Tree regeneration and seedling survival patterns in old-growth lowland tropical rainforest in Namdapha National Park, north-east India

Forest Ecology and Management 255 (2008) 3995–4006

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Tree regeneration and seedling survival patterns in old-growth lowland tropical rainforest in Namdapha National Park, north-east India Panna Deb a,b, R.C. Sundriyal b,* a b

G.B. Pant Institute of Himalayan Environment and Development, North East Unit, Vivek Vihar, Itanagar, Arunachal Pradesh 791113, India G.B. Pant Institute of Himalayan Environment and Development, Kosi-Katarmal, Almora, Uttarakhand 263643, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 11 December 2007 Received in revised form 17 March 2008 Accepted 21 March 2008

The tree structure and regeneration was studied in the buffer zone area comprising lowland evergreen and semi-evergreen forests in the Namdapha National Park, one of the largest remaining tract of pristine rainforests in the Eastern Himalayan biodiversity hotspot in India. The investigations were conducted in the three forest stands, viz. Altingia-mixed species, Shorea-Dipterocarp, and Albizia forests that are most dominant forest types in the lowland areas of the park. A total of 98, 54 and 20 species have been recorded at tree stratrum, while 87, 44 and 15 species at regeneration stratum for three stands, respectively. The cumulative regenerating density (seedlings + saplings) was estimated 17,648, 16,110 and 768 individual ha1 for respective stands. It was interesting to note that of the total regenerating species, 44–47% species were new to different stands, which mainly comprised middle storey species. Low-dominant and rare species also contributed significantly in the regeneration of the forest stands. The expanding population structure of forest stands indicated higher survival of the mid- and the low-canopy species than the topcanopy species. The data revealed that the future composition of these stands will highly depended on the potential regenerative status of species in each of the stand and such information would be crucial for forest management. Since the park contributed significantly to the regional biodiversity by depicting species assemblages for both wet evergreen and semi-evergreen biomes, such last remnants of rainforest should be integrally protected from anthropogenic disturbances. ß 2008 Elsevier B.V. All rights reserved.

Keywords: Tropical lowland evergreen and semievergreen stands Canopy cover Saplings Seedlings Dominant and rare species Eastern Himalaya

1. Introduction The potential regenerative status of tree species often depicts the future composition of forests within a stand in space and time (Henle et al., 2004). An understanding of the processes that affect regeneration of forest species is of crucial importance to both ecologists and forest managers (Slik et al., 2003). As floristic and structural composition changes from one community to another, the competitive relationship of species may change with corresponding changes in opportunity for regeneration (Barker and Kirkpatrich, 1994). Studies on population behavior of tree seedlings in different forests showed that their recruitment, growth and survival are influenced by a variety of microclimatic and edaphic factors (Augspurger, 1984a,b; Scholl and Taylor, 2006). In Southeast Asia, rainforest represents centre of high species diversity (Van Steenis, 1957; Whitmore, 1996). An increasing interest in the development and management of such forests give

* Corresponding author. Tel.: +91 5962 241041. E-mail addresses: [email protected] (P. Deb), [email protected], [email protected] (R.C. Sundriyal). 0378-1127/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2008.03.046

rise to the need to understand the regeneration process that ensures maintenance of the community structure and ecosystem stability (Moravie et al., 1997). The north-east region of India also comprises a few such forests; however, despite of the high biological significance these areas are not appropriately investigated (Proctor et al., 1998). Namdapha National Park in the northeast India sustains tropical rainforests at about 278 north of the equator; the ruggedness, inaccessibility, and remoteness have facilitated conservation of diverse forest types in this area (Proctor et al., 1998; Deb and Sundriyal, 2005). The area was declared as a National Park in 1983 and subsequently the taxonomic inventories were conducted here (Chauhan et al., 1996). It, however, lacks quantitative inventories on structural and functional aspects of different forest types that are essential to maintain species composition and recruitment, enhance forest productivity, limit financial input and develop prescription for park management and conservation of plant diversity (Bhat et al., 2000). An information on the tree structure in buffer zone area of the park is just reported (Deb, 2006). In this paper an attempt is being made to study the seedling and sapling demography that is crucial for providing important clues for species establishment phase, thus useful in the interpretation of future composition of the forest stands.

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1.1. Study area The study was conducted in Namdapha National Park (278230 3000 –278390 4000 N to 968150 200 –968580 3300 E longitude) located in the Changlang district of Arunachal Pradesh state (268280 –298360 N and 918300 –978300 E), in the eastern most part of India (Fig. 1). The park covers an area of 1985.25 km2 having 177.43 km2 in buffer zone and 1807.82 km2 in its core zone. The area falls under the Eastern Himalaya (2D) biogeographic province of Indian origin, which covers the Paleartic and the Indo-Malayan (Oriental) realms (Rodgers and Panwar, 1988). The park is wedged between the Dapha Bum range of Mishmee Hills, an outspur at the tail end of North Eastern Himalaya and the Patkai range with an elevational variance of 200–4571 m above sea level. General topography of the park is rugged with steep hills and narrow valleys that are intersected by several streams. Geologically the park is of recent origin and owes its formation to the upheaval of the Himalaya in Pleiocene period of the tertiary age (Chauhan et al., 1996). Namdapha National Park exhibits high floral and faunal diversity (Chauhan et al., 1996; Ghosh, 1987). The area exhibits

