Journal of Integrative Plant Biology 2006, 48 (6): 642−653

Effect of Hillslope Gradient on Vegetation Recovery on Abandoned Land of Shifting Cultivation in Hainan Island, South China Yi Ding, Run-Guo Zang* and You-Xu Jiang (Key Laboratory of Forest Ecology and Environment, The State Forestry Administration; Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry, Beijing 100091, China)

Abstract In the present study, we investigated the effect of hillslope gradient on vegetation recovery on abandoned land of shifting cultivation in Hainan Island, south China, by measuring community composition and structure of 25-year-old secondary forest fallows along a hillslope gradient (up-, middle-, and down-slope position). A total of 49 733 free-standing woody plant stems higher than 10 cm and belonging to 170 species, 112 genera, and 57 families was found in the three 1-hm2 investigation plots. Stem density was highest in the down-slope stand and lowest in the up-slope stand. Species richness and the Shannon-Wiener index were both highest in the middle-slope stand, and lower in the down-slope and up-slope stands. The recovery forest fallows on different hillslope positions were all dominated by a few species. The five most abundant species accounted for 70.1%, 58.8%, and 72.9% of total stem densities in stands in the up-, middle-, and down-slope positions, respectively. The five species with the greatest basal areas accounted for 74.5%, 84.3%, and 74.7% of total stem basal area for the up-, middle-, and down-slope positions, respectively. The number of low-density species (stem abundance less than five) increased from the up-slope position downward. Of the nine local common species among three different functional groups, the short-lived pioneer species dominated the up-slope position, but long-lived pioneer species dominated the middle- and down-slope positions. The climax species of primary tropical lowland rain forest was found in the downslope position. Both the mean diameter at breast height (DBH) and mean height of the trees increased with decreasing hillslope gradient. The stem density and basal area in different size classes were significantly different in stands in different slope positions. Our results indicated that the rate of secondary succession varies, even over small spatial scales caused by the hillslope gradient, in early vegetation recovery. Key words: community structure; functional groups; Hainan Island; hillslope gradient; shifting cultivation; species diversity; tropical rain forest; vegetation recovery. Ding Y, Zang RG, Jiang YX (2006). Effect of hillslope gradient on vegetation recovery on abandoned land of shifting cultivation in Hainan Island, South China. J Integrat Plant Biol 48(6), 642−653.

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Shifting cultivation or slash-and-burn agriculture is a widespread form of land use that exists in almost all tropical regions (Brady 1996; Hauser and Norgorver 2001). It is one of the main

Received 21 Oct. 2005

Accepted 15 Feb. 2006

Supported by the National Natural Science Foundation of China (30340047 and 30430570). *Author for correspondence. Tel: +86 (0)10 6288 9546; Fax: +86 (0)10 6288 4972; E-mail: .

causes of annual tropical deforestation (Richards 1996) and accounts for 35% in all New World, 70% in Africa and 49% in Asia (Whitmore 1998). In the process of shifting cultivation, all vegetation is slashed and almost no remnant trees are left, except for very few large individuals at the end of the dry season. After drying for several weeks, the slash is burnt. With the onset of the rainy season, the land is planted with crops. It is usually abandoned to natural recovery after 2–3 yr of cultivation, when crop production declines. The fallow period is usually 7–15 yr. Three or four cycles of cultivation did

