Structural and compositional variations in undisturbed and disturbed tropical forests of Bhuban hills in south Assam, India

Abstract

The present study describes the structure, composition and regeneration of tree species in undisturbed and disturbed tropical forests in Bhuban hills in southern Assam, India. The study area is part of one of the globally recognized biodiversity hot spots and in spite of the increasing human activities in these forests, so far no information is available on their impacts on plant assemblages in the area. In each forest, the vegetation was analysed through 50 quadrats (10 m × 10 m) in a belt transect of 500 m × 10 m. A total of 49, 47 and 42 tree species were recorded in disturbed forests and 62, 61 and 60 species in undisturbed forests. The average tree density and basal area were significantly higher in undisturbed forests (708.67 and 42.01 m² ha⁻¹, respectively) than disturbed forests (443.33 and 18.44 m² ha⁻¹, respectively). The population structure of tree species in disturbed forests showed a fluctuating curve, and in undisturbed forests, it was a reversed J-shaped curve. The present study reveals that the species richness, diversity, density and basal area have significantly declined (P < 0.01) due to anthropogenic disturbances. As reflected from the population structure, most of the tree species in disturbed forests showed poor regeneration status. If degradation of these forests continues at the present pace, several of these species which are providing various ecosystem services to the local human communities may disappear in the very near future.

Keywords:

• tropical forests
• disturbance index
• density
• diversity
• basal area
• seedling
• sapling

Introduction

Although tropical forests harbour the world’s most species rich plant communities, they are also vulnerable to deforestation and degradation (World Conservation and Monitoring Centre 1992; LaFrankie et al. 2006; Lu et al. 2010). The degradation of tropical forests and habitat loss due to anthropogenic activities are the major causes of decline in biodiversity (Pickett & White 1985). To meet the increasing demands of rapidly increasing human population, natural forest resources are being utilized far beyond their regenerative capacity. This has resulted in decreasing size of natural forests at alarming rates in India and other parts of the world. The effect of human disturbances on species diversity is an issue that has engaged ecologist since long (Pickett & White 1985; Tilman 1988; Vetaas 1997; Sheil 1999; Lawes et al. 2007; Sapkota et al. 2009; Lu et al. 2010). For the management of both undisturbed and disturbed forest communities, it is very important to understand forest structure and composition (Congdon & Herbohn 1993; Ssegawa & Nkuutu 2006). Such an understanding helps the forest managers to assess the potential impacts, the amelioration of effects of disturbance, optimization of productivity and rehabilitation of degraded ecosystems (Congdon & Herbohn 1993). Assessment of floristic composition and structure of forest communities is also useful in identifying important elements of plant diversity, protecting threatened and economic species, and monitoring the forest communities, among others (Tilman 1988; Ssegawa & Nkuutu 2006).

Human disturbance in tropical forests is not simply a phenomenon of the colonial and modern eras, but dates back to early human occupation in tropical region (Devenan 1976; Fairhead & Leach 1998). Disturbances of both natural and human origin influence forest dynamics and tree diversity at local and regional scales (Sheil 1999; Sapkota et al. 2009). Disturbances not only influence diversity, but also post disturbance regeneration and dominance of tree species (Lawes et al. 2007). Several workers (Vetaas 1997; Sheil 1999; Venkateswaran & Parthasarathy 2003; Lawes et al. 2007) have explained the relationship between disturbance and species richness, but the studies that elucidate how disturbances influence stand structure, species composition and regeneration of tree species are very limited (Sapkota et al. 2009).

Tropical forests in southern part of Assam are one of the biodiversity rich regions of northeast India. The tropical forests in this region still retain significant proportion of its phytodiversity, possibly due to long years of isolation and difficult terrain. However, they are now under increasing anthropogenic pressure (such as timber extraction, firewood collection, cattle grazing, etc.) to unleash its resources for economic development (Borah & Garkoti 2011). In spite of the increasing human activities in the forests, no studies have been conducted to assess their impacts on plant assemblages in the area. Past studies indicate that due disturbance in the forests; conditions become unfavourable for existing species and fail to re-establish themselves (Kramer 1933; Singh & Singh 1992). Based on this understanding, we hypothesized that the disturbance would help light demanding opportunist species to get established in the gaps created due to forest biomass extraction and hence would result in changes in forest community structure and regeneration patterns. The present study determined the impact of anthropogenic activities on the diversity, composition and structure of the tropical forests in this region.

