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).种子扩散是植物生活史的核心环节之一，在当前人为生境破碎化与气候变化的多重威胁下，其重要性愈发凸显。对于多数湿地植物类群而言，水体是关键的种子扩散媒介：水流可将种子携带至远距离的适宜定植生境。种子扩散距离受溪流特征与种子性状的共同调控，但诸如种子尺寸这类关键性状的具体作用机制仍未被充分阐明。 本研究通过野外受控实验，探讨了种子尺寸对低地溪流（lowland streams）中植物种子扩散距离的影响。我们在荷兰境内4条低地溪流的修复段与渠化段（restored and channelized sections），投放不同尺寸的软木种子模拟物（cork seed mimics）。投放24小时后，记录各模拟物的滞留位置、滞留机制（entrapment mechanism）以及捕获模拟物的植被类型。 实验结果显示，大尺寸种子的平均扩散距离普遍长于小尺寸种子。种子尺寸的这一效应，大概率与三类不同的滞留机制相关：网捕（net trapping）、表面张力捕集（surface tension trapping）与尾流捕集（wake trapping），上述机制均与种子尺寸存在显著关联。 其中，网捕（net trapping）机制是多数小尺寸种子模拟物被滞留的主要原因，这类模拟物多滞留于水生、河岸草本植物、漆姑草（starwort）以及芦苇（reed）等植被群落中。由于这类植被在低地溪流中广泛分布，尤其在夏季，小尺寸种子更易被植被滞留，因此其扩散距离相对更短。 我们对现有种子数据集的分析显示，依赖水体扩散的河岸植物普遍拥有较大的种子，这是其对长距离扩散的适应性进化结果。 此外，实验结果表明，夏季低地溪流中植物种子的扩散中位距离为0.02~1.8千米（99分位数<8.5千米）。冬季时，溪流及周边区域的植被覆盖率更低，因此种子扩散中位距离可达0.12~14.2千米（99分位数<65千米）。 研究总结：本研究证实两项核心结论：其一，低地溪流中的大尺寸种子普遍比小尺寸种子扩散更远；其二，种子扩散距离与溪流植被紧密相关。该研究结果可为未来的生态修复工作提供科学参考，例如规划修复方案时可选择开阔水体占比更高的时段或环境，以更利于目标植物（尤其是种子尺寸<10毫米的物种）的有效扩散。