Land characteristics and plant resources in relation to agricultural land-use planning in a humid tropical strand plain, southeastern Thailand

Landscape and Urban Planning 65 (2003) 133–148

Land characteristics and plant resources in relation to agricultural land-use planning in a humid tropical strand plain, southeastern Thailand Satoru Okubo a,∗ , Kazuhiko Takeuchi a , Benjaporn Chakranon b , Apichart Jongskul b a

Department of Ecosystem Studies, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, Japan b Pikunthong Royal Development Study Center, Kaluwor Nua, Muang, Narathiwat 96000, Thailand Received 12 June 2002; received in revised form 10 December 2002; accepted 16 December 2002

Abstract Recently in humid tropical Asia, human activities have begun to extend toward the coastal lowland areas where widespread peat swamps are distributed. Draining peat swamp areas for agricultural uses creates a high risk of sulfuric acidification. The research site in this paper was a strand plain comprising beach ridges, swales on inter-ridges, and peaty backswamps in southeast Thailand. Thus, the actual and potential acid sulfate soil in the peaty backswamps is a major problem there, but sandy podzols on older beach ridges are also considered an agricultural problem soil. To establish an agricultural landscape plan encompassing the whole of the strand plain, we studied the land characteristics associated with the landforms and soils, their spatial variability related to landscape evolution, and the status of present plant resources in terms of biomass and species diversity for local farmers on peaty backswamps and beach ridges. We found that all spatial attributes of the two soils, such as the locations, profiles, and chemical and physical properties, were closely related to the geomorphic formation process or chronosequence: the landward or older backswamp has ripe clay and the seaward or younger one partly has unripe clay; the degree of spodic horizon development with ortstein is highest in the most landward beach-ridge system and decreases seaward; and the profile positions of the spodic horizons were related to the fluctuations of marine sand sediments, as revealed in a fine-scale survey. Finding these spatial regularities is useful for predicting land characteristics in areas outside of detailed research sites, reducing time and cost as well as forming the basis of landscape planning. Plant resources presently utilized by local farmers on the strand plain are Melaleuca cajuputi forests on both the peaty backswamps and the beach ridges, and home gardens of tropical fruit trees around settlements. Ecological carrying capacity in terms of biomass of M. cajuputi was not significantly different among soil types in the strand plain, but the growth rate and potential biomass of the species appears to be highest in thick, water-saturated peaty backswamps with acid sulfate soils. By comparing the species composition of home gardens in different areas, we found that on the beach ridges, although the ecological carrying capacity in terms of biomass for several types of fruit trees appears lower, a great number of species particular to the shrub layer are maintained. Based on the spatial variability of land characteristics and the ecological carrying capacity in terms of biomass, we identified separate land units and suggested future land-management and land-use systems appropriate to each unit. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Acid sulfate soils; Agricultural landscape planning; Ecological carrying capacity; Humid tropics; Podzols; Strand plain

∗ Corresponding author. Tel.: +81-3-5841-5050; fax: +81-3-5841-5072. E-mail address: [email protected] (S. Okubo).

0169-2046/$20.00 © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-2046(03)00011-2

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1. Introduction Recently in humid tropical Asia, human activities have increased in magnitude and have begun to extend toward the coastal lowland areas where widespread peat swamps are distributed. Peat swamp forests are one of the ‘last frontiers’ in southeast Asia’s challenge to produce as much food as possible to satisfy the demands of an increasing population (Rijksen and Persoon, 1991). According to Kyuma (1992), peat swamps in southeast Asia cover an area of about 20 million hectares: two-thirds of the total area of the world’s tropical peat swamps. Most of these peat swamps have developed on marine clay sediments that were deposited during periods of high sea level and contain high concentrations of pyrite (FeS2 ). When the pyrite is oxidized upon exposure to air, sulfuric acid is formed and the pH value of the soil falls to below 4, and sometimes even to below 3 (Dent and Pons, 1995; van Mensvoort and Dent, 1998). In southeast Asia, there are more than 5 million ha of acid sulfate soils, including human-induced acid sulfate soils (van Breemen and Pons, 1978); these soils are most commonly found in swampy coastal lowlands. Since that study, the area of acid sulfate soils is likely to have increased as a result of artificial drainage for agricultural development carried out without careful consideration of the consequences. Most farmland developed in this manner is affected by aluminum and iron toxicity, salinity, low quantities of major nutrients, and low base status, as well as by soil acidification, and is consequently abandoned (Attanandana and Vacharotayan, 1986; Phillips, 1998). Considerable attention, therefore, has been paid to the problems of acid sulfate soils in the process of land reclamation for agricultural use (Bloomfield and Coulter, 1973; Dent and Pons, 1995). Additionally, actual amelioration methods have been presented (Dent, 1992; Minh et al., 1998; Mathew et al., 2001). However, under land conditions that are potentially highly vulnerable to human impact, the first step in establishing an agricultural landscape plan should be to recognize the status of land characteristics associated with landforms and soil types and their distributions, and to find thresholds of land degradation (Young, 1975, 1998). In this respect, several studies have been conducted in the Mekong Delta (Dent and van Mensvoort, 1993) and other coastal lowlands in Malaysia, Indone-