Materials methods

Study area

The southern part of Assam, popularly known as Barak Valley, consists of three districts (viz. Cachar, Hailakandi and Karimganj). The region experiences a warm and humid climate having mean annual rainfall of 2508 mm (Athokpam & Garkoti 2013). According to Champion and Seth (1968), vegetation of the region is dominated by Cachar tropical evergreen forests (1/1/B/C3) and Cachar tropical semi-evergreen forests (2/2B/C3). The forests of this region are relatively unexplored harbouring rich plant diversity. The vegetation of this region has been free from anthropogenic disturbances over centuries. But due to rapid population growth and development activities, some parts of the forests are under huge anthropogenic pressure such as over exploitation of species for timber, fuelwood, fodder, bamboo harvesting and settlement.

Methodology

For the present study, a total of six forest sites (three undisturbed and three disturbed) were selected based on species density, canopy cover, intensity and frequency of anthropogenic activities, etc., in the Bhuban Hills of Assam. Study Sites I, II and III were at the foothill and closer to the human settlements (Table 1). Due to close proximity, people frequently visit these forests for collection of various provisioning services from the forests like timber, fuelwood, bamboo and other non-timber forest products (NTFPs). Study Sites IV, V and VI were at higher elevations and far from the human settlements. People rarely visit IV, V and VI forest sites to collect forest resources due to the distance, difficult terrains and also availability of resources in nearby forests. Consequently, they have in general remained free from anthropogenic pressures.

Table 1. Study sites with disturbance index and GPS coordinates, altitude, disturbance index, nearest village and distance of the forest from the village.

Study sites	Latitude	Longitude	Altitude (m)	Forest category	Disturbance index (%)	Nearest village	Distance from the human settlements (km)
Site I	N24^o37′07.7″	E92^o59′26.6″	71	Disturbed	26.84	Ganganagar	0.81
Site II	N24^o33′39.7″	E92^o55′27.6″	37	Disturbed	28.3	Moniarkhal	0.84
Site III	N24^o33′49.3″	E92^o58′21.6″	79	Disturbed	34.3	Nagakhal	0.47
Site IV	N24^o37′15.5″	E93^o03′24.8″	621	Undisturbed	0	Krishanagar	3.05
Site V	N24^o38′53.7″	E92^o59′26.4″	433	Undisturbed	5.4	Motinagar	2.58
Site VI	N24^o38′46.8″	E93^o00′07.5″	506	Undisturbed	0.28	Bhuban Khal	3.44

The vegetation was analysed through a belt transect of 500 m × 10 m in each forest following Ganesh et al. (1996) and Borah and Garkoti (2011). Each belt transect was again subdivided into 50 quadrats of 10 m × 10 m size along its length. Girths at breast height (gbh at 1.37 m from the ground) of all trees in each quadrat were measured. Arbitrarily an individual with ≥10 cm gbh was categorized as tree. The cut stumps and lopped trees in each quadrat were counted and the girth measured at ground height (i.e. 10 cm from the ground). The saplings and seedlings were sampled at each 50 m along the belt transect using a 5 m × 5 m quadrat. Specimens of each species were brought to the laboratory and prepared for the herbarium. All the plant species were identified with the help of ‘Flora of Assam’ (Kanjilal 1934–1940), ‘Assam Flora’ (Chowdhury et al. 2005) and the herbarium of the Botanical Survey of India, Shillong. The phytosociological data were quantitatively analysed for density, frequency, basal area (Curtis & McIntosh 1950), relative density, relative frequency and relative dominance (Phillips 1959; Garkoti & Singh 1994). To analyse and interpret species population dynamics, gbh structure of all tree species was stressed. Trees were categorized into nine gbh classes starting with 10 cm and ending with >240 cm class for the analysis of population structure. All the individuals of ≥3 to <10 cm girth were considered as tree saplings and <3 cm girth considered as tree seedlings. The total number of individuals belonging to these classes was calculated for undisturbed and undisturbed forest following Muller-Dombois & Ellenberg (1974).

A disturbance index for each forest site was calculated following Kanzaki and Kyoji (1986), Rao et al. (1990) and Bhuyan et al. (2003). The disturbance index (DI) was calculated as the basal area of cut trees measured at the ground level expressed as fraction of total basal area of all trees:

Structure of the present study forests were assessed quantitatively by calculating density, basal area and importance value index (IVI) following Magurran (2004).

Diversity index was calculated following Shannon and Wiener (1963) as follows:

where p_i is the proportion of individuals of ith species and total number of individuals of all species.

The concentration of dominance was calculated following Simpson (1949) as follows:

where p_i is the proportion of individuals of ith species and total number of individuals of all species.