Vegetation Recovery of Shifting Cultivation 643

not slow biomass accumulation of secondary forests (Steininger 2000; Lawrence 2005). However, with increasing pressure from population growth and a shortage of agricultural land, the fallow period has been shortened excessively and the period of cultivation has been extended for too long (Whitmore 1998). Unlike other agricultural practices, the secondary forest fallow itself is one part of the shifting cultivation process (Lawrence 2004). With the decline of tropical primary forest, the succession rate and trajectory of secondary forest fallow could affect tropical forest biodiversity conservation, global environmental change, and ecosystem services that are essential to humans (Brown and Lugo 1990; Guariguata and Ostertag 2001). From the 1980s, there has been increased interest in the secondary succession on abandoned lands of shifting cultivation (Ewel 1981; Uhl et al. 1981; Uhl 1987; Lawrence 2004; 2005; Ding and Zang 2005; Gehring et al. 2005; Lawrence et al. 2005). The environmental heterogeneity in space is regarded as one of the most important factors in maintaining tropical forest biodiversity (Wright 2002; Tuomisto et al. 2003). Topography, as an indirect ecological factor (Fang et al. 2004), can affect the distribution and regeneration of plants (Thompson et al. 2002). With increasing knowledge on tropical forest biodiversity, many researchers have recognized that seed dispersal and germination, seedling recruitment and survival, and community composition and structure are all affected by topography (Clark et al. 1999; Bellingham and Tanner 2000; Svenning 2001; Robert and Moravie 2003). As an important part of topography, the hillslope gradient can influence vegetation recovery by changing biotic and abiotic environmental conditions. After high-intensity disturbance, up-slope sites would experience more soil erosion, less seed input, and harsher microclimate than downslope sites. However, there is little information available on the effect of hillslope gradient on vegetation recovery on abandoned lands of shifting cultivation. The deforestation rate in Hainan Island, South China, is known to be higher than the average deforestation rate of the world (Zang et al. 2004). The great pressure of population increase and shortage of agricultural lands led to the shifting cultivation in Hainan Island more extensive than other tropical regions. On Hainan Island, the mountainous areas occupy the main part of the region, resulting in variable topography (Jiang et al. 2002). Most primary lowland tropical forests in low-elevation areas of Hainan Island have been converted to shifting cultivation lands. Natural recovery of the vegetation is extremely difficult on some abandoned agricultural lands owing to past high-intensity land use. Realizing the effects of dominant factors on secondary succession will assist us to understand the recovery mechanism vegetation on abandoned shifting cultivation lands, which is important of the determination of proper management activities in tropical forest regions. In the present study, we investigated

and compared the composition and structure of a naturally regenerated secondary forest fallow along a hillslope gradient (up-, middle-, and down-slope positions) in Hainan Island. The following issues were examined: (i) were there any differences in stem abundance, species richness, and community structure in stands along the hillslope gradient; (ii) how did the species composition of vegetation change along the hillslope gradient; (iii) did the rate of secondary succession vary along the hillslope gradient; and (iv) did the hillslope gradient affect the distribution patterns of different functional groups?

Results Species diversity In total, 49 733 free-standing woody plant stems higher than 10 cm in the three 1-hm2 secondary forest stands were recorded, which belong to 170 species, 112 genera, and 57 families. Stems were most abundant in the down-slope stand, and decreased from the middle- to the up-slope stand. The species and family richness of stands varied along the hillslope gradient. Stands in the up-slope position contained fewer species and exhibited less family richness than stands located in the middleand down-slope positions (Table 1). Species richness was highest in the middle-slope stand, whereas family richness was highest in the down-slope stand. Stems in the small size classes (small seedlings, large seedlings, and saplings) contributed more to the percentage of the species and family richness than those in the large size classes (small trees and large trees) in the three sites (Table 1). The Shannon-Wiener index was highest in the middle-slope stand, and lowest in up-slope stand (Table 1). The accumulative species-area curves showed that the rate of species accumulation was similar for stands in the middle- and downslope positions, and these rates were both higher than that for the stand in the up-slope position (Figure 1A). The speciesindividual accumulation rate on different slope positions differed significantly (Figure 1B). The stand in the middle-slope position exhibited the greatest rate of species-individual accumulation among the three sites. Species dominance Only a few species dominated in the forest fallow on different hillslope positions (Figure 2A, B). The five most abundant species of the secondary forest fallow accounted for 70.1%, 58.8%, and 72.9% of the total stem abundance in stands in the up-, middle- and down-slope positions, respectively. However, the dominance of some species were mainly contributed by their high densities in the low size classes. There were more

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Table 1. Biodiversity and structural characteristics of the 25-year-old stands along the hillslope gradient in Bawangling Nature Reserve Slope gradient a

Indices

Size class

Middle-slope

Down-slope

Species richness

Small seedlings

47 (0.69)

72 (0.60)

66 (0.57)

