1 Introduction

The Ooyamazawa riparian forest is a representative riparian forest in a cool-temperate zone in Japan. Many researchers have visited the forest to conduct studies, and numerous papers (Sakio 1993, 1997; Sakio et al. 2002, 2013; Kubo et al. 2000, 2001a, b, 2005, 2008, 2010; Kawanishi et al. 2004, 2006; Sato et al. 2006) and books (Sakio and Tamura 2008; Sakio 2017) have been published. This research site attracts attention not only as a riparian forest in Japan but also due to its status as a long-term ecological research (LTER ) site. Ecological research at the site began in 1983 and has been ongoing ever since. During this period, substantial data on tree life histories and coexistence have accumulated. Major changes have occurred in the forest; for example, forest floor vegetation has decreased dramatically as deer populations have increased. The reproductive characteristics of trees have also shown long-term changes, possibly due to global warming.

2 History of Long-Term Research in the Ooyamazawa Riparian Forest

The Ooyamazawa riparian forest is part of the forest land owned by Saitama Prefecture. It is located in Ooyamazawa, Nakatsugawa, Chichibu, Saitama, Japan (Fig. 1.1). The forest was donated to Saitama Prefecture in 1930 by Seiroku Honda , PhD. The Saitama Prefecture forest is made up of 12 forest stands, including old-growth forest, coppice forest, and plantations of Cryptomeria japonica and Chamaecyparis obtusa. Ooyamazawa comprises the 106th and 107th forest stands.

Fig. 1.1
figure 1

Map of the experimental area within the research site

Wood production by selective cutting of old-growth forest was conducted in the downstream part of Ooyamazawa from 1936 to 1941, and there are no logging records prior to 1936. Timbers were transported downstream by water discharged from a log dam known as a “Teppou zeki” (Fig. 1.2). At that time, the upper basin of Ooyamazawa had not been cut down and the natural forest was conserved.

Fig. 1.2
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A log dam, known as a Teppou zeki, in the Ooyamazawa stream. Photograph courtesy of Saitama Prefecture

Construction of a forest road along the Ooyamazawa stream began in 1964 through this forest stand (Fig. 1.3). However, public opposition to the felling of the national forest of the Chichibu Mountains increased, such that construction of the forest road was stopped mid-way through 1969. Most of the natural riparian forests in mountain regions in Japan have been lost to clear cutting , and were replaced by conifer plantations after World War II; thus, there are few natural riparian forests left. The upper part of the Ooyamazawa riparian forest is valuable because it has not been affected by human activities such as logging or erosion control work s.

Fig. 1.3
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The entrance of a forest road along the Ooyamazawa stream

In 1950, this area was designated as a Class II special zone (i.e., an area in which agriculture, forestry, and fishery activities must be coordinated to suit the environment) of Chichibu-Tama National Park. In 2000, the area was renamed Chichibu-Tama-Kai National Park.

In 2013, a portion of these stands (109.12 ha), including the riparian zone (5 ha), was designated as a Natural Monument of Saitama Prefecture. This area was designated as the Kobushi Biosphere Reserve in 2019.

In October 1983, a riparian area research plot (core plot: 60 m × 90 m) was demarcated, and was subsequently extended to 4.71 ha along the mountain stream from 1991 to 1998. In December 2006, the research site was registered as an associate site of the Japan LTER Network (JaLTER ; In 2008, a 1-ha plot, including a 0.54-ha core plot in the research site (Fig. 1.4), was also registered as a Core Site of the Monitoring Sites 1000 Project by the Ministry of the Environment, Japan (

Fig. 1.4
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Core Site of the Monitoring Sites 1000 Project, as designated by the Ministry of the Environment, in the Ooyamazawa riparian forest

3 Site Description

The Ooyamazawa riparian forest research site (35°57′48″ N, 138°45′22″ E) is located in the Chichibu Mountains of the Kanto region of central Japan (Fig. 1.1). This site is located in a riparian zone along a small stream (Ooyamazawa) of the Nakatsugawa River branch of the Arakawa River in Saitama Prefecture, central Japan. The site is situated within the Chichibu-Tama-Kai National Park and ranges from 1210 to 1530 m above sea level (a.s.l.). The 106th and 107th forest stands in Ooyamazawa cover approximately 512 ha.

