Destruction of vegetation due to geo-hazards and its environmental impacts in the Wenchuan earthquake areas

Ecological Engineering 44 (2012) 61–69

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Destruction of vegetation due to geo-hazards and its environmental impacts in the Wenchuan earthquake areas Peng Cui a,∗ , Yong-ming Lin b , Can Chen b a Key Laboratory of Mountain Hazards and Earth Surface Process, Chinese Academy of Sciences/Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, China b College of Forestry, Fujian Agricultural and Forestry University, Fuzhou 350002, China

a r t i c l e

i n f o

Article history: Received 26 October 2011 Received in revised form 5 March 2012 Accepted 26 March 2012 Available online 2 May 2012 Keywords: Wenchuan earthquake Geo-hazards Vegetation destruction Restoration and reconstruction Environmental impacts

a b s t r a c t Geo-hazards induced by earthquakes have caused ecosystem degradation and vegetation destruction. Little, however, is known about the consequences of geo-hazards due to a lack of research data. We have undertaken a study in the Wenchuan earthquake-affected area of China in order to identify and characterize vegetation destruction and its consequent environmental impact. The Wenchuan earthquake on 12th May, 2008 induced numerous geo-hazards (including rock avalanches, landslides, landslide-dammed lakes and debris flows) that caused vegetation destruction up to 1249.5 km2 , of which shrub comprised the largest proportional area with 338.559 km2 . The vegetation coverage decreased by 4.76% in 9 severely damaged cities and counties and by 12.37% in the Subao river, Beichuan county. Rock avalanches and landslides were the most common destructive types, resulting in 98.73% of all types of geo-hazards, whereas debris flows and landslide-dammed lakes accounted for 1.27%. Vegetation destruction was distributed along both sides of rivers causing erosion, formation of debris flows and landslides. Hydrologic progress was changed and hydrological adjusting function diminished due to vegetation deterioration resulting in bare rock (infiltration reduced, runoff increased and flow concentration expedited) and deposit region (infiltration increased and runoff reduced) in catchment. Soil erosion was intensified causing increased sediment transportation of rivers, decreased storage capacities of reservoirs downstream, a significantly increased area that has suffered severe erosion and aggravated magnitude and damage capability of debris flows and landslides. Ecosystem function declined and vegetation restoration and reconstruction was difficult due to co-degradation of vegetation-soil system in the earthquake-affected areas. Finally, we summarized the challenges faced in the future for vegetation restoration and reconstruction. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The devastating Wenchuan Earthquake struck Sichuan Province, Southwestern of China, with a magnitude of 8.0 at 14:28:01.42 CST on May 12, 2008 and killed at least 68,000 people. The energy source of the earthquake and Longmenshan’s southeast push came from the strike of the Indian Plate onto the Eurasian Plate and its northward push. This finally caused a sudden dislocation in the Yingxiu-Beichuan fracture, leading to the violent earthquake of Ms 8.0 with great intensity, long endurance, huge destructive force, wide affected area, heavy loss, frequent geological disasters, and serious ecological degradation (Bao, 2008; Huang et al., 2009).

∗ Corresponding author. Tel.: +86 28 85214421; fax: +86 28 85238460. E-mail address: [email protected] (P. Cui). 0925-8574/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ecoleng.2012.03.012

Data from the forestry department of Sichuan Province indicated that the direct economic loss of forest system was estimated to be as much as 23 billion RMB in Sichuan province. Forest land deteriorated by 32.867 × 104 ha and forest coverage decreased from 30.7% to 30.2% (Forestry Department of Sichuan Province, 2008). In 45 severe calamity cities and counties, the losses of Ecological Public Welfare Forest (EPWF) and Conversion of Cropland to Forest Project (CCFP), accounted for 21.112 × 106 and 204.12 × 106 RMB with an area of 1.652 × 104 ha and 1.595 × 104 ha respectively (Deng and Li, 2009). According to the standard investment of Construction of EPWF and CCFP, the loss of stocking volume reached 2098.63 × 104 m3 , accounting for 3.6% of the total area with potential economic loss of 8.395 billion RMB (Forestry Department of Sichuan Province, 2008). Ecological restoration after a catastrophic disaster has been previously reported for the Chi-Chi earthquake in 1999 at the Jou-Jou Mountain area in the Wu-Chi basin, Taiwan (Lin et al., 2001, 2004, 2005, 2006). There are many factors that affect the restoration