Large seedlings Saplings Small trees Large trees Total stems

50 (0.74) 46 (0.68) 31 (0.46) 19 (0.28) 68

83 (0.70) 84 (0.70) 51 (0.43) 43 (0.36) 119

79 (0.68) 81 (0.70) 49 (0.42) 36 (0.31) 116

Small seedlings Large seedlings Saplings Small trees Large trees Total stems

23 (0.70) 26 (0.79) 22 (0.67) 19 (0.58) 12 (0.36) 33

35 34 36 27 23 44

30 38 38 26 23 49

Shannon-Wiener index

2.49

2.9

2.64

Density (No. stems /hm )

Small seedlings Large seedlings Saplings Small trees Large trees Total stems

5 2 1 3

217 (0.41) 286 (0.18) 352 (0.11) 687 (0.29) 209 (0.02) 12 750

7 3 3 1

466 (0.48) 239 (0.21) 049 (0.20) 170 (0.08) 513 (0.03) 15 437

10 841 (0.50) 4 791 (0.22) 4 021 (0.19) 1 023 (0.05) 870 (0.04) 21 546

Basal areab (cm2/hm2)

Saplings Small trees Large trees

23 847.5 (0.26) 39 539.7 (0.42) 30 012.9 (0.32)

13 234 (0.08) 36 959.4 (0.23) 109 856 (0.69)

16 526.0 (0.07) 34 704.9 (0.14) 197 496.6 (0.79)

Total stems

93 400.1

160 049.4

248 727.4

Family richness

Diversity 2

Up-slope

(0.80) (0.77) (0.82) (0.61) (0.52)

(0.61) (0.78) (0.78) (0.53) (0.47)

For size class categorizations, refer to Materials and Methods. a

Data show the number of stems, with percentages given in parentheses. Note, that percentages are given as a percentage of total stems.

b

Only for stems

1.5 m in height.

low-density species in stand on middle-slope and down-slope positions than the stand on up-slope position (Figure 2A). The number of species with less than five stems was 19 (27.5%), 48 (40%), and 49 (41.9%) in up-, middle-, and down-slope stands, respectively (Table 2). The five most dominant species represented 74.5%, 84.3%, and 74.7% of the basal areas of all stems in the up-, middle-, and down-slope stands, respectively (Table 3). Moreover, the species composition of the 10 most dominant species in terms of density or basal area varied along the hillslope gradient (Tables 2, 3). The population structures of the nine local common species among the three different functional groups varied along the hillslope gradient (Figure 3). The short-lived pioneer species Melastoma sanguineum Sims, Cratoxylum cochinchinense (Lour.) Bl., and Glochidion sphaerogynum (Muell.-Arg.) Kurz were most abundant in the up-slope stand. The long-lived pioneer tree species Engelhardtia roxburgiana Lindl., Schima superba Gardn. et Champ., and Lithocarpus elaeagnifolius (Seem.) Chun showed the opposite distribution pattern, which all flourished in the down-slope stand. However, the shortlived pioneer tree species Aporosa chinensis (Champ.) Merr.

was well represented in each of the stands in the three different slope positions. Vatica mangachapoi Blanco, the climax canopy species of tropical lowland rain forest, was mainly found in the down-slope stand. The climax shrub species of tropical forest Psychotria rubra (Lour.) Poir. dominated the understory of the down-slope stand, but it was also found in the middle-slope stand. Community structure The mean diameter at breast height (DBH; non-parametric oneway ANOVA, F = 34.704, P < 0.001) and height (non-parametric one-way ANOVA, F = 82.049, P < 0.001) for stems 1.5 m in height differed significantly among stands along the hillslope gradient (Figure 4). Although no difference in mean stem DBH existed between the middle- and down-slope stands (P = 0.213), there were significant differences in stem DBH distributions (χ2 = 484.382, P < 0.001) and height distributions (χ2 = 2 541.716, P < 0.001) among the stands in different hillslope positions. The diameter or height distribution was skewed towards the small DBH or height classes (Figure 5). In each

Vegetation Recovery of Shifting Cultivation 645

Table 2. Density of the 10 most dominant species in the 25-year-old stands in different hillslope positions in Bawangling Nature Reserve Species

Density (No. stems/hm2)

Up-slope stand

Cratoxylum cochinchinense (Lour.) Bl.

3 600

Melastoma sanguineum Sims

2 876

Aporosa chinensis (Champ.) Merr.

