Managing forests for watershed protection in Taiwan

Forest Ecology and Management 143 (2001) 77±85

Managing forests for watershed protection in Taiwan Shiang-Yue Lua,*, Jack D. Chengb, Kenneth N. Brooksc a

b

Taiwan Forestry Research Institute, 53 Nan Hai Road, Taipei, Taiwan Department of Soil and Water Conservation, Research Center for Conservation of Water Resources and Disaster Prevention, National Chung-Hsing University, 250 Kou-Kuang Road, Taichung, Taiwan c Department of Forest Resources, 115 Green Hall, University of Minnesota, St. Paul, MN 55108, USA

Abstract Few places in the world experience the severity of watershed management problems faced by Taiwan. The island is 74% mountainous with steep slopes and weak geologic formations. Each typhoon season brings torrential rainfall, resulting in frequent ¯ooding, debris torrents, and landslides. On the other hand, seasonal water shortages occur in parts of the island, a problem that will become more severe as Taiwan's population expands from its current 590 people per square kilometer. Despite forest exploitation earlier in this century, Taiwan now manages its 58% forest cover primarily for watershed protection with an emphasis on slope stabilization. Watershed protection in the past has relied heavily on engineering structures on hillslopes and along stream channels, which raises some concern about unwanted downstream effects. Forest clearing for crops, road construction and various development schemes are also of concern because of reduced slope stability, increased sediment and pollutant delivery downstream, and increased peak ¯ows. This paper discusses watershed management needs for the coming century, considering cumulative effects of past land use changes on Taiwan's mountainous watersheds, and the issue of non-structural versus structural engineering solutions to watershed problems. Watershed management implications of institutional and policy changes related to forest lands administration are also discussed. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Watershed management; Taiwan; Disaster prevention

1. Introduction Few places in the world experience the hydrometeorological extremes and watershed management challenges faced by Taiwan. The densely populated island is 74% mountainous with weak geologic formations stressed by frequent earthquake activity and annual typhoons. Annual precipitation averages 2500 mm in Taiwan, but exceeds 6000 mm in the mountains of the north. Nearly 80% of annual

* Corresponding author. E-mail address: [email protected] (S.-Y. Lu).

precipitation falls during the May±October typhoon season, when torrential rainfall results in landslides, debris ¯ows, and ¯ooding. For example, 1987 mm of rainfall was measured over a 42 h period during Typhoon Herb in 1996 at the high elevation Alishan Climate Station. This rainfall led to serious debris ¯ows and ¯ooding (Table 1). By contrast, water shortages, common in central and southern parts of the island during the dry season, are projected to become more severe in the coming century for Taiwan's expanding population, which already exceeds 590 people per square kilometer. In response to the frequency and magnitude of typhoons in Taiwan, extensive engineering works have

0378-1127/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 0 ) 0 0 5 0 7 - 7

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S.-Y. Lu et al. / Forest Ecology and Management 143 (2001) 77±85

Table 1 Selected debris flow events in central Taiwan during 1990±1996 (Cheng et al., 1997) Debris flow location

Date

Tung-Men, Hualin Chun-Keng, Nantou Er-Bu-Keng, Nantou Tung-Fu, Nantou Shen-Mu, Nantou

23 31 31 31 31

a

June 1990 July±1 August July±1 August July±1 August July±1 August

1996a 1996a 1996a 1996a

Rainfall event

Impacts on life and property

475 mm for 3 h >600 mm in less than 2 days >700 mm in less than 2 days >1300 mm in less than 2 days >1600 mm in less than 2 days

29 deaths, six missing, 24 houses and road destroyed four deaths, severe damage to houses and roads five deaths, 10 houses and 3.8 ha fruit orchard destroyed two deaths, 18 houses destroyed five deaths, six injured, eight houses and 3 ha orchard damaged

Typhoon Herb.

