Tree species composition, dispersion and diversity

Forest Ecology and Management 186 (2003) 61–71

Tree species composition, dispersion and diversity along a disturbance gradient in a dry tropical forest region of India R. Sagar, A.S. Raghubanshi*, J.S. Singh Department of Botany, Banaras Hindu University, Varanasi 221005, India Received 16 July 2002; received in revised form 31 March 2003; accepted 22 April 2003

Abstract Forest inventory data were collected in 1998–2000 from fifteen 1 ha permanent plots along a disturbance gradient in a dry tropical forest region of India. A total of 4033 stems, 49 species, 44 genera and 24 families of adult trees (30 cm CBH), occurred in the 15 ha of forest area. The study indicated that the dry tropical forest is characterised by a patchy distribution of species and individuals with mixed species composition, and the sites are represented by different combinations of the dominants and co-dominant species. A PCA ordination indicated that the variation in species composition of the sites is explained by the variation in soil nitrogen as well as the degree of disturbance. About half the analysed species showed changing nature in dispersion along the disturbance gradient. The distribution of Boswellia serrata, Holarrhena antidysenterica and Lannea coromandelica changed from clumped to uniform and the distribution of Butea monosperma, Cassia fistula and Elaeodendron glaucum changed from uniform to clumped as the degree of disturbance increased. The mean stem density was highest (419 stems ha1) at the least disturbed site and lowest (35 stems ha1) at the highly disturbed site, and for basal area, the highest value (13.78 m2 ha1) was for the second least disturbed forest site and the lowest value (1.30 m2 ha1) was for the most disturbed site. The total number of stems, indices of species richness, evenness and a-diversity decreased with disturbance. A strong influence of number of species per individual on b-diversity suggests that for resisting change in floristics due to disturbance, a site must have low species-individual ratio. # 2003 Elsevier B.V. All rights reserved. Keywords: b-Diversity; Density; Distribution; Evenness; Species richness

1. Introduction Biodiversity is essential for human survival and economic well being and for the ecosystem function and stability (Singh, 2002). Political and scientific concerns have been raised as we are experiencing an increase in species extinction rates caused by anthropogenic activities (Ehrlich and Wilson, 1991). Many kinds of environmental changes influence or * Corresponding author. Fax: þ91-542-2368174. E-mail address: [email protected] (A.S. Raghubanshi).

determine processes that can both augment and erode diversity (Sheil, 1999). In India, habitat destruction, over exploitation, pollution and species introduction are identified as major causes of biodiversity loss (UNEP, 2001). The disturbances created by these factors determine forest dynamics and tree diversity at the local and regional scales (Burslem and Whitmore, 1999; Hubbell et al., 1999); these disturbance has been considered as an important factor structuring communities (Sumina, 1994). Sheil (1999) opined that the disturbance of a suitable intensity will increase species richness in

0378-1127/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0378-1127(03)00235-4

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R. Sagar et al. / Forest Ecology and Management 186 (2003) 61–71

old-growth communities in consonance with intermediate disturbance hypothesis of Connell (1978), however, others believe that disturbance cannot increase diversity in genuine old-growth forest (e.g. Phillips et al., 1997). The diversity disturbance debate needs further work, and the detailed information on the disturbance gradient of species distribution, dispersion, forest stand structure and species diversity from dry tropical forest is lacking. Prior to forest management operations, biodiversity inventories are used to determine the nature and distribution of biodiversity resources of the region being managed. Such biodiversity inventories are best integrated with the timber resource inventories in order that forest management operations can be planned (Rennolls and Laumonier, 2000). In these inventories, quantification of tree species diversity is an important aspect as it provides resources and habitat for many species (Cannon et al., 1998). Being a dominant life form, trees are easy to locate precisely and to count (Condit et al., 1996) and are also relatively better known, taxonomically (Gentry, 1992). The dry tropical forest accounts for 38.2% of the total forest cover of India (MoEF, 1999) which is largely threatened by lopping, burning, overgrazing and clearing for cultivation (Jha and Singh, 1990). Because of these threats, since past several decades the dry deciduous forest cover in most part of the central India is being converted into dry deciduous scrub, dry savanna and dry grasslands which are progressively species-poor (Champion and Seth, 1968). Therefore, a detailed study of impact of the disturbance on dry deciduous forest plant diversity was long overdue. The objective of the present study was to understand: (i) the impact of different degree of disturbance on species composition and on dispersal pattern of selected species occurring in a dry tropical environment, and (ii) the changes in over-storey tree diversity along a disturbance gradient.