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of vegetation cover e.g. climate, soil erosion, human activities, as well as geo-hazards (Honnay et al., 1999; Rodriguez et al., 2005; Wang et al., 2007; Lin et al., 2006) and an ecological approach and engineering countermeasures are both needed (Mitsch and JØrgensen, 2004). Geo-hazards are the main limiting factor for vegetation recovery in severely affected area, but their influences are still poorly understood and documented. In earthquake-affected area vegetation can be badly damaged by subsequent, frequent geo-hazards that then threaten the ecological security and sustainable development of the economy and society. This paper analyzes the types and characteristics of geohazard induced vegetation destruction and discusses their effect on the environment with regard to disaster prevention, vegetation restoration and reconstruction of an earthquake-affected area. 2. Geo-hazards induced by the earthquake Wenchuan is an area with great topographical relief and so the earthquake produced many geo-hazards. The ground motion from the earthquake destabilized many mountain slopes causing rock avalanches and landslides. These, in turn, produced an abundant amount of unconsolidated materials that became the source of subsequent debris flows. 2.1. Rock avalanches and landslides Rock avalanches and landslides rank as the most common geohazard induced by the earthquake. They caused many fatalities directly and badly disrupted the transportation system. A geological survey in Sichuan Province figured out 3286 landslides and 1218 rock avalanches happened soon after the earthquake. These events caused heavy losses by destroying roads, smashing vehicles, burying villages, blocking rivers, and claiming the lives of many local people (Cui et al., 2011). The earthquake caused unstable mountainsides with correspondingly increased difficulty for vegetation restoration and reconstruction due to the high possibility of rock avalanches and landslides occurring during rainstorm. In addition, the landslides and rock avalanches produced abundant unconsolidated materials that could be then the source of debris flows. Some large landslides traveled down into river valleys blocking drainage to create large landslide-dammed lakes. Fig. 1a (Tangjiashan

landslide destroyed and buried vegetation which grow up on it) and Fig. 1b (the photo showed Wenjiaba landslide buried vegetation and farmland) illustrate vegetation and farmland destruction from the Tangjiashan Landslide in Beichuan county, and Wenjiaba Landslide in Pingwu county. 2.2. Landslide-dammed lakes Blocked drainage from landslides in river valleys will make water levels upstream rise, resulting in flooding of roads, villages, towns, farmlands and forest. The Wenchuan Earthquake created about 256 landslide-dammed lakes, distributed in clusters along rivers. The blockages are commonly characterized by loose material, low soil strength, and huge volume. Collapse is highly instable when water overflows the blockage surface. It is possible that a series of successive dam collapses could create a huge flood surge that would be able to threaten towns far downstream (Cui et al., 2009). Within 14 km of the Jian River eight landslide lakes have been formed with the density of 0.57 lakes/km (Cui et al., 2010a). The Tangjiashan Lake, located on the upper reaches of the Tongkou River (tributary to the FuJiang River), was the largest landslidedammed lake (Fig. 1a, Tangjiashan landslide destroyed and buried vegetation which grow up on it). If the dams upstream collapse, it is probable that this will cause the lakes downstream to collapse in a chain reaction due to the volume of flood water. The lives and property and vegetation downstream are thereby greatly threaten in the rainy season (Cui et al., 2009). 2.3. Debris flows Landslides and rock avalanches produce abundant unconsolidated materials that are the source of debris flows in the affected areas. In addition, the earthquake has widely increased slope instability, providing more rock debris for the initiation of debris flows. The large volume of loose materials in valleys, about 2.5 billion cube meters in earthquake-affected area, is easily swept away during rainstorms providing a potential source of abundant unconsolidated materials that frequently turn into debris flows for a relatively long period (Cui et al., 2010b). Debris flows destroy vegetation distributed along the channel and valley sides that, in turn, also limits potential vegetation restoration.