been developed on hillslopes, within steep mountain drainages, and along the ¯oodplains of stream and river channels. Despite these efforts, Taiwan's citizens continue to experience the ravages of typhoons, landslides, debris ¯ows, and ¯oods with a corresponding loss of life and revenues. Land scarcity compounds the problems caused by too much or too little water. Natural resource managers and engineers are under increasing pressure to cope with natural disasters, but at the same time provide for sustainable management of land and water. Furthermore, the highly educated and af¯uent population is becoming more concerned about environmental issues of water quality, ecosystem management, and protection of native ¯ora and fauna. The challenge presented to managers and planners is to develop comprehensive watershed management that meets the multiple-use objectives of its citizens. Consequently, several questions arise: (1) to what extent can the disasters resulting from water excesses be prevented or mitigated through protection of upland forested watersheds? (2) What are the cumulative watershed effects of development activities, including the extensive use of engineering structures designed to mitigate effects of natural hydrometeorological disasters? (3) Which institutions and policies can best address problems caused by recurring extreme events in Taiwan? These questions are addressed in this paper. 2. Institutional arrangements for forested watersheds in Taiwan Since 1901, Taiwan has promoted forest cover in its high elevation watersheds to control soil erosion and

mitigate water-related disasters (Koh et al., 1988). A policy of designating protection forests in selected headwater areas for stream¯ow regulation and erosion control was established during the Japanese occupation (1895±1945) and continues today. This policy was based on the ®rst Forest Act in Japan promulgated in 1897 and was similar to policies of several European countries at that time. The expansion of protection forests in Taiwan after 1945 re¯ects the public's historic appreciation of the value of forests in protecting watersheds. This appreciation is in part indicated by Chinese proverbs such as `green mountains yield clean and steady water' and `whoever controls the mountains governs the stream'. Watershed management in Taiwan, as in many other parts of the world, involves several agencies (Table 2). The Soil and Water Conservation Act and the Statute on the Conservation and Use of Slopeland Resources provide the legislative basis for watershed management. However, the management of forested and downstream non-forested watersheds is the responsibility of several organizations, each with their own agendas and without a central coordinating group. 3. Forest management 3.1. Taiwan's public forests Even with the history of concern for forest protection, Taiwan's forested watersheds have experienced periods of exploitation. Economic pressures led to an aggressive timber harvesting program through the 1970s that was essentially halted with the national forestry management policy of 1976 (Wang, 1997).

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Table 2 The key managers of forested watersheds at their respective levels of government, Taiwan Agencies

Roles and responsibility

National Government Level The Council of Agriculture Park Service, Ministry of Interior

Financial and technical support; promotional and advisory role Conservation and protection of National Parks

Provincial Government Level The Soil and Water Conservation Bureau Water Conservancy Bureau Taiwan Forestry Bureau (TFB) Taiwan Forestry Research Institute (TFRI) Reservoir Administration Offices Taiwan Power Company Universities

Slope conservation and stabilization Channel protection and improvement works Upland forest management Forestry and watershed management research Watershed management in their respective reservoir catchment areas Watershed management in their respective reservoir catchment areas Forestry research and education

About 74% (1,559,728 ha) of the total forested lands are classi®ed as national forests and managed by the Taiwan Forestry Bureau (TFB), a provincial governmental agency (TFB, 1997). Timber harvesting on these lands peaked during the Japanese occupation and immediately following World War II. Large areas of valuable timber, primarily cypress, spruce, and camphor, were cut and shipped primarily to Japan. From 1965 to 1975, an average of 1,552,600 m3 timber volume, corresponding to about 18,000 ha, were cut annually. These levels of harvesting brought about petitions from environmental protection groups urging forest protection. In 1975, sustained yield, conservation, and public welfare became the guiding principles for the management of national forests. Since then, the TFB's emphasis has shifted almost entirely from timber production to forest protection. After 1977, the annual volume cut was reduced to less than 1,000,000 m3, mainly from forest plantations (Wang, 1997). By 1990, 99% of the timber supply in Taiwan was imported. Currently, national forest lands are almost exclusively managed for purposes of stream¯ow regulation, erosion control, and conservation of biological diversity. Coincident with the changes in timber harvesting policy, an active soil and water conservation program was initiated in the 1970s that focused on agricultural practices on hillslopes. Many formerly forested hillslopes had been converted to fruit tree orchards, vegetable crops, tea, and betel nut. There was, and continues to be, concern about the effects of hillslope conversions on landslides, debris ¯ows, and ¯ooding.

Today, forests cover about 58% (2,091,274 ha) of Taiwan and consist of hardwood, conifer, mixed conifer±hardwood, bamboo, and mangrove forest types (Table 3). More than 3800 species of vascular plants from tropical to temperate regions can be found in Taiwan's forests, forming unique and varied ecosystems (Eu, 1986). The Council of Agriculture (COA) has overall responsibility of all natural resources, but the TFB actually manages these lands, including more than 35 natural protected areas and nature preserves, totaling 150,000 ha (TFB, 1994). In 1996, a special designation of protection forests was instituted for speci®c purposes as indicated in Table 4. Headwater protection and erosion control above reservoirs and in unstable areas along roadways were targeted in 94% of the areas. Timber harvesting is strictly prohibited in these protection forests; management focuses only on ®re prevention and reforestation. Although these designations have been made operationally, research has not identi®ed species or forest communities that are best suited for soil and Table 3 Forest resources of Taiwan, 1996 (TFB, 1997) Forest type

Area (ha)

Percent of forested land

Growing stock volume (100 m3)

Hardwood Conifer Mixed forest Bamboo

1111459 438251 392722 148842

53 21 19 7

132594 125383 99128 ±

Total

2091274

100

357105

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S.-Y. Lu et al. / Forest Ecology and Management 143 (2001) 77±85

Table 4 Types and areas of protection forests in Taiwan, 1996 (TFB, 1997) Purpose of protection a

Area (ha)

Percent

Headwater protection Erosion controlb Sant stabilization Aesthetics/landscape Windbreak Tide protection Others

289732 134635 5997 12335 3558 405 5484

64 30 1 3 1 <1 1

Total

452146

100

a b

Mostly located in the reservoir watersheds. Mainly in the potential unstable areas or along roads.

water protection (Hsia and Koh, 1982; Cheng et al., 1987; Lu and Tang, 1995). 3.2. National parks and protected areas National Parks comprise about 10% of the land area of Taiwan (303,500 ha) and are almost exclusively forested mountainous areas. The ®ve National Parks, Kenting, Sheipa, Taroko, Yangmingshan, and Yushan, are managed by the Park Service, Ministry of Interior, to enhance and protect ecological and cultural values, to promote environmental research and education, and to provide quality recreational opportunities. As they encompass headwater areas of several major streams and rivers, they represent key watersheds that also function to enhance and protect water ¯ow. Roads and recreational infrastructure are the principal forms of land disturbance. Outside the National Parks, the COA administers 35 natural protected areas totaling more than 150,000 ha (TFB, 1994). The actual forest and wildlife management of these areas, however, is the responsibility of the Taiwan Forest Bureau. 3.3. Other forest lands The management responsibility for the remaining forested watersheds (46,864 ha) in Taiwan include reservoir management agencies, the Forest Development Agency, Aborigine Administration Bureau, Taiwan Forest Research Institute, and universities (TFB, 1996). Privately owned forest lands constitute about 8.8% of the total forest land area (185,255 ha) and are

found largely in low elevation areas. Agencies at all levels have limited authority over land use practices on private forests. Presently, a subsidy of NT$ 4700/ha (NT$ 35 was approximately US$ 1 in 1998) per year on an average is available to encourage the conversion of farm lands to forest lands in reservoir watershed areas (Wang et al., 1998). 4. Issues of forest land use and cumulative watershed effects Land scarcity and favorable economic conditions for development have resulted in a population of rural and urban inhabitants living in areas vulnerable to the hazards of landslides, debris ¯ows, and ¯oods. To cope with these problems, a watershed perspective is needed that considers cumulative effects of forest conversions, road construction and maintenance, urban and recreational expansion on hillslopes, and other types of development that remove forest cover and disturb the soil. In recent years, market prices have encouraged cultivation of high-elevation tea, betel nuts, fruit tree orchards and mountain vegetables, resulting in the removal of forest cover from many mid- to highelevation hillslopes. During the initial conversion, soils are exposed and often mechanically altered, leading to rates of soil erosion that can exceed 220 t/ha per year (Wu, 1998). The replacement of dense native forest cover by agricultural crops also leads to greater instability of hillslopes, which contributes to landslides, debris ¯ows and ¯ooding. Roads pose serious slope stability problems, and also open previously inaccessible areas to greater economic development. Due to land scarcity, development quickly follows road construction into new areas. Road construction accelerates surface erosion, gully erosion, and soil mass movement. Although now prohibited, earlier constructed logging roads continue to affect watershed protection. For example, the 285 logging roads constructed in 1984 totaled more than 3680 km in length (TFB, 1985), and most are continued to be used as mountain trails. These areas remain unstable, however, and many have been blocked by landslides. Outdoor recreational development has expanded in coincidence with Taiwan's economic growth.