2. Materials and methods 2.1. Study area The study was conducted on five sites, viz. Hathinala, Khatabaran, Majhauli, Bhawani Katariya, and Kota (24860 5200 –248260 1600 N and 83810 8600 –83890 6000 E) in

the Vindhyan hill ranges (dry tropical forest) of India in the year 1998–2000 (Fig. 1). Physiographically, the area is characterised by hillocks, escarpments, east– west trending gorge-like valleys, flat basins and flattopped ridges. The altitude varies from 313 to 483 m a.s.l. (Jha and Singh, 1990). The soils are Ultisols, sandy loam in texture and reddish to dark grey in colour, and are extremely poor in nutrients (Singh et al., 1989). The area experiences a tropical monsoon climate with mean annual rainfall of 821 mm, of which about 86% is received from the southwest monsoon during June– August. The potential natural vegetation of the region is dry tropical deciduous forest. Shorea robusta, Anogeissus latifolia, Lagerstroemia parviflora, Terminalia tomentosa, Hardwickia binata, Boswellia serrata, Buchanania lanzan, Acacia catechu, etc. are the important tree species (Champion and Seth, 1968). These species exhibit local dominance. The region is undergoing rapid changes in vegetation and is facing large scale anthropogenic forcing in the form of mining, thermal power generation, cement industry, etc. Besides illegal sporadic tree felling, widespread lopping and extraction of non-timber resources is occurring. The forested area is continuously decreasing and the remnant forest cover exists in the form of non-contiguous patches of varying sizes (Singh et al., 1991). 2.2. Sampling For sampling, five sites were selected on the basis of satellite images and field observations to represent the entire range of conditions. The sites were ranked according to the degree of disturbance they experience. These forest sites witnessed natural as well as anthropogenic disturbances with varying intensity. On Kota site, the intensity of chronic anthropogenic disturbances is highest. These disturbances include removal of ground cover by grazing animals, and scraping by native people for the collection of grasses in summer season, cutting and lopping of trees and shrubs for fodder and fuel wood. The disturbance regimes with estimated relative impact, on each of the five dry tropical forest sites are shown in Table 1. Among the criteria used to delineate the disturbance levels, soil erosion and rockyness belong to natural disturbances category, and others are biotic. The sites


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Fig. 1. Location of the study area, numbers 1, 2, 3, 4, and 5, respectively, indicate approximate location of Hathinala, Khatabaran, Majhauli, Bhawani Katariya and Kota sites.

nearer to roads, agricultural lands, human habitations and market experience enhanced utilisation pressure. The site with maximum distance from road, agricultural land, habitation or market was given the impact factor 1. Impact factors for other sites were calculated as ratios of the distance of this site to the distance of the other sites, e.g. distance of the Khatabaran site from the road was maximum, i.e. 6000 m. The distance of the Hathinala site from the road was 1000 m. The impact of road for Khatabaran was 1, and that for Hathinala was 6 (6000/1000). In a similar way, impact of cutting and lopping was relativised with the help of tree basal area, and that of grazing and browsing by

sapling density. Other impacts were determined through visual estimation. Over all, the disturbances gradually increase in degree from Hathinala to Kota site (Table 1). The sites differed in the physico-chemical characters of soil (Table 2). These physico-chemical characters were not related to the disturbance gradient, however. At each of the five sites, three 1 ha contiguous permanent plots were established. Each hectare (100 m 100 m) was gridded into 100 sub-plots, each 10 m 10 m in size, as workable units. All individuals 30 cm circumference overbark at breast height

