季开展4~6次测定，并报告穿透冠层的光照比例。