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Even though legislation was enacted in 1983 to limit forest removal on hillslopes, during the next 10 years new golf courses and recreational facilities resulted in 5183 ha of forest land being cleared. Construction activities increased waterborne sediment. The resulting golf courses do not have the same hydrologic attributes of the original forest cover, resulting in higher rates of storm¯ow and diminished water quality. Landslides, debris ¯ows, and ¯ooding have, without doubt, been aggravated by the cumulative effects of past and present land use (Cheng et al., 1997). Coping with these problems has involved a combination of forest protection and extensive construction of hillslope stabilization structures, debris ¯ow structures, and concrete channels with energy dissipaters. Two issues arise here. First, the effects of engineering modi®cations to hillslopes and stream channels accumulate as one moves from headwaters to ¯oodplain areas, raising questions about the ultimate effects on downstream ¯ooding in contrast to natural watersheds and stream systems with intact riparian vegetation. Second, the emphasis on structural solutions to protect people from landslides, debris ¯ows, and ¯oods has sometimes resulted in a false sense of security by downstream communities. This concern has been expressed by Davies (1997), who emphasized that modifying human behavior on the land is needed along with measures that modify natural systems. In Taiwan, contemporary solutions will likely entail modifying natural systems and human behavior on the watershed. 5. Scientific basis for watershed management To address the questions posed in the introduction of this paper, we need a sound scienti®c basis for watershed management in Taiwan. Limitations in our understanding help identify both research needs and operational measures that are required to cope with hydrologic extremes. 5.1. Existing knowledge The hydrologic role of forests has long been of interest in Taiwan (Koh et al., 1988), as substantiated by more than 30 years of experimental forest

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watershed research by the Taiwan Forestry Research Institute (TFRI). Following the recommendations of Dils (1964), the ®rst gauged watershed was established in the Shanping Experimental Forest of southern Taiwan in 1964. Since 1964, TFRI has added experimental watershed studies in Lienhuachi, Piluchi, and Fushan stations (Fig. 1). Currently there are 15 gauged watersheds representing several forest types and climatic conditions from the northern subtropical, temperate hardwood, and conifer forests to southern tropical hardwood forests. These sites are administratively controlled by TFRI, the Taiwan Forestry Bureau, and National Taiwan University. The TFRI conducts four categories of research in upland watersheds: (1) basic forest hydrology and tree±water relationships; (2) forest management affects on stream¯ow quantity and quality, sediment yield, landslides, debris ¯ows, and ¯oods; (3) soil and water conservation methods and (4) ecosystem management, supported by ®ve long-term ecological research (LTER) sites established in 1994. Basic studies are underway that focus on processes of canopy interception, in®ltration, evapotranspiration, soil erosion, groundwater, stream¯ow regime and models for predicting stream¯ow discharge and the frequency of occurrence of rainfall and stream¯ow extremes. Recent emphasis has been placed on comparing the hydrology of betel nut palms to native forest (Wu, 1998) because of the exponential rise in betel nut plantations over the last two decades. The effects of converting native forests to betel nut plantations on evapotranspiration, stream¯ow, landslides and debris ¯ows are of particular interest. Taiwan's interest in reducing betel nut production, and thereby reducing the availability of the drug, has prompted this research initiative as much as hydrologic concerns. Paired watershed experiments in the Lienhuachi watershed in central Taiwan have shown that clearcutting a hardwood forest and subsequent conversion to pasture and crops increased water yields by 240 mm (27%) during the initial 4-month wet season (Table 5) (Hsia and Koh, 1982). Expanding this research to determine effects of conversions on the frequency of ¯ooding and on stream¯ow regimes during periods of low rainfall requires long-term studies and the development of improved hydrologic models. Soil erosion control studies have provided insight into bioengineering alternatives for watershed