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Table 1 Disturbance regime (estimated, relative impact factors) at each of the five dry tropical forest sites Source of impact

Relative impact Hathinala

Road Agricultural land Habitation Market Cutting and lopping Grazing Scraping Soil erosion Rockyness Wild animals Total

Khatabaran

Majhauli

Bhawani Katariya

Kota

6 1 1 1 2 2 0 2 3 4

1 2 3 3 1 5 0 3 5 5

8 3 3 1 2 1 1 4 4 3

75 3 3 1 2 3 0 5 2 2

60 8 8 9 11 6 5 1 1 1

22

28

30

96

108

(CBH) were enumerated by species and the circumference of all the individuals was measured at nearest mm with Freeman’s tape. The CBH values were converted to basal cover values. The relative basal cover of a species on a site was calculated as the basal cover of a species divided by total basal cover of the site and multiplied with 100. 2.3. Data analysis The dominant and co-dominant species of each site were identified on the basis of relative basal cover. The species having highest relative basal cover was defined as dominant and that having the second highest relative basal cover was defined as co-dominant species.

The dispersion of species was studied by varianceto-mean ratio (Greig-Smith, 1983) at the scale of 3 ha plot. A ratio of 1.0 indicates a random dispersion, less than 1.0 a uniform dispersion and greater than 1.0 an increasingly clumped dispersion. The species richness (number of species per unit area), evenness (distribution of abundances among the species) and unified indices (exponential Shannon–Wiener index and Simpson’s diversity) as measures of a-diversity, were calculated for each site (3 ha sample size) using Biodiversity Pro (vers. 2) Software (1997). Number of species, stem density, basal area for each hectare as well as data pooled for 3 ha sample size for each site were also calculated. b-Diversity for each site was calculated on the basis of data from

Table 2 Physico-chemical properties of soil form the five sites (based on unpublished soil analysis by Anshuman Tripathi and Ekta Khurana under ISRO and MoEn Projects, Department of Botany, Banaras Hindu University)a Soil parameters

Hathinala

Khatabaran

Majhauli

Bhawani Katariya

Kota

Physical characteristics Texture (%) Sand Silt Clay Water holding capacity (%)

68.0 29.2 2.8 40.1

87.0 11.0 2.0 30.6

76.7 20.5 3.5 53.9

54.7 39.0 6.3 42.6

60.7 35.0 4.0 36.8

Chemical characteristics Organic carbon (%) Total nitrogen (%) Total phosphorus (%) a

(0.82) (0.72) (0.72) (2.14)

2.79 (0.218) 0.10 (0.012) 0.007 (0.001)

The values in parentheses are 1S.E.

(2.08) (1.53) (0.58) (3.01)

1.38 (0.210) 0.15 (0.010) 0.008 (0.001)

(3.71) (5.11) (1.32) (3.50)

2.28 (0.151) 0.13 (0.011) 0.007 (0.0004)

(1.76) (2.08) (0.33) (1.19)

1.67 (0.248) 0.11 (0.029) 0.011 (0.001)

(4.70) (4.04) (0.57) (1.11)

1.18 (0.140) 0.12 (0.014) 0.008 (0.0004)


the three 1 ha plots. The b-diversity values thus reflect habitat heterogeneity within a site. Different diversity indices were calculated using the following equations of Shannon and Weaver (1949) (Eq. (1)); Margalef (1958) (Eq. (2)); Whittaker (1972) (Eq. (3)); Hill (1973) (Eq. (4)); Greenberg (1956), Berger and Parker (1970) (Eq. (5)); Whittaker (1972) (Eq. (6)): s X pi ln pi

H0 ¼

to ecologist (Krebs, 1989). ANOVA procedure of SPSS package (1997) was used to see the effect of degree of disturbance on basal area, number of stems and number of species at different sites, using the three 1 ha plots as replicates for each site. The sites were ordinated using basal area for tree species by Principal Component Analysis (PCA). For this, PC-ORD software was used (McCune and Mefford, 1999). The relationships of soil nitrogen and disturbance with PCA axes were determined using SPSS package (1997).

