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Effects of pasture development on the ecological functions of riparian forests in Hokkaido in northern Japan
Ecological Engineering 24 (2005) 539–550
Effects of pasture development on the ecological functions of riparian forests in Hokkaido in northern Japan Futoshi Nakamura ∗ , Hiroyuki Yamada Department of Forest Science, Graduate School of Agriculture, Hokkaido University, Sapporo 060-8589, Japan Accepted 10 January 2005
Abstract In the summer, the forest canopy lowers the water temperature, which is very important for anadromous fish, and its population density is significantly lower in grassland streams. Leaf litter and terrestrial invertebrates are the critical food resources for stream organisms. In a basin where the riparian forest is preserved, but other areas have been cut, the amount of leaf litter is almost equivalent to that in an intact natural basin. The annual input of terrestrial invertebrates falling into the forested reaches was 1.7 times greater than that in the grassland reaches, and fish biomass was significantly less in the grassland reaches. In-stream large woody debris creates storage sites for organic and inorganic matter and enhances habitat diversity for aquatic biota. However, the volume and number of large wood pieces decreased significantly with pasture development, because it clears the riparian forests and covers the riverbanks with grass. Fine sediment is a prominent by-product of agricultural development and adversely impacts periphyton productivity, the density and diversity of aquatic invertebrates, fish feeding, fish spawning and egg survival. We also examine the adequate width of a riparian buffer if it is to be able to satisfy its ecological functions. © 2005 Elsevier B.V. All rights reserved. Keywords: Riparian buffer; Ecological function; Pasture; Agriculture; Riparian zone
1. Introduction In Japan, agricultural development and urbanization has resulted in the rapid disappearance of riparian buffers from floodplain rivers (Nagasaka and Nakamura, 1999). In Hokkaido, in northern Japan, rivers were channelized and floodplains have been altered intensively for cattle grazing and crop cultivation ∗ Corresponding author. Tel.: +81 11 706 2510; fax: +81 11 706 4935. E-mail address: [email protected] (F. Nakamura).
0925-8574/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2005.01.010
since the 1960s. This has resulted in several environmental problems such as sediment pollution (Nagasawa et al., 1987; Sato et al., 2002), eutrophication, and elevated water temperatures (Sugimoto et al., 1997). There has been a loss of aquatic and terrestrial habitats (Inoue et al., 1997), and river ecosystems have been severely damaged. Wetlands such as the Kushiro Mire in Hokkaido, the largest wetlands in Japan, have suffered the same fate as the rivers. They have been affected by pasture development in the upper watershed, due to the deposition of fine sediment and nutrients (Nakamura et al., 1997). The original vegetation, such
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Fig. 1. Structure and function of riparian zone (modified from Nakamura, 1995).
as reeds (Phragmites australis) and sedges, has been replaced by alder trees (Alnus japonica), which have been spreading since the 1970s. First we will examine the history of river and floodplain alternation, based on a series of aerial photos, and look at the social conditions that led to these policies being adopted. Secondly, we will discuss the effects of pasture development on riparian buffers and the deterioration of their ecological functions, such as the attenuation of sunlight, leaf and terrestrial invertebrate inputs, the input of large woody debris, the retention of fine sediment, the transport of particulate and dissolved material, and the importance of connecting fragmented forest patches (corridor) (Gregory et al., 1991; Nakamura, 1995) (Fig. 1). Finally, we propose reasonable buffer widths that can be applied to the Japanese pasture-dominated landscape, and the subsequent management implications for rivers and riparian zones.
2. Historical changes since the 1960s in floodplain environments with progressive pasture development and channelization After World War II, the population in Hokkaido was intended to increase in proportion to the number of coal mining and agricultural reclamation projects.
However, the projects did not proceed as planned, and wheat, oats and potatoes continued to be produced by poor individual farmers on small plots of land. In 1961, the Fundamental Law of Agriculture was established. The aim of this law was to increase agricultural productivity by mechanizing production and consolidating land holdings. The Japanese government’s policy has been to make Hokkaido the center of crop and cattle production, and most agricultural land has been converted to pastures as agricultural modernization projects have continued. The population of local town has rapidly decreased because mechanized farming encouraged poor farmers to move to urban areas. In contrast, the number of livestock and the amount of pasture has sharply increased since the 1960s, resulting in the alteration of the floodplains and rivers (Fig. 2). Floodplains were cultivated and dikes were built to protect farmers, livestock and crops from flooding. Riparian forests were almost completely removed to prevent log-laden floods. Channel courses and the thalweg were straightened and fixed with dikes and revetments. As a result, degraded riverbeds have become a prominent feature of channelized rivers, the hydrologic and geomorphic interactions between floodplains and streams have been severely restricted and the aquatic river system is currently independent from the terrestrial floodplain system.
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Fig. 2. Historical changes in riparian and river landscape following progressive agricultural development in Shibetsu River. Pasture area and milk cow population have been greatly increased since 1961.
3. Effects of pasture development on the ecological functions of riparian forests 3.1. Increase in stream temperature When the surface of a stream is shaded by a riparian forest canopy, the stream surface receives only a percentage of the sunshine filtered through the foliage. In broad-leaved tree forests in Hokkaido, approximately 85% of the sunlight is intercepted, and therefore the solar energy that directly reaches the stream surface is reduced to approximately 15% in summer (Nakamura and Dokai, 1989). It has also been reported that in coniferous forests in the United States, approximately 97% of sunlight is intercepted, and only 3% reaches the stream surface. The riparian forests have been almost completely removed for the development of pastures, which
requires channelized streams and banks stabilized by revetments. Based upon the heat budgets of water mass in a small stream (second order), net solar radiation contributes the majority of energy once the canopy of trees is removed, although other energy fluxes such as sensible and latent heat contribute more under canopy cover. Thus, summer maximum water temperature (SMWT) can be easily estimated by the channel lengths where no canopy cover is provided. Based upon multi-variable statistical analysis, Sugimoto et al. (1997) found that an 80% variation in SMWT in small tributaries of the Toikanbetsu River (drainage area is 276 km2 ) can be explained by the square root of the sum of the channel lengths without riparian covers. These lengths were identified and measured from aerial photographs. We applied the same methods and techniques and expanded them to 68 sites in tributaries of 12 rivers distributed
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throughout Hokkaido (83,453 km2 ). Although the interception of regression curves shift according to the average air temperature, about 80% of SMWT variation can be explained by the sum of channel lengths without canopy cover. However, sites near the confluence of the forested tributary and the main stream should be separated for accurate estimation of SMWT, because the large mass of cool water from tributaries affects the SMWT (Fig. 3). The temporal changes in SMWT were ascertained from a series of aerial photographs taken on different dates. Sugimoto et al. (1997) estimated that the SMWT in a tributary of the Toikanbetsu River used to be about 22 ◦ C in 1947, but increased to about 28 ◦ C because of pasture development. The shading described above suppresses the growth of periphyton, which attaches itself to stones or gravel in the streams, as well as helping to maintain a stable, low-temperature environment even in summer. This is very important for anadromous migratory fish such as masu salmon (Oncorhynchus masou), which favor cold water. Water temperature is an important factor in restricting their habitat. Inoue et al. (1997) found that salmon densities were significantly higher in forested streams because SMWT were kept below 16 ◦ C, whereas in pasture-dominated streams it reached above 17 ◦ C. Sato et al. (2001) examined the influence of high water temperature on the feeding activity of juvenile masu salmon,
and concluded that appetites decreased drastically at 24 ◦ C. 3.2. Low leaf and terrestrial invertebrate input In mountain streams, where sunlight is notably intercepted by the riparian forest canopy, the amount of photosynthesis provided by aquatic plants is very low. Energy must therefore be obtained from organic matter produced outside the streams. Leaves falling into streams in autumn are the most important source of energy. Branches, trunks, flowers, seeds, insects, dead fish and other debris can also be a source of energy. In a study examining leaf and twig litter input among three small disparate basins, clear-cut (CC: 43.8 ha), preserved riparian forest (RF: 51.8 ha), and intact natural forest (NF: 280.5 ha) (Fig. 4), it was shown that leaf and twig litter supplied from RF was almost equivalent to that from NF. The amount of litter in the CC basin, however, was significantly lower, ranging from 1/4 to 1/3 for leaves and 1/8 to 1/5 for twigs. Leaves entering the streams decompose through the following process: first, soluble substances dissolve; then, microbes, especially bacteria, attach themselves to the leaves; lastly, they are ingested by aquatic insects. Not only leaves but also terrestrial invertebrates fall into streams from the branches in riparian forests, and are an important food source for fish. In summer,
Fig. 3. The maximum water temperature estimated by total lengths without riparian canopy cover and average air temperature in August: (a) no forested tributaries were met above the gauging site, and (b) a large forested tributary was joined above the gauging site. OCL is total lengths without riparian covers (km), and AT is average air temperature in August.