tropical climate having typical monsoon pattern with an annual rainfall of 2500–3000 mm. The temperature and relative humidity remains high throughout the season (Fig. 2). The present study was confined to the buffer zone area, which mainly comprised the lowland tropical evergreen forest type that is also called as South Bank Tropical Wet Evergreen forest and a small portion under the Riverine Semi-Evergreen Forests (Kaul and Haridasan, 1987). Altingia-mixed species and the Shorea-Dipterocarp stands are most dominating forms of Tropical Wet Evergreen forest that are often intermingled together (Deb and Sundriyal, 2005). Both of these stands are peculiar to north-east region that are under high biotic pressure in other areas. In Namdapha National Park, however, they exhibit more or less stable status. As such both the stands occupy 132 ha that comprise 2/3 area of the buffer zone. The Albizia procera is found in riverine areas in combination with other species and such areas are often flooded during rains because of heavy rainfall. Lisu, Lama and Chakma communities live in and around the park (Deb and Sundriyal, 2005). Soils comprise sandy-loam in tropical wet evergreen forests and loamy-sand in the riverine Albizia stand. The physico-chemical characteristics of the soils of the study sites are given in Table 1.

Fig. 1. Location map of the study area (buffer zone) in Namdapha National Park, Arunachal Pradesh, north-east India (stand I, II and III are shown for Altingia-mixed species, Shorea-Dipterocarp, and Albizia stands, respectively).

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Fig. 2. Climatic condition at Namdapha National Park (Deban Range) during 2002 and 2003.

(Champion and Seth, 1968; Kaul and Haridasan, 1987). The tree structural analysis comprising species composition, diversity, and other phytosociological parameters was assessed from 50 m  20 m size transects (n = 12, 10, 18 for three stands, respectively) laid randomly using grid maps in each stand that were further sub-divided into ten 100 m2 quadrats for detailed study (Deb, 2006). All individuals 10.00 cm DBH (diameter at breast height) were considered trees. For the regenerating individuals (<10.0 cm DBH), which comprise saplings

2. Methods 2.1. Natural regeneration of tree species Three forest stands undertaken in tree structural analysis were considered for assessing the tree regeneration, viz. Altingia-mixed species stand, Shorea-Dipterocarp stand, and Albizia stand (Deb, 2006). The stands were named based on physiognomic characteristics that are adopted in the forest classification of the region

Table 1 Comparative account for the three forest stands of the buffer zone area of Namdapha National Park, Arunachal Pradesh Parameters

Forest stands Altingia-mixed species

Shorea-Dipterocarp

Albizia

300–600 South Bank Tropical Wet Evergreen (Dipterocarpus)

350–400 South Bank Tropical Wet Evergreen (Dipterocarpus)

350–400 Riverine Semi-Evergreen Forest

Species richness Tree Sapling Seedling

98 72 36

54 36 20

20 20 3

Total no. of genera Total no. of families Dominant tree family

54 31 Euphorbiaceae

34 22 Dipterocarpaceae

19 15 Leguminosae

Density (ha1) Trees Saplings Seedlings

418 4464 13,184

390 3860 12,250

245 369 423

Dipterocarpaceae Dipterocarpus macrocarpus, Shorea assamica

Dominant shrub Dominant herb

Lauraceae Altingia excelsa, Dipterocarpus macrocarpus Beilschmiedia assamica, Dysoxylum procerum Strobilanthus sp. Forrestia sp.

Strobilanthus sp. Forrestia sp.

Euphorbiaceae Albizia procera, Dalbergia pinnata, Bombax ceiba Wrightia coccinia, Glochidion lanceolarium Citrus medica Mikenia mikrantha

Soil Soil texture N (%) P (%) C (%)

Sandy loam 0.37  0.09 0.23  0.02 2.69  1.0

Sandy loam 0.10  0.014 0.21  0.014 0.98  0.14

Loamy sand 0.20  0.05 0.19  0.02 1.11  0.11

Elevational range Forest typea

Dominant regenerating family Dominant tree species Dominant regenerating species

Shorea assamica, Dipterocarpus macrocarpus

a South Bank Tropical Wet Evergreen (Dipterocarpus) and Riverine Semi-Evergreen Forest types (as classified by Kaul and Haridasan, 1987) are corresponded to the Assam valley Tropical Wet Evergreen Forest (1B/C1) and Eastern Hollock Forests as classified by Champion and Seth (1968).

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(DBH < 10.0 cm, height >20 cm) and seedlings (height < 20 cm), the sampling was done from same transects used for tree structure analysis by sub-dividing them in 5 m  5 m size quadrats (n = 160) and 2 m  2 m quadrats for seedlings (n = 160) at each stand separately (Sundriyal and Bisht, 1988; Sundriyal and Sharma, 1996). Samples of all plant species were collected and later identified to species level with the help of experts. Based on the status of trees in each stand, i.e. dominant (density 10 tree ha1), co-dominant (5 but <10 tree ha1), less dominant (<5 but >1 tree ha1) and rare species (1 tree ha1), all the regenerating individuals were also grouped into such tree-categories to depict the future status of these species in a given stand (Whitmore, 1996). Furthermore, considering the canopy status of different trees, the seedling and sapling were also categorized to topcanopy, mid-canopy and low-canopy status to depict the recruitment behaviors of different species (Swaine, 1996). The regeneration status of all species in a given stand was considered ‘good’ if number of seedlings > saplings > adult trees; ‘fair’ if seedlings > saplings  adult; ‘Poor’, if a species survives only in sapling stage but not as seedlings though saplings may be more or equal to adults; ‘none’, if a species is absent both in sapling and seedling stages but present as adult trees; and ‘new’ if a species has no adults but presents only in sapling and/or seedling stage (Sukumar et al., 1992). 2.2. Tree regeneration and DBH class distribution Selected most common species of the three forest stands were assessed for their regeneration along with the mature trees in different girth classes. Such an expression of species is important to predict the status of these species in the forest stand, for example if a species show ‘reverse J’ shaped distribution with higher number of individuals in seedling stage and the number gradually decreased in saplings, small trees, old trees categories, such distribution shows that these species are most dominant form in the stand at present. The ‘J’ shaped distribution depicts pioneer status while ‘reverse J’ expresses successional status of the species. 2.3. Relationship of species regeneration with tree density and basal cover The species regeneration data (tree seedlings, saplings and total regeneration) were also analyzed in view of their overall relationship to species number, tree density and total basal area in each forest stand. The pooled data for all three stands was further subjected to Pearson correlation matrix analysis to show the relationship among various parameters, viz. tree density, total basal area, species number, and seedling and sapling density. 2.4. Seedling survival of species An experiment was designed to know as to how different canopy species perform after germination in the natural forest stands by assessing their seedling survival and mortality. The study was conducted on few important species occupying different canopy-stratum in the two stands of the lowland tropical evergreen forest only. Tree seedlings with <20 cm height were tagged with labeled aluminum foil in identified locations. The selection was made on the basis of their dominance in each stratum, comprising three top-canopy species, six mid-canopy species, and two low-canopy species. These sites were visited at quarterly intervals for 1 year (May 2002 to May 2003) to record the survival and mortality of the tagged plants. Efforts were also made to assess the cause of death of any given seedling.

2.5. Data analysis The statistical analysis (ANOVA and correlation) were conducted among the plots laid within a stand, and between different stands for regeneration and seedling survival using SYSTAT computer package (Wilkinson, 1986). Significant differences among regeneration means, obtained from plot data in each condition were separated by using least significant difference (LSD) following Snedecor and Cochran (1968). 3. Results 3.1. Tree species richness, density and other characteristics of the stands A comparative account of the three forest stands of the buffer zone area of Namdapha National Park is presented in Table 1. A total of 130 tree species varying in 75 genera and 44 families were recorded from all the three stands studied. Of these, Altingia-mixed species stand had 98 tree species in 54 genera and 31 families, Shorea-Dipterocarp stand comprised 54 tree species in 34 genera and 22 families, and Albizia stand recorded 20 tree species in 19 genera and 15 families (Table 1). In Altingia-mixed species stand 11 families were represented by more than one genus and 21 by a single species. In Shorea-Dipterocarp stand 7 families included more than one genus while 16 had one genus. In Albizia stand, 3 families were represented by more than one genus and the remaining 12 by a single genus. Tree species rarity with greater number of singletons at a site, which contributed significantly to the stand diversity, seems to be an important characteristic of the evergreen forest stands. Overall stand tree density (DBH  10 cm) was highest in the Altingia-mixed forest stand, followed by the Shorea-Dipterocarp stand, and minimum in the Albizia stand (Table 1). The Altingia-mixed species stand was dominated by Altingia excelsa (35 trees ha1), Dipterocarpus macrocarpus (18 trees ha1) and Ostodes paniculata (63 trees ha1) (Table 1). The Shorea-Dipterocarp stand was dominated by Dipterocarpus macrocarpus (40 trees ha1) and Shorea assamica (36 trees ha1). The Albizia-stand was dominated by Albizia procera (126.2 trees ha1). 3.2. Tree regeneration status A total of 87, 44 and 15 species were found regenerating in the Altingia-mixed species, Shorea-Dipterocarp, and Albizia stands, respectively. Altogether a total of 104 tree species were found regenerating in three studied stands (Table 1, Annexure I). Of these 13 species were top canopy species, 66 were the mid storey species while 24 species were lower canopy species. Regenerating individuals varied significantly among different stands (P < 0.05) of which high variation were recorded between seedlings (P < 0.001) than the saplings (P < 0.01). The overall regeneration was high at Altingia-mixed species stand, followed by ShoreaDipterocarp and minimum at Albizia stands (Table 1). Of the total regeneration at each stand, the relative contribution of top, mid and lower canopy tree species was recorded 12, 70, and 18%, respectively, for Altingia-mixed species stand; 36, 38, and 26% at Shorea-Dipterocarp stand; and 7, 48, and 45% for respective canopy layers at Albizia stand (Table 2). The data clearly show that the relative contribution of mid canopy species was higher than the top and lower canopy layers. A total of 87 tree species were found regenerating in the Altingia-mixed species stand, of which 49 species (56%) represented the existing tree species, while 41 species (44%) were new to the site that were not recorded in the tree stratum (>10 cm DBH). A total of 24 species were represented in all stages, i.e. in tree, sapling and seedling stages and of these 18

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Table 2 Regeneration of tree species as influenced by canopy cover in the buffer zone of Namdapha National Park, north-east India (seedling and sapling density in ha1 basis) Canopy cover

Altingia-mixed spp. stand No. of species

Sapling density

Top canopy Mid canopy Lower canopy

12 55 21

325 2445 1694

All

87

4464

Shorea-Dipterocarp stand Seedling density

Albizia stand

No. of species

Sapling density

1,800 9,931 1,453

5 19 10

640 1200 2020

5,125 5,000 2,125

13,184

34

3860

12,250

species showed good regeneration (seedling > sapling > tree) that comprised Beilschmiedia assamica, Cinnamomum sp., Dysoxylum procerum, Saprosma ternatum, Syzygium macrocarpum and Ostodes paniculata (Annexure I). Of the 54 tree species in Shorea-Dipterocarp stand, 34 species were found regenerating that comprised 17 main tree species of the site. Nearly 19 species were recorded new in the stand as they were represented only in regenerating stage without any individual in tree stratum (DBH > 10 cm). A total of 10 species were having good regeneration as they represented with the number of seedling > sapling > adult tree, of which Shorea assamica and Dipterocarpus macrocarpus showed highest regeneration (Annexure I). In the Albizia stand, a total of 15 species were regenerating in the stand of which only 9 species (45%) were already represented in the tree stratum. Glochidion lanceolaria was the only species, which showed good regeneration with presence in all the three stages in the order seedling > sapling > adult. At least 11 new species were recorded regenerating at this stand that was not found growing in tree stratum (Annexure I). Albizia procera, the most dominating species in the tree stratum, was not recorded regenerating in the stand. The species richness for sapling layer was recorded 72, 36 and 15 tree species for Altingia-mixed species, Shorea-Dipterocarp, and Albizia stands, respectively; while for seedlings it was 41, 30 and 3 tree species for the respective stands (Table 1, Annexure I). The dominant regenerating families were Lauraceae, Dipterocarpaceae and Euphorbiaceae for Altingia-mixed species stand, ShoreaDipterocarp stand and Albizia stand, respectively. 3.3. Regeneration status of dominant and rare species The Altingia-mixed species stand (stand I) recorded nearly 2/ 3 of the total seedlings and saplings of the stand contributed by

Seedling density

No. of species

Sapling density

Seedling density

3 9 4

55 191 123

0 192 231

15

369

423

low-dominant and rare species (Table 3). Amongst the rare species category nearly 60% regenerating individuals contributed by new species. In the Shorea-Dipterocarp stand, about 62, 6, 5 and 27% of total regenerating individuals fall under dominant, co-dominant, low-dominant and rare species category (Table 3). A major share of the regeneration of the rare species was contributed by new species (83%) at this stand. At Albizia stand, a total of 5, 1, 5 and 9 tree species were recorded under dominant, co-dominant, low-dominant and rare species category based on tree density demography (Table 3). This stand showed that 55% of total tree density was contributed by new species, while dominant, co-dominant and rare species category contributed 20, 14, and 9%, respectively, of the total regeneration (Table 3). A comparison of three stand showed that new species contributed 12, 23 and 55% of total regeneration in the respective stands. When species of each stand were categorized as pioneer and top canopy species, the regeneration of the later was significantly higher (P < 0.05) than the former group showing that the forest was an old growth type. 3.4. Tree DBH class distribution and regeneration Most of the studied species showed ‘reverse J’ shaped distribution with higher number of seedlings, followed by saplings, small trees, and only a few individuals as old trees. At Altingiamixed species stand, Dysoxylum procerum, Dipterocarpus macrocarpus, Talauma hodgsonii and Ostodes paniculata showed good to fair status as these species were represented in all stages from seedling to adult trees (Fig. 3A). Dysoxylum procerum had 81% of its individuals in seedling stage, 18% in saplings and just 1% in trees. Ostodes paniculata showed 74% individuals in seedling stage, 17% in saplings, 8% in trees with 10–30 cm girth sizes.

Table 3 Regeneration status of dominant, co-dominant and rare tree species in the three studied stands of buffer zone area of Namdapha National Park Stands

Dominant trees, >10 individuals

Co-dominant trees, 5–10 individuals

Less dominant trees, <5–>1 individuals

Rare trees, <1 individuals

New trees, no mature individuals

7 54.5 5 507

31 77.5 21 1590

53 53 16 289

0 0 38 429

19 52 3 172

16 66 1 128

0 0 17 748

9 9 5 14

0 0 7 86

I: Altingia-mixed spp. stand No. of tree species Tree density (individuals/ha) No. of regenerating sp. Density of seedlings + saplings (/0.20 ha)

9 174 7 781

II: Shorea-Dipterocarp stand No. of tree species Tree density (individuals/ha) No. of regenerating sp. Density of seedlings + saplings (/0.20 ha)

10 198 8 1984

9 74 6 188

III: Albizia stand No. of tree species Tree density (individuals/ha) No. of regenerating sp. Density of seedlings + saplings (/0.20 ha)

5 217 3 32

1 9 1 22

5 12.51 0 0

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Fig. 4. Tree density, total basal area, saplings, seedlings and species number in different transects of the three studied stands.

Fig. 3. Number of tree individuals (%) at different DBH classes at (I) Altingia-mixed species stand, (II) Shorea-Dipterocarp stand, and (III) Albizia stand (species girth class categories show DBH classes: Sd = seedling, <10 cm = saplings, 10–30 cm = small trees, 31–60 cm = medium trees, 61–90 cm = adult trees, and >90 = mature trees).

In Shorea-Dipterocarp stand, Dipterocarpus macrocarpus was regenerating most efficiently with 91% individuals as seedlings, 8% saplings and just 1% as trees, which signifies a good regenerating status of the species (Fig. 3B). Shorea assamica showed 89% individuals as seedlings and 11% as saplings. Ostodes paniculata was having 68% individuals as seedlings, 27% sapling, and 4% as trees. Terminalia myriocarpa had maximum representation in 60– 90 cm DBH class sizes (57%) (Fig. 3B). In the Albizia stand, Albizia procera was recorded in poor regeneration status without any seedling and sapling. For this species nearly 51% individuals were recorded in 10–30 cm DBH class, 47% in 31–60 cm and 2% in 61–90 cm DBH class (Fig. 3C). Bombax ceiba was regenerating in sapling stage only. No regeneration was recorded for Dalbergia sericia and Gmelina arborea.

variation among the parameters in different transects within Altingia-mixed species or Shorea-Dipterocarp stands except for sapling density in the later stand (Fig. 4). The Albizia stand, however, showed high fluctuation in the all the parameters studied across the quadrats mainly in the sapling density (Fig. 4). The Pearson correlation matrix for the pooled data for all three stands showed positive relationship among various parameters (Table 4). Tree density showed a strong positive correlation with species number followed by total basal area and total regeneration of the stand (Table 4). Species number had significant high correlation with seedling and sapling numbers. Though the overall correlation between seedling and sapling was positive, it was weak (r2 = 0.2) among these two parameters. 3.6. Regeneration status of tree species at different canopy stratum Thirteen species were found in the top canopy strata at the Altingia-mixed species stand, in which Cinnamomum bejolghota Table 4 Pearson correlation matrix among regeneration and other parameters of the forest

3.5. Relationship of species regeneration with tree density and basal cover The regenerating individuals (seedlings + saplings) showed positive relationship with species number (F24,12 = 5.46, P < 0.01), tree density (F27,9 = 4.49, P < 0.01) and total basal area (F31,5 = 16.09, P < 0.05). Individually, number of seedlings (F28,9 = 10.61, P < 0.001) and saplings (F28,9 = 3.207, P < 0.05) also showed significant relationship with tree density though it was more strong for seedlings than for saplings. At a given stand, there was no significant

Tree TBA Sapling Seedling Species Total reg.

Tree

TBA

Sapling

Seedling

Species

Total reg.

1.000 0.760 0.557 0.527 0.896 0.695

1.000 0.471 0.550 0.774 0.658

1.000 0.212 0.625 0.752

1.000 0.561 0.804

1.000 0.759

1.000

Tree = tree density; TBA = total basal area; Sapling = sapling density; Seedling = seedling density; Species = number of species per transect; Total reg. = total regeneration (seedling + sapling density).

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Fig. 6. Seedling survival of top canopy (A), mid canopy (B) and lower canopy (C) species at Namdapha National Park, north east India.

3.7. Tree seedling survival and mortality

Fig. 5. Regeneration status of top canopy, mid canopy and lower canopy species in the three studied stands in Namdapha National Park (for detailed species names, please refer to Annexure I).

showed highest regeneration followed by Dipterocarpus macrocarpus. Altingia excelsa, the most dominant species in the tree stratum in this stand, was found regenerating in seedling stage only (10 individuals ha1). At the Shorea-Dipterocarp stand, both Shorea assamica and Dipterocarpus macrocarpus performed satisfactorily (Fig. 5). In the Albizia stand, Bombax cieba showed highest regeneration though represented in sapling stage only among the top canopy species (Fig. 5). In the mid canopy stratum a total of 66 species were found regenerating when all the three stands taken together. Beilschmiedia assamica was the most prominently regenerating species, followed by Dysoxylum procerum at Altingia-mixed species stand (Fig. 5). Castanopsis indica recorded highest regenerating individuals at Shorea-Dipterocarp stand. At the Albizia stand, Dysoxylum procerum was recorded regenerating in the sapling stage only. In the lower canopy stratum, a total of 24 species were found regenerating in the three stands. Saprosma ternatum recorded maximum regeneration at the Altingia-mixed species stand. Strobilanthes sp., an under canopy shrub, was also found regenerating profusely at this stand. The Shorea-Dipterocarp stand recorded Ardisia sp. and Saprosma ternatum as two most profusely regenerating species (Fig. 5). At the Albizia stand, Morus indica was regenerating most efficiently among the lower canopy species.

The seedling survival and mortality varied significantly among the species (P < 0.05). In the top canopy layer Cinnamomum bejolghota (56%) showed highest seedling survival followed by Dipterocarpus macrocarpus (49%) and Shorea assamica (41%) (Fig. 6A). The pioneer species, viz. Altingia excelsa, Cedrella toona and Sapium baccatum recorded a marked mortality of over 95%. For mid-canopy layer, Beilschmiedia assamica showed highest survival (73%), followed by Talauma hodgsonii (70%) (Fig. 6B). In the lower canopy species, Saprosma ternatum showed highest survival (83.33%) (Fig. 6C). Further, it was observed that most of the mortality was observed during the winter months for species of all the strata. 4. Discussion The wealth of forest depends on the potential regenerative status of species composing the forest stand, in space and time (Jones et al., 1994). An understanding of the processes that affect regeneration of tropical forest species is of crucial importance to both ecologists and forest managers. In this study an attempt was made to study the tree regenerative behavior of the lowland tropical evergreen (two stands) and semi-evergreen forest types of the globally known Namdapha National Park that falls in the eastern Himalayan region. The results showed significant differences among seedling and sapling demography of the tree species in the three studied stands. Altingia-mixed species and ShoreaDipterocarp stands in lowland tropical evergreen forest had good

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population of seedlings and saplings, though it was fairly low at Albizia stand that falls under lowland tropical semi-evergreen forest category. Of the total 98, 54, and 20 tree species in Altingiamixed species (stand I), Shorea-Dipterocarp (stand II) and Albizia (stand III) stands, respectively, only 56, 32 and 45% species of the respective stands were found regenerating either in seedling or sapling or in both stages. Different species differed in abundance of their seedlings and saplings, which could be attributed to the canopy cover and micro-climatic conditions at different stands. The poor regeneration at Albizia stand, however, could be attributed to frequent floods, poor soil depth, frequent grazing and trampling of the area. The overall regeneration status was recorded fairly high for the lowland tropical evergreen forests in Namdapha National Park in comparison to many other similar forests elsewhere (Swaine, 1996; Rose and Poorter, 2003). Being a protected forest there is restriction of human access to the park, thus tree felling and other forest operations are totally banned, which ultimately favored the regeneration of the species. Besides, high rainfall, moderate temperature and wide variation in altitude and soil characteristics provide a favorable environment for the luxuriant growth of humid tropical forest at the lowland areas of Namdapha National Park, which is a well known characteristic of the tropical rainforests to maintain its heterogeneity (Richards, 1976; Whitmore, 1996; Proctor et al., 1998). Our findings are contrary to those of Tripathi and Khan (1992), who reported that survival of tree seedling is greater in disturbed forest compared to protected forests and attributed it to the silvicultural characters of the studied species. The top (emergent) canopy in the studied forests was dominated by pioneer species (Altingia excelsa) in one stand and climatic-climax (Dipterocarpus macrocarpus and Shorea assamica) in the other. Both these stands showed mixed forest types with diverse species at top, mid and low-canopy levels. Mixed Dipterocarp forest comprises an important forest type that covers much of the upland hill, and lowland regions of South East Asia (Whitmore, 1990). The overall regeneration trees in the primary forest had a greater contribution of middle and understorey species as they are better adapted to grow under the shady condition. There were more than half of the regenerating species new to the stand, which were not recorded in the tree stratum. Such feature is common for tropical forest trees (Swaine, 1996). The top canopy species like Altingia excelsa, Ailanthus grandis, Bombax ceiba, Cedrella toona, and Canarium strictum registered poor regeneration in different forests stands. Contrarily, seedlings and saplings were recorded for some other top canopy species, such as Artocarpus chaplasa, Chikrussia tabularis, Mangifera sylvatica and Sapium baccatum, which were not recorded in tree stratum at any of the studied stands. Such differences in frequency and density of seedling recruitment and longevity occur between species, and is a characteristic features of the tropical forests (Whitmore, 1996). Varied floristic composition of the top canopy and seedling & sapling stratum are reported for several tropical forests (Richards, 1976; Uhl and Murphy, 1981; Jones et al., 1994). The niche differentiation of seedling establishment sites may be a key for the maintenance of tree species diversity in tropical forest communities. Tree species of tropical rainforest have been classified into pioneer and top canopy species particularly with reference to species light requirements for germination and establishment (Swaine and Whitmore, 1988). A comparison between the regeneration of pioneer species and top canopy species in the studied stands showed that the later group of species was significantly more abundant. In the present study, pioneer species as Altingia excelsa, which was dominating in the tree stratum in the stand I had no seedling under the close canopy and was recorded

germinating only in the canopy gaps that too in low density. However, the dominant top-canopy climatic-climax species, particularly Dipterocarpus macrocarpus and Shorea assamica had adequate regeneration though it was low at stand I than stand II. It can be attributed to gentle slope at Shorea-Dipterocarp stand that makes soil well drained, which is considered better for the germination and initial growth of dipterocarps and both these species can thrive well under the close canopy forest in shade and also recorded to survive in suppressed condition (Ashton, 1988). Pioneer species require high light for germination (Bazzaz, 1991) and survival (Agyeman et al., 1999). The regeneration of pioneer species is limited to the early stages of succession when light availability is high and such species survive only in gaps (Uhl and Murphy, 1981). On the other hand, majority of the species are shade tolerant that are able to establish and grow under low light conditions (Pompa and Bongers, 1988) and regenerate in situ by establishing seedlings under the closed forest canopy (Hubbell and Foster, 1987; Augspurger, 1984a; Brokaw, 1985; Whitmore, 1989). The juvenile compositions also differ from stand to stand in a forest at different locations as was recorded in this investigation. Abundant populations of juveniles in tropical forests provide an idea of the regeneration status of different species. Less representation for certain primary tree species in forest stands was reported in Kade, Ghana (Swaine and Hall, 1988) and Panama forests (Hubbell, 1979). Canopy dominant late successional tree species are site specialist restricted to particular topographic positions of the rainforest (Ashton et al., 2001). Establishment stage is very important in determining the distinct site specialization of a species (Ashton and Berlyn, 1991; Jurik and Pleasant, 1990). The tree seedling recruitment is often limited by low and uncertain seed supply and establishment, it is also limited by lack of suitable microsites and factors, which affect early seedling growth and mortality (Clark et al., 1999). The regeneration of shade intolerant species continuously increases as gap size increases (Brokaw, 1985, 1987; Whitmore, 1989). The proportions of different life stages (seedlings, saplings, small trees and large trees) in a given population may help in predicting possible future status of the forest stands (Sundriyal and Sharma, 1996). In a forest stand, generally the most dominant species are represented by all diameter classes (Khan et al., 1987; Sundriyal and Bisht, 1988; Sukumar et al., 1992). The dominance of tree individuals in medium to lower girth classes suggests that the forest is still in evolving stage (Campbell et al., 1992). In this investigation many tree species showed a ‘reverse J’ shaped population structure having more number of small tree individuals (juveniles), few large number of medium sized individuals, and very few large tree individuals. Some other species devoid of their regeneration showed their poor status, while a few were represented in seedling and sapling stage only and such species seem to be new intruders in the studied stands. Such a discontinuous population structure of species is also reported for a number of other tropical tree species (Brokaw, 1987; Itow and Mueller-Dombois, 1988; Whitmore, 1996). The strata wise regeneration observation showed the dominance of middle strata species, which accounted for maximum regeneration. In the present investigation of the total number of individuals (seedling + sapling + tree), a total of 73, 25 and 2% individuals at stand I, 73, 23, and 2% at stand II and 50, 40 and 10% individuals at stand III were recorded in respective stages. The study recorded a net loss of 10–50% seedlings during developmental stage from seedling to sapling, and a further loss of 21–30% individuals in developmental stage from sapling to tree stage. The individual lapse rate from seedling to sapling to tree stratum also varied for different species. The establishment phase gives the biggest ‘demographic squeeze’ to seedling population (Whitmore, 1996;

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The importance of soil moisture in influencing tree seedling survival and growth in forests has been well reported (Mcleod and Murphy, 1977; Mueller-Dombois et al., 1980; Schulte and Marshall, 1983). Microclimate and edaphic condition play a key role in recruitment and survival pattern of tree seedling populations in these forests since there was not enough evidence of the damage to seedling populations due to leaf predation, grazing and pathogen attack (Barik et al., 1992). High survival was recorded for shade tolerant species than light demanding pioneers. Similar finding have been reported for other forests as well (Lieberman et al., 1990). Tree seedlings and saplings of many species can survive in the undergrowth of tropical forest for several years and wait for favorable light conditions to start growing (Halle et al., 1978; Whitmore, 1990). Tiny seedling suffers the heaviest mortality (Lieberman, 1996). Progressively lower mortality with increasing sizes has been shown in Penang (Turner, 1990; Brown and Whitmore, 1992; Swaine, 1996). There is a complex relationship between seedling ecology and canopy species composition and more study over a larger area and longer span of time are needed to expose more of the dynamics and to elucidate whether a forest is in equilibrium or not.

Alvarez-Buyalla and Martinez-Ramos, 1992; Flores, 1992). In tropical trees regeneration is often difficult and it varies greatly in timing, duration and intensity of flowering and fruition, and the high germination is followed by high mortality due to lack of microhabitats, at times over 90% of the new seedlings die before establishment (Harper, 1977; Cook, 1979). Of the successful ones, only a very small fraction grows up to mature trees (Peters, 1994). The seedling survival and mortality over shorter time spans could provide important clues for future composition of a forest stand, and results of the present study indicated important differences among seedling groups. It was clearly seen that the shade tolerant mid-canopy and the understorey species germinated abundantly and survived well. The fast growing top canopy species other than baring pioneers had very abundant germination, but extremely low survivorship. Of all top canopy species, the pioneer species recorded remarkably low seedling survival rates (5–6%) than the top canopy species (41–56%), the mid (49–73%), and the lower canopy species (30–83%). In the Dipterocarp forests of the Malaya, an enormous number of seedlings die in their first year from various causes (Swaine, 1996). In African rainforest, the seedling and sapling mortality was recorded 38–50%, it was much higher for liana (52–72%), though it was low for shrubs (25%) (Hladik and Mitja, 1996). In Panama a loss of up to 67% of newly germinated seedlings during the first 2 weeks of age was recorded (Garwood, 1982). In Costa Rica, a mean seedling mortality rate of 86% was recorded in the first year after germination (Li et al., 1996). Early survivorship has been shown to be good indicator for subsequent survivorship (Li et al., 1996). Survival of the seedling is also greatly affected by the ambient environment, biotic and abiotic factors (Harper, 1977). Higher seedling mortality of species during winter could be attributed to the prevailing low soil moisture and low temperature conditions.

Acknowledgements This study was funded by the Ministry of Environment and Forests, Government of India, New Delhi under the Biosphere Reserve Programme (F. NO.10/35/98-CS/BR). We thank the PCCF, Additional PCCF (Wildlife and Biodiversity), and Field Director (Namdapha), Department of Forests and Wildlife, Government of Arunachal Pradesh for permission to work in the park, and Director, G.B. Pant Institute of Himalayan Environment and Development for providing facilities.

Appendix A. Regeneration status (saplings and seedlings) of the three studied forest stands in the Namdapha National Park, northeast India Species

Canopy status

Ailanthus grandis Prain Altingia excelsa Noronha Artocarpus chaplasa Roxb. Bombax ceiba Linn. Canarium strictum Roxb. Cedrella toona Roxb. Chikrussia tabualris (Smith) A. DC. Cinnamomum bejolghota (Buch-Ham) Sweet Dipetrocarpus macrocarpus Vesque Mangifera sylvatica Roxb. Sapium baccatum Shorea assamica Dyer Syzygium cumini Aesculus assamicus Griff. Aphania rubra (Roxb.) Radlk. Aporosa dioica Muell. Arg Beilschmiedia assamica Meissn. Beilshcmeidia sp. 2 Castanopsis indica (Roxb.) A. DC. Castanopsis sp. 2 Castanopsis sp. 3 Castanopsis sp. 4 Castanopsis tribuloides A. DC. Chisocheton paniculatus Hiern Dalbergia pinnata Balakr. (Lour.) Prain. Dalbergia sp.

Top Top Top Top Top Top Top Top Top

Altingia-mixed species stand Trees

Shorea-Dipterocarp stand

Albizia stand

Saplings

Seedlings

Trees

Saplings

– – 10.82 – 32.47 – –

– 10 33.78 – 67.56 33.78 135.13

2 6 – – – – –

– 20 – – 20 – –

– – – – – – –

102.81

1013.45

–

60

125

–

–

–

18.33

140.69

168.91

40

160

1875

–

–

–

– – 1.67

33.78 33.78 202.68 67.56 – 135.13 0 5269.88

–

–

–

–

–

–

32

380

3125

–

–

–

– – – –

20 – 40

250 – –

– – – – – –

– 35 – – 0.83 1.67 0 5

Seedlings

Top Top Top Top Middle Middle Middle Middle

– 2.5 – 0.83

16.23 – 21.64 – – 5.41 21.64 551.95

Middle Middle

2.5 3.33

10.82 156.90

– 270.24

– 14

– 120

– 1625

Middle Middle Middle Middle Middle Middle

– 2.5 – 2.5 11.67 –

5.41 – – 27.06 5.41 –

67.63 33.78 – – – –

– 2 4 2 2 –

– – 20 – – –

– – – – – –

54.12

202.47

–

60

625

Middle

–

Tree

Saplings

0.41 – – 18.32 – – –

6.15 – – 49.23 – – –

– – – – – 31.03 –

12.31 – – – – – – – – – – 92.31 –

Seedlings – – – – – – –

– – – – – – – – – – – – –

P. Deb, R.C. Sundriyal / Forest Ecology and Management 255 (2008) 3995–4006

4004

Appendix A (Continued ) Species

Diospyros sp. Diplospora sp. Dysoxylum binecteriferum Dysoxylum procerum Hiern Dysoxylum sp. 1 Dysoxylum sp. 2 Elaeocarpus aristatus Roxb. Elaeocarpus sp. Euonymous sp. 1 Euonymous sp. 2 Ficus sp. Garcinia lanceolata Glycosmis sp. Gmelina arborea Roxb. Goniothalamus sp. Grewia disperma Rottl. Griffithianthus fuscus Merr. Knema angustifolia (Roxb.) Warb. Knema linifolia Hiern Laportea sp. Lauraceae Lindera latifolia Hk. f. Linociera macrophylla Wall. Litsea monopetala Pers. Litsea salicifolia Hk. f. Litsea sp. 1 Litsea sp. 2 Magnolia griffithii Hk. f. and T. Melia dubia Cav. Meliaceae Mesua ferrea Linn. Milletia sp. Olea dentata Wall. ex. DC. Ostodes paniculata Bl. Persea sp. 2 Persea sp. Premna barbata Wall. Premna sp. Pterygota alata Roxb. R. Br. Quercus lamellosa Sm. Quercus semiserrata Roxb. Randia sp. Rhamnaceae Sterculia sp. Stereospermum chelonoides (Linn. F.) DC. Styrax serruletum Roxb. Syzygium macrocarpum (Roxb.) Balakr. Syzygium sp. Talauma hodgsonii Hk. f. and Th. Turpinia pomifera DC. Vitex sp. Wrightia coccinia Smith Xerospermum glabratum Actinodaphne obovata Blume Alangium chinense (Lour.) Rehder Allophyllus sp. Antidesma acuminatum Linn. Ardisia sp. Baccaurea ramiflora Lour. Calicarpa rubella Lindl. Camelia caudata Wall. Capparis acutifolia Cayratia pedata Juss. Cledion sp. Combretum sp. Croton roxburghii A. Juss. Glochidion lanceolarium (Roxb.) Voigt. Helicia robusta Hk. f. and T Hymenopogon sp.

Canopy status

Altingia-mixed species stand

Shorea-Dipterocarp stand

Albizia stand

Trees

Trees

Seedlings

Tree

Saplings

– – – – – – – – – – – – – 18.37 – 0.41 – –

– – – 18.46 – – – – – – – – – 18.46 – 12.31 – –

Saplings

Seedlings

Saplings

Seedlings

Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle

1.67 – 10 15.83 – – 2.5 0.83 1.67 4.17 0.83 – – – – – 2.5 1.67

16.24 5.41 – 243.50 10.82 – 27.06 10.82 5.41 5.41 37.88 5.41 10.82 – 5.41 – 48.70 5.41

– – – 1081.01 – – 236.47 67.46 – – 33.78 – – – – – 33.78 –

– – – 4 10 – 12 – – