Duration of a flood along a plant diversity gradient in the Jena Experiment (Main Experiment, June 2013)

The Jena Biodiversity Experiment is located on a Central European mesophilic floodplain on the banks of the Saale River (see further details below). In the main experiment, 82 grassland plots of 20 x 20 m were established from a pool of 60 species belonging to four functional groups (grasses, legumes, tall and small herbs). In May 2002, varying numbers of plant species from this species pool were sown in the plots to create a gradient of plant species richness (1, 2, 4, 8, 16 and 60 species) and functional richness (1, 2, 3, or 4 functional groups). Plots were maintained by bi-annual weeding and mowing.
In June 2013, a natural 200-year flood event occurred at the field site. Rainfall in May 2013 in Jena was ~150mm, constituting >25% of annual precipitation at the site that year. Overall the flood affected the entire Elbe River Basin and much of Europe and was one of the largest natural flooding events in the past two centuries. The flood lasted for a total of 24 days at the site (30 May-24 June) and led to anaerobic soil conditions. Due to small topographical differences among the plots in the experiment (<1m), there was variation in the duration of flooding and the proportion of each plot that was flooded. This variation was well-distributed across the diversity gradient. To assess the importance of flood severity, the proportion of each plot that was flooded was estimated by eye (using five classes: 0 completely dry, 0.25 up to a quarter under water, 0.5 half, 0.75 up to three quarters under water, and 1 more than three quarters under water up to completely submerged). These values, for each of the 24 days that the flood lasted, were summed up to calculate a flooding index. The resulting flooding index is given for each plot of the Main Experiment.

耶拿生物多样性实验（Jena Biodiversity Experiment）选址于中欧温带洪泛平原，紧邻萨勒河（Saale River）岸畔（详细背景详见下文）。主实验中，研究团队从涵盖4个功能群（functional groups）的60个物种种库中，布设了82块20m×20m的草原样地，这4个功能群分别为禾本科植物、豆科植物、高大草本与小型草本。2002年5月，研究人员于各块样地中播种该物种库内不同数量的植物物种，以此构建植物物种丰富度（species richness）与功能群丰富度（functional richness）的梯度系列：物种丰富度梯度涵盖1、2、4、8、16及60个物种，功能群丰富度梯度涵盖1、2、3及4个功能群。样地通过每年两次的除草与刈割进行维护。

2013年6月，该实验场地发生了一场200年一遇的自然洪灾。2013年5月耶拿地区的降雨量约为150毫米，占该观测点当年年降水量的25%以上。此次洪灾整体波及易北河流域（Elbe River Basin）及欧洲大部分区域，是近两个世纪以来规模最大的自然洪灾事件之一。该场地的洪灾持续时长总计24天（5月30日至6月24日），并导致土壤处于厌氧状态。由于实验样地间存在微小的地形差异（落差小于1米），各块样地的淹水时长与淹水占比均存在差异，且该差异在多样性梯度上分布均匀。为评估洪灾严重程度的影响，研究人员通过目视估算各块样地的淹水占比，共分为5个等级：0代表完全干燥，0.25表示淹水面积不超过四分之一，0.5表示淹水面积达一半，0.75表示淹水面积不超过四分之三，1表示淹水面积超过四分之三直至完全浸没。将洪灾持续的24天内每日的估算值累加，即可计算得到淹水指数（flooding index）。主实验的所有样地均配有对应的淹水指数。