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Fig. 4. Seasonal input of leaf and twig litter among the three small disparate basins, clear-cut (CC), preserved riparian forest (RF), and intact natural forest (NF). Spring: 15 April–1 July, summer: 16 July–24 September, autumn: 25 September–9 December, and winter: 10 December–14 April.
when most leaves in streams have been washed out and the number of aquatic insects living in the streams has declined, the amount of terrestrial invertebrates falling into the streams reaches its peak (Nakano and Murakami, 2001). During the period when streams become oligotrophic, terrestrial invertebrates can serve as an important and nutritious food source for fish. The annual input of terrestrial invertebrates falling into the forest reaches was 1.7 times greater than in the grassland reaches (Kawaguchi and Nakano, 2001). Stream salmonids, both in the forest and grassland reaches, are highly dependent on terrestrial invertebrates (falling plus drift) in summer, representing 68% and 77% of total prey, respectively, although fish biomass was significantly less in the grassland reaches (Kawaguchi and Nakano, 2001). In addition, Nakano et al. (1999) demonstrated a strong relationship between terrestrial invertebrate input and trophic interactions among fish, benthic invertebrates and periphyton. When terrestrial invertebrate input was reduced by greenhouse-type cover, predation pressure by fish (Dolly Varden [Salvelinus malma]) shifted dramatically from terrestrial invertebrates (drift) to benthic invertebrates. Consequently, the depletion of benthic invertebrates resulted in an increase in periphyton biomass.
3.3. Loss of geomorphic complexity in streams Pasture development over the floodplains has resulted in channelized rivers and streams, so they are now monotonous straight channels without riparian forests. In the Toikanbetsu River, the length of the main stream has been shortened from 33.8 km in 1947, 25.0 km in 1972, to 20.6 km in 1980, about a 40% reduction in 33 years. This shortening increased stream currents, leaving very few of the pools or riffle structures that are usually created by thalweg migration. The lateral migration of the stream channel is not the only factor in creating pools and habitat cover, but obstacles such as large boulders, fallen trees and driftwood, also play an important role. Fallen trees or driftwood in streams are known as large woody debris (LWD) or large organic debris (LOD). LWD pieces obstruct and divert stream flow to form various microtopographies, and trap sediment and leaf litter (Nakamura and Swanson, 1993). They have been intensively investigated in the Pacific Northwest of the United States (Harmon et al., 1986), because riparian forests consist of large conifers, and their effect on river morphology are remarkable. The results of research in the United States show that, in general, when there is more LWD, there are more fish. It is thought that environments formed by
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Fig. 5. Comparison of coarse woody debris (CWD) abundance among the three stand age classes. The different letter of the alphabet above the bar graphs indicates a significant difference at 5% level statistically (from Nagasaka and Nakamura, 1999).
LWD are an essential component of fish habitats. Most of these results, however, were reported from regions where large-diameter coniferous trees of approximately 1m in diameter at breast height and over 50 m in height grow in riparian zones. It is not clear whether or not the same function can be expected of Japanese riparian forests, which are intensively managed, and where small-diameter broad-leaved trees are the norm. The removal of LWD initiates substantial changes in channel morphologies, sediment transport, and thereby fish (Abe and Nakamura, 1996, 1999). Nagasaka and Nakamura (1999) investigated the distribution and the amount of LWD in the Toikanbetsu River basin (Fig. 5). They classified the riparian zones along nine tributaries into areas without forest cover, second-growth forest, and old-growth forest. The volume and number of LWD differed significantly between the three classes: very low in areas of no forest cover, significantly greater in old-growth forests and intermediate in second-growth forests. River regulations such as channelization and revetment impede the regeneration processes of riparian trees and a source of woody debris is lost. Inoue and Nakano (1998) clarified that a decrease in LWD in pasture-bordered streams resulted in a smaller population of masu salmon because of less habitat cover in which to hide from predators.
3.4. Effects of fine sediment production on stream biota, and the filtering function of riparian buffers Fine sediment produced by soil and gully erosion associated with cattle grazing and crop cultivation increases the turbidity of stream water and has a negative impact on stream biota. Murakami et al. (2001) clarified that the percentage of openwork gravel and the hydraulic conductivity of bed material, measured using the packer test, decreased with an increase in the proportion of fine sediment (<2 mm) (Fig. 6a). Moreover, turbidity and fine sediment discharge increases as agricultural areas in the catchment increase, resulting in less openwork gravels and low hydraulic conductivity (Fig. 6b). Turbidity may impede the light penetration to the surface of bed stones, resulting in low periphyton productivity (Rivier and Sequier, 1985; Davis-Colley et al., 1992), and also hinder fish feeding (Burton, 1985). Furthermore, the fine sediment deposits on streambeds lower the quality of fish spawning habitat and egg survival rates (salmonids; Bjornn and Reiser, 1991; Yamada and Nakamura, 2001), physically hinder aquatic insects’ respiration (Lemly, 1982) and decrease the organic matter content of periphyton (Graham, 1990). In addition to turbidity, the accumulation of fine sediments on stone surfaces, which takes place under conditions of low current velocity, reduces the light
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Fig. 7. Seasonal variation of suspended solid discharge among the three small disparate basins, clear-cut (CC), preserved riparian forest (RF), and intact natural forest (NF). Spring: 15 April–30 July, summer: 16 July–24 September, and autumn: 25 September–9 December.