The species evenness was calculated following Pielou (1975) as follows:

where H′ is the Shannon diversity index and H_max is the Shannon maximum diversity index, which was calculated as follows:

where S = the total number of species in the forest site.

Statistical analyses were performed by using SPSS 16.0 (SPSS Inc., Chicago, IL, USA). To determine the effects of anthropogenic disturbances on structure and composition of forest sites, comparisons of different phytosociological attributes (such as species richness, density, basal area and diversity indices) between undisturbed and disturbed forests were done by paired t-test. Pearson bivariate correlation was performed to determine the correlations between different phytosociological attributes and disturbance index. The nearest distance from the human settlements to the study sites were estimated with the help of Google map and Google Earth software (Google Inc., Santa Clara County, CA, USA).

Results

Species richness and diversity

Both tree species richness and diversity have decreased from undisturbed forests to undisturbed forests of the present study. A total of 88 tree species were recorded in the present study, of which 75 and 56 tree species were recorded from undisturbed and disturbed forests, respectively. Several species were common to both sites. The highest number of tree species (62 species from 35 families) was recorded in the undisturbed forest Site V while the lowest number of species (42 species from 26 families) was recorded in the disturbed forest Site III (Table 2). In the undisturbed and disturbed forests, the average tree species number per 0.5 ha area were 61 species (from 32 families) and 46 species (26 families), respectively (Table 3). The t-test shows that species richness and family richness were significantly different (P < 0.05) in the undisturbed and disturbed forests (Table 3). The Shannon diversity index (H′) ranged from 1.44 to 1.98 (Table 2). The average value of Shannon diversity index was also significantly higher (P < 0.001) in the undisturbed forests (1.95) than the disturbed forests (1.49) (Table 3). The average species evenness (J′) and H_max values (Table 3) were higher in undisturbed forests (56.03 and –0.029, respectively) than that of disturbed forests (41.11 and 0.036, respectively).

Table 2. Different plant community attributes of the forest study sites.

Forest category	Study sites	Disturbance index (%)	Family richness	Species richness	Density (no. ha^−1)	Basal area (m^2 ha^−1)	H′	Cd	Hmax	J′
Disturbed	Site I	26.84	24	49	396	16.96	1.52	0.039	−0.034	44.08
	Site II	28.3	27	47	590	21.14	1.44	0.063	−0.036	40.37
	Site III	34.3	26	42	344	17.21	1.50	0.040	−0.038	38.9
Undisturbed	Site IV	0	32	61	614	38.44	1.97	0.017	−0.029	67.31
	Site V	5.4	35	62	718	42.54	1.89	0.021	0.029	65.38
	Site VI	0.28	30	60	794	45.07	1.98	0.019	0.030	66.81

Table 3. Comparison of different plant community attributes between undisturbed and disturbed forests.

Parameters	Forest categories		Paired t-test
	Disturbed	Undisturbed	t	P
Family richness	25.67 ± 0.88	32.33 ± 1.45	3.922	P < 0.05
Species richness	46 ± 2.08	61 ± 0.58	6.943	P < 0.05
Density (no. ha^−1)	443.33 ± 74.86	708.67 ± 52.17	2.908	P < 0.05
Basal (m^2 ha^−1)	18.44 ± 1.35	42.01 ± 1.93	0.996	P < 0.001
Shannon index (H′)	1.49 ± 0.03	1.95 ± 0.03	12.343	P < 0.001
Simpson index of concentration of dominance (Cd)	0.047 ± 0.008	0.019 ± 0.001	3.576	P < 0.05
Evenness (J′)	41.11 ± 1.54	56.03 ± 0.57	15.420	P < 0.001
Hmax	−0.036 ± 0.001	0.029 ± 0.002	2.354	ns
Sapling species richness	24.67 ± 1.76	34.33 ± 1.45	4.230	P < 0.05
Sapling density (no. ha^−1)	4066.67 ± 570.56	5786.67 ± 209.56	2.829	P < 0.05
Sapling basal (m^2 ha^−1)	2.28 ± 0.36	3.06 ± 0.20	1.894	ns
Seedling species richness	15.67 ± 1.33	22.33 ± 2.03	2.747	ns
Seedling density (no. ha^−1)	3880.00 ± 302.88	3586.67 ± 288.76	−0.7009	ns
Seedling basal (m^2  ha^−1)	0.07 ± 0.001	0.06 ± 0.002	−0.1799	ns

Note: ns, not significant.