Fig. 1. (a) Tangjiashan landslide destroyed and buried vegetation which grow up on it (Image from State Bureau of Surveying and Mapping). (b) The photo showed Wenjiaba landslide buried vegetation and farmland (Image from State Bureau of Surveying and Mapping).

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Fig. 2. (a) Cross sectional view of slope before earthquake, an illustration of vegetation cover. (b) Cross sectional view of slope after earthquake, which shows the vegetation demolition.

3. Types, characteristics and distribution of vegetation damage 3.1. Driven force of vegetation damage Geo-hazards induced by the Wenchuan Earthquake caused a significant degree of geographical change to the landscape. These changes resulted in intensive surface material movement that led to a large area of vegetation that was either destroyed, removed or diminished. When a giant landslide happens, land moves in high speed and any vegetation on the sliding mass is destroyed and incorporated quickly with the landslide. Farmland, houses and highways at the bottom of slope are buried as shown by Fig. 1a (Tangjiashan landslide destroyed and buried vegetation which grow up on it) and Fig. 2b (Cross sectional view of slope after earthquake, which shows the vegetation demolition). Fig. 2 illustrates the destruction of slope. On the top of the slope, there is a denuded area created by

landslide that is hard to be restored with the original vegetation. In the middle of the slope, a weak weathered layer or slightly disturbed soil layer caused by unconsolidated materials’ movement is restored relatively easily by vegetation. At the bottom of the slope, materials from the top of the slope are deposited but are unstable because of basal erosion, therefore, need to be protected by engineering measures (Fig. 2a (Cross sectional view of slope before earthquake, an illustration of vegetation cover) and Fig. 2b (Cross sectional view of slope after earthquake, which shows the vegetation demolition)). Moreover, a huge landslide can continue up the opposite bank if the gully is narrow and the trend and direction of movement is near perpendicular, as show by Fig. 1b (The photo showed Wenjiaba landslide buried vegetation and farmland). Vegetation and farmlands along the river valley will be submerged by water from the landslide-dammed lakes. The largest landslide-dammed lake, for example, is located at Tangjiashan, 3.2 km upstream from Beichuan township (Fig. 3 (Tangjiashan landslide-dammed lake)). It had a submerged area over

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Table 1 Areas of different vegetation deterioration types in 9 worst-hit cities and counties (km2 ). Vegetation types Evergreen broadleaf forest Deciduous broadleaf forest Mixed evergreen and deciduous broadleaf forest Broadleaf and coniferous forest Deciduous coniferous forest Evergreen coniferous forest Economic forest Bamboo forest Shrub Meadow Total Vegetation loss rate

Anxian

Bei chuan

16.398 17.024

14.124 42.919 43.958

2.970 21.150 2.210 59.752 4.26%

1.295 16.866 45.867 0.135 63.803 11.136 240.103 8.39%

Jiang you 17.919 7.588

2.645 34.531 1.727 0.701 65.111 2.40%

Mao xian

Mian zhu

17.525 8.164 2.448

1.362 5.202 4.196

17.831 0.243 24.664 29.485 0.599 53.718 42.141 196.818 5.11%

12.481

65.02 × 104 m2 in Xuanping town on May 18, 2008 (An et al., 2011). The water level rose and submerged farmlands and forests with the area of 17.79 × 104 m2 and 1.549 km2 along stream terrace and valley side in Xuanping town and Yuli towns (Fan et al., 2008). Fig. 4 (Niujuangou debris flow buried and destroyed vegetation during its movement in Wenchuan county) shows vegetation destroyed by a debris flow. When debris flows move along a channel and came across a sharp bend or meet a blockage or obstacle, they climb up the sides of the valley and destroys any vegetation, usually forests, that it comes into contact with. In addition, debris flows will bury vegetation when it slowed down or stopped on the downstream watercourse. Rock avalanches removed large areas of surface soil from mountain sides with the result that the vegetation cover became patchy on the slopes and this cover was no longer continuous. Rockfalls destroyed trees over a small area. They toppled trees, bent branches and caused gaps in the forests. In addition, ruptures and loose soil materials can directly destroy or topple a large area of forest.

27.207 7.237 2.183 43.577 0.490 103.905 8.30%

Ping wu 0.462 3.590 16.462

0.001 5.912 48.140 20.876 0.964 96.407 1.62%

Qing chuan

Shi fang

Wen chuan

8.071 3.761 24.829

5.638 5.034 0.326

22.554 19.656 3.314

11.903

0.020 3.594 20.428 4.776 0.236 20.131 1.290 61.473 7.20%

5.363 76.272 27.828 2.315 121.584 29.739 308.625 7.56%

8.027 49.107 0.241 10.973 0.329 117.241 3.59%

Total 69.736 122.643 120.145 42.235 10.496 184.991 268.121 5.709 338.599 86.79 1249.465 4.76%

Due to geo-hazards induced by earthquake, land coverage changed significantly. As an indication of land coverage, vegetation coverage seriously decreased by the disturbance of geo-hazards. For instance, the proportion of barren land in Dujiangyan city increased from 5.24% to 22.22% after the earthquake (Ni et al., 2009). In the upper Mianjiang River, forest coverage decreased from 28.44% in 2005 to 24.45% in 2008 after the earthquake (Huang et al., 2009). 3.2. Distribution of vegetation damage Nine severely damaged cities and counties, were used as case studies: Beichuan county, Anxian county, Jiangyou city, Maoxian county, Mianzhu city, Pingwu county, Qingchuan county, Shifang city and Wenchuan county. Remote sensing images with 2 m resolution were taken by Aerial Digital Senso 40 (ADS40) from 16 to 28 May 2008, and used to interpret the distribution of geo-hazards (Cui et al., 2011). The data of damaged vegetation in all 9 cities and counties were extracted from interpretation of the images of the China-Brazil Earth Resources Satellite (CEBRS-02B) by Sichuan

Fig. 3. Tangjiashan landslide-dammed lake (The blue line represents the channel of Jianjiang River before earthquake. Image from State Bureau of Surveying and Mapping). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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Fig. 4. Niujuangou debris flow buried and destroyed vegetation during its movement in Wenchuan county (Image from China Ministry of Land and Resources).

Forest Inventory and Planning Institution in earthquake-affected areas from 14 to 28 May 2008 (Zhang et al., 2008). Subsequently, all data were projected and processed using ERDAS IMAGINE 9.1 and ARC/INFO 9.2 for rapid evaluation of damaged vegetation. The GIS overlay analysis of damaged vegetation and geo-hazards for pre- and post-earthquake was used to determine the distribution of damaged vegetation. This data is presented in Fig. 5 (The distribution of damaged vegetation by Geo-hazards in 9 severely damaged cities and counties). The total area of damaged vegetation was 1249.465 km2 , accounting for 4.76% of the study area. Among 10 vegetation types, as seen in Table 1 (Areas of different vegetation types in 9 worst-hit cities and counties), shrub is mainly distributed between 1800 and 3400 m altitude and plays an important role on headwater conservation and animal habitat (Liu et al., 2011). Shrub contributes as the largest proportional area with 338.559 km2 , which is 27.10% of the study area. Economic forest accounts for 21.46%, evergreen coniferous forest accounts for 14.81%, deciduous broad-leaved forest accounts for 9.82%, mixed evergreen and deciduous forest accounts for 9.62%, meadow accounts for 6.95%, evergreen broad-leaved forest accounts for 5.58%, mixed broad-leaved and coniferous forest accounts for 3.38%, deciduous coniferous forest accounts for 0.84%, and bamboo forest accounts for 0.46%. After the earthquake, as given in Table 1 (Areas of different vegetation types in 9 worst-hit cities and counties), the total area of damaged vegetation in Wenchuan county reached 308.625 km2 (7.56% of Wenchuan county), in Beichuan county reached 240.103 km2 (8.39% of Beichuan county), in Maoxian county reached 196.818 km2 (5.11% of Maoxian county), in Qingchuan county reached 117.241 km2 (3.59% of Qingchuan county), in Mianzhu city reached 103.905 km2 (8.30% of Mianzhu city), in Pingwu county reached 96.407 km2 (1.62% of Pingwu county), in Jiangyou city reached 65.111 km2 (2.40% of Jiangyou

city), in Shifang city reached 61.473 km2 (7.56% of Shifang city), and in Anxian county reached 59.752 km2 (4.26% of Anxian county). Compared with other counties and cities, vegetation in Pingwu county and Jiangyou cities was affected mildly with the decreased vegetation coverage by 1.62% and 2.40% respectively. Moreover, the main type of damaged vegetation in Pingwu and Jiangyou was economic forest, accounting for 49.93% of Pingwu county, and 53.03% of Jiangyou city. Table 2 (Areas of destroyed vegetation caused by geo-hazards in 9 worst-hit cities and counties) describes the distribution of geohazards. Vegetation was mainly destructed by rock avalanches and landslides, accounting an area of 1233.650 km2 , occupying 98.73% of the whole damaged vegetation; The remainder was destroyed by debris flows and landslide-dammed lakes. These were mainly distributed in Beichuan county, Wenchuan county and Shifang city with an area of 2.813, 2.477 and 1.809 km2 respectively. Due to 7 blockages in Shiting River, which flows through Shifang city, landslide-dammed lakes were created submerging vegetation and resulting in a greater area of damage (2.94%) than other cities and counties (Cui et al., 2009). 3.3. Characteristics of vegetation damage in typical catchment With the aim of demonstrating the extent of damage and loss of vegetation, the catchment of the Subao river in Beichuan county was taken as a typical example. Eleven aerophotographs (with total area of 5882.7 ha) were taken on 19, May, 2008 by Chinese State Bureau of Surveying and Mapping and used for interpreting geohazards, vegetation cover and land use. The GIS overlay analysis for pre- and post-earthquake was used to determine the distribution of damaged vegetation and the results shown in Fig. 6 (The distribution of vegetation damaged by Geo-hazards in Subao river basin).

Table 2 Areas of destroyed vegetation caused by geo-hazards in 9 worst-hit cities and counties (km2 ). Geo-hazards

Anxian

Beichuan

Jiang you

Mao xian

Mian zhu

Ping wu

Qing chuan

Shifang

Wen chuan

Total/rate

Debris flow Dammed lake Landslide

0.642 0.561 58.548

1.875 2.477 235.752

0.001 0.082 65.027

0.052 1.057 195.708

0.391 1.075 102.468

0.252 0.017 96.137

0.042 0.748 116.451

0.053 1.809 59.611

1.865 2.813 303.946

5.173 (0.41%) 10.639 (0.85%) 1233.650 (98.73%)

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Fig. 5. The distribution of damaged vegetation by Geo-hazards in 9 severely damaged cities and counties.

Subao River is a second tributary of Anchang River, having a catchment area of 4377.4 ha of forest (not including grass) before the earthquake, accounting for 74.41% of the study range (5882.7 ha). After the earthquake this area decreased by 12.37% to 3649.7 ha. Among the damaged vegetation types, coniferous forest lost 175.2 ha, which accounted for 24.08% of the decreased forest, broad-leaved forest lost 448.6 ha, shrub lost 103.9 ha, and grass lost 30.6 ha. Earthquake created back scarp with an area of 39.1 ha, rock avalanches with 213.2 ha, locomotion area of debris flows and landslides with 379.3 ha and deposit area of landslides and debris flows with 122.2 ha. Most of farmland was on gentle slope and so suffered less destruction, totaling only 21.5 ha in area.

