Data_Sheet_2_An Indirect Impact of Sika Deer Overpopulation on Eutrophication of an Aquatic Ecosystem via Understory Vegetation: An Individual-Based Approach Using Nitrate Reductase Activity.PDF

Eutrophication of aquatic ecosystems is a serious global issue. Stream nitrate concentrations at the University of Tokyo Chichibu Forest have increased since 2000 after the opening of the new highway in 1998. Nitrogen oxide emissions from automobile exhausts were the most likely source of increased nitrate input in the forest ecosystem. Around the area, the sika deer Cervus nippon Temminck population has greatly increased since around 2000 and intensively browsed the understory vegetation. We hypothesized that the degradation of the understory vegetation caused by the deer overpopulation was one of the causes of increased nitrate output. Monthly observations were carried out from April to October 2013 to investigate the understory vegetation at the heights of 0–30 and 100–150 cm above the ground inside (without deer) and outside (with deer) of a deer exclusion fence. Plant taxa and % coverage of each taxon at each layer were recorded. The in vivo nitrate reductase activity (NRA) (≈ nitrate assimilation rate) was determined for each plant taxa each month. Compared to inside the fence, the understory vegetation outside was poor with smaller % coverage and less diverse community structure, and was occupied by unpalatable plant taxa that were uncommon or absent inside the fence. Contrary to our expectation, the phylogenetic diversity of the community assemblage outside the fence showed greater evenness (less clustering) than inside. The NRA peaked in early in the season or late in the season. In contrast to a previous report, no significant difference in the NRA was found between woody and herbaceous plants. Although the difference was no more than that of vegetation coverage, the estimated community-level NRA inside the fence was 5.6 times higher than that of the outside. The difference was greatest early in the season. These results support our hypothesis.

水生生态系统富营养化是一项严峻的全球性环境问题。1998年新高速公路开通后，东京大学秩父森林的溪流硝酸盐浓度自2000年起持续上升。汽车尾气排放的氮氧化物，最有可能是该森林生态系统中硝酸盐输入量增加的来源。该区域周边的梅花鹿（Cervus nippon Temminck）种群自2000年左右起大幅增长，并对林下植被造成高强度啃食。我们提出假说：鹿种群过度增殖引发的林下植被退化，是硝酸盐输出量上升的诱因之一。2013年4月至10月期间，我们开展了月度观测，对防鹿围栏内部（无鹿）与外部（有鹿）样地中，地面0–30 cm和100–150 cm高度层的林下植被进行调查，记录各高度层的植物类群及其盖度百分比。每月测定各植物类群的活体硝酸还原酶活性（NRA，约等于硝酸盐同化速率）。与围栏内部样地相比，外部样地的林下植被状况更差，盖度更低、群落结构多样性更低，且被围栏内部罕见或不存在的非适口性植物类群占据。与我们的预期相反，围栏外部群落组合的系统发育多样性均匀度更高（系统发育聚集度更低）。NRA在季节早期或晚期达到峰值。与既往研究报道不同，木本植物与草本植物的NRA并无显著差异。尽管该差异幅度不及植被盖度的差异，但围栏内部估算得到的群落水平NRA是外部的5.6倍，且该差异在季节早期最为显著。上述结果支持了我们提出的假说。