Species composition

Although both undisturbed and disturbed forests have several common species but disturbance in the forests resulted arrival and establishment of many new species adapted to disturbance only. Therefore, disturbance has resulted in the change of species composition in the present study forests. Cynometra polyandra was the most dominant tree species in the undisturbed forests (with an average IVI value 25.59) followed by Mesua ferrea, Stereospermum personatum, Artocarpus chama, Palaquium polyanthum, Mesua floribunda, Dysoxylum binectariferum, Trewia nudiflora, Pterygota alata, etc. (Table 4). In the disturbed forests, Artocarpus chama was the most dominant tree species (IVI value 32.27) followed by Schima wallichii, Tetrameles nudiflora, Mitragyna rotundifolia, Toona ciliata, Castanopsis purpurella, Callicarpa arborea, etc. (Table 4). Some species such as Holarrhena pubescens, Grewia nervosa, Maesa indica, Streblus asper, Albizia procera, Bridelia monoica, Maesa Montana, Alangium chinensis, Cassia fistula, Symplocos cochinensis were unique to the disturbed forests. On the other hand, some important tree species such as Cinnamomum glaucescens, Crysophyllum roxburghii, Dillenia pentagyna, Dysoxylum binectariferum, Garcinia cowa, Hydnocarpus kurzii, Mangifera sylvatica, Mesua ferrea, Michelia champaca, Pterygota alata, Podocarpus nerifolia, Saraca asoca, Xerospermum glabratum, Zanthoxylum rhetsa, etc., found only in the undisturbed forests.

Table 4. Tree species recorded in the disturbed (D) and undisturbed (UD) forests with their average IVI.