Six distance ranges were defined to investigate the relationship between the damage and the rivers: <0.5 km, 0.5–1 km, 1–2 km, 2–3 km, 3–4 km, 4–5 km. Some changes with distance were corrected from GIS overlays. As seen in Fig. 7 (The relationship between damaged vegetation and the rivers in 9 severely damaged cities and counties), 25.39% of the damaged forest was within the distance <0.5 km; 13.51% between 0.5 and 1 km, 11.45% between 1 and 2 km, 8.87% between 2 and 3 km, 8.27% between 3 and 4 km, and 6.20%between 4 and 5 km. Clearly the damage decreased with the increase of distance, following an exponential relationship with the correlation coefficient of 0.9577.

3.4. Relationship between damaged vegetation and the rivers

Vegetation in earthquake-affected areas changed its characteristics as follows: (1) Injured trees were subjected susceptible to infection by plant diseases and insect pests, therefore, the regrowth of the trees and the quality of their wood declined by a wide margin. (2) Forest fires were more prevalent as the restoration of vegetation became more difficult due to non-clearage of wastes on its lands. (3) Gaps in the forest canopy, which arose after

The damaged vegetation distribution was presented in Fig. 7 (The relationship between damaged vegetation and the rivers in 9 severely damaged cities and counties). The damaged vegetation was mainly distributed along the river within 5 km, reaching 317.177 km2 (73.69% of the total area of damaged vegetation).

3.5. Subsequent damage to vegetation

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Fig. 6. The distribution of vegetation damaged by Geo-hazards in Subao river basin.

trees were toppled, allowed enabled a change in the species composition of the vegetation to occur. (4) Some areas, formerly with low plant coverage may have been convert to areas with a higher coverage due to soil upheaval and change of seed pool. The damaged area was always an important region for soil and water conservation and a significant part of ecological defense in the upper reaches of Yangtze River. Landslides and debris-flows have a crucial effect on forest and soil in this mountainous region, which not only give rise to local variation in the vegetation, but also result in physicochemical property changes of the soil. Moreover, it has special effect on the condition of the downstream river, as well as a direct and indirect influence on the other regions. Devastating geo-hazards have a negative effect on vegetation growth and accelerate the destruction of rocks and soils. This destruction provides more soil material that threatens the viability

of rivers through increased sediment and, in turn, may affect the water supply to people downstream. 4. Environmental impact of vegetation damage 4.1. Potential impact of vegetation damage to hydrologic progress in watershed Hydrologic progress and hydrodynamic condition of slopes in earthquake-affected areas were changed due to vegetation

Percent age/%

0.3 0.25

25.39%

0.2

-0.2508x

0.15

y = 0.265e 2 R = 0.9172

13.51% 11.45% 8.87%

0.1

8.27% 6.20%

0.05 0

<0.5

0.5-1

1-2

2-3

3-4

4-5

Distance to river/km Fig. 7. The relationship between damaged vegetation and the rivers in 9 severely damaged cities and counties.

Fig. 8. The destroyed plant debris piled up in gully. Drift woods blocked gully and broke into large scale debris flow after earthquake in forest park, Anxian county (Photo from You Yong).

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damage. Crown canopy, litter layer, root system and well-formed soil in slopes were destroyed, leading to the function decline of rainfall redistribution. Infiltration runoff processes were changed as follows: bare rock with steep slope and small roughness caused by rock avalanches and landslides has little ability to control surface water infiltration. This has the effect of increasing runoff and flow concentration leading to an increased danger from erosion. Deposit regions have an increased infiltration capacity to fill water and become unstable with high water saturation. Consequently, the hydrological adjusting function is diminished. Since 1940, the runoff coefficient and mean annual runoff have decreased while forest coverage has reduced in the upper stream of Mingjiang River (Zheng et al., 2010). In fact, the runoff rate decreased by 0.27 m3 /km2 as forest coverage reduced 1% from 1930 to 1970 in the upper stream of Mingjiang River (Man et al., 2007). After the earthquake, the runoff rate and mean annual runoff will decline further due to the reduction of forest coverage.