(1)

i¼10

SR ¼

S1 lnðNÞ

(2)

Ew ¼

S ln Ni ln Ns

(3)

N1 ¼ expH

0

3. Results The five sites yielded a total of 4033 stems (Table 3) and 49 species of trees 30 cm CBH. These species represent 44 genera and 24 families (Table 4). The number of species and individuals varied from 1 to 9 species and 1 to 12 individuals per quadrat of 10 m 10 m size, and a majority of quadrats had 1–3 species and 1–4 individuals (Table 5) indicating a patchy distribution of species and individuals in the forest at each site. Table 6 shows that the majority of species were represented by very few individuals which varied site to site. Total number of species on different sites varied from 7 to 31 (Table 7). The per cent species represented by a single individual varied from 18 (Bhawani Katariya) to 30 (Khatabaran) and the per cent species represented by 1–10 individuals varied from 42 (Kota) to 53 (Khatabaran) site. On the basis of relative basal cover, the five sites differed in the combination of dominant and co-dominant species (Table 4). S. robusta was dominant at the Hathinala and Majhauli sites and co-dominant at the Bhawani Katariya site. H. binata dominated at the

(4)

D¼1l

(5)

Sc bw ¼ S

(6)

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In the above equations, SR is the Margalef index of species richness, S the number of species, N the total number of individuals, Ew the Whittaker’s index of evenness, Ni the abundance of most important species, Ns the abundance of least important species, pi the proportion of individuals belonging to species i, H0 the Shannon–Wiener index, ln the natural log (i.e. 2.718), D the Simpson’s diversity,P l the Simpson’s concentration of dominance (1 p2i ), bw the Whittaker’s index of b-diversity, Sc the total number of species, S the average number of species per sample and N1 the number of equally common species which would produce the same diversity as H0 . Hill (1973) recommends using N1 rather than H0 , because the units (number of species) are more clearly understandable

Table 3 The stand structure of the dry tropical forest along a disturbance gradienta Variables Mean Mean Mean Mean

2

1

basal area (m ha ) no. of stems ha1 no. of species ha1 no. of genera ha1

Hathinala

Khatabaran

Majhauli

Bhawani Katariya

Kota

8.50 419.00 23.00 22.33

13.78 279.33 22.00 22.00

8.72 395.67 18.33 17.67

6.17 215.33 16.67 16.00

1.30 (0.56) 35 (10.69) 3.67 (0.33) 3.67 (0.33)

Total no. of unique species Total no. of species per individual a

The values in parenthesis are 1S.E.

(1.50) (64.01) (2.89) (2.33)

6 0.025

(2.10) (50.61) (0.00) (0.00)

13 0.036

(0.66) (17.68) (0.67) (0.88)

1 0.019

(0.72) (23.13) (1.73) (0.58)

1 0.034

1 0.067

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Table 4 Relative basal cover and dispersion pattern of tree species in a dry tropical forest regiona Species cc

A. catechu Adina cordifolianc Aegle marmeloscr Albizia odoratissimacr A. latifolianc Bauhinia racemosanc Bombax ceibacr B. serratanc Briedelia retusacr B. lanzancc B. monospermanc Carissa spinarumcc Casearia ellipticacc C. fistulanc Cassia siameacc Chloroxylon swieteniacr Dalbergia sissoocr D. melanoxyloncc E. glaucumnc Emblica officinalisnc Eriolaena quinqueoloculariscu Ficus benghalensisnc Flacourtia indicacu Gardenia latifoliacr Gardenia turgidacr Grewia serrulatacu H. binatacc H. antidysentericanc Holoptelia integrifoliacr Hymenodicyon excelsumcu L. parvifloracc L. coromandelicanc Miliusa tomentosanc Mimosa himalayanacr Nyctanthes arbortristisnc Ougeinia oojensiscu Pterocarpus marsupiumnc Randia dumetorumcr Schrebera swietenioidescr Semecarpus anacardiumcu S. robustacc Soymida febrifugacc Syzygium heyneanumcr T. grandiscc Terminalia chebulanc T. tomentosacc Zizyphus glaberrimanc Unidentified (Papara)cu Unidentified (Rij)cu