sia, and Australia (Paramananthan and Gopinathan, 1981; Willet and Walker, 1982; Diemont et al., 1993). However, as Dent and Pons (1995) and Diemont et al. (1993) mentioned, there are still few detailed systematic surveys in southeast Asia. The research site selected for this study is a strand plain comprising beach ridges, swales on inter-ridges, and peaty backswamps in Narathiwat Province, southeast Thailand. The peat swamp in the study area has been developed and drained for agriculture since 1975. Although much amelioration of the soil has been applied to reclaim agricultural land from the swamp, so far almost none of the reclaimed area has been utilized (Jongskul and Sittibush, 1999). The actual and potential acid sulfate soils in the peaty backswamps are clearly a major problem, but the sandy podzols on the older beach ridges also constitute agricultural problem soils. Therefore, our final objective was to establish an agricultural landscape plan encompassing the whole of the strand plain with its various land characteristics, each of which pose difficulties for agricultural use. As a part of the process of developing such an agricultural landscape plan, we also present in this paper the land characteristics associated with the landforms and soils, and the status of present plant resources (in terms of biomass and species composition) available to local farmers on the peaty backswamps and beach ridges. The research objectives of this paper are: 1. to determine how land characteristics and soil properties vary spatially, 2. to determine the risk of sulfuric acidification in the peaty backswamps, as determined by the depth and ripeness of the marine clay sediments underlying the peat (Ahmed and Dent, 1997; van Mensvoort and Dent, 1998; Husson et al., 2000), 3. to obtain the strength of podzolization and the depth of the spodic horizon in the beach ridges, 4. to explain the links between the spatial variability of the soils mentioned above and the landscape evolution of the strand plain following Holocene sealevel changes (Teh, 1980; Vijarnsorn, 1992), and 5. to identify typical plant resources utilized by local residents and to compare ecological carrying capacities under different land conditions in terms of biomass and species diversity in the strand plain and its surroundings, in order to find implications for future land uses.

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Fig. 1. Location of study area and elevation map.

2. Materials and methods

nally from weathered granite. Alluvial lowlands exist between the hills and the coastal lowland.

2.1. Study site The study area is located on the southeastern coast of the Malay Peninsula, Thailand (latitude 6◦ 30 N, longitude 101◦ 45 E; Fig. 1). According to observation data from 1973 to 1995 at the Narathiwat meteorological station, the mean annual temperature is approximately 27 ◦ C and the mean annual rainfall is 2560 mm. About a half of the annual rainfall falls in November and December. In contrast, there are several months (from February to April) in which the monthly rainfall is <100 mm; thus, it is close to a tropical monsoon climate. The inland area approximately 15 km west of the seashore has landforms of hills (up to 600 m a.s.l.) formed on granite bedrock. The coastal lowland area consists of a strand plain of beach ridges and peaty backswamps on abandoned lagoons. The beach ridges, which are several kilometers long and parallel to the seashore, formed in response to sea-level fluctuations after the last glacial period. Similar strand plains are common features along the eastern shoreline in places where embayments form protected environments and a large supply of sandy sediments exists (Soil Survey Division, 1975; Vijarnsorn, 1992). Most of the beach ridges are covered by fine eolian sands underlain by backshore and foreshore deposits, origi-

2.2. Spatial variability of land characteristics related to geomorphic formations in the strand plain We surveyed landforms and soils in the strand plain, focusing on two problem soils for agricultural development: podzols in the beach ridges and marine clay sediments with high pyrite content under the peat in the backswamps. A survey transect was established on the strand plain approximately perpendicular to the shoreline, but in one place the transect was displaced because of inaccessibility (Fig. 1). Along the transect, the spatial distributions of the land units associated with landforms and soils as well as the land uses were described. Landforms were delineated using approximately 1:50 000 aerial photographs taken in 1975 by the Royal Thai Survey Department. Actual profiles of landforms were measured with a hand level. 2.2.1. Podzols on the beach ridges In humid tropical areas, soils strongly affected by podzolization are extensive in freely drained areas of siliceous sands, such as coastal sandy areas and weathered sandstone plateaus (Richards, 1941). These soils are generally thought to be infertile because of the

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leaching of nutrients. Even under undisturbed conditions, therefore, the predominant vegetation on these soils is originally heath-like forest, with a representative species composition strikingly different from that of the typical mixed dipterocarp forest of the climatic climax vegetation in the region (Katagiri et al., 1991; Newbery, 1991). Following the deforestation that occurred as part of the reclamation of the backswamps, the present dominant vegetation on all the beach ridges except the most seaward one is Melaleuca cajuputi trees (see below). The rate of podzol formation has been investigated in chronosequences (Lundström et al., 2000); the degree of podzol development increases as a log-linear function of time (Barrett, 2001). Podzol development—including the differentiation of albic and spodic horizons—and especially the occurrence of ortstein cemented by spodic materials, such as iron, aluminum, and organic matter, are considered to be the main factors in the evolution of the unique landscape in the humid tropics as well as in the subtropics (Thompson, 1981). To compare the development of the spodic horizons among the beach-ridge systems, we constructed detailed soil profiles from samples taken with a soil auger at 25 m intervals along the transect. Soils for laboratory analysis were sampled from each horizon in several pedons taken from each beach-ridge system. All samples were analyzed to determine particle size, pH in a 1:1 soil–water mixture, extractable Al (KCl extract), extractable Fe (diethylene-triamine-pentaacetic acid extract (DTPA)), and organic matter content. Additionally, we conducted a survey on a finer scale in one of the older beach ridges to determine the spatial variability in the depth of the spodic horizon. Three parallel survey lines 500 m apart were generated across the beach ridge. The relative elevation of each line was calibrated using a hand level. In each line, the depth of eolian sands and the starting and ending depths of the spodic horizon (up to a depth of 2 m) were measured at 2 m intervals. 2.2.2. Peat and underlying soils on the backswamps We used a soil auger at 10–25 m intervals along the transect to measure the spatial distribution and depth from the surface (up to a depth of 2 m) of marine clay sediments in the two backswamps. Like pod-

zolization, pyrite-induced acidification is also considered to proceed over time. Unripe marine clay soils mature when exposed during periods of regression in the Holocene, and the accumulation of pyrite differs significantly under different conditions during the geomorphic formation process (Lin et al., 1995). Thus, we could compare the degree of ripeness between the two backswamps. Marine clay sediments were sampled with a hand sampler and the amount of pyrite and total sulfur was analyzed by X-ray diffraction. 2.2.3. Chronological analysis of landscape evolution We carried out a chronological analysis to understand the process of landscape evolution of the strand plain and to explain the differences between the properties of the two soil types. Five specimens for radiocarbon dating were collected from the bottom of the peat layer in three swales and two backswamps. To avoid sample contamination, we used a peat sampler (DIK-105A; Daiki Rika Kogyo Co. Ltd., Japan; resale material from Eijkelkamp Agrisearch Equipment, The Netherlands). The collected specimens were sieved to reduce coarse fractions, and then pretreated with acid washing and alkali treatment. After being thus treated and dried, the specimens were sent to the Institute of Geography, Tohoku University and measured by the liquid scintillation counting method (Polach, 1987). 2.3. Plant resources on the peaty backswamps and beach ridges During our landform and soil surveys we also described the present plant resources actually utilized by local farmers on the strand plain. We found two major land-use types: (1) M. cajuputi forests, which grow on both the peaty backswamps and the beach ridges, even where pyrite-induced acidification occurs (Phikul Thong Study Center, 1991), and which are utilized by the local farmers for charcoal-making, fuel wood, building materials, and medicines extracted from the leaves; and (2) home gardens, which are located around settlements on the beach ridges as well as on other soil types outside of the strand plain. These home gardens comprise tropical fruit trees and timber trees providing useful products for daily use, such as food, fuel wood, and building materials.

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2.3.1. Melaleuca cajuputi forests on the backswamps and beach ridges Melaleuca (Myrtaceae) is a genus naturally distributed from Australia to Vietnam and southern Burma (Brinkman and Xuan, 1991). M. cajuputi is normally found growing in pure, same-age stands. In such stands, self-thinning occurs because of intra-species competition for resources such as soil nutrition, water, and light (Yoda et al., 1963): as the trees in a stand grow, the finite space of the stand is occupied by progressively fewer trees as competitively disadvantaged trees die from crowding and suppression. Thus, stands approach a limiting number of trees (Johnson and Shifley, 2002). Under these conditions, when the mean tree biomass is plotted against the individual density on a double logarithmic graph, the relation approaches an asymptotic line of negative slope varying around −3/2 (Yoda et al., 1963). The model is called the ‘self-thinning rule’ and is well known to hold true in various vegetation types (Silvertown, 1987). The rule can be expressed as: log w = a log p + K, where w represents the mean biomass of individuals (usually substituted by stem volume for practical reasons in forestry; Bégin et al., 2001), and p represents the population density. The slope a (usually around −3/2) and the intercept K are constant values. The value of the intercept K reflects site quality (Westoby, 1984; Wirth et al., 1999)—the higher the value, the higher ecological carrying capacity of the land for that species, in terms of biomass carried at a given stand density. We estimated the ecological carrying capacity in terms of biomass for M. cajuputi so that we could compare the intercept values among three soil types: thick peat soil (more than 1 m deep), shallow peat soil (<1 m deep), and sandy podzol. Quadrats of 10 m×10 m were established in forest stands where population density was high and self-thinning had occurred. We sampled eight stands in thick peat soil, five in shallow peat soil, and 13 in sandy podzols. In each quadrat, tree height and diameter at breast height (DBH) were measured for all individuals, and the number of individuals was counted. For the measure of biomass, we estimated individual stem volume as a circular cylinder, using stem area at breast height and tree height. The regressions between population density and mean tree volume were calculated for each soil type to estimate the constant values of slope and intercept. To investi-