Species	IVI
	D	UD
Actinodaphne angustifolia Nees	–	0.58 ± 0.08
Actinodaphne obovata (Nees) Bl.	–	0.59 ± 0.13
Alangium chinensis (Lour.) Rehder	0.15 ± 0.09	–
Albizia lebbeck (L.) Benth.	3.16 ± 0.16	0.34 ± 0.01
Albizia lucidior (Steud.) Nielson ex Hara	6.80 ± 2.35	1.08 ± 0.15
Albizia procera (Roxb.) Benth.	5.81 ± 1.08	–
Alstonia scholaris (L.) R.Br.	1.02 ± 0.39	1.38 ± 0.19
Aporusa sp	–	0.07 ± 0.01
Artocarpus chama Buch-Ham.	32.27 ± 4.71	17.58 ± 4.71
Artocarpus lacucha Buch-Ham.	2.21 ± 1.53	4.92 ± 1.53
Baccaurea ramiflora Lour.	–	3.89 ± 0.41
Bombax ceiba L.	2.25 ± 0.28	–
Bridelia monoica (Lour.) Merr.	3.45 ± 0.62	–
Callicarpa arborea Roxb.	8.29 ± 0.59	0.83 ± 0.19
Carallia branchiata (Lour.) Merr.	0.32 ± 0.14	2.44 ± 0.73
Cassia fistula L.	0.23 ± 0.08	–
Castanopsis purpurella (Miq.) Balak.	9.28 ± 2.34	2.04 ± 0.34
Cinnamomum glaucescens (Nees) Hand-Mazz.	–	2.49 ± 0.54
Cordia dichotoma Forst. f.	–	0.58 ± 0.23
Crysophyllum roxburghii G. Don	–	0.69 ± 0.01
Cynometra polyandra Roxb.	1.63 ± 0.48	25.59 ± 2.48
Desmos longiflorus (Roxb.) Safford.	–	1.26 ± 0.26
Dillenia indica L.	2.20 ± 0.51	0.49 ± 0.03
Dillenia pentagyna Roxb.	–	2.74 ± 0.10
Diospyros toposia Buck-Ham.	0.28 ± 0.11	0.70 ± 0.02
Duabanga grandiflora (Roxb. ex DC.) Walp	6.93 ± 2.89	9.76 ± 1.89
Dysoxylum binectariferum (Roxb.) Hook. f.	–	14.15 ± 2.89
Elaeagnus sp.	4.11 ± 0.73	0.80 ± .03
Eurya acuminata DC.	2.79 ± 0.78	1.92 ± 0.78
Excoecaria oppositifolia Griff	–	0.47 ± 0.11
Ficus hirta Vahl.	3.23 ± 0.19	–
Ficus hispida Vahl.	6.17 ± 0.75	1.77 ± 0.35
Ficus racemosa L.	3.18 ± 0.72	3.87 ± 0.72
Flacourtia indica (Burm.f.) Merr.	–	0.01 ± 0.001
Garcinia cowa Roxb. ex DC.	–	0.07 ± 0.01
Garuga pinnata Roxb.	2.93 ± 0.62	1.21 ± 0.14
Gmelina arborea Roxb.	2.93 ± 0.34	0.55 ± 0.02
Grewia nervosa (Lour.) Panigr.	6.25 ± 0.81	–
Gyclosmis arborea (Roxb) Corr.	–	0.76 ± 0.03
Gynocardia odorata R. Br.	–	4.87 ± 1.21
Hibiscus macrophyllus Roxb	–	0.34 ± 0.14
Holarrhena pubescens (Buch-Ham) Wall	6.61 ± 1.41	–
Hydnocarpus kurzii (King) Warb.	–	6.72 ± 1.71
Knema linifolia Roxb	–	1.68 ± 0.38
Lagerstroemia reginae Roxb.	5.91 ± 0.19	1.23 ± 0.15
Litsea laeta Wall.	–	0.10 ± 0.01
Litsea salicifolia (Roxb. ex Nees) Hook. f.	3.03 ± 0.63	0.70 ± 0.10
Macaranga peltata Roxb.	7.63 ± 1.35	1.67 ±.0.16
Maesa indica (Roxb) A.DC.	5.63 ± 2.04	–
Maesa montana A. DC.	0.06 ± 0.01	–
Mallotus ferrugineus (Roxb.) Muell.Arg.	2.15 ± 0.86	2.69 ± 0.86
Mangifera sylvatica Roxb.	–	0.61 ± 01
Memecylon umbellatum f.	–	1.42 ± 0.09
Mesua ferrea L	–	23.11 ± 1.21
Mesua floribunda (Wall.) Koste.	0.67 ± 0.41	14.20 ± 2.41
Meyna spinosa Roxb.	0.26 ± 0.12	0.06 ± 0.01
Michelia champaca L.	–	1.77 ± 0 .14
Mitragyna rotundifolia (Roxb.) O. Kuntze	19.26 ± 1.23	0.07 ± 0.02
Oroxylum indicum (L.) Vent.	3.44 ± 0.57	3.99 ± 0 .57
Palaquium polyanthum Benth.	1.12 ± 0.16	16.50 ± 1.16
Podocarpus nerifolia D. Don	–	1.04 ± 0.09
Pterospermum lanceaefolium Roxb.	3.00 ± 1.04	9.69 ± 1.44
Pterygota alata (Roxb.) R.Br.	–	11.48 ± 1.47
Sapindus attenuatus wall.	–	0.52 ± 0.06
Sapindus mukorossi Roxb.	–	0.06 ± 0.01
Sapium baccatum Roxb.	11.34 ± 1.71	4.21 ± 171
Saraca asoca (Roxb.) de Wilde.	–	0.43 ± 0.12
Saurauia roxburghiia Wall.	2.67 ± 0.45	1.75 ± .45
Schima wallichii (DC.) Kuntze.	28.16 ± 3.33	0.36 ± 0.11
Semecarpus anacardium L.	–	6.79 ± 1.72
Spondias pinnata (L.f.) Kurz.	2.91 ± 0.90	10.82 ± 1.90
Sterculia villosa Roxb.	0.70 ± 0.23	1.10 ± 0.23
Stereospermum personatum (Hassk.) Chatterjee	4.18 ± 1.86	19.15 ± 2.86
Streblus asper Lour.	5.37 ± 1.37	–
Symplocos cochinensis (Lour.) Moore	0.24 ± 0.10	–
Syzygium cumini (L.) Skeels.	3.56 ± 0.22	1.97 ± 0.26
Syzygium oblatum (Roxb.) Wall. ex A.M & Cowan	3.7 ± 0.24	0.11 ± 0.03
Syzygium praetermissum (Gage) Balak.	–	1.83 ± 0.74
Syzygium syzygioides (Miq.) Mer	2.14 ± 0.93	1.36 ± 0.22
Terminalia bellirica (Gatertn.) Roxb.	4.16 ± 0.86	1.02 ± 0.24
Terminalia chebula Retz.	2.61 ± 0.23	0.44 ± 0.09
Tetrameles nudiflora R.Br.	27.33 ± 2.19	5.04 ± 1.19
Toona ciliata M. Roem.	11.00 ± 1.07	0.68 ± 0.01
Trewia nudiflora L.	0.38 ± 0.11	13.53 ± 2.36
Vatica lancaefolia (Roxb.) Bl.	3.97 ± 0.88	7.65 ± 1.08
Vitex peduncularis Wall. ex. Schauer	8.92 ± 1.73	1.55 ± 0.73
Xerospermum glabratum (Kurz.) Radlk	–	7.80 ± 1.21
Zanthoxylum rhetsa (Roxb.) DC	–	2.27 ± 0.64