Family

Hathinala

Khatabaran

Majhauli

Bhawani Katariya

Kota

Mimosiaceae Rubiaceae Rutaceae Mimosiaceae Combrataceae Caesalpiniaceae Bombacaceae Burseraceae Euphorbiaceae Anacardiaceae Fabaceae Apocynaceae Flacourtiaceae Caesalpiniaceae Caesalpiniaceae Rutaceae Fabaceae Ebenaceae Celastraceae Euphorbiaceae Sterculiaceae Moraceae Flacourtiaceae Rubiaceae Rubiaceae Tiliaceae Caesalpiniaceae Apocynaceae Ulmaceae Rubiaceae Lythraceae Anacardiaceae Anonaceae Mimosiaceae Oleaceae Fabaceae Fabaceae Rubiaceae Oleaceae Anacardiaceae Dipterocarpaceae Miliaceae Myrtaceae Verbenaceae Combrataceae Combrataceae Rhamnaceae

11.94 – – 0.08 3.09 0.14 – 5.57 5.10 6.79 – – – 0.10 – – – 3.47 0.11 2.57 0.24 – 0.16 0.04 0.06 0.11 11.71 0.12 – – 2.75 8.22 0.80 0.07 0.46 0.21 0.79 – – – 19.63 2.41 – – 0.06 10.55 2.32 0.09 –

5.18 2.95 0.18 – 14.93 – 0.19 – – 5.24 2.17 1.07 0.48 0.12 – 0.20 – 3.07 1.64 2.07 – 0.02 – – 0.02 – – 4.55 0.39 1.72 5.07 4.75 – – 0.04 – – 0.05 0.03 0.52 9.63 – 0.02 26.20 – 7.47 0.02 – –

8.74 0.13 – – 2.48 0.13 – 2.90 0.07 2.49 – – – 1.58 – – – 6.71 1.84 2.62 0.08 – – – – – – 1.87 – – 16.34 3.92 2.28 – – – 1.15 – – – 29.46 0.05 – – 0.55 14.53 0.03 – 0.11

5.88 – – – 1.55 – – 3.17 – 3.32 – – – – 1.42 – – 6.48 3.56 0.05 – – 0.30 0.43 0.05 – 43.39 0.26 – – 2.10 0.15 1.23 – – – 0.48 – – – 19.83 2.41 – – 0.05 3.81 0.08 – –

13.27 – – – 29.54 – – – – – 15.82 – – – – – 2.79 0.27 – – – – – – – – 37.97 – – – – 0.34 – – – – – – – – – – – – – – – – –

a cc, cu, cr and nc, respectively, stand for consistently clumped, consistently uniform, consistently random and dispersion not consistent along the disturbance gradient.

R. Sagar et al. / Forest Ecology and Management 186 (2003) 61–71 Table 5 Number of quadrats with varying number of adult tree species and individuals at each of the five dry tropical forest sites arranged according to increasing level of disturbance

8

Sites

2

No. of individuals

1–3

4–6

7–9

1–4

5–8

9–12

194 228 230 246 75

91 44 65 14 0

5 0 1 0 0

167 221 185 234 75

110 51 106 25 0

13 0 5 1 0

6 KT 4 PCA Axis 2

Hathinala Khatabaran Majhauli Bhawani Katariya Kota

No. of species

67

BK MJ

0 KH

-2 -4

HT

-6 -8 -10

Table 6 Number of species represented by 1, 1–10, 11–20, 21–30, 31–40, 41–50 and more than 50 individuals at each of the five dry tropical forest sites Sites