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gate the differences in the thinning relation among the three soil types, we applied a weighted Chi-squared test to both the estimated slopes and the intercept values (Bégin et al., 2001). 2.3.2. Home-garden structure on the beach ridges Home gardens have been studied as a sustainable land-use type (Wojtkowski, 1993). Gillespie et al. (1993) showed that the structure of home gardens varied with land conditions—the structures were limited in size, diversity, and complexity in areas of nutrient-poor, well-drained soils. In the present study area, home gardens are also distributed on beach ridges with sandy podzols. To understand whether their structure is poor, we compared species composition and diversity with those in other home gardens situated on foot slopes and high uplands located in the area landward of the strand plain. In this area, there are rubber plantations and orchards and soil properties are comparatively better. Sixteen home gardens were selected on four different land types: foot slope, high upland, and two beach ridges. In each garden, tree height, DBH, species names, and numbers of individuals were recorded for all trees taller than 1.5 m. Species composition and diversity by tree height class were then analyzed. Multidimensional scaling analysis (MDS; Gower, 1966) as an ordination technique was applied to compare the differences in species composition; a correlation matrix among sites for the analysis was made using percentage similarity (Whittaker, 1952). 3. Results 3.1. Landform–soil development in the strand plain 3.1.1. Podzol chronosequences From the interpretation of aerial photographs, we separated the beach ridges into five systems (Fig. 2). Each system was named beach-ridge systems 1–5, from landward to seaward. The age of each system determined by radiocarbon analysis was, respectively, estimated as from 3950 ± 50 to more than 5000 years BP, 1070 ± 30 to 3950 ± 50 years BP, 620 ± 20 to 1070 ± 30 years BP, 340 ± 30 to 620 ± 20 years BP, and the present to 340 ±30 years BP. This implies that

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Fig. 2. Landform classification map of the lowland area.

the strand plain was generated in intermittent periods in a seaward direction. System 1 is located between the two peaty backswamps, and has a slight undulating relief. Inselberg of weathered intrusive granite appears within the beach-ridge system, as shown in Fig. 2, and land use is quite different to that on the other beach-ridge systems. Rubber plantations are established in the area. System 2 consists of numerous beach ridges and swales, all of which are narrow. System 3 also consists of several beach ridges, but it includes one wider and relatively higher beach ridge. System 4 is situated between wide and deep swales where the peat horizon is found at a depth of about 50 cm. It has a profile typical of a relict barrier bar. The last, system 5 has prograded berm ridges; the backshore

plain of the second landward berm ridge is capped by eolian sands. According to the World Reference Base for Soil Resources (WRB; Driessen, 2001), the soils of the first four systems are densic podzols and the soils of system 5 are albic or haplic Arenosols. A comparison between the degrees of podzol development in the beach-ridge systems, except system 5 where no spodic illuvial horizon was found, is summarized in Table 1, and typical soil properties of each system are presented in Table 2. The presence/absence of a spodic horizon was significantly different among the systems (Chi-squared test, P < 0.01). The degree of spodic horizon development was highest in the most landward beach-ridge system and decreased seaward. The occurrence of ortstein cemented by spodic materials containing organic matter also followed the

Table 1 Degree of podzolization in the beach-ridge systems Site of pedon

Beach-ridge Beach-ridge Beach-ridge Beach-ridge

system system system system

Number of sites

1 2 3 4

12 26 17 62

Occurrences of spodic horizon With ortstein (%)

Without ortstein (%)

None (%)

Depth (S.D.) (cm)

100.0 96.2 47.1 24.2

0.0 3.8 47.1 48.4

0.0 0.0 5.8 27.4

47.1 55.9 54.6 54.9

(8.7) (20.6) (12.6) (16.7)

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Table 2 Chemical soil profiles of the beach-ridge systems Site of pedon

Depth (cm)

pH (1:1 H2 O)

Organic matter (%)

DTPA Fe (ppm)