Fig. 6. (a) Relationship between hydraulic conductivity and weight percentage of 0.125–1.0 mm sized sediment in substrate. The solid line indicates the regression line. (b) Mean and S.E. (bar) of hydraulic conductivity of the study reach in five rivers. The white and black solid circles indicate after a flood and normal flow, respectively. Asterisk (*) indicates no data (from Murakami et al., 2001).
penetration required for photosynthesis and increases non-living periphyton and the autotrophic index (APHA, AWWA, WEF, 1992; Yamada and Nakamura, 2002). Nagasaka et al. (2000) investigated the effects of fine sediment production from agricultural lands on macroinvertebrates, and clarified that fine sediment was the most important variable in explaining low density and low species diversity in an agricultural catchment. Many studies suggest that riparian buffers filter fine sediment eroded from adjacent agricultural lands and effectively decrease fine sediment discharge (Peterjohn and Correll, 1984; Whitworth and Martin, 1990). Forest soils are characterized by a thick organic litter layer which reduces raindrop energy causing splash erosion, and provides a high permeability preventing surface
runoff (Brown, 1980). Fine sediment discharge in the three small basins where litter input was examined (CC, RF and NF), was compared for three seasons. There were distinctive differences between the three basins (Fig. 7). The specific suspended solid discharges in the CC basin were more than ten times greater than those in the NF, particularly in summer, when there are heavy rains. The riparian buffer in the RF basin efficiently traps fine sediment and reduces specific total annual discharge by 38%. Thus, the riparian buffer effectively traps fine sediment produced not only by agricultural lands but also by logging (Trimble and Sartz, 1957; Haupt, 1959) and urban development (USEPA, 1973; Hartung and Kress, 1977). 3.5. Water pollution in association with cattle manure and fertilization Cattle manure and pasture fertilization are the main sources of water pollution in Hokkaido. A riparian buffer removes nitrogen and phosphorus efficiently (Pinay and Decamps, 1988; Vought et al., 1994). Research indicates that groundwater that had flowed through a 30 m long riparian forest zone had greatly reduced nitrate content. The riparian forests’ function as a buffer or a filter that preserves water quality is presently being studied. Most of the recent studies, however, assume a one-dimensional direction of flow from hillslope to river channel, but a two- or three-dimensional mass balance of subsurface flows was not satisfied. We
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Fig. 8. Water budgets of subsurface and hyporheic flows.
believe that at least a two-dimensional mass balance model should be examined, or else accurate nitrogen changes such as dilution, uptake by plants, nitrification and denitrification cannot be evaluated. Groundwater modeling, such as MODFLOW and PLASM, is a powerful tool to simulate flow path and advective transport (Anderson and Woessner, 1992). Fig. 8 illustrates the flow path and mass balance between a stream channel, gravel bar, floodplain and hillslope in a small stream under steady state low flow conditions in Karuushinai Creek, Japan. The real water balance in riparian zone is not as simple as many studies assume using a onedimensional flow. About half of the discharge out of the gravel bar and floodplain was supplied by hillslope groundwater, but the other 50% was provided by the stream channel. They merged in the floodplain and gravel bar as hyporheic flows, and flowed out at the end of the gravel bar. Among riparian forests, wetland forests, sedges (Carex Cladium mariscus) and reeds (Phragmites australis) that grow in sloughs and low marshes have long received attention because of their water purification function, although there are limitations to this function. For instance, the Kushiro Mire situated at the bottom
of agricultural watersheds is now facing problems of soil eutrophication and variations in marsh vegetation associated with the deposition of excessive amounts of nutrients (Kameyama et al., 2001; Nakamura et al., 2002). 3.6. Loss of habitat connectivity (corridors) A riparian zone may only occupy 5% of a watershed but foster a variety of plant and animal species (Naiman et al., 1993) because river dynamics and sedimentation create mosaic patterns of heterogeneous habitat conditions in a narrow space. Some studies have reported that vertebrate animal species using riparian zones during their life stages were about 70% of all vertebrate species in the district (Raedeke, 1989; Arizona Riparian Council, 1990; Fry et al., 1994). The structural and compositional complexity of riparian forest stands creates intricate edges and ecotones. The topographic, edaphic and floral complexity provide essential habitats for many birds, bats, marten, otter, beaver, and salamander. Ezo deer (Cervus nippon yesoensis) in Hokkaido use lakesides or riparian zones as wintering locations. The Blakiston’s fish owl (Ketupa blakistoni)
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needs riparian forests with large-diameter trees for itsheir nest building. Not only terrestrial animals but also a large fish such as Sakhalin taimen (Hucho perryi) require a large amount of riparian foliage that overhangs the water (Sagawa et al., 2002). Since the 1960s, the Sakhalin taimen and Blakiston’s fish owl have disappeared rapidly from rivers and riparian zones (Fig. 2), and are now endangered species. Unfortunately, very little research into the effects of pasture development on riparian passage and dwelling species has been conducted in Japan.
4. The required width of a riparian buffer to satisfy the ecological functions in Japanese streams and rivers The required width of a riparian buffer to provide shading is about 30 m in the western U.S. (Brazier and Brown, 1973; Spence et al., 1996), and is relatively greater than that in the eastern U.S., which is about 10–20 m (Aubertin and Patric, 1974). This is because giant conifer species such as the Douglas fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla) dominate the western landscape, and the east is covered mainly by broad-leaved trees. The Forest Ecosystem Management Assessment Team (FEMAT) proposed that the shading effect can be fulfilled by a buffer 75% of a site potential tree height (SPTH). As riparian trees in Japan are predominantly broadleaved species such as Pterocarya rhoifolia and Aesculus turbinate, we propose that a 15–20 m buffer be preserved. In regard to the leaf litter and invertebrate input function, Coughlin et al. (1992) (Fry et al., 1994) recommended an 8–15 m wide buffer in Pennsylvania, and FEMAT proposed one 50–100% of SPTH. The crown diameter of mature riparian trees in Japan ranges from 10 to 15 m and they lean over the stream surface. Thus, a 10–15 m buffer would be adequate to preserve this function. Van Sickle and Gregory (1990) reported that in coniferous forests all LWD were from a 50 m riparian zone. McDade et al. (1990) investigated the source of LWD in reference to vegetation type, and concluded that 100% of LWD were from a 25 m wide buffer of mature broad-leaved forest, 45–50 m in mature conifers, and 55 m in old-growth conifers. Clearly buffer width
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should be determined from the view of supply sources, and it is reasonable to consider a width 100% of SPTH to maintain this function. Assuming SPTH in Japanese riparian trees is about 25 m, a 20–25 m wide buffer should be preserved in LWD production areas. The fine sediment filtering effect varies with slope gradient, the roughness element of floor vegetation, and particle size (Nanba et al., 1976; Cooper et al., 1987; Spence et al., 1996). Among the research results in Japan and other countries, Trimble and Sartz (1957) proposed a wider buffer width which includes most other results, and thus the following two equations were proposed (SABO Society, 2000). Here interception and inclination of the equation for general streams were doubled for the important streams, in accordance with Trimble and Sartz (1957): riparian width (m) = 8 m + slope gradient (%) × 0.6 (general streams) riparian width (m) = 16 m + slope gradient (%) × 1.2 (important streams) Regarding the density of floor vegetation, a 20 m riparian buffer can trap 90% of fine sediment if it consists of sand and silt, whereas a more than 100 m wide buffer is required to filter clay-size particles (Johnson and Ryba, 1992). FEMAT proposed one SPTH to trap fine sediment. Obviously we cannot determine a fixed width for a fine sediment buffer, but if floor vegetation can be kept as a dense cover, we suggest a 10–20 m buffer for <20% (slope gradient), 20–30 m for 20–40%, and 30–50 m for >40%. A riparian buffer also filters nutrients. The majority of phosphorus attaches to fine particles, so the buffer width should be similar to that of fine sediment. Studies performed abroad indicate that a 10–20 m buffer effectively removes nitrate from shallow groundwater (Lowrance, 1992; Vought et al., 1994). We support this guideline because very few Japanese studies have been conducted on this subject. Research on the corridor function of a riparian zone is also limited in Japan. Rudolph and Dickson (1990) reported that amphibians and reptiles inhabit a 30 m riparian zone in Texas. Birds need not only width, but also a specific structure and composition of riparian vegetation, and the requirements vary from one species
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to another. Spackman and Hughes (1995) report that 90–95% of bird species inhabit a 150–175 m riparian zone in Vermont. Small mammals such as squirrels seem to require buffers of about 100 m in width.