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Fig. 1. Location of experimental watersheds and long-term ecological research sites in Taiwan.

restoration. From these studies, bahia grass (Paspalum notatum) and subcostate crape myrtle (Lagerstroemia subcostata), a local tree species that is easily cut and used as vegetative stakes, have emerged

as excellent treatments to control erosion and promote slope stability, respectively (Liao et al., 1991). Bioengineering measures may provide effective, and more environmentally friendly alternatives

S.-Y. Lu et al. / Forest Ecology and Management 143 (2001) 77±85 Table 5 Water yield increases after clearcutting a 5.86 ha natural hardwood forest watersheda (Hsia and Koh, 1982) Year and season 1979 1979 1980 1980

Water yield increases c

wet season dry seasond wet season dry season

mm

%b

240 46 184 20

27 91 41 70

a

No. 4 watershed at Lienhuachi, central Taiwan. Percent increase based on the discharge predicted from the control watershed (representing uncut conditions). c From May to September. d From October to April. b

to concrete structures for stabilizing hillslopes and channels. Based on research, ®eld experience, and the worldwide literature, several tentative conclusions can be reached: (1) forests provide the most desirable cover on upland watersheds to meet most soil and water conservation objectives. (2) Physiographic, geologic, and climatic conditions render the mountain slopes of Taiwan extremely sensitive to disturbance (Koh et al., 1988; Wu et al., 1995). Road construction and forest clearing followed by improper cultivation of hillslopes accelerate the occurrence of landslides and debris ¯ows (Cheng et al., 1997). (3) Forest clearing in headwater watersheds to augment water supplies during the dry season is not realistic, given the above concerns (Hsia and Koh, 1982; Koh et al., 1988). (4) Forests represent the best vegetative cover type for producing high quality surface water and groundwater. Nutrient budget studies have helped clarify water quality relationships for natural hardwood stands (Liu and Sheu, 1997). 5.2. Research challenges and opportunities A major challenge to researchers is represented by the questions of the hydrologic roles of forests in mitigating water-related disasters and in `regulating' water ¯ow from watersheds. Typhoons are an annual occurrence in Taiwan, and droughts occur periodically. There is a need to recognize the practical extent to which forest cover and other vegetative and structural technologies can mitigate adverse effects of

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¯oods, landslides, debris ¯ows, and droughts (Brooks, 1998). As in other parts of the world, many professionals misunderstand the hydrologic role of forests and trees. On one hand, some believe forests somehow act as storage reservoirs that store water during the ¯ood season (somewhat true for deeper soils, but with a limited effect on major ¯ood events) and then release water during the dry season (which is false). Furthermore, some still attribute forest cover with the ability to attract rainfall, which has been shown not to be the case worldwide (Bosch and Hewlett, 1982; Whitehead and Robinson, 1993). On the other hand, ¯ooding disasters are often attributed only to improper land use, suggesting that natural forests can prevent ¯oods. While we recognize that forest cover has limitations in attenuating major ¯ood peaks and debris ¯ows, the extent to which different forest types can mitigate such effects is not well understood in Taiwan. Given the diversity of climatic zones, forest communities, and watershed conditions on the island, a key research objective is to determine which forest and other species are best suited for speci®c locations and purposes. For example, trees with strong root systems and high consumptive use would be well suited for steep, unstable slopes. In other areas, combinations of understory grasses and shrubs with or without trees might be more suitable for surface erosion control. Although fruit tree orchards and betel nut plantations are blamed for many ills in upland watersheds, usually it is the process of establishment that causes slope instability. Under certain conditions, there may be combinations of vegetative and structural methods that allow for production on moderate slopes. Methods of predicting the cumulative effects of engineered hillslopes and channels on ¯ows downstream need to be developed. Comparisons of storm¯ow between natural riparian stream systems and structurally altered stream channels need to be made from headwater streams to large rivers that ¯ow through downstream urban centers. Research programs should be developed to support integrated watershed management (Dils, 1964, 1977; Eu, 1986; Koh et al., 1988) if realistic solutions are to be found. Interdisciplinary studies that characterize social, economic, and political aspects of watershed