1 624

Glochidion sphaerogynum (Muell.-Arg.) Kurz

Figure 1. Randomized cumulative species-area (A) and speciesindividual (B) curves for the 25-year-old stands in different hillslope positions in Bawangling Nature Reserve. U, up-slope; M, middle-slope; D, down-slope.

777

Toxicodendroh succedaneum (Linn.) O. Kuntze

418

Cinnamomum camphora (Linn.) Presl

346

Dodonaea viscose (Linn.) Jacq.

278

Phyllanthus emblica Linn.

248

Breynia fruticosa (Linn.) Hook. f.

233

Lannea coromandelica (Houtt.) Merr.

188

Middle-slope stand

Aporosa chinensis

4 747

Psychotria rubra (Lour.) Poir.

1 804

Elaeocarpus sylvestris Poir.

934

Ardisia quinquegona Bl.

837

Cratoxylum cochinchinense

763

Decaspermum gracilentum (Hance) Merr. et Perry

431

Adinandra hainanensis Hayata

423

Machilus suaveolens S. Lee

421

Canthium horridum Bl.

419

Melastoma sanguineum

409

Down-slope stand

Psychotria rubra

5 511

Aporosa chinensis

3 665

Ardisia quinquegona

3 027

Prismatomeris tetrandra (Roxb.) K. Schum.

1 627

Lithocarpus elaeagnifolius (Seem.) Chun

1 277

Engelhardtia roxburgiana Lindl.

965

Syzygium hancei Merr et Perry

639

Figure 2. Species dominance-rank curves.

Diospyros eriantha Champ. ex Benth.

512

(A) Log(relative abundance) versus abundance rank and (B) Log

Cratoxylum cochinchinense

508

(relative basal area) versus basal area rank for 25-year-old stands in

Canthium horridum

478

different hillslope positions in Bawangling Nature Reserve.

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Table 3. Basal areas of the 10 most dominant species in the 25-year-old stands in different hillslope positions in Bawangling Nature Reserve Up-slope stand Species

Middle-slope stand Basal area (m 2/hm2)

Species

Down-slope stand Basal area (m2 /hm2)

Species

Basal area (m 2/hm2)

Cratoxylum cochinchinense

2.70

Syzygium cumini (Linn.) Skeels

2.22

Engelhardtia roxburgiana

9.91

Glochidion sphaerogynum

2.32

Elaeocarpus sylvestris

2.14

Lithocarpus elaeagnifolius

3.03

Ilex rotunda Thumb.

1.10

Aporosa chinensis

1.48

Castanopsis hystrix

2.36

Aporosa chinensis

0.50

Engelhardtia roxburgiana

1.26

Schima superba Gardn. et Champ.

1.72

Melastoma sanguineum

0.34

Liquidambar formosana Hance

1.15

Cyclobalanopsis kerrii (Craib) Hu

1.55

Engelhardtia roxburgiana

0.33

Glochidion sphaerogynum

1.09

Cratoxylum cochinchinense

0.98

Cinnamomum camphora

0.31

Toxicodendroh succedaneum

0.98

Glochidion sphaerogynum

0.65

Elaeocarpus sylvestris

0.25

Cratoxylum cochinchinense

0.57

Aporosa chinensis

0.65

Phyllanthus emblica

0.23

Albizia procera (Roxb.) Benth.

0.38

Psychotria rubra

0.41

Adinandra hainanensis

0.17

Castanopsis hystrix A. DC.

0.37

Ilex rotunda

0.32

Figure 3. Population structures of nine common species in three different functional groups in the 25-year-old stands in different hillslope positions in Bawangling Nature Reserve. For size class and functional groups categorizations, see Materials and Methods.

Vegetation Recovery of Shifting Cultivation 647

Figure 4. Mean (± SE) values for (A) diameter at breast height (DBH) and (B) height for stems

1.5 m in 25-year-old stands in different

hillslope positions in Bawangling Nature Reserve. Different letters at the tops of columns indicate significant differences (P < 0.05).