Extractable Al (meq./100 g)

Sand (%)

Silt (%)

Clay (%)

Beach-ridge system 1

0–2 2–54 54–60 >60

3.9 5.8 4.3 4.8

7.47 0.11 6.14 2.69

143 12 25 3

0.58 – 12.45 2.48

94.6 97.6 89.4 94.8

3.8 1.9 9.5 4.7

1.6 0.5 1.1 0.5

Beach-ridge system 2

0–10 10–30 30–51 51–56 >56

4.4 5.6 5.3 5.4 5.7

3.28 0.14 3.58 2.47 0.65

13 6 5 5 2

0.68 0.10 11.47 1.68 0.32

96.3 97.9 96.0 97.5 99.0

2.2 1.6 3.5 2.0 0.5

1.5 0.5 0.5 0.5 0.5

Beach-ridge system 3

0–5 5–18 18–50 50–70

5.5 5.7 6.1 6.3

1.63 0.56 1.10 0.04

38 4 5 18

0.25 0.16 0.06 0.01

94.2 94.2 96.2 97.2

2.3 3.6 3.3 2.3

3.5 2.0 0.5 0.5

Beach-ridge system 4

0–10 10–30 30–50

4.8 5.8 6.2

1.94 0.27 0.45

52 17 18

0.11 0.15 0.14

91.8 97.4 97.9

5.2 2.1 1.6

3.0 0.5 0.5

same trend. Unlike previous studies (Thompson, 1981; Lichter, 1998), the depth to the illuvial horizon did not change (Mann–Whitney’s U-test, P > 0.05). In addition to this result, the soil chemical properties also support these differences in podzol development. Similar to previous soil chronosequence studies in strand plains (Singleton and Lavkulich, 1987; Bowman, 1989; Barrett, 2001) and in other coastal dune areas in the subtropics (Skjemstad et al., 1992; Thompson, 1992), the following trends were observed as the beach-ridge systems increased in age: increasing total organic matter in the pedons with increasing accumulation in the lower profiles, and increasing extractable Al in the illuvial horizon. The accumulation of DTPA-extractable Fe was highest in the surface horizon for all pedons and decreased in the lower profiles. This means that organic chelates of Fe exist in the surface horizons in both younger and older pedons. This result supports the description by Walker et al. (1981) of the illuvial horizon being low in Fe and comparatively rich in organic and Al compounds. However, we should keep in mind that Fe oxides that are not extracted by DTPA also accumulate in the illuvial horizon. The depths and thicknesses of the spodic horizon measured in our fine-scale survey of a beach ridge in system 2 are shown in Fig. 3. The eastern end of each

of the three cross-sections started at the same swale; thus, these diagrams are arranged so that the right ends are aligned. Although the land surfaces are not parallel along the three lines because some parts of the surface have been artificially flattened, the upper boundary of the marine sand sediments, which are the original beach-ridge deposits and have coarse grains of a bright brown color (7.5YR 5/8), undulates consistently along each of the lines. The most interesting point is that the spodic horizon is developed at around the boundary between fine eolian sands and coarse marine sands; thus, the depth and occurrence of the spodic horizon are also parallel in each of the survey lines. 3.1.2. Marine clay sediments The soils of the peaty backswamps are classified as fibric Histosols in the WRB system. Fig. 4 shows the spatial variability of the marine clay sediments. There is considerable difference in the sediment types underlying the peat. In the landward backswamp, weathered granite intrusions like those in beach-ridge system 1 occur. Although most of the sediment is marine clay in the landward backswamp, sand and clay sediments mingle and their upper boundaries undulate in the seaward backswamp. A buried beach ridge is located in the center of the seaward backswamp. Between the two backswamps, there are differences not only in

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Fig. 3. Fine-scale variability of podzol profiles in one of the beach ridges of beach-ridge system 2. The profiles were taken along three parallel survey lines, established 500 m apart. The eastern end of each cross-section begins at the same swale; therefore, the right margins of the profiles are aligned.

sediment types, but also in the ripeness of the marine clay sediments. The amount of pyrite in the marine clay sediments under the peat horizon was markedly greater in the seaward backswamp than in the land-

ward backswamp (Table 3). Furthermore, field observations revealed that yellow brown-colored jarosite (KFe3 (SO4 )2 (OH)6 ), which is formed during the process of pyrite oxidation and is a good indicator of

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Fig. 4. Cross-section of soil profiles along the research transect in the peaty backswamps.