Fig. 9. Debris flow contained abundant drift woods on August, 13, 2010, in Qingping town, Mianzhu city (Photo from You Yong).

4.2. Increase of soil erosion area and sediment delivery Destructive geo-hazards induced large area of collapsed woodland, toppled trees, and buried plants. Numerous unstable bare slopes and unconsolidated materials were created and easily sputtered during rainstorms. Subsequent runoff confluence on slopes resulted in scouring erosion, which cuts the slopes and form gullies. For example, in the Yuleishan area of Longmenshan, 20 km from Wenchuan county, the total area of erosion and land movement due to vegetation loss increased by 86.3 km2 (19.2% of the study area). Compared with pre-earthquake figures, the area of severe and very severe soil erosion were substantially increased by 45.8 km2 and 99.2 km2 (Di et al., 2010). Intensive soil erosion (including landslides, rock avalanches, debris flows) transport large quantities of sediment into rivers and threaten the safety of natural river channels and hydraulic and hydro-power engineering. High sedimentation rates in river channels have occurred previously in earthquake-affected areas. For example, on August, 13, 2010, debris flows transported large amount of materials into the section between Gengda and Yingxiu of Yuzixi River and Qingping section of Mianyuan River after a rainstorm. With the highest accumulation of river bed level-30 m in Yuzixi River, the average accumulation of level of river channel is about 15 m in the section of Yuzixi River and 6 m in the section of Mianyuan River. 4.3. Aggrandizement of debris-flow destructivity and landslide susceptibility The frequency of geo-hazards has caused serious vegetation damage and produced a large volume of plant debris in earthquakeaffected areas. Plant debris is piled up on landslides and rock avalanches or mixed with rock debris and increases the porosity of the deposit region. Landslides and rock avalanches with high porosity are particularly unstable as rapid rainfall infiltration decrease the strength of soil and increase the weight of the deposit. Continuous rainfall can cause landslides, rock avalanches and drift wood to move down as debris flows into the gully. Drift woods are usually too long to pass through the channel and so can easily form blockages across the channel creating a series of dams/barriers composed of soil and woods in main channel and tributary (Fig. 8 (The destroyed plant debris piled up in gully)). Often initial dams are relatively weak and can be destroyed by stronger, subsequent flows. This step by step increase due to chain reaction creates a large scale debris flow. Presently, the sequence of block and break is: drift woods on slope → moving into gully → creating dams/barriers → breaking → breaking step by

step → chain reaction → creating large scale debris flow. Moreover, due to the shock of drift woods, destructive effect of debris flow increases (Fig. 9 (Debris flow contained abundant drift woods on August, 13, 2010, in Qingping town, Mianzhu city)). In addition, although some vegetation areas would appear to be normal from a superficial inspection, the ground contains a large number of cracks. For instance, in the upper stream of Weijiagou watershed, Beichuan county, the ground of Alnus cremastogyne forest which is undestroyed has 12 cracks (width: 30–50 cm). The anchorage action of forest roots cannot fix the cracks on the ground of slope so the strength of weak structural plane will be reduced due to the water wedge impact induced by runoff, which infiltrate down from cracks. Combined with the weight of trees and the action of wind-swing, the slope through time becomes unstable and can become susceptible to landslide. 4.4. Landscape fragmentation and soil degradation Damaged vegetation leads to the decline of ecosystem function in earthquake-affected areas. Existence and breeding of the flora and fauna are seriously affected due to habitat fragmentation and the reduction of total habitat area and connectedness. Vegetation patches before the earthquake are presented as mosaic distribution with other patches. After the earthquake, vegetation patches become displaced by bare land patches and vegetation coverage correspondingly decreased, within these patches it is difficult to restore and reconstruct vegetation. A case study of vegetation patches change was performed in Longxi-hongkou and Baishuihe nature reserve (Wang et al., 2008). Before the earthquake there were 2123 forest and grass patches (3 patches (1000–10,000 ha) and 35 patches (100–1000 ha)). After the earthquake, forest and grass patches increased to 11,673 and the area of the biggest patch decreased from 22,981 to 10,462 ha. Moreover, the biggest patch was divided into several patches and aggravated landscape fragmentation. Due to landscape fragmentation, marginal habitat increased and interior habitat and it is connectedness declined. Surface soil tends to be eroded without vegetation cover and so nutrients and organic matter lose due to soil and water loss. Additionally, special mass movement types (rock avalanches, landslides, rockfalls, debris flows) destroyed or buried nutritious and well-formed soil, which was then replaced by crude soil. Following soil degradation, the vegetation growth conditions deteriorate and inhibit vegetation reconstruction. Therefore, ecological restoration is difficult due to co-degradation of vegetation-soil system in the