1

1–10 11–20 21–30 31–40 41–50 >50


6 9 6 4 2

16 16 10 12 3

1 3 2 2 2

– 1 2 2 1

1 2 3 1 –

5 2 1 2 1

8 6 5 3 –

Bhawani Katariya and Kota sites, and Tectona grandis at the Khatabaran site. A. latifolia co-dominated at the Khatabaran and Kota sites. A. catechu and L. parviflora, respectively, co-dominated at the Hathinala and Majhauli sites. Thus, the species exhibited local dominance. These data revealed that the Hathinala site represented Shorea-Acacia community; Khatabaran site, Tectona-Anogeissus community; Majhauli site, Shorea-Lagerstroemia community; Bhawani Katariya, Hardwickia-Shorea community; and the Kota site, Hardwickia-Anogeissus community. A. catechu, A. latifolia, Diospyros melanoxylon and Lannea coromandelica were common tree species on all sites. The Table 7 Number of tree species with adult population showing different dispersion patterns at each of the five sites in the dry tropical forest arranged according to increasing level of disturbance Sites


Species distribution pattern Clumped

Random

Uniform

Total

13 13 12 10 4

8 11 5 6 3

10 6 6 6 0

31 30 23 22 7

-8

-6

-4

-2

0

2

4

6

8

10

PCA Axis 1

Fig. 2. PCA ordination of dry tropical forest sites. PCA axis 1 and 2, respectively, represent for first and second coordinates (scores) of sites. The axis 1 explained 43% and axis 2 explained 28% of total variance in species composition.

Khatabaran site had highest number of unique species (27%) while Majhauli, Bhawani Kataria and Kota each had only 2% species as unique (Table 4). The average total basal area varied from 1.30 to 13.78 m2 ha1 and mean species presence and abundances per hectare ranged from 3.67 to 23.00 and 35 to 419 stems, respectively (Table 3). ANOVA revealed that the degree of disturbance caused significant differences in basal area (F4;10 ¼ 12:467; P ¼ 0:001), total number of species (F4;10 ¼ 32:182; P ¼ 0:000) and abundances (F4;10 ¼ 15:784; P ¼ 0:000) of the species. There was a general trend of decline in stem density as the level of disturbance increased. The PCA ordination of the five permanent plots on the basis of species basal cover is presented in Fig. 2. The PCA axis 1 accounted for 43% variation in species composition while PCA axis 2 accounted for 28% variation. The PCA axis 1 was related with soil nitrogen (r ¼ 0:912,P ¼ 0:031), and the PCA axis 2 represented the disturbance gradient (r ¼ 0:887; P ¼ 0:045). An analysis of dispersion pattern indicated that 12 species were consistently clumped, 14 consistently random and 8 had consistently uniform distribution on all sites, where they occurred (Table 4). As many as 15 species showed a change in the distribution pattern from site to site. The number of species showing different dispersion patterns at each of the five sites is given in Table 7.

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Table 8 Species distribution and dispersion with their abundance along a disturbance gradienta Variables A. catechu A. latifolia B. serrata B. lanzan B. monosperma C. fistula D. melanoxylon E. glaucum E. officinalis H. binata H. antidysenterica L. parviflora L. coromandelica M. tomentosa S. robusta S. febrifuga T. tomentosa

Hathinala

Khatabaran

C

Majhauli

C

C

72 63C – 73C 24U 5U 59C 8U 19C – 125C 37C 45C – 45C – 35C

240 46U 42C 79C – 1 54C 1 43R 105C 3 43C 113C 9U 188C 32C 89C

105 27C 11C 51C – 31C 50C 7U 36C – 28C 168C 71U 38C 317C 1 216C

Bhawani Katariya C

81 15U 10U 45C – – 43C 10C 1 207C 5U 26C 3 9U 105C 28C 31C

Kota 16C 12C – – 25C – 1 – – 48C – – 1 – – – –

a The superscripts U, R and C stands for uniform, random and clumped dispersion, respectively. The dispersion of only those species which are present at minimum two site with 5 individuals at a site are shown.