5. Buffer width from the perspective of catchment and riparian dynamics A river changes its form, starting as a mountain headwater stream, changes to an alluvial fan, then a rear slough and finally a delta. The ecological functions of the riparian forest vary with these river morphologies. For example, the shading function of the tree canopy has limitations, and its effect decreases downstream. Supposing that the tree height limit of a species composing riparian forests in Japan is approximately 25 m, it is believed that tree crowns can fully shade the surface of rivers approximately 15 m in width, and mostly shade one 30 m in width, but they have little effect on wider rivers (Nakamura and Dokai, 1989). As the ratio of sunlight reaching rivers increases, the effect of litter input decreases and photosynthesis by aquatic plants and diatoms becomes more important. Leaves decomposed by upstream aquatic insects, however, become food for the ones living downstream (Vannote et al., 1980). The function of LWD in changing the microtopography of channels also varies depending on stream size (Nakamura and Swanson, 1993). When a large-diameter tree falls into a stream’s upper reaches in the mountains, it collects driftwood delivered from upstream to form a natural dam. This formation of natural dams by LWD is most noticeable in streams 5–10 m in width, and is not often seen in larger rivers. In large rivers, LWD are generally retained near banks or on gravel bars, oriented longitudinally according to water flow and river landforms. In the future, it will be important for agriculture and river managers to establish riparian zones so as to allow their functions to reach their full potential. Having management policies that cover an entire catchment ecosystem is equally important. The expected functions of riparian zones will vary depending on the drainage basin landforms and the need to be compatible with surrounding land use patterns. Riparian zones feature a distinct flow of material in their streams, and this tightly links the upstream and downstream ecosystems. It is therefore important to locate riparian zones continuously
throughout the drainage basin, and treat upstream riparian zones in the mountains with special care, since their impact on the lower reaches is so great. In order to maintain the riparian forests, which are the main components of riparian zones, the original dynamic environments of natural streams such as the deposition of sand and gravel, as well as scouring and variation in channel or riverbed, should be preserved as much as possible. This view is essential to the preservation of rare pioneer species such as Chosenia arbutifolia, which require dynamic site environments (Nakamura and Shin, 2001). If the riverbed is fixed, these species will be replaced by other species. River management that allows for variations in the riverbed is technically difficult, but is a challenge that demands further examination. Few primeval riparian forests remain along streams or rivers in Japan. These existing riparian forests will be very important models of species composition for restoring lost forests and for creating future management policies, and therefore measures must be taken to protect them. We should be prudent enough not to destroy them through pasture development and river channelization.
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Lowrance, R., 1992. Groundwater nitrate and denitrification in a coastal plain riparian forest. J. Environ. Qual. 21, 401– 405. McDade, M.H., Swanson, F.J., MaKee, W.A., Franklin, J.F., Van Sickle, J., 1990. Source distance for coarse woody debris entering small streams in western Oregon and Washington. Can. J. For. Res. 20, 326–330. Murakami, M., Yamada, H., Nakamura, F., 2001. Hydraulic conductivity of substrate and openwork gravel rate associated with fine sediment deposition in mountain streams, southern Hokkaido. Ecol. Civil Eng. 4 (2), 109–120 (in Japanese with English abstract). Nagasaka, A., Nakamura, F., 1999. The influences of land-use changes on hydrology and riparian environment in a northern Japanese landscape. Landscape Ecol. 14, 543–556. Nagasaka, A., Nakajima, M., Yanai, S., Nagasaka, Y., 2000. Influences of substrate composition on stream habitat and macroinvertebrate communities: a comparative experiment in a forested and an agricultural catchment. Ecol. Civil Eng. 3, 234–254 (in Japanese with English abstract). Nagasawa, T., Umeda, Y., Mizutani, T., 1987. Behaviors of suspended sediment loads from agro-forestry basin in Hokkaido. Recollect. Fac. Agric. Hokkaido Univ. 15 (4), 352–362 (in Japanese with English abstract). Naiman, R.J., Decamps, H., Pollock, M., 1993. The role of riparian corridors in maintaining regional biodiversity. Ecol. Appl. 3, 209–212. Nakamura, F., Dokai, T., 1989. Estimation of the effect of riparian forest on stream temperature based on heat budget. J. Jpn. For. Soc. 71, 387–394 (in Japanese with English abstract). Nakamura, F., Swanson, F.J., 1993. Effects of coarse woody debris on morphology and sediment storage of a mountain stream system in western Oregon. Earth Surf. Proc. Land. 18, 43–61. Nakamura, F., 1995. Structure and function of riparian zone and implications for Japanese river management. Trans. Jpn. Geomorph. Union 16 (3), 237–256. Nakamura, F., Sudo, T., Kameyama, S., Jitsu, M., 1997. Influences of channelization on discharge of suspended sediment and wetland vegetation in Kushiro Marsh, northern Japan. Geomorphology 18, 279–289. Nakamura, F., Shin, N., 2001. The downstream effects of dams on the regeneration of riparian tree species in northern Japan. Geomorphic Processes and Riverine Habitat. AGU Water Sci. Appl. 4, 173–181. Nakamura, F., Jitsu, M., Kameyama, S., Mizugaki, S., 2002. Changes in riparian forests in the Kushiro Mire, Japan, associated with stream channelization. River Res. Appl. 18, 65–79. Nakano, S., Miyasaka, H., Kuhara, N., 1999. Terrestrial-aquatic linkages: riparian arthropods inputs alter trophic cascades in a stream food web. Ecology 80, 2435–2441. Nakano, S., Murakami, M., 2001. Reciprocal subsidies: dynamic interdependence between terrestrial and aquatic food webs. Proc. Natl. Acad. Sci. U.S.A. 98, 166–170. Nanba, S., Kitamura, Y., Yanase, H., 1976. Prevention of soil erosion by buffer forests. The 1975 Report by Forestry and Forest Products Research Institute, Gijyutsu Kaihatsu Shiken Seiseki Houkoku-sho, pp. 783–789 (in Japanese).