Figure 5. (A) Diameter at breast height (DBH) and (B) height class proportion distributions of stems in 25-year-old stands in different hillslope positions in Bawangling Nature Reserve. Stems with DBH = 0 refer to those individuals in size classes 1 (small seedlings, height between 0.1 and <0.5 m) and 2 (large seedlings,

stand in different hillslope positions, the greater proportion of trees was represented by stems with small DBH and height. In general, there was a greater proportion of small trees (DBH 5.0–9.9 cm) in the up-slope stand (Table 1). There were 13 stems with DBH 20 cm in the up-slope stand (Figure 5A). In contrast, the number of stems with DBH 20 cm in middle- and down-slope stands was 101 and 224, respectively. There were 317 and 85 stems that were 15 m in height in the down- and middle-slope stands, respectively, whereas no stems of this size were found in the up-slope stand (Figure 5B). The densities and basal areas of stems in different size classes and in total all varied significantly (non-parametric oneway ANOVA, all P < 0.01) among the stands in different hillslope positions (Figure 6). Except for small trees, the densities of stems within the other four size classes in the down-slope stand were higher than those in middle- and up-slope stands (Figure 6A). The stand in up-slope position had the greatest stem basal areas in the sapling and small tree size classes. However, the basal areas for large trees and for all stems in total were greatest in the down-slope stand. Stem density (P = 0.988) and basal area (P = 0.679) of small trees was not

height between 0.5 and <1.5 m).

significantly different between the middle- and down-slope stands.

Discussion Species diversity In tropical regions, the recruitment limitation of seeds is regarded as an important factor that determines the succession rate of secondary forest fallow (Uhl 1987; Hooper et al. 2004). On abandoned agricultural lands that have experienced highintensity disturbance, the regeneration of forest is mainly dependent on seed dispersal (Uhl 1987; Miller and Kauffman 1998). So, factors that influence seed dispersal and recruitment would affect vegetation recovery. For example, the distance to remnant primary forest and species composition of seed sources, which are impacted on by the landscape pattern around the

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Figure 6. Mean (± SE) values for (A) density and (B) basal area for stems in different size classes in 25-year-old stands in different hillslope positions in Bawangling Nature Reserve. Different letters at the tops of columns indicate significant differences (P < 0.05). For size class categorizations, see Materials and Methods.

abandoned lands (Endress and Chinea 2001), could influence the rate and trajectory of secondary succession of forest fallow (Holl 1999; Duncan and Duncan 2000; Guariguata and Ostertag 2001). The regrowth of secondary forest fallow is also influenced by weed competition (Zimmerman et al. 2000), remnant vegetation (Finegan and Delgado 2000; Guariguata and Ostertag 2001; Mesquita et al. 2001), and edaphic conditions (Uhl and Jordan 1984; Brown and Lugo 1990). Although the effect of topography on the species distribution in primary forest is limited (Wright 2002; Valencia et al. 2004), it should become obvious in the early secondary succession process. In the present study, we found that secondary forest fallow in the up-slope position had less species richness, stem density, basal area, and mean height than stands in the middleand down-slope positions after 25 yr of succession (Table 1; Figure 4). Most of the seeds of early established pioneer species recruited on the abandoned agricultural lands were dispersed by wind (Richards 1996; Finegan and Delgado 2000). In