4 Climate Characteristics in the Ooyamazawa Riparian Forest

There is a large difference in climate between the Pacific Ocean and Japan Sea sides. On the Japan Sea side, snow and rain are abundant in the winter due to seasonal northwest winds. On the Pacific side, a significant amount of rain falls during the summer due to southeast seasonal winds blowing from the Pacific Ocean. The Ooyamazawa research site is on the Pacific Ocean side. Japanese forest zones are divided into four categories: subarctic forest, cool-temperate forest, warm temperate forest, and subtropical forest; Ooyamazawa is a cool-temperate forest.

4.1 Air Temperature

The mean annual temperature at the study site (1450 m a.s.l.) is 7.1 °C. The mean air temperature is 18.3 °C in the warmest month (August) and −5.2 °C in the coldest month (January) (Fig. 1.5). The monthly average maximum temperature is 20.3 °C in August and the monthly average minimum temperature is −8.6 °C in January (2008–2018). The diurnal ranges of air temperature from January to May are larger than in other months due to the development of canopy leaves (Fig. 1.6).

Fig. 1.5
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Monthly mean air temperatures in the Ooyamazawa research site from 2008 to 2018

Fig. 1.6
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Diurnal ranges of air temperature in the Ooyamazawa research site from 2008 to 2018

4.2 Relative Air Humidity

The mean annual air humidity was 81.9% from 2014 to 2018. The maximum mean air humidity was 97.4% in September and the minimum was 68.3% in January (Fig. 1.7). The air humidity is higher in the summer season than in the winter season. The diurnal ranges of air humidity were larger from January to May than in other months (Fig. 1.8).

Fig. 1.7
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Monthly mean relative air humidity in the Ooyamazawa research site from 2014 to 2018

Fig. 1.8
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Diurnal ranges of relative air humidity in the Ooyamazawa research site from 2014 to 2018

4.3 Precipitation

The mean annual precipitation at the Mitsumine automated weather station in Saitama Prefecture, located 15 km from the research site at 975 m a.s.l., was 1611.7 mm from 1980 to 2018 (Fig. 1.9). Precipitation is higher in summer than in winter due to the rainy season and the occurrence of typhoon s (Fig. 1.10). In particular, due to a large typhoon, 717.5 mm of precipitation was recorded in August 2016. On 14 August 1999, we recorded 440 mm of daily precipitation. The mean maximum snow depth at the research site was approximately 30 cm between January and March. However, in 2014, there were heavy snowfalls and the snow depth was estimated to exceed 3 m in the valley bottom due to avalanches.

Fig. 1.9
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Changes in precipitation measured at the Mitsumine automated weather station in Saitama Prefecture from 1980 to 2018. This station is 975 m above sea level (a.s.l.) and 15 km from the Ooyamazawa research site

Fig. 1.10
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Monthly mean precipitation at the Mitsumine automated weather station in Saitama Prefecture from 1980 to 2018

5 Geology, Topography, Natural Disturbances, and Soils

The study area is covered by graywacke and sandstone of the Paleozoic era . The Chichibu Mountains have a complex topography with steep slopes (greater than 30°) and a network of mountain streams. This area is registered as a Japanese Geopark .

The topography of the Ooyamazawa riparian research plot along the stream (1.2 km) can be divided into two parts: the downstream area is a V-shaped valley with a steep slope of 30° (Fig. 1.11), and the upstream area is a wide floodplain characterized by debris flow s and landslide s with a slope of 12° (Fig. 1.12).

Fig. 1.11
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The V-shaped valley in the downstream area. The hillside is very steep with a slope of >30°

Fig. 1.12
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The wide floodplain characterized by debris flows and landslides in the upstream area

In the Chichibu Mountains, large typhoons accompanied by >300 mm of diurnal precipitation have occurred once every decade throughout the twentieth century (Saitama Prefecture & Kumagaya Local Meteorological Observatory 1970). These heavy rains result in debris flow s (Fig. 1.13), surface landslides, and channel movement s that do not improve light conditions on the mountains, whereas another type of disturbance, involving the destruction of large areas of canopy trees, improves light conditions. For example, large earthquake s and typhoons can cause large mass movements through landslides (Fig. 1.14). Sedimentation and erosion of sand and gravel occur on a small-scale every year in the active channels during the rainy and typhoon seasons. Flooding during these times has resulted in the emergence of abandoned channel s and large deposits (Sakio 1997).

