Data from: Seed size regulates plant dispersal distances in flowing water

Dispersal is an essential component of plant life, especially under the current threats of anthropogenic habitat fragmentation and climate change. For many wetland species, water is a key dispersal vector, as it can presumably disperse seeds long distances and towards suitable sites for establishment. Seed dispersal distance is affected by stream characteristics and seed traits. Yet, the effect of relevant seed traits, such as size, remains largely unknown. Here, we report on an experimental field study examining the effect of seed size on dispersal distance in lowland streams. We released cork seed mimics of different sizes in four Dutch lowland streams in restored and channelized sections. After 24 hours, we recorded their entrapment location, entrapment mechanism, and the vegetation type in which they were caught. Large seeds generally dispersed over longer distances than smaller seeds. This effect of seed size is likely caused by the different entrapment mechanisms – net trapping, surface tension and wake trapping – which were highly correlated with seed size. Especially net trapping was responsible for the capture of a large proportion of small seed mimics in vegetation such as aquatic and riparian grasses, starwort, and reed. Due to the prevalent occurrence of these vegetation types in lowland streams, particularly during summer, smaller seeds are more likely to become entrapped and, hence, disperse less far. Our analysis on existing seed data reveals that water-dispersed riparian plants have relatively large seeds and are thereby evolutionarily adapted to long-distance dispersal. Furthermore, our results indicate that median dispersal distances are 0.02-1.8 km (99-percentile <8.5 km) in lowland streams in summer. In winter, less vegetation is present in and surrounding the streams, which leads to median dispersal distances of 0.12-14.2 km (99-percentile <65 km). Synthesis: This study demonstrates that (1) large seeds generally disperse further than smaller seeds in lowland steams and (2) distances depend strongly on stream vegetation. This information should inform future restoration by, for instance, planning efforts to coincide with times or conditions of open water which are more favourable for the dispersal of target plant species – especially those with small seeds (<10 mm).

种子扩散（seed dispersal）是植物生活史中不可或缺的关键环节，在当前人为活动导致的栖息地破碎化与气候变化的多重威胁下，其重要性愈发凸显。对于诸多湿地植物而言，水流是核心的扩散媒介，理论上可将种子长距离运输至适宜的定植位点。种子扩散距离受溪流特征与种子性状共同影响，但诸如种子大小这类关键性状的具体作用仍有待探明。
本研究通过野外控制实验，探究了种子大小对低地溪流中植物种子扩散距离的影响。我们在荷兰四条低地溪流的修复段与渠化段中，投放了不同尺寸的软木种子模拟物（cork seed mimics）。24小时后，记录下模拟物的截留位置、截留机制以及其所附着的植被类型。
总体而言，大尺寸种子的扩散距离显著大于小尺寸种子。这种种子大小的效应可能源于三类与种子大小高度相关的截留机制：网格截留（net trapping）、表面张力截留（surface tension）与尾流截留（wake trapping）。其中网格截留尤其易捕获大量小尺寸种子模拟物，这类捕获多发生在水生与河岸草本、星花草（starwort）以及芦苇（reed）等植被中。由于这类植被在低地溪流中广泛分布，尤其在夏季，小尺寸种子更易被截留，因此扩散距离更短。
我们对现有种子数据的分析表明，以水流为扩散媒介的河岸植物种子尺寸普遍偏大，这是其在演化中适应长距离扩散的结果。此外，研究结果显示，夏季低地溪流中种子扩散的中位距离为0.02~1.8千米（99分位数<8.5千米）。冬季溪流及周边植被覆盖率较低，此时种子扩散的中位距离可达0.12~14.2千米（99分位数<65千米）。
综合分析表明：本研究证实了两点结论，其一，在低地溪流中，大尺寸种子的扩散距离普遍大于小尺寸种子；其二，扩散距离与溪流植被密切相关。该研究结果可为后续的湿地修复工作提供参考，例如规划修复时机与方案时，可选择开阔水域占比更高的时段或环境，以利于目标植物物种（尤其是种子尺寸小于10毫米的物种）的扩散。