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earthquake-affected areas. For example, in Wenchuan earthquake areas, the areas of severe soil erosion of forests and grasslands were substantially increased by 8.3–12.7% and 3.3–20.3% (Wang et al., 2008). The amount of loose materials produced in vegetationdamaged areas was increased to 3.422 billion tons, which delivers 1.540 billion tons sediment into the rivers with a sediment transport ratio of 0.45. As a result of vegetation destruction, the financial loss of water conservation function of vegetation reached 2.197 billion RMB per year (Wang et al., 2009). 5. Conclusions The Wenchuan Earthquake triggered numerous geo-hazards, including landslides, rock avalanches, debris flows and landslidedammed lakes. These geo-hazards caused heavy losses by destroying roads, smashing vehicles, burying villages, blocking rivers and damaging vegetation. In some regions, vegetation was ruined by landslides, rock avalanches and debris flows. Geo-hazards seriously damaged and buried large area of vegetation. The area of damaged vegetation reached 1249.465 km2 and vegetation coverage decreased by 4.76% in 9 severely damaged cities and counties. Induced by geo-hazards, forest coverage (not including grass) decreased by 12.37% in typical catchment-Subao river, Beichuan county. In earthquake-affected areas, damaged vegetation was distributed along both sides of river bank and decreased sharply with distance from river, obeying an exponential relation. Due to vegetation damage, crown canopy, litter layer, root system and well-formed soil in slopes were destructed, which decreased the regulating action of the hydraulic process. Bare rock caused by rock avalanches and landslides has little ability to control surface water infiltration, so runoff increased rapidly and flow concentration expedited. Rubble and unconsolidated materials with porous structures from deposited rock avalanches and landslides have increased infiltration capacity to fill water and so become unstable with high water saturation. Drift woods from damaged vegetation create a series of dams composed of soil and woods in the main channels and tributaries. The dams break up step by step due to chain reaction, intensifying the scale and destructive effect of debris flow. Some slopes with a large number of cracks in ground become unstable due to increased porosity and weight in rainy reason. The damaged vegetation leads to landscape fragmentation and soil degradation. Farther more, the ecosystem function declined and vegetation restoration and reconstruction was difficult due to co-degradation of vegetation-soil system in the earthquakeaffected areas. Thus the restoration and rehabilitation of the earthquakeaffected areas are made more difficult through the challenges of the environmental issues arising from the vegetation damage in earthquake-affected areas. Characteristics of damaged vegetation and controlling factors of restoration, the mechanisms of interaction between vegetation and geo-hazards, the ability of vegetation to resist geo-hazards and the recovery potential, methods and technologies of vegetation restoration in geo-hazard affected area should be systematically studied to aid restoration and rehabilitation strategies. Acknowledgements This work is supported by the National Basic Research Program of China (973 Program) (Grant No. 2011CB409902), Key laboratory of Mountain Hazard & Surface Processes, CAS (Grant No. 2009), and the Fujian Provincial Natural Science Foundation (No. 2009J05092).

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