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Peterjohn, W.T., Correll, D.L., 1984. Nutrient dynamics in an agricultural watershed: observations on the role of a riparian forest. Ecology 65, 1466–1475. Pinay, G., Decamps, H., 1988. The role of riparian woods in regulating nitrogen fluxes between the alluvial aquifer and surface water: a conceptual model. Regul. Rivers: Res. Manage. 2, 507–516. Raedeke, K., 1989. Streamside Management: Riparian Wildlife and Forestry Interactions, Contribution no. 59. Institute of Forest Resources, University of Washington, Seattle, WA, USA. Rivier, B., Sequier, J., 1985. Physical and biological effects of gravel extraction in river beds. In: Alabaster, J.S. (Ed.), Habitat Modification in Freshwater Fisheries. Butterworths, London, pp. 131–146. Rudolph, D.C., Dickson, J.G., 1990. Streamside zone width and amphibian and reptile abundance. South. Nat. 35 (4), 472–476. SABO Society (Ed.), 2000. Riparian Management (Mizubeiki Kanri). Kokon Syoin, Tokyo, 329 pp. (in Japanese). Sagawa, S., Yamashita, S., Nakamura, F. 2002. Summer habitat use of adult Sakhalin taimen in a tributary of the Teshio River, Hokkaido, Japan: management implications for habitat conservation. Jpn. J. Ecol. 52, 167–176 (in Japanese with English abstract). Sato, H., Nagata, M., Takami, T., Yanai, S., 2001. Shade effect of riparian forest in controlling summer stream temperature: impact on growth of masu salmon juveniles (Oncorhynchus masou Brevoort). J. Jpn. For. Soc. 83 (1), 22–29 (in Japanese with English abstract). Sato, H., Yanai, S., Nagasaka, Y., Nagasaka, A., Sato, H., 2002. Influence of land use on suspended sediment discharge from watersheds emptying into Funka Bay, southwestern Hokkaido, Northern Japan. J. Jpn. Soc. Hydorol. Resour. 15 (2), 117–127 (in Japanese with English abstract). Spackman, S.C., Hughes, J.W., 1995. Assessment of minimum stream corridor width for biological conservation: species rich-
ness and distribution along mid-order streams in Vermont, USA. Biol. Conserv. 71, 325–332. Spence, B.C., Lomnicky, G.A., Hughes, R.M., Novitzki, R.P., 1996. An ecosystem Approach to Salmonid Conservation. ManTech Environmental Research Services Corp., Corvallis, OR. Sugimoto, S., Nakamura, F., Ito, A., 1997. Heat budget and statistical analysis of the relationship between stream temperature and riparian forest in the Toikanbetsu River basin, northern Japan. J. For. Res. 2, 103–107. Trimble Jr., S.W., Sartz, R.S., 1957. How far from a stream should a logging road be located. J. For. 55, 339–341. U.S. Environmental Protection Agency (USEPA), 1973. Processes, producers, and methods to control pollution resulting from all construction activity. EPA Report 430/9-73-007. U.S. Environmental Protection Agency, Washington, DC. Van Sickle, J., Gregory, S.V., 1990. Modelling inputs of large woody debris to streams from falling trees. Can. J. For. Res. 20, 1593–1601. Vannote, R.L., Minshall, G.W., Cummins, K.W., Sedell, J.R., Cushing, C.E., 1980. The river continuum concept. Can. J. Fish. Aquat. Sci. 37, 130–137. Vought, L.B.M., Dahl, J., Pedersen, C.L., Lacoursiere, J.O., 1994. Nutrient Retention in Riparian Ecotones. Ambio 23 (6), 342– 348. Whitworth, M.R., Martin, D.C., 1990. Instream benefits of CRP filter strips. Trans. N Am. Wildl. Nat. Resour. Conf. 55, 40–45. Yamada, H., Nakamura, F., 2001. Effect of fine sediment deposition on Masu Salmon (Oncorhynchus masou) embryo associated with a decrease in permeability. Trans. Meet. Hokkaido Branch Jpn. For. Soc. 49, 112–114 (in Japanese). Yamada, H., Nakamura, F., 2002. Effect of fine sediment deposition and channel works on periphyton biomass in the Makomanai River, northern Japan. River Res. Appl. 18 (5), 481– 493.
Effects of pasture development on the ecological functions of riparian forests in Hokkaido in northern Japan Futoshi Nakamura ∗ , Hiroyuki Yamada Department of Forest Science, Graduate School of Agriculture, Hokkaido University, Sapporo 060-8589, Japan Accepted 10 January 2005
Abstract In the summer, the forest canopy lowers the water temperature, which is very important for anadromous fish, and its population density is significantly lower in grassland streams. Leaf litter and terrestrial invertebrates are the critical food resources for stream organisms. In a basin where the riparian forest is preserved, but other areas have been cut, the amount of leaf litter is almost equivalent to that in an intact natural basin. The annual input of terrestrial invertebrates falling into the forested reaches was 1.7 times greater than that in the grassland reaches, and fish biomass was significantly less in the grassland reaches. In-stream large woody debris creates storage sites for organic and inorganic matter and enhances habitat diversity for aquatic biota. However, the volume and number of large wood pieces decreased significantly with pasture development, because it clears the riparian forests and covers the riverbanks with grass. Fine sediment is a prominent by-product of agricultural development and adversely impacts periphyton productivity, the density and diversity of aquatic invertebrates, fish feeding, fish spawning and egg survival. We also examine the adequate width of a riparian buffer if it is to be able to satisfy its ecological functions. © 2005 Elsevier B.V. All rights reserved. Keywords: Riparian buffer; Ecological function; Pasture; Agriculture; Riparian zone
1. Introduction In Japan, agricultural development and urbanization has resulted in the rapid disappearance of riparian buffers from floodplain rivers (Nagasaka and Nakamura, 1999). In Hokkaido, in northern Japan, rivers were channelized and floodplains have been altered intensively for cattle grazing and crop cultivation ∗ Corresponding author. Tel.: +81 11 706 2510; fax: +81 11 706 4935. E-mail address: [email protected] (F. Nakamura).
0925-8574/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2005.01.010
since the 1960s. This has resulted in several environmental problems such as sediment pollution (Nagasawa et al., 1987; Sato et al., 2002), eutrophication, and elevated water temperatures (Sugimoto et al., 1997). There has been a loss of aquatic and terrestrial habitats (Inoue et al., 1997), and river ecosystems have been severely damaged. Wetlands such as the Kushiro Mire in Hokkaido, the largest wetlands in Japan, have suffered the same fate as the rivers. They have been affected by pasture development in the upper watershed, due to the deposition of fine sediment and nutrients (Nakamura et al., 1997). The original vegetation, such
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Fig. 1. Structure and function of riparian zone (modified from Nakamura, 1995).
as reeds (Phragmites australis) and sedges, has been replaced by alder trees (Alnus japonica), which have been spreading since the 1970s. First we will examine the history of river and floodplain alternation, based on a series of aerial photos, and look at the social conditions that led to these policies being adopted. Secondly, we will discuss the effects of pasture development on riparian buffers and the deterioration of their ecological functions, such as the attenuation of sunlight, leaf and terrestrial invertebrate inputs, the input of large woody debris, the retention of fine sediment, the transport of particulate and dissolved material, and the importance of connecting fragmented forest patches (corridor) (Gregory et al., 1991; Nakamura, 1995) (Fig. 1). Finally, we propose reasonable buffer widths that can be applied to the Japanese pasture-dominated landscape, and the subsequent management implications for rivers and riparian zones.