the present study, the abandoned land in the up-slope position may have received relatively little seed input. In contrast, land in the middle- and down-slope positions not only may have experienced less severe soil erosion, but may also have received a greater seed input from surface runoff or animal dispersal activity. A long period of disturbance could make any contribution of seeds of primary species in the soil bank less important to the forest regrowth (Ewel 1981; Uhl 1987). One study has demonstrated that the cycle number of shifting cultivation is related to soil nutrients (Lawrence 2005), but the edaphic condition itself is influenced by hillslope gradient (Valencia et al. 2004). Moreover, soil erosion could be more severe in the up-slope position during the process of cultivation and early vegetation recovery. Because of these reasons, the differences in community composition of our investigated plots were probably mainly caused by the different environmental conditions along the hillslope gradient, such as soil water content, organic matter, and nutrient conditions. However, identifying the main limiting factors requires further investigation on the physical and chemical characteristics of the soil along the hillslope gradient. Unlike stem density and basal area, among the three sites species richness was highest in the middle-slope stand (Table 1; Figure 1). This may be caused by the species Syzygium cumini (Linn.) Skeels, which dominated in remnant trees in the middle-slope position site (Table 3). This species is usually distributed on agricultural fields in Bawangling Nature Reserve (BNR) and its fruits and seeds are eaten by birds (Y Ding et al.; pers. obs., 2004). Many studies on tropical secondary succession in abandoned agricultural lands have demonstrated that the remnant trees play an important role in vegetation recovery (Finegan 1996; Guariguata and Ostertag 2001; Ferguson et al. 2003). These trees could serve as perches and food resources for seed dispersers (Carrière et al. 2002), provide shade for the regeneration of shade-tolerant species (Uhl 1987; Ferguson et al. 2003), or directly input seeds to accelerate succession (Zahawi and Augspurger 1999). So, the remnant trees of S. cumini had possibly attracted more seed dispersers and increased the species richness consequently in our middle-slope secondary forest fallow. Dominance of different functional groups In the forest fallow investigated in the present study, different functional groups dominated in stands in different hillslope positions, which implies that functional group replacement may occur during the process of secondary succession in tropical lowland rain forest of Hainan Island. The most dominant species in the up-slope position were short-lived pioneer species. However, long-lived pioneer species dominated in the middleand down-slope positions. Furthermore, the climax species of

Vegetation Recovery of Shifting Cultivation 649

lowland tropical rain forest was mainly found in the downslope position. The community composition of stands in different hillslope positions reflected, to some degree, the different stages of succession in the tropical region: the shorter-lived species dominated early succession, whereas the longer-lived species were abundant in the middle phase and were eventually replaced by other, more shade-tolerant and long-lived species (Finegan 1996; Richards 1996; Guariguata and Ostertag 2001). Like other early secondary succession on abandoned agricultural land (Ewel 1981; Uhl et al. 1982; Aide et al. 2000), the light-demanding pioneer species dominated on the abandoned lands in BNR. Moreover, it was very difficult to find lowland tropical primary tree species in the present study sites, even after 25 yr of succession. Interestingly, we found more primary shrub species of lowland tropical forest than primary canopy tree species in the sites examined, indicating different responses of different species within the same functional group to the secondary successional process. For instance, P. rubra and Ardisia quinquegona Bl., the primary forest shrub species, established early in the secondary succession and dominated in the understory of the secondary forest fallow. These shrubs have a higher ability to adapt to stress, a relatively early maturation, and their seeds are used by birds as food. So they can recruit and establish successfully in the early period of succession and dominate in the understory of later succession as well. However, the canopy species of the primary forest were only able to gradually colonize and dominate the site until the microenvironment recovered to one that was more suitable for their survival and growth. Community structure The burning of slash would result in an increase of the input of nutrients into the soil and the release of nutrients in soil during the process of shifting cultivation (Ewel 1981). However, some studies have indicated that soil erosion was markedly increased after slash-and-burn (Lu and Zeng 1981; Tang et al. 1997) and different nutrient elements showed different degrees of loss in the crop cultivation process (Ewel 1981; Uhl and Jordan 1984; Lawrence and Schlesinger 2001). The low soil nutrient levels and changed composition of soil nutrient elements in the abandoned fields could markedly reduce the growth of plants (Jordan 1989). The abundant stems with small DBH and low height in the three stands of our investigated sites demonstrated the slow diameter and height growth rate caused by the low level of soil nutrients (Figure 5). In this area, the soil organic matter of shifting cultivation lands was markedly less than selectively logged and old-growth forests (FY Deng and RG Zang, unpubl. data, 2006). In the present study, we found a low density of large trees in