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S.-Y. Lu et al. / Forest Ecology and Management 143 (2001) 77±85

management programs must be stressed. Protecting people against the hazards of landslides, debris ¯ows, and ¯ooding depends upon appropriate institutions and policies as much as ®nding appropriate vegetative cover and bioengineering methods to mitigate impacts.

pate in setting goals and priorities, in formulating policies, and in implementing programs. With public support, adequate resources, a clearly de®ned mission, and well-de®ned roles and responsibilities, the coordination and management of Taiwan's forested watersheds can be improved.

5.3. Policies and institutional arrangements

6. Conclusions

The goal of forest and watershed management in Taiwan is to meet the diverse needs of people while protecting them from the effects of natural disasters. Accomplishment of this goal requires that appropriate policy and institutional arrangements be in place to provide the necessary incentives for implementation by individuals and agencies alike. Several issues need to be addressed before more effective watershed management can be implemented. First, the number of different agencies involved with watershed management, but often with non-watershed related missions, is problematic. A protection forest policy exists, but it cannot be implemented in isolation from policies governing use and management of all watershed lands. Protection forests must be managed in concert with production forests, National Parks, and other lands to achieve environmental protection. This conforms with Article 13 of ROC Forest Act (revised in 1985) which stipulates forest management and watershed management should be integrated. The means for integration, however, are not identi®ed. Presently, the role of the TFB in watershed management is not de®ned, even though it is responsible for managing the majority of forested watersheds on the island. Management guidelines are needed that provide for protection forests as well as silvicultural systems for more active management. Given the diverse nature of resource management agencies, their missions, priorities, responsibilities, and organizational structures, one of the two approaches is needed to achieve integrated watershed management. Either some type of effective coordinating organization is needed or one agency should be charged with the overall responsibility for forest and watershed management. This decision and its implementation must be made at the highest level of central government, but with active public participation. A well-informed, knowledgeable public should partici-

Mountainous Taiwan is subjected to frequent typhoons, heavy rainfall, and related disasters that make the management of its forested watersheds an issue of national concern. Protecting people from landslides, debris ¯ows, and ¯ooding requires an active and integrated research and management program. In addressing the questions posed in Section 1: (1) upland forested watersheds are currently managed primarily for protection purposes, but typhoons will continue to cause watershed disasters as long as people occupy hazardous areas on the landscape. Maintaining good forest cover can mitigate the effects of typhoons to a limited extent, but this alone is not suf®cient. (2) The cumulative effects of agricultural and recreational development, and the associated roads in mountainous areas, result in greater runoff, and exacerbate the frequency of landslides and debris ¯ows. It is also suggested, however, that the engineering works designed to stabilize upstream channels and hillslopes, may as well have unintended cumulative effects on low lying areas. These structures transfer water more quickly to downstream areas and have the potential to accelerate ¯ooding. Such effects need to be studied in a watershed context so that appropriate forest management, bioengineering, and combinations of non-structural and structural conservation measures can be developed. (3) A key to more effective watershed protection depends on policies and institutions that support a coordinated and integrated watershed management program. Such a program must recognize that disaster prevention must concentrate on in¯uencing human behavior and use of watersheds as much as attempting to control the ¯ow of water through biophysical conservation measures. Regulatory measures, ®scal measures, and direct public investment should be considered collectively to provide the resources, improve knowledge, and provide incentives needed for effective implementation of an integrated watershed management program in Taiwan.

S.-Y. Lu et al. / Forest Ecology and Management 143 (2001) 77±85

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