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Table 3 Major components of marine clay analyzed by X-ray diffraction Component

Seaward backswamp

Landward backswamp

Total content of Silicon (%) Sulfur (ppm) Pyrite (%) Acid-extractable sulfur (ppm) Residual sulfur (ppm) Loss on ignition (450 ◦ C) (%)

25.60 31755 5.53 15 31740 18.60

37.30 4705 0.79 335 4370 6.31

raw acid sulfate soils (see Dent and Pons, 1995), was abundant in the profile of the landward pedon but, by contrast, was scarce in the seaward profile. These observations support the supposition that the degree of ripeness of marine sediments is related to age. However, despite the drainage in the seaward backswamp,

the marine clay sediments still retain a surprising amount of pyrites showing little oxidation. Jongskul and Sittibush (1999) observed that the water table drops to a maximum of only 0.8 m below the surface even in the driest month; thus, acid sulfate would be produced along the edges where the backswamps meet the beach ridges (including the buried one), where marine clay sediments are closest to the surface. 3.2. Plant resources and ecological carrying capacities in different sites 3.2.1. Melaleuca forests The relationships between population density and averaged individual tree volume in each stand of M. cajuputi are given in Fig. 5. Significant double

Fig. 5. Relationships between population density and averaged individual tree volumes for Melaleuca cajuputi on three different soil types.

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Fig. 6. Similarity analysis by MDS of species composition in home gardens under different land conditions.

logarithmic regressions were obtained in each soil type (thick peat: adjusted r 2 = 0.934, PANOVA < 0.01; shallow peat: adjusted r2 = 0.827, PANOVA < 0.05; sandy podzol: adjusted r2 = 0.905, PANOVA < 0.01). The weighted Chi-squared test revealed no significant differences in the slopes of the three regression models (weighted average slope = −1.99, P > 0.05). The intercept values were 6.32 (95% confidence interval (CI): 4.12–8.52) in thick peat stands, 6.75 (95% CI: 0.12–13.38) in shallow peat stands, and 5.30 (95% CI: 3.73–6.87) in sandy podzol stands. These values were also not significantly different by the weighted Chi-squared test (weighted average intercept = 5.94, P > 0.05); however, excluding the model of shallow peat stands in which the standard errors of both the slope and the intercept were highest because of the lack of observation samples, the intercept values of thick peat stands seem to be slightly higher than those of sandy podzol stands. 3.2.2. Home gardens The similarity analysis (MDS) of species composition in home gardens revealed a marked difference

between the samples on the two beach ridges we examined and those in other locations (Fig. 6). Actual compositions also show a marked difference (Table 4). Several fruit trees, including mango, guava, and jackfruit, were common in every site. Home gardens on the foot slopes and uplands are characterized by longkong (a local fruit tree) and durian, both of which are economically valuable. In contrast, the home gardens on the beach ridges have several species particular to them, including coconut and cashew. On the more seaward of the two beach ridges (system 4) in particular, lime trees, sugar apple trees, and two timber trees (locally called ‘payom’ and ‘mao’) are found. Home gardens are so artificial that the species compositions strongly depend on the decisions and cultural backgrounds of the landowners. Interviews with elder householders in each village during the survey revealed that the villagers living on the beach ridges had originally transmigrated from the foot slope areas around the 1950s and that in the earlier phase of their settlement they attempted to plant the same fruit trees as grew in the foot slope areas, but these failed because of ‘poor’ soils and seasonal floods. Therefore, the present species compositions have been

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Table 4 Species composition of home gardens under different land conditions Name

Species

F

U

B2

B4

Mango Guava Jackfruit Tamarind Longkong

Mangifera indica Psidium guajava Artocarpus heterophyllus Tamarindus indica Lansium domesticum var. typicum Musa sp. Durio zibethinus Lansium domesticum Manihot sp. Nephelium lappaceum Carica papaya Cocos nucifera Anacardium occidentale Shorea sp. Citrus aurantifolia Annona squamosa Strychnos sp.

II III II III IV

I I I I II

II II II I –

II I I II I

IV IV IV V V IV I I – I – –

II II II – I I I I – – – –

I – – I – I III III II I I

I – – I I I II II IV V V V

Banana Durian Duku Cassava Rambutan Papaya Coconut Cashew Mao Lime Sugar apple Payom

F: foot slope, U: high upland, B2: beach-ridge system 2, B4: beach-ridge system 4, I: 1–20%, II: 21–40%, III: 41–60%, IV: 61–80%, V: 81–100%.

established on the basis of local farmers’ experiences over five decades. When the Shannon–Wiener’s species diversity index (Pielou, 1966) is calculated for the four sampling units, no significant difference is found among the units. However, applying the diversity index by tree height class shows a marked difference (Fig. 7). The species diversity among the taller classes and the complexity of vertical structure was greater in the home gardens on the foot slopes and high uplands than in the home gardens on the beach ridges. We interpreted this to mean that the soils of the beach ridges cannot support the various taller tree species. However, from another viewpoint, the species diversity of the shrub layer was similar for all of the home gardens. Although the ecological carrying capacity in terms of biomass for several fruit trees might be lower on the beach ridges, a great number of species are maintained that are particular to the shrub layer. 4. Discussion and conclusion 4.1. Spatial variability of two problem soils associated with landscape evolution

Fig. 7. Boxplots of species diversity in home gardens by tree height classes.