Tree density and basal area

The density and basal area of the tree species were significantly lower in the disturbed forest sites than the undisturbed forest sites. The highest tree density (794 trees ha⁻¹) was recorded in the undisturbed forest Site VI and the lowest (344 trees ha⁻¹) in the disturbed forest Site III (Table 2). The average tree density was significantly higher (P < 0.05) in undisturbed forests (708.67 trees ha⁻¹) compared to the disturbed forests (443.33 trees ha⁻¹). The tree basal area in this study ranged from 17.21 to 45.07 m² ha⁻¹ (Table 2). The average tree basal area was also significantly higher (P < 0.001) in the undisturbed forests (42.01 m² ha⁻¹) than the disturbed forests (18.44 m² ha⁻¹) (Table 3).

Population structure

Distribution of tree densities in different girth classes in undisturbed forests revealed reversed J-shaped curve (Figure 1a). In undisturbed forests, population densities decreased with increased girth classes. But in disturbed forest, a fluctuating pattern of population was recorded. In disturbed forests, trees from the girth class 150–180 cm and >210 trees were absent.

Figure 1. Distribution of (a) population density and (b) tree basal area in different girth classes in undisturbed (UD) and disturbed forests (D).

The distribution of basal area in different girth classes is shown in the Figure 1(b). In undisturbed forests, the basal area increased with increase in girth class up to 150–180 cm girth class and then declined. But in disturbed forests, 30–60 cm contributed more basal area than the other girth classes and followed a fluctuating pattern of distribution curve.

Relationship between different community attributes and disturbance index

In the present study, plant community attributes such as species richness, Shannon diversity index, tree density and tree basal area were negatively correlated with disturbance index and positively correlated with the distance from the human settlements (Table 5). All the studied community characteristics, except for tree density, were positively correlated (P < 0.01) with altitude. Also disturbance index was positively correlated with the distance from human settlements (P < 0.001) and altitude (P < 0.01) (Table 5). The present study also examined the relationship between standing tree basal area and cut stump basal area in different girth classes. A positive and significant (P < 0.01) correlation was recorded between the standing tree basal area and cut stump basal area in different girth classes (Figure 2).

Figure 2. Relationship between standing tree basal area and cut stump’s basal area of different girth classes.

Table 5. Relationships of different community attributes with disturbance index, distance from the human settlements and altitude.

Parameters	Disturbance index		Distance from human habitation		Altitude
	r	P	r	P	r	P
Disturbance index	–	–	–0.991	P < 0.01	–0.970	P < 0.01
Species richness	–0.976	P < 0.01	0.944	P < 0.01	0.921	P < 0.01
Tree density	–0.840	P < 0.05	0.860	P < 0.05	0.724	Ns
Tree basal area	–0.962	P < 0.01	0.971	P < 0.01	0.920	P < 0.01
Shannon diversity index	–0.980	P < 0.01	0.981	P < 0.01	0.984	P < 0.01

Seedlings and saplings

The species richness and densities of saplings and seedlings were also lower in the disturbed forest sites than those of the undisturbed forest sites. The maximum numbers of sapling species (37 species) was recorded in undisturbed forest Site V and minimum number of species (22 species) in disturbed forest Site II. The highest number of seedling species (26 species) was recorded in the undisturbed forest Site IV and lowest number of seedling species (13 species) occurred in disturbed forest Site I (Table 6). The sapling density ranged from 3160 to 6200 individuals ha⁻¹ and seedling density ranged from 3080 to 4360 individuals ha⁻¹ (Table 6). The highest sapling basal area (3.32 m² ha⁻¹) was recorded in the undisturbed forest Site V and the lowest sapling basal area (1.65 m² ha⁻¹) in the disturbed forest Site I (Table 6). The seedling basal area ranged from 0.047 to 0.089 m² ha⁻¹ (Table 6). The average number of sapling species, density and basal area were higher in undisturbed forests (34.33 species, 5786.67 individuals ha⁻¹ and 3.06 m² ha⁻¹, respectively) than in the disturbed forests (24.67 species, 4066.67 individuals ha⁻¹ and 2.28 m² ha⁻¹, respectively). Sapling species richness and densities were significantly (P < 0.05) different in undisturbed and disturbed forests (Table 3).

Table 6. Average species richness, density and basal area of seedlings and saplings in different forest sites of the present study.