Approximately 42–57% species on each site had clumped distribution. Interestingly, on the least disturbed Hathinala site as many as one-third species showed a regular distribution. For further analysis of dispersion pattern, species which were present at minimum of two sites with 5 individuals at a site, were considered (Table 8). Of the total analysed species, 41% showed changing nature in dispersion along the disturbance gradient, half of these species changed from clumped to uniform dispersion (B. serrata, Holarrhena antidysenterica and L. coromandelica) and their abundances generally declined with increasing disturbance. On the other hand, Butea monosperma, Cassia fistula and Elaeodendron glaucum changed from uniform to clumped dispersion with increased abundances along the disturbance gradient (Table 8). The number of individuals summed

across the species showing clumped behaviour declined with increasing disturbance except for the Majhauli site, while the number of individuals summed for the species showing uniform dispersion did not follow any pattern (Table 8). Table 9 shows the pattern of diversity along the disturbance gradient indicating that richness, evenness 0 and a-diversity parameters (D and eH ) decreased as the level of disturbance increased. Margalef index of species richness ranged from 1.289 to 4.308, highest values were recorded at the Khatabaran site and the lowest at the Kota site. A similar pattern was found for evenness, which ranged from 1.808 to 5.876. However, a-diversity indices were highest at the least disturbed Hathinala site and lowest at the most disturbed Kota site, values being 0.698–0.904 for D and 0 4.047–13.860 for eH . b-Diversity was highest for the

Table 9 Pattern of tree species diversity in dry tropical forest sites arranged according to increasing level of disturbance Variables

Hathinala

Khatabaran

Majhauli

Bhawani Katariya

Kota

Species richness (Margalef index) Evenness (Whittaker index) Simpson’s diversity (D ¼ 1 l) 0 expH b-Diversity

4.204 5.656 0.904 13.86 1.348

4.308 5.876 0.902 13.695 1.364

3.108 3.994 0.857 9.826 1.255

3.245 4.126 0.838 9.347 1.320

1.289 1.808 0.698 4.047 1.909

R. Sagar et al. / Forest Ecology and Management 186 (2003) 61–71 2.0

Beta diversity

1.8 1.6 1.4 1.2 1.0 0.01

0.02

0.03

0.04

0.05

0.06

0.07

Species / individual

Fig. 3. Relationship between beta diversity (bw ) and species/ individual (Sn) according to bw ¼ 0:940 þ 13:724Sn , r 2 ¼ 0:913; P ¼ 0:011.

drastically disturbed Kota site. b-Diversity ranged from 1.255 to 1.909. In the present study, b-diversity was not a very sensitive indicator of disturbance as, four sites had somewhat similar values although number of species per individual had a direct positive influence on b-diversity (Fig. 3).

4. Discussion It has been argued that if environmental change produced by disturbance is large, it may become lethal to greater numbers of established species than are, or can be, immediately replaced by immigrants (Sheil, 1999). Disturbance such as logging usually causes an immediate decline in biodiversity followed by a recovery, although not necessarily of the same species (Noble and Dirzo, 1997). Species richness of a site experiencing disturbance, therefore, will be a cumulative outcome of differential responses of species to disturbance. Some species may tolerate the disturbance and the others may disappear. This study indicated a marked patchiness in the distribution of species and individuals, a characteristic of the dry tropical forest which has drawn little attention in the past (Jha and Singh, 1990). The presence of maximum number of species with only one or 1–10 individuals at all the forest sites may indicate the mixed nature of the forest (Richards, 2002), and a marked diversity. In the present study, the species represented