In this paper, we focused on two soil types with limiting properties for intensive agricultural utilization: sandy podzols on older beach-ridge systems and actual/potential acid sulfate soils on peaty backswamps. Numerous studies referred to in this paper have considered these two soil types individually; however, they cannot be discussed separately when considering the establishment of an agricultural landscape plan for the strand plain. Integration of our findings revealed that all of the spatial attributes of the two soil types, such as the locations, profiles, and chemical and physical properties, were closely related to the geomorphic processes or chronosequence. Knowledge of this spatial regularity will be helpful and useful for predicting the land characteristics in areas outside of detailed research sites, thereby reducing time and costs as well as forming the basis of future landscape planning. To determine the risk of acid sulfate soils in particular, it is most important to know the spatial distributions of marine clay sediments containing pyrites.

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In previous research in the Mekong Delta, Vietnam (Husson et al., 2000), the Pulau Petak tidal swamp, southern Kalimantan (Bregt et al., 1993), and the tidal floodplain of the River Gambia, Senegal (Ahmed and Dent, 1997), attempts to map the soils showed a high short-range variation without a significant relationship to the depth of the pyritic layer or to the ripeness of the clay. Fortunately, in the strand plain examined in the present study, we were able to obtain both vertical and spatial distribution patterns of ripe/unripe marine clay sediments in association with landscape evolution stages: the landward or older backswamp has ripe clay and the seaward or younger one partly has unripe clay. Although the distribution pattern was the result of a single transect, Nagano et al. (unpublished data) surveyed the depth to marine clay sediments throughout the whole of the backswamps at approximately 100 m grid points and found that the depths varied almost parallel to the shoreline. We, thus, conclude that the younger backswamp is potentially vulnerable to sulfuric acidification. Although this acidification occurs naturally, it must be recognized that unsuitable artificial drainage accelerates this process. Podzol development on the beach ridges was also closely related to landscape age. In addition, the fine-scale survey showed that the occurrence and profile position of the spodic horizons paralleled the shoreline and were related to the fluctuation of marine sand sediments. Compared to the rates of podzolization found in previous studies of temperate strand and lacustrine plains, and also even in subtropical coastal dune areas, the podzolization rate revealed in our study seems to be high, especially when comparing the formation of ortstein. In the beach ridges in our study, ortstein formed later than 620 years BP; in contrast, in temperate areas it is reported to have formed around 700–1000 years BP or earlier (Barrett, 2001). A spodic horizon with ortstein presents a strong physical barrier to plant root extension and also involves difficult chemical conditions, such as little nutrition in the albic horizon and Al toxicity and acidity in the spodic horizon. Therefore, from the viewpoint of agricultural planning, when creating land-use zones, the spatial variability of podzol development can be reflected at two scales— that of the strand plain and that of a single beach ridge.

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4.2. Ecological carrying capacities of the two problem soils in terms of biomass In order to establish a sustainable land-use system in humid tropical lowlands, it is not sufficient to compare land productivities by using only biomass products, a single crop, or pedocentric aspects (van Mensvoort and Dent, 1998). For instance, the evaluation criteria for the ‘land capability classification’ developed by the United States Department of Agriculture consist only of general soil properties, such as cation exchange capacity and permeability; thus, acid sulfate soils and sandy podzols are traditionally classified as infertile soils. If such land is evaluated only by pedological aspects, soil amelioration requiring a great deal of time and cost is the only method by which the impoverished land units can be utilized for agriculture, and unsuitable implementation usually leads to further land degradation. The United Nations Food and Agriculture Organization (FAO, 1993) has set out a rational framework for maximizing the benefits of the specific properties inherent in individual land units, even in impoverished lands. This framework involves understanding the characteristics of the land in detail, selecting land-use types appropriate to the particular land requirements by observing the practices of local residents, and evaluating land productivity on the basis of a number of criteria. Truly, the two problem soils can hardly be utilized for economically valuable agricultural products at present. Nevertheless, both M. cajuputi trees and representative shrub species found in home gardens on beach ridges are useful resources utilized by local residents for food, firewood, and medicines; this has important implications for determining relevant land-use types during land evaluation. M. cajuputi can grow well in both acid sulfate soils and sandy podzols. Yamanoshita et al. (2001) reported that the potential biomass of the species in thick, sulfuric acid peat is not as high as the biomass of other species in a typical tropical rain forest, but that the growth rates are similar. Osaki et al. (1998) examined the growth of several native tree species, including M. cajuputi, in the sulfuric acid peat of the study area under different Al and pH treatments. They concluded that the species was tolerant and well adapted to high concentrations of Al as well as to low pH conditions. Yamanoshita et al. (2001) also concluded that

