Stream water quality in relation to watershed-scale practical forest management in a cool-temperate natural forest in northern Japan

The water chemistry of a stream reflects the biogeochemical processes occurring in upstream forests. Anthropogenic disturbances in forests, such as cutting of trees, altering the nitrogen (N) cycle, and increase in N leaching from the soil to streams, potentially cause acidification or eutrophication downstream. In forests with dense understory vegetation, mechanical site preparation following tree cutting is commonly used to improve the early establishment of tree seedlings. In cool-temperate forests in northern Hokkaido, Japan, dense understory vegetation (mainly comprising Sasa dwarf bamboo) inhibits forest regeneration after tree cutting. Soil scarification is a common site preparation technique for eliminating Sasa bamboo and improving forest regeneration. Long-term data are useful for examining the temporal changes in stream water chemistry exposed to different specific forest management practices under changing environment (e.g., climate change and atmospheric N deposition). For 14 years (2003–2016), we observed the stream water chemistry in naturally forested watersheds and at one point after the confluence of all streams in Uryu Experimental Forest of Hokkaido University (North Hokkaido Experimental Forests Site of JaLTER) in northern Japan. We also monitored stream discharge, water level, and stream water temperature in each watershed. Water samples were collected from the outlets of ten watersheds. The forest management practice in each watershed includes clear-cutting, soil scarification in sparse forest with dense understory Sasa, and clear-cutting and soil scarification followed by soil replacement. Long-term data in the six unmanaged watersheds are also valuable as a background information to analyze the effect of long-term climate, environment, and vegetation changes on stream water chemistry. The measured water quality data of 1,873 water samples include the ion concentrations (Cl-, NO3-, SO42-, Na+, NH4+, K+, Mg2+, and Ca2+), pH, and electrical conductivity (EC) in the stream water. The range of the concentrations of Cl-, NO3-, SO42-, Mg2+, and Ca2+ in the stream water across all the watersheds throughout the observed periods (minimum to maximum) were 3.35 to 23.67, 0.01 to 8.68, 0.83 to 4.01, 0.45 to 2.55, and 0.72 to 6.16 mg L-1, respectively. Similarly, the stream pH and EC ranged from 6.04 to 7.53 and 3.14 to 9.47 mS m-1, respectively.

溪流的水化学特征可反映上游森林内发生的生物地球化学过程。森林中的人为干扰活动，如林木采伐、改变氮（N）循环，以及氮从土壤向溪流的淋溶量增加，均可能在下游引发水体酸化或富营养化。在林下植被茂密的森林中，林木采伐后通常会采用机械整地措施，以提升林木幼苗的早期定植成功率。在日本北海道北部的冷温带森林中，以矮竹（Sasa）为主的茂密林下植被会抑制采伐后的森林更新，而土壤松耙（soil scarification）是一种常用的整地技术，可用于去除矮竹并改善森林更新效果。

长期监测数据有助于探究在气候变化、大气氮沉降等环境变化背景下，受不同森林经营措施影响的溪流水化学的时间演变特征。本研究于2003—2016年的14年间，在日本北海道北部的北海道大学雨龙实验林场（JaLTER北部北海道实验林场站点）的天然林集水区，以及所有溪流汇合后的一处点位开展了溪流水化学监测。同时，我们还对每个集水区的溪流流量、水位及水温进行了同步监测。

研究人员从10个集水区的出水口采集了水样。各集水区采用的森林经营措施包括：皆伐、在林下矮竹茂密的稀疏林分中实施土壤松耙，以及皆伐结合土壤松耙后再进行土壤置换。6个未受人为干扰的集水区的长期监测数据，可作为背景参考信息，用于分析长期气候变化、环境变化及植被变化对溪流水化学的影响。

本次测得的1873个水样的水质数据涵盖了溪流水体中的离子浓度（Cl⁻、NO₃⁻、SO₄²⁻、Na⁺、NH₄⁺、K⁺、Mg²⁺及Ca²⁺）、pH值及电导率（EC）。在整个监测周期内，所有集水区溪流水体中Cl⁻、NO₃⁻、SO₄²⁻、Mg²⁺及Ca²⁺的浓度范围分别为3.35~23.67 mg·L⁻¹、0.01~8.68 mg·L⁻¹、0.83~4.01 mg·L⁻¹、0.45~2.55 mg·L⁻¹及0.72~6.16 mg·L⁻¹；溪流的pH值及电导率范围分别为6.04~7.53及3.14~9.47 mS·m⁻¹。