2. Historical changes since the 1960s in floodplain environments with progressive pasture development and channelization After World War II, the population in Hokkaido was intended to increase in proportion to the number of coal mining and agricultural reclamation projects.
However, the projects did not proceed as planned, and wheat, oats and potatoes continued to be produced by poor individual farmers on small plots of land. In 1961, the Fundamental Law of Agriculture was established. The aim of this law was to increase agricultural productivity by mechanizing production and consolidating land holdings. The Japanese government’s policy has been to make Hokkaido the center of crop and cattle production, and most agricultural land has been converted to pastures as agricultural modernization projects have continued. The population of local town has rapidly decreased because mechanized farming encouraged poor farmers to move to urban areas. In contrast, the number of livestock and the amount of pasture has sharply increased since the 1960s, resulting in the alteration of the floodplains and rivers (Fig. 2). Floodplains were cultivated and dikes were built to protect farmers, livestock and crops from flooding. Riparian forests were almost completely removed to prevent log-laden floods. Channel courses and the thalweg were straightened and fixed with dikes and revetments. As a result, degraded riverbeds have become a prominent feature of channelized rivers, the hydrologic and geomorphic interactions between floodplains and streams have been severely restricted and the aquatic river system is currently independent from the terrestrial floodplain system.
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Fig. 2. Historical changes in riparian and river landscape following progressive agricultural development in Shibetsu River. Pasture area and milk cow population have been greatly increased since 1961.
3. Effects of pasture development on the ecological functions of riparian forests 3.1. Increase in stream temperature When the surface of a stream is shaded by a riparian forest canopy, the stream surface receives only a percentage of the sunshine filtered through the foliage. In broad-leaved tree forests in Hokkaido, approximately 85% of the sunlight is intercepted, and therefore the solar energy that directly reaches the stream surface is reduced to approximately 15% in summer (Nakamura and Dokai, 1989). It has also been reported that in coniferous forests in the United States, approximately 97% of sunlight is intercepted, and only 3% reaches the stream surface. The riparian forests have been almost completely removed for the development of pastures, which
requires channelized streams and banks stabilized by revetments. Based upon the heat budgets of water mass in a small stream (second order), net solar radiation contributes the majority of energy once the canopy of trees is removed, although other energy fluxes such as sensible and latent heat contribute more under canopy cover. Thus, summer maximum water temperature (SMWT) can be easily estimated by the channel lengths where no canopy cover is provided. Based upon multi-variable statistical analysis, Sugimoto et al. (1997) found that an 80% variation in SMWT in small tributaries of the Toikanbetsu River (drainage area is 276 km2 ) can be explained by the square root of the sum of the channel lengths without riparian covers. These lengths were identified and measured from aerial photographs. We applied the same methods and techniques and expanded them to 68 sites in tributaries of 12 rivers distributed
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throughout Hokkaido (83,453 km2 ). Although the interception of regression curves shift according to the average air temperature, about 80% of SMWT variation can be explained by the sum of channel lengths without canopy cover. However, sites near the confluence of the forested tributary and the main stream should be separated for accurate estimation of SMWT, because the large mass of cool water from tributaries affects the SMWT (Fig. 3). The temporal changes in SMWT were ascertained from a series of aerial photographs taken on different dates. Sugimoto et al. (1997) estimated that the SMWT in a tributary of the Toikanbetsu River used to be about 22 ◦ C in 1947, but increased to about 28 ◦ C because of pasture development. The shading described above suppresses the growth of periphyton, which attaches itself to stones or gravel in the streams, as well as helping to maintain a stable, low-temperature environment even in summer. This is very important for anadromous migratory fish such as masu salmon (Oncorhynchus masou), which favor cold water. Water temperature is an important factor in restricting their habitat. Inoue et al. (1997) found that salmon densities were significantly higher in forested streams because SMWT were kept below 16 ◦ C, whereas in pasture-dominated streams it reached above 17 ◦ C. Sato et al. (2001) examined the influence of high water temperature on the feeding activity of juvenile masu salmon,
and concluded that appetites decreased drastically at 24 ◦ C. 3.2. Low leaf and terrestrial invertebrate input In mountain streams, where sunlight is notably intercepted by the riparian forest canopy, the amount of photosynthesis provided by aquatic plants is very low. Energy must therefore be obtained from organic matter produced outside the streams. Leaves falling into streams in autumn are the most important source of energy. Branches, trunks, flowers, seeds, insects, dead fish and other debris can also be a source of energy. In a study examining leaf and twig litter input among three small disparate basins, clear-cut (CC: 43.8 ha), preserved riparian forest (RF: 51.8 ha), and intact natural forest (NF: 280.5 ha) (Fig. 4), it was shown that leaf and twig litter supplied from RF was almost equivalent to that from NF. The amount of litter in the CC basin, however, was significantly lower, ranging from 1/4 to 1/3 for leaves and 1/8 to 1/5 for twigs. Leaves entering the streams decompose through the following process: first, soluble substances dissolve; then, microbes, especially bacteria, attach themselves to the leaves; lastly, they are ingested by aquatic insects. Not only leaves but also terrestrial invertebrates fall into streams from the branches in riparian forests, and are an important food source for fish. In summer,
Fig. 3. The maximum water temperature estimated by total lengths without riparian canopy cover and average air temperature in August: (a) no forested tributaries were met above the gauging site, and (b) a large forested tributary was joined above the gauging site. OCL is total lengths without riparian covers (km), and AT is average air temperature in August.
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Fig. 4. Seasonal input of leaf and twig litter among the three small disparate basins, clear-cut (CC), preserved riparian forest (RF), and intact natural forest (NF). Spring: 15 April–1 July, summer: 16 July–24 September, autumn: 25 September–9 December, and winter: 10 December–14 April.
when most leaves in streams have been washed out and the number of aquatic insects living in the streams has declined, the amount of terrestrial invertebrates falling into the streams reaches its peak (Nakano and Murakami, 2001). During the period when streams become oligotrophic, terrestrial invertebrates can serve as an important and nutritious food source for fish. The annual input of terrestrial invertebrates falling into the forest reaches was 1.7 times greater than in the grassland reaches (Kawaguchi and Nakano, 2001). Stream salmonids, both in the forest and grassland reaches, are highly dependent on terrestrial invertebrates (falling plus drift) in summer, representing 68% and 77% of total prey, respectively, although fish biomass was significantly less in the grassland reaches (Kawaguchi and Nakano, 2001). In addition, Nakano et al. (1999) demonstrated a strong relationship between terrestrial invertebrate input and trophic interactions among fish, benthic invertebrates and periphyton. When terrestrial invertebrate input was reduced by greenhouse-type cover, predation pressure by fish (Dolly Varden [Salvelinus malma]) shifted dramatically from terrestrial invertebrates (drift) to benthic invertebrates. Consequently, the depletion of benthic invertebrates resulted in an increase in periphyton biomass.
3.3. Loss of geomorphic complexity in streams Pasture development over the floodplains has resulted in channelized rivers and streams, so they are now monotonous straight channels without riparian forests. In the Toikanbetsu River, the length of the main stream has been shortened from 33.8 km in 1947, 25.0 km in 1972, to 20.6 km in 1980, about a 40% reduction in 33 years. This shortening increased stream currents, leaving very few of the pools or riffle structures that are usually created by thalweg migration. The lateral migration of the stream channel is not the only factor in creating pools and habitat cover, but obstacles such as large boulders, fallen trees and driftwood, also play an important role. Fallen trees or driftwood in streams are known as large woody debris (LWD) or large organic debris (LOD). LWD pieces obstruct and divert stream flow to form various microtopographies, and trap sediment and leaf litter (Nakamura and Swanson, 1993). They have been intensively investigated in the Pacific Northwest of the United States (Harmon et al., 1986), because riparian forests consist of large conifers, and their effect on river morphology are remarkable. The results of research in the United States show that, in general, when there is more LWD, there are more fish. It is thought that environments formed by
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Fig. 5. Comparison of coarse woody debris (CWD) abundance among the three stand age classes. The different letter of the alphabet above the bar graphs indicates a significant difference at 5% level statistically (from Nagasaka and Nakamura, 1999).
LWD are an essential component of fish habitats. Most of these results, however, were reported from regions where large-diameter coniferous trees of approximately 1m in diameter at breast height and over 50 m in height grow in riparian zones. It is not clear whether or not the same function can be expected of Japanese riparian forests, which are intensively managed, and where small-diameter broad-leaved trees are the norm. The removal of LWD initiates substantial changes in channel morphologies, sediment transport, and thereby fish (Abe and Nakamura, 1996, 1999). Nagasaka and Nakamura (1999) investigated the distribution and the amount of LWD in the Toikanbetsu River basin (Fig. 5). They classified the riparian zones along nine tributaries into areas without forest cover, second-growth forest, and old-growth forest. The volume and number of LWD differed significantly between the three classes: very low in areas of no forest cover, significantly greater in old-growth forests and intermediate in second-growth forests. River regulations such as channelization and revetment impede the regeneration processes of riparian trees and a source of woody debris is lost. Inoue and Nakano (1998) clarified that a decrease in LWD in pasture-bordered streams resulted in a smaller population of masu salmon because of less habitat cover in which to hide from predators.
3.4. Effects of fine sediment production on stream biota, and the filtering function of riparian buffers Fine sediment produced by soil and gully erosion associated with cattle grazing and crop cultivation increases the turbidity of stream water and has a negative impact on stream biota. Murakami et al. (2001) clarified that the percentage of openwork gravel and the hydraulic conductivity of bed material, measured using the packer test, decreased with an increase in the proportion of fine sediment (<2 mm) (Fig. 6a). Moreover, turbidity and fine sediment discharge increases as agricultural areas in the catchment increase, resulting in less openwork gravels and low hydraulic conductivity (Fig. 6b). Turbidity may impede the light penetration to the surface of bed stones, resulting in low periphyton productivity (Rivier and Sequier, 1985; Davis-Colley et al., 1992), and also hinder fish feeding (Burton, 1985). Furthermore, the fine sediment deposits on streambeds lower the quality of fish spawning habitat and egg survival rates (salmonids; Bjornn and Reiser, 1991; Yamada and Nakamura, 2001), physically hinder aquatic insects’ respiration (Lemly, 1982) and decrease the organic matter content of periphyton (Graham, 1990). In addition to turbidity, the accumulation of fine sediments on stone surfaces, which takes place under conditions of low current velocity, reduces the light
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Fig. 7. Seasonal variation of suspended solid discharge among the three small disparate basins, clear-cut (CC), preserved riparian forest (RF), and intact natural forest (NF). Spring: 15 April–30 July, summer: 16 July–24 September, and autumn: 25 September–9 December.
Fig. 6. (a) Relationship between hydraulic conductivity and weight percentage of 0.125–1.0 mm sized sediment in substrate. The solid line indicates the regression line. (b) Mean and S.E. (bar) of hydraulic conductivity of the study reach in five rivers. The white and black solid circles indicate after a flood and normal flow, respectively. Asterisk (*) indicates no data (from Murakami et al., 2001).
penetration required for photosynthesis and increases non-living periphyton and the autotrophic index (APHA, AWWA, WEF, 1992; Yamada and Nakamura, 2002). Nagasaka et al. (2000) investigated the effects of fine sediment production from agricultural lands on macroinvertebrates, and clarified that fine sediment was the most important variable in explaining low density and low species diversity in an agricultural catchment. Many studies suggest that riparian buffers filter fine sediment eroded from adjacent agricultural lands and effectively decrease fine sediment discharge (Peterjohn and Correll, 1984; Whitworth and Martin, 1990). Forest soils are characterized by a thick organic litter layer which reduces raindrop energy causing splash erosion, and provides a high permeability preventing surface
runoff (Brown, 1980). Fine sediment discharge in the three small basins where litter input was examined (CC, RF and NF), was compared for three seasons. There were distinctive differences between the three basins (Fig. 7). The specific suspended solid discharges in the CC basin were more than ten times greater than those in the NF, particularly in summer, when there are heavy rains. The riparian buffer in the RF basin efficiently traps fine sediment and reduces specific total annual discharge by 38%. Thus, the riparian buffer effectively traps fine sediment produced not only by agricultural lands but also by logging (Trimble and Sartz, 1957; Haupt, 1959) and urban development (USEPA, 1973; Hartung and Kress, 1977). 3.5. Water pollution in association with cattle manure and fertilization Cattle manure and pasture fertilization are the main sources of water pollution in Hokkaido. A riparian buffer removes nitrogen and phosphorus efficiently (Pinay and Decamps, 1988; Vought et al., 1994). Research indicates that groundwater that had flowed through a 30 m long riparian forest zone had greatly reduced nitrate content. The riparian forests’ function as a buffer or a filter that preserves water quality is presently being studied. Most of the recent studies, however, assume a one-dimensional direction of flow from hillslope to river channel, but a two- or three-dimensional mass balance of subsurface flows was not satisfied. We
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Fig. 8. Water budgets of subsurface and hyporheic flows.
believe that at least a two-dimensional mass balance model should be examined, or else accurate nitrogen changes such as dilution, uptake by plants, nitrification and denitrification cannot be evaluated. Groundwater modeling, such as MODFLOW and PLASM, is a powerful tool to simulate flow path and advective transport (Anderson and Woessner, 1992). Fig. 8 illustrates the flow path and mass balance between a stream channel, gravel bar, floodplain and hillslope in a small stream under steady state low flow conditions in Karuushinai Creek, Japan. The real water balance in riparian zone is not as simple as many studies assume using a onedimensional flow. About half of the discharge out of the gravel bar and floodplain was supplied by hillslope groundwater, but the other 50% was provided by the stream channel. They merged in the floodplain and gravel bar as hyporheic flows, and flowed out at the end of the gravel bar. Among riparian forests, wetland forests, sedges (Carex Cladium mariscus) and reeds (Phragmites australis) that grow in sloughs and low marshes have long received attention because of their water purification function, although there are limitations to this function. For instance, the Kushiro Mire situated at the bottom
of agricultural watersheds is now facing problems of soil eutrophication and variations in marsh vegetation associated with the deposition of excessive amounts of nutrients (Kameyama et al., 2001; Nakamura et al., 2002). 3.6. Loss of habitat connectivity (corridors) A riparian zone may only occupy 5% of a watershed but foster a variety of plant and animal species (Naiman et al., 1993) because river dynamics and sedimentation create mosaic patterns of heterogeneous habitat conditions in a narrow space. Some studies have reported that vertebrate animal species using riparian zones during their life stages were about 70% of all vertebrate species in the district (Raedeke, 1989; Arizona Riparian Council, 1990; Fry et al., 1994). The structural and compositional complexity of riparian forest stands creates intricate edges and ecotones. The topographic, edaphic and floral complexity provide essential habitats for many birds, bats, marten, otter, beaver, and salamander. Ezo deer (Cervus nippon yesoensis) in Hokkaido use lakesides or riparian zones as wintering locations. The Blakiston’s fish owl (Ketupa blakistoni)
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needs riparian forests with large-diameter trees for itsheir nest building. Not only terrestrial animals but also a large fish such as Sakhalin taimen (Hucho perryi) require a large amount of riparian foliage that overhangs the water (Sagawa et al., 2002). Since the 1960s, the Sakhalin taimen and Blakiston’s fish owl have disappeared rapidly from rivers and riparian zones (Fig. 2), and are now endangered species. Unfortunately, very little research into the effects of pasture development on riparian passage and dwelling species has been conducted in Japan.
4. The required width of a riparian buffer to satisfy the ecological functions in Japanese streams and rivers The required width of a riparian buffer to provide shading is about 30 m in the western U.S. (Brazier and Brown, 1973; Spence et al., 1996), and is relatively greater than that in the eastern U.S., which is about 10–20 m (Aubertin and Patric, 1974). This is because giant conifer species such as the Douglas fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla) dominate the western landscape, and the east is covered mainly by broad-leaved trees. The Forest Ecosystem Management Assessment Team (FEMAT) proposed that the shading effect can be fulfilled by a buffer 75% of a site potential tree height (SPTH). As riparian trees in Japan are predominantly broadleaved species such as Pterocarya rhoifolia and Aesculus turbinate, we propose that a 15–20 m buffer be preserved. In regard to the leaf litter and invertebrate input function, Coughlin et al. (1992) (Fry et al., 1994) recommended an 8–15 m wide buffer in Pennsylvania, and FEMAT proposed one 50–100% of SPTH. The crown diameter of mature riparian trees in Japan ranges from 10 to 15 m and they lean over the stream surface. Thus, a 10–15 m buffer would be adequate to preserve this function. Van Sickle and Gregory (1990) reported that in coniferous forests all LWD were from a 50 m riparian zone. McDade et al. (1990) investigated the source of LWD in reference to vegetation type, and concluded that 100% of LWD were from a 25 m wide buffer of mature broad-leaved forest, 45–50 m in mature conifers, and 55 m in old-growth conifers. Clearly buffer width
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should be determined from the view of supply sources, and it is reasonable to consider a width 100% of SPTH to maintain this function. Assuming SPTH in Japanese riparian trees is about 25 m, a 20–25 m wide buffer should be preserved in LWD production areas. The fine sediment filtering effect varies with slope gradient, the roughness element of floor vegetation, and particle size (Nanba et al., 1976; Cooper et al., 1987; Spence et al., 1996). Among the research results in Japan and other countries, Trimble and Sartz (1957) proposed a wider buffer width which includes most other results, and thus the following two equations were proposed (SABO Society, 2000). Here interception and inclination of the equation for general streams were doubled for the important streams, in accordance with Trimble and Sartz (1957): riparian width (m) = 8 m + slope gradient (%) × 0.6 (general streams) riparian width (m) = 16 m + slope gradient (%) × 1.2 (important streams) Regarding the density of floor vegetation, a 20 m riparian buffer can trap 90% of fine sediment if it consists of sand and silt, whereas a more than 100 m wide buffer is required to filter clay-size particles (Johnson and Ryba, 1992). FEMAT proposed one SPTH to trap fine sediment. Obviously we cannot determine a fixed width for a fine sediment buffer, but if floor vegetation can be kept as a dense cover, we suggest a 10–20 m buffer for <20% (slope gradient), 20–30 m for 20–40%, and 30–50 m for >40%. A riparian buffer also filters nutrients. The majority of phosphorus attaches to fine particles, so the buffer width should be similar to that of fine sediment. Studies performed abroad indicate that a 10–20 m buffer effectively removes nitrate from shallow groundwater (Lowrance, 1992; Vought et al., 1994). We support this guideline because very few Japanese studies have been conducted on this subject. Research on the corridor function of a riparian zone is also limited in Japan. Rudolph and Dickson (1990) reported that amphibians and reptiles inhabit a 30 m riparian zone in Texas. Birds need not only width, but also a specific structure and composition of riparian vegetation, and the requirements vary from one species
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to another. Spackman and Hughes (1995) report that 90–95% of bird species inhabit a 150–175 m riparian zone in Vermont. Small mammals such as squirrels seem to require buffers of about 100 m in width.
5. Buffer width from the perspective of catchment and riparian dynamics A river changes its form, starting as a mountain headwater stream, changes to an alluvial fan, then a rear slough and finally a delta. The ecological functions of the riparian forest vary with these river morphologies. For example, the shading function of the tree canopy has limitations, and its effect decreases downstream. Supposing that the tree height limit of a species composing riparian forests in Japan is approximately 25 m, it is believed that tree crowns can fully shade the surface of rivers approximately 15 m in width, and mostly shade one 30 m in width, but they have little effect on wider rivers (Nakamura and Dokai, 1989). As the ratio of sunlight reaching rivers increases, the effect of litter input decreases and photosynthesis by aquatic plants and diatoms becomes more important. Leaves decomposed by upstream aquatic insects, however, become food for the ones living downstream (Vannote et al., 1980). The function of LWD in changing the microtopography of channels also varies depending on stream size (Nakamura and Swanson, 1993). When a large-diameter tree falls into a stream’s upper reaches in the mountains, it collects driftwood delivered from upstream to form a natural dam. This formation of natural dams by LWD is most noticeable in streams 5–10 m in width, and is not often seen in larger rivers. In large rivers, LWD are generally retained near banks or on gravel bars, oriented longitudinally according to water flow and river landforms. In the future, it will be important for agriculture and river managers to establish riparian zones so as to allow their functions to reach their full potential. Having management policies that cover an entire catchment ecosystem is equally important. The expected functions of riparian zones will vary depending on the drainage basin landforms and the need to be compatible with surrounding land use patterns. Riparian zones feature a distinct flow of material in their streams, and this tightly links the upstream and downstream ecosystems. It is therefore important to locate riparian zones continuously
throughout the drainage basin, and treat upstream riparian zones in the mountains with special care, since their impact on the lower reaches is so great. In order to maintain the riparian forests, which are the main components of riparian zones, the original dynamic environments of natural streams such as the deposition of sand and gravel, as well as scouring and variation in channel or riverbed, should be preserved as much as possible. This view is essential to the preservation of rare pioneer species such as Chosenia arbutifolia, which require dynamic site environments (Nakamura and Shin, 2001). If the riverbed is fixed, these species will be replaced by other species. River management that allows for variations in the riverbed is technically difficult, but is a challenge that demands further examination. Few primeval riparian forests remain along streams or rivers in Japan. These existing riparian forests will be very important models of species composition for restoring lost forests and for creating future management policies, and therefore measures must be taken to protect them. We should be prudent enough not to destroy them through pasture development and river channelization.
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