Study sites	Disturbance index (%)	Sapling			Seedling
		Species richness	Density (no. ha^−1)	Basal area (m^2 ha^−1)	Species richness	Density (no. ha^−1)	Basal area (m^2 ha^−1)
Site I	26.84	28	3160	1.65	13	3320	0.051
Site II	28.3	22	3920	2.29	17	4360	0.089
Site III	34.3	24	5120	2.90	17	3960	0.068
Site IV	0	34	5520	2.66	26	3600	0.079
Site V	5.4	37	6200	3.32	22	4080	0.074
Site VI	0.28	32	5640	3.22	19	3080	0.047

The seedling species richness in undisturbed forests ranged from 19 to 26 species with an average of 22.3 species while in disturbed forests, it ranged from 13 to 17 species with average seedling species richness of 15.7 species (Tables 3 and 6). But the statistical analyses did not show any significant difference in seedling richness between disturbed and undisturbed forest (Table 3). The seedling density (3880 individuals ha⁻¹) and basal area (0.07 m² ha⁻¹) were higher in disturbed forest than the values recorded in the undisturbed forests (3586.67 individuals ha⁻¹ and 0.06 m² ha⁻¹, respectively) (Table 3).

Discussion

Species richness of a disturbed forest is a cumulative outcome of differential responses of species to disturbances (Sagar et al. 2003). In the present study, tree species richness were lower in disturbed forest sites (49, 47 and 42 species, respectively, in Sites I, II and III) than that of undisturbed sites (61, 62 and 60 species, respectively, in Sites IV, V and VI). This may be due to more frequent and heavy extraction of forest resources (such as timber, fuelwood and other minor forest products) by the local community near the disturbed sites. A large number of cut stumps recorded in the disturbed sites indicate tree extraction in those sites. Local people still depend heavily on different ecosystem services (such as fuelwood, timber, fodder and various NTFPs) provided by the forests for their subsistence. The absence of a number of species (recorded in the undisturbed sites) from the disturbed sites may be due to increased human demand and overexploitation of certain resources. Earlier studies (Noble & Dirzo 1997; Sheil 1999; Sagar et al. 2003) have also shown that environmental change due to the anthropogenic disturbances result in loss of species from different plant communities. The highest species richness and Shannon diversity were recorded in the Site V (undisturbed forest) which is located at comparatively higher and difficult terrain than disturbed sites. Due to the availability of resources in nearby forest ecosystems, people do not prefer to go long distances. The difficult terrain also deterred people from exploiting the forests at those sites for timber or other plant resources. Hence they have remained undisturbed and contributed to the maintenance of plant diversity in the area. Lack of human disturbance may create more conducive environmental conditions for growth and establishment of tree species, which in turn may result in higher species richness in undisturbed forests compared to the disturbed forests. Leigh (1965) reported that ecosystems with higher species richness are more complex, with higher number of interactions among them, which gives the ecosystem more stability than ecosystems with lower species richness.

Shannon diversity index values for the present study forests fall within the range (0.70–3.57) reported for the tropical forests of the region (Bhuyan et al. 2003; Mishra et al. 2004; Deb et al. 2009; Tripathi et al. 2010; Borah & Garkoti 2011; Borah et al. 2012). The lower Shannon diversity index in disturbed forest sites may be due to the human disturbances which occurred in them. According to Noble and Dirzo (1997), anthropogenic disturbances such as logging usually cause an immediate decline in species diversity. This explains why in the present study, a positive and significant correlation was recorded between diversity index and disturbance.

The present study revealed that human disturbances affected not only the species richness and diversity, but also the species composition of the forests. The most remarkable change in species composition due to disturbance was the change of dominance (based on IVI) in plant communities. Cynometra polyandra, Mesua ferrea, Stereospermum personatum, Artocarpus chama, Palaquium polyanthum, Mesua floribunda, Dysoxylum binectariferum, Trewia nudiflora, Pterygota alata, etc., were dominant tree species in the undisturbed forests; on the other hand A. chama, Schima wallichii, Tetrameles nudiflora, Mitragyna rotundifolia, Toona ciliata, Castanopsis purpurella, Callicarpa arborea, etc., have dominated in disturbed forest sites. Continuous removal of tree biomass from the forests resulted in formation of canopy gaps of variable sizes which enabled increased light intensity to reach on the forest floor. Disturbances in the forest ecosystems can alter the environmental condition in the forests by changing light availability and soil conditions which ultimately change the species composition of the forests (Singh et al. 1997; Fredericksen and Mostacedo 2000). Several light demanding successional species (such as Artocarpus chama, Schima wallichii, Mitragyna rotundifolia, Toona ciliata, Castanopsis purpurella and Callicarpa arborea) occupied the gaps and dominated plant community after the disturbance. The over extraction of important timber species from the disturbed forests may have led to the local disappearance of several species and establishment of light demanding tree species (Borah & Garkoti 2011). Some earlier workers (Thorington et al. 1982; Mishra et al. 2004) reported that the shift in tree species dominating plant communities seems to be linked with the level of anthropogenic disturbances.