the secondary forest fallow in the up-slope position (Figure 6). The low level of soil nutrients and light inhibition may be the main reason for these observations. However, small trees were more abundant in the stand in up-slope position than in stands in either the middle- or down-slope positions (Table 1; Figure 6). This DBH frequency distribution was caused, in part, by differences in the life history (Finegan 1996; Finegan and Delgado 2000) of dominant species in different hillslope positions. In the up-slope position, the dominant species were short-lived pioneer species, which normally have limited ability of diameter growth. In contrast, the long-lived pioneer species with large DBH dominated and shaded out the small, short-lived pioneer species as a result of a taller canopy in the middle- and down-slope positions. We found significantly different stem DBH and height frequency distributions in stands along the hillslope gradient (Figure 5), implying that the differences in community structure were caused by the hillslope position. After 25 yr of succession, the mean basal area of secondary forest fallow was 16.74 m 2/hm 2, which varied between 9.34 and 24.87 m2/hm2. If we only consider stands in the middleand down-slope positions, the mean basal area of 25-year-old secondary forest fallow was 20.44 m2/hm2, which is less than half of the lowland old-growth forest (43 m 2/hm2) in BNR (Y Ding and RG Zang, unpubl. data, 2006). The rate of basal area accumulation in BNR was lower than in other tropical regions (Brown and Lugo 1990). Some studies have demonstrated that biomass accumulation follows the form of an asymptote curve, indicating a very rapid initial rate of biomass accumulation, followed by a slowing down in rate latter in the succession process (Guariguata and Ostertag 2001; Gehring et al. 2005). Moreover, the past disturbance in BNR was quite severe, so the recovery of community structure was very slow in the region investigated and the recovery time required may be 100 yr or more. With the age of the stand, the fallow in the middleand down-slope positions may become more similar to that in the up-slope position. Ultimately, the stands in the different slope positions may develop into the same vegetation type, but the time needed for this to occur could be extremely long. Implications for vegetation recovery In tropical regions, the species accumulation rate of secondary succession after anthropogenic disturbance is fast (Uhl 1987). To reach species richness comparable with the primary forest, normally approximately 70–100 yr is required (Brown and Lugo 1990; Finegan 1996; Guariguata and Ostertag 2001), although it has been reported that only 40 yr are needed in some regions (Aide et al. 2000). The accumulation rate of ecological functioning, such as primary production and nutrient cycle, is faster than the rate of species diversity (Uhl and Jordan 1984; Read and Lawrence 2003; Lawrence 2005). But the rate of

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species accumulation declined with the increasing of severity and duration of disturbance (Finegan 1996). Moreover, species composition is markedly changed (Brown and Lugo 1990; Finegan 1996; Guariguata and Ostertag 2001), especially the primary forest species has lost after several cycles of shifting cultivation (Lawrence 2004), which makes the restoration to primary forest more difficult. It would take several centuries to recover the species composition before the disturbance (Richards 1996), but some people have argued that it is impossible to achieve this even after only one cycle of shifting cultivation (Lawrence 2004). After abandonment of shifting cultivation fields, the changed abiotic environment, such as the physical and chemical characteristics of the soil, result in difficulties in seed germination, seedling recruitment, survival, and growth (Guariguata and Ostertag 2001; Hooper et al. 2002). In addition, seed predation is high in abandoned fields (Uhl 1987). The strong feedback between biotic factors and the physical environment can alter the efficacy of these succession-based management efforts (Suding et al. 2004). In BNR, the area of primary lowland tropical rain forest has been markedly reduced as a result of extensive commercial logging and long-term shifting cultivation (Ding and Zang 2005) and most of the areas in BNR are now dominated by secondary forest. The species composition of secondary forest is different from that of the remnant primary or old-growth forest. So, the secondary forest cannot provide the seeds of primary forest species for recovery. Furthermore, the pine plantations distributed in BNR have increased the distance between abandoned lands and remnant old-growth forest. Difficulties in vegetation recovery have increased accordingly because the seed resources have decreased or disappeared within the changed landscape (Endress and Chinea 2001). It is almost impossible to restore the climax lowland tropical rain forest using natural regeneration only. So, it is necessary to accelerate the succession rate by management activities when biodiversity conservation is the chief goal, which could ameliorate the community environment and further accelerate regeneration (Aide et al. 2000; Chazdon 2003). However, we must realize that different recovery practices and guidelines should be applied to different topographical sites, such as on different hillslope positions. After 25 yr of succession, the secondary forest fallow in the down-slope position in the present study had a more complex community structure, where the understory environment was also ameliorated. It will be helpful to increase biodiversity by planting some primary lowland tropical rain forest species on such a site. In contrast, severely degraded soil covered many parts of the site in the up-slope position. Moreover, herbs still dominated in some subplots on the floor. Under such conditions, natural succession without further human disturbance may be a better choice for the recovery of community structure.