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by a single individual varied from 18 to 30%. Black et al. (1950) from Amazonia rain forests found that among trees of at least 10 cm dbh, over one-third of the species were represented by a single individual. Phytosociological analysis of the present forest indicated that the five sites studied represented different combinations of species with different dominants and co-dominants. Significant relationships between PCA axis 1 and soil nitrogen and between PCA axis 2 and disturbance intensity indicated that the habitat conditions as well as disturbance regimes are important in determining the constitution and distribution of the dry tropical forest communities. Soil N is supposed to be the most limiting nutrient in a majority of terrestrial ecosystems (Fenn et al., 1998). An earlier study suggested that the heterogeneity of the environment as well as disturbance are the prime cause for patch formation in these forests (Jha and Singh, 1990). A small number of unique species on the more disturbed sites and a decrease in the total number of species along the disturbance gradient may reflect high utilisation pressure (Bhat et al., 2000). The recurrent human interventions for collection of fuel wood and minor forest products and the practices of grazing and trampling may change the habitat fitness for many species (Pandey and Shukla, 1999). Occurrence of A. catechu, A. latifolia, D. melanoxylon and L. coromandelica at all the sample locations along the disturbance gradient suggests their tolerance to biotic pressure and a wide ecological amplitude. For analysis of variability in dispersion, only species which are present at minimum two sites and have 5 individuals at one site were considered. About half of the analysed species in this study showed no effect of disturbance on dispersal behaviour and were characterised by clumped distribution. Clumping in these species may be due to coppice forming habit, and patchy distribution of microhabitats suitable for plant growth in dry tropical forest soils (Roy and Singh, 1994). According to Odum (1971), the clumped distribution is common in nature while random distribution is found only in very uniform environments. The clumping of individuals of a species may be due to insufficient mode of seed dispersal (Ashton, 1969; Richards, 1996), or when death of tree creates a large gap encouraging recruitment and growth of numerous saplings (Armesto et al., 1986; Newbery et al., 1986; Richards, 1996). Vegetative reproduction by suckers

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and coppice also encourages clumpiness (Lieberman, 1979). A. latifolia, B. monosperma, C. fistula, D. melanoxylon, H. antidysenterica and L. parviflora are the species which form coppice and as a result of stem poaching, they either recover or increase in number through coppice when the disturbance is moderate. Of these coppice forming species, only A. latifolia and B. monosperma are able to tolerate high degree of disturbance. Half of the analysed species changed dispersion behaviour as a result of disturbance. Species changing from clumped distribution to uniform distribution included B. serrata, H. antidysenterica and L. coromandelica. Opposite to this, B. monosperma, C. fistula and E. glaucum shifted their distribution behaviour from uniform to clumped. Connell (1971) suggested that the uniform dispersion patterns of species in tropical forests largely enables the maintenance of high levels of diversity. The changes in the dispersion pattern may reflect the reactions of species to disturbance as well as to changes in the habitat conditions. For example, the stem density of species changing from clumped to uniform dispersion was lower, and that of the species changing from uniform to clumped dispersion was higher on the more disturbed sites. The study of Ramirez-Marcial et al. (2001) showed decreasing density and basal area with disturbance intensity, and Smiet (1992) correlated the basal area with the rate of disturbance. Our study also indicated that stem density declined with disturbance. The decline in stem density along the disturbance gradient may be due to a gradual increase in the extraction of timber, debarking and rotting of boles. The a-diversity (exponential Shannon–Wiener index and Simpson’s diversity) and its components (species richness and evenness) followed an inverse trend with disturbance gradient which may be due to decreased resource availability with increasing disturbance (Brokaw, 1985). Collins et al. (1995) found a significant monotonic decline in species diversity with increasing frequency of experimental disturbance. The b-diversity did not decrease with disturbance gradient, but was extremely high for the most disturbed site, because of increased species:individual ratio. It can be argued that in order to resist change in floristics, the dry tropical forest site must have a low species individual ratio. According to the intermediate disturbance hypothesis (Connell, 1978; Huston, 1979), with no or little

disturbance, only the competitive dominants can survive, while at sufficiently high level of disturbance only fugitive species can survive, therefore, the diversity is maximum at the intermediate level of disturbance (Abugov, 1982). In contrast, in the present study species diversity declined with increasing level of disturbance. Leigh (1965) suggested that stability increases with the complexity of the ecosystems, that is with the number of species and with the number of interactions between them. MacArthur (1955) pointed out that diversity is a function of the number of species. The stability has been reported to increase with diversity (Safi and Yarranton, 1973). Therefore, disturbance in the dry deciduous forest can potentially lead to a decrease in stability and complexity of the ecosystems.

Acknowledgements Ministry of Environment and Forests, Government of India, New Delhi, is acknowledged for financial support.

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