Fig. 1.13
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Debris flow after heavy rain (Sakio 2008)

Fig. 1.14
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A deep-seated landslide after a typhoon, which caused large mass movements and changes in light conditions

A typical soil for this area is a moderately moist brown forest soil (BD). However, riparian zones have complex microtopography with many soil types. The substratum of the active channel is sand and/or gravel, while that of the hillslope is mature soil. In the active channel, the ground surface is covered with large rocks, gravel, and sand. In the winter season, only groundwater flow is present. In the abandoned channel , no movement of sand or gravel occurs due to stream flow and there is a dense Fraxinus platypoda and Pterocarya rhoifolia sapling bank. In the floodplain , there are two A horizon layers containing plant roots due to repeated sedimentation. Meanwhile, on the hillslope, a thick litter layer and a humus layer can be observed in the soil profile (Sakio 1997; Fig. 1.15).

Fig. 1.15
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Soil profiles at the core plot in the Ooyamazawa research site (Sakio 1997). The soil of the active channel was immature and divided into three layers. Floodplain 1 showed two A horizons due to repeated sedimentation

6 Vegetation

The Ooyamazawa riparian forest (4.71 ha) is a species-rich natural forest. This forest is situated in the upper areas of a cool-temperate, deciduous broad-leaved forest zone that ranges from 700 to 1600 m a.s.l; it is a typical mountain region riparian forest classified as a Dryopterido—Fraxinetum commemoralis type (Maeda and Yoshioka 1952) originally reported by Suzuki (1949). However, Ohno (2008) suggested that it should be classified as Cacalio yatabei—Pterocaryetum rhoifoliae, as per the riparian forest communities in the mountain belts of the inland Chubu and Kanto regions.

The Ooyamazawa river basin contains at least 230 species of vascular plant s (Kawanishi et al. 2006). Forty-six woody tree species were reported in a 4.71-ha research plot (Sakio 2008); of these, nineteen were canopy tree species. Canopy layer species are over 30 m in height and include F. platypoda, P. rhoifolia, and Cercidiphyllum japonicum. The dominant plants in the subcanopy layer are Acer shirasawanum and Acer pictum, while the lower layer is dominated by Acer carpinifolium and Acer argutum. The main herbaceous species in the understory are Mitella pauciflora, Asarum caulescens, Meehania urticifolia, and Dryopteris polylepis.

7 History of the Ground Design of the Research Plot

We demarcated a permanent plot of 60 × 90 m (0.54 ha), in the Ooyamazawa riparian forest in October 1983 (Fig. 1.16), and researched the forest structure and soil profile (Sakio 1997). This plot was extended to 4.71 ha along the mountain stream from 1991 to 1998. The plot was long (1170 m) and narrow (30–60 m), and comprised 30 × 30-m subplots that covered the lower part of a hillside adjacent to the riparian area (Sakio et al. 2002).

Fig. 1.16
figure 16

History of the study design of the Ooyamazawa research site. The core plot (60 m × 90 m) was demarcated in 1983. The research site was extended to 4.71 ha along the mountain stream from 1991 to 1998. The core plot was extended to a 1-ha plot and registered as a Core Site of the Monitoring Sites 1000 Project in 2008

All living trees ≥4 cm diameter at breast height (DBH) were numbered and identified to the species level, and divided into canopy trees, subcanopy trees (DBH ≥ 10 cm), and small young trees (DBH < 10 cm). Canopy trees reached the top stratum of the forest. All trees were mapped using a compass survey.

Seed and leaf production have been measured from 1987 to the present. We set 20 conical litter traps approximately 1 m above the ground in a regular pattern within the core plot (0.54 ha) in May 1987 (Fig. 1.17). Litter trap s were made of nylon netting (~1.0-mm mesh) with 0.5 m radius openings. When the plot became a Monitoring Sites 1000 Project Core Site (1 ha, including the 0.54 ha core plot) in April 2008, we added five new litter traps, giving 25 traps in total (Fig. 1.17). The litter trap size was changed to a 0.4-m radius in 2010. We collected the contents of the litter traps and brought them back to the laboratory every month between May and November. In the winter, the traps were left in the plot. In the laboratory, we divided the litter into seed s, flower s, fruits , fallen leaves, and branches/bark, measured the weight of each litter type and counted the seeds. From 1987 to 1994, only F. platypoda was measured; after 1995, F. platypoda, P. rhoifolia, C. japonicum, Tilia japonica, Ulmus laciniata, Carpinus cordata, Abies homolepis, and some Acer species were also measured.

Fig. 1.17
figure 17

Layout of litter traps in the Monitoring Sites 1000 Project Core Site

Studies of ground beetle s have been in progress at the Monitoring Sites 1000 Project Core Site since 2008. Five subplots (5 m × 5 m) have been established within the Core Site and four pitfall trap s were installed in each subplot. In addition, a bird survey project conducted at five fixed points in Ooyamazawa was established in 2010.