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the species grew well under conditions of prolonged flooding. In our study, we found no difference in ecological carrying capacity in terms of biomass among soil types. However, potential biomass seemed to be slightly higher in thick peat soils. Considering the results of the studies mentioned above, the growth rate and biomass capacity of M. cajuputi would be highest in water-saturated, thick peat swamps, even if an acid sulfate soil had developed. Sandy podzol soils cannot carry the biomass of many home-garden species, but can support several species of shrub-like fruit trees. Furthermore, species compositions differed between the home gardens on the two beach ridges. In the younger beach ridge, economically valuable fruit trees, such as lime and sugar apple, and the timber tree ‘payom’ were found to be distinctly common. Walker et al. (1981) showed that, for dune systems developed during the Holocene, younger and older dune systems support different species of vegetation. We consider that the degree of podzolization affects the capabilities of the soil to support vegetation. Although Walker et al. (2001) mentioned that vegetation and biomass successions were progressive in dune system landscapes up to 40 000 years old, in the present study area, rapid podzol formation with ortstein occurring at an earlier stage in horizons near the surface would accelerate the progression and change the process to retrogressive succession. In landscapes that are in retrogression, human impacts such as forest clearing will make the vegetation collapse, with no chance of recovery (Walker et al., 2001). Therefore, further monitoring is needed to ascertain how vulnerable old beach ridges are to human activities. 4.3. Identification of separate land units and implications for future land uses and management practices To establish an agricultural landscape plan in an inherently vulnerable landscape, responses of plants to soil properties after amelioration or degradation must be included in any land-evaluation system. Additionally, not only land-use types, but also land-management practices by local farmers should be considered. In a case study in the Mekong Delta, Tri et al. (1993), van Mensvoort et al. (1993), and van Mensvoort and Dent (1998) showed and established

comprehensive frameworks for assessments based on farmers’ land use and soil–water management practices. Our study presents only the early stages of assessing land for sulfuric acidification and podzolization, describing present plant resources utilized by local farmers, and evaluating ecological carrying capacities in terms of biomass. There needs to be further research into technological developments by which to improve the land, checks on the possibility of growing other crops, feasibility assessments of whether local farmers can manage the land by themselves, and economic evaluations. Nevertheless, on the basis of risk assessment research, we can identify the following separate land units and summarize their implications for future land use and management practices. 1. Old peaty backswamp underlain by ripe marine clay sediments: This land unit would have high reclamation potential because further acidification can be considered unlikely. However, the 2 m deep peat soils and high water table prevent the growth of most cash crops. Water table management and high raised-bed cultivation systems (Tri et al., 1993) should be adopted. 2. Young peaty backswamp underlain by unripe marine clay sediments: This land unit is highly vulnerable to considerable sulfuric acidification following drainage. Therefore, a system of M. cajuputi cultivation that maintains the high water table should be established. 3. Young peaty backswamp underlain by sandy sediments: This land unit is more useful than the first land unit, because the peat layer is shallower. However, this land unit is interleaved with the second land unit; thus, acid water from unripe marine clay might affect the unit. Therefore, it should not be drained. High raised-bed cultivation with a mixture of sandy sediments and peat is recommended. 4. Old beach-ridge systems with well-developed podzols: In this land unit, the podzolization process is well under way, and a highly cemented spodic horizon is found near the surface. Deep cultivation to break up the albic and spodic horizons and to admix the eolian and marine sands could allow shrub-like fruit trees such as lime to be planted. 5. Young beach-ridge systems with little-developed podzols: This land unit also has nutrition

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deficiencies, but the above-mentioned shrub-like fruit trees could be cultivated.

Acknowledgements This research was supported by a Grant-in-Aid for Creative Basic Research from the Ministry of Education, Science, Sports and Culture of Japan (Project #09NP0901). We wish to thank Mr. Chaiwat Sitthibush (Director of the Pinkunthong Royal Development Study Center, Thailand) and Mr. Tanit Nuyim (Royal Forest Department, Thailand) and his research staff for their assistance in conducting field surveys. Special appreciation is due to Dr. Pisoot Vijarnsorn (soil expert of Thailand from the Department of Land Development, Thailand), Dr. Toshihide Nagano (Professor of Tokyo University of Agriculture, Japan), Dr. Takeshi Tange (Professor of the University of Tokyo), and Dr. Katsumi Kojima (Associate Professor of the University of Tokyo) for their helpful suggestions and comments while this research was being conducted. The manuscript owes much to the thoughtful and helpful comments of Dr. Amal Kar of the Central Arid Zone Research Institute, India, who was in our laboratory as a Visiting Professor.

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