The negative correlation of tree density and basal area with disturbance index in the present study indicates that increased disturbance lowered the tree density and basal area of the forest. The heavy and rapid extraction of tree species from disturbed forest sites led to lower tree density and basal area in these forest sites than that of undisturbed forest sites. The tree density recorded in the present study is comparable with findings of some earlier reports from the tropical forests of northeast India (245–1620 trees ha⁻¹) by Bhuyan et al. (2003), Mishra et al. (2004, 2005), Deb et al. (2009), Tripathi et al. (2010), Borah and Garkoti (2011) and Borah et al. (2012). The tree basal area recorded in the present study is comparable to the values reported from other tropical forest ecosystems (Parthasarathy & Sethi 1997; Upadhaya et al. 2004; Nath et al. 2005; Kumar et al. 2006; Deb et al. 2009; Tripathi et al. 2010). In the present study, a strong and significant (P < 0.01) negative correlation was recorded between standing tree basal area and cut stump’s basal area in different girth classes. Distribution of basal area in different girth classes showed a fluctuating pattern of curve in disturbed forests. Lower basal area in higher girth classes indicates the removal of those trees from the forest in past. This trend supports the findings of Bhuyan et al. (2003) and Borah et al. (2012) for tropical wet evergreen forests of northeast region.

Tree size class distribution is an important indicator of changes in population structure and species composition of a forest ecosystem (Newbery & Gartlan 1996). Similar to earlier reports from different tropical forests of India (Kadavul & Parthasarathy 1999; Nath et al. 2005; Rajkumar & Parthasarathy 2008; Deb et al. 2009; Page et al. 2010; Borah & Garkoti 2011; Borah et al. 2012), population structure of tree species in the undisturbed forests showed a reverse J-shaped curve pattern. The reverse J-shaped distribution curves are characteristics of the forests with continuous regeneration (Khamyong et al. 2004). However, the disturbed forests showed a fluctuating pattern of distribution curve which is generally common in disturbed forests (Borah & Garkoti 2011). Fluctuating pattern of tree size class distribution may indicate the presence of unplanned and unsustainable exploitation of trees in the forest (Alelign et al. 2007). The number of large and old trees was low in the disturbed forest sites of the present study, indicating the heavy removal of mature trees from these sites. The low abundance of large and mature trees in the disturbed forests might have had adverse effects on tree natural regeneration due to a reduction in the number of seeds produced by the forests (Singh et al. 1997).

Lower sapling and seedling richness, sapling density and basal area in disturbed forests in the present study may be due to damage and trampling of seedlings and saplings during the biomass extraction. Furthermore, this trend may be due to decline in number of adult individuals of several species of reproductive age that could produce sufficient number of seeds for regeneration. Contrary to the findings of Pare et al. (2009) and Borah et al. (2012), which showed significant positive relationship between seedling and adult tree density, seedling density and basal area were higher in disturbed forests. It seems that increased light intensity, decreased inter-specific and intra-specific competition for available soil resources in the gaps created by disturbances may have favoured seedlings of light demanding successional species (such as Callicarpa arborea, Grewia nervosa, Holarrhena pubescens, Mallotus ferrugineus, Mitragyna rotundifolia, Schima wallichii) in the disturbed sites. However, due to excessive disturbance, even the seedlings of those light demanding species failed to reach the sapling stage (Chapman & Chapman 1997; Eilu & Obua 2005; Garkoti 2008).

Consistent with the findings of Singh et al. (1997) and Shaheen et al. 2011, in the present study, disturbance index was negatively correlated with the distance from human settlements (P < 0.001) and altitude of study sites (P < 0.01) (Table 5). From these correlations, it is clear that the forests near the human habitation and at lower altitude are more disturbed than those located far and higher at altitude.

Conclusion

Anthropogenic pressures due to the unsustainable exploitation of the forests resources from the sites closer to the human settlements have resulted in decline in plant diversity and density, and have affected regeneration of tree species. Species population structure indicates that most of the tree species in the disturbed forests show poor regeneration. Several important species that were typical to undisturbed forests were absent in the disturbed forests which has affected community structure and species composition. It appears that if degradation of these forests continues at present pace several of these species which are providing various ecosystem services to the local human communities may disappear locally in very near future.

Acknowledgements

We thank anonymous reviewers for their comments and valuable suggestions. The work was funded by Department of Biotechnology, Government of India.