Materials and Methods Study site The present study was conducted in BNR (18°50'–19°05' N, 109°05'–109°25' E), which is located on the boundary between Changjiang County and Baisha County in Hainan Island, south China. The topography varies from flat terraces to undulating hills and the altitude varies between approximately 120 and 1 475 m. The climate is tropical monsoon. The mean annual precipitation is 1 750 mm with a distinct wet season (from May to October) and a dry season (from November to April). The mean annual temperature is 23.6 °C (Zang et al. 2005). The parent material is granite and the soil is latosols. The main vegetation types in BNR include tropical lowland rain forest, tropical montane rain forest, and montane evergreen forest (Jiang et al. 2002). The natural vegetation in low elevation areas is classified as “tropical lowland rain forest”, in which the dominant species are V. mangachapoi, Litchi chinensis Sonn., and Homalium hainanense Gagnep. (Hu and Li 1992), but most of the species have disappeared owing to commercial timber logging and long-term shifting cultivation (Ding and Zang 2005). The field investigation site was located in Nanchahe in BNR. There were two small villages in Nanchahe, but the villagers emigrated to other places before 1950. The practice of shifting cultivation has stopped since then. The shifting cultivation had been conducted for at least 10 cycles before 1950 (personal communication with local foresters and local residents). Natural secondary succession began on the abandoned shifting cultivation lands in the early 1950s. In 1978, the naturally regenerated forest fallow was cleared in order to make plantations. As in the case of shifting cultivation, the slash was burnt and tree plantation was made on burnt sites. The species planted were Cunninghamia lanceolata (Lamb.) Hook., Cinnamomum camphora (Linn.) Presl, and Camellia oleifera Abel. After the planting, no further management measures have been conducted in such plantation lands. But the plantation in this area was found unsuccessful 5 yr later, since more than 99% of the planted trees died and replaced by the natural regenerated tree species (BNR historical recordings). When we conducted the field investigation in 2004, almost no planted trees remained. At present, the naturally regenerated secondary forest fallow covers most of the low-elevation (< 800 m) hills in Nanchahe of BNR. There was no further human disturbance since 1978 when natural vegetation recovery began. Data collection We selected three secondary forest stands that developed from the same type of soil and experienced the same abovementioned manner of shifting cultivation, but on different

Vegetation Recovery of Shifting Cultivation 651

Table 4. Summarized conditions at the three study sites in Bawangling Nature Reserve Condition

Hillslope gradient Up-slope

Middle-slope

Down-slope

Elevation (m)

600

550

450

Slope (°)

15

15

15

Aspect

SW

SW

SW

Soil type

Latosols

Latosols

Latosols

Soil depth (m)

0.5

0.8

1.1

Stand age (yr)

25

25

25

Crown density

0.7

0.9

0.9

0

8

11

No. remnant trees surviving the slash and burn

hillslope positions (up-slope, middle-slope and down-slope) in Nanchahe in BNR (Table 4). The distance between all three sites was less than 1 km and the elevation varied from 450 to 600 m. At each site, one plot (100 m × 100 m) was designed for vegetation investigation and each plot was further divided into 400 contiguous subplots of 25 m2 (5 m × 5 m). For all freestanding woody plants (excluding lianas), species name, height, and DBH (height 1.5 m) of stems in each subplot were recorded and measured. The species name and abundance for stems between 0.1 and 1.5 m in height were recorded. The nomenclature followed Wu (1994).

significant difference. Taking each subplot (5 m × 5 m) of the stand in the different slope positions as one repetition, n = 400; stem density and basal area of different size classes were compared by non-parametric one-way ANOVA. The DBH and height frequency distribution in stands along the slope gradient were compared using the Chi-squared test. All analyses were performed using SPSS version 13.0 (SPSS 2004). Significance was set at P < 0.05.

Acknowledgements The authors thank Mr Xiu-Sen Yang, Jing-Qiang Wang, Yang Wang, Yu-Cai Li, and Da-Dong Feng (Bawangling Forestry Bureau of Hainan Province) for their assistance in the field. The authors also thank Mr Guo-Ai Fu (Forestry Bureau of Hainan Province) for his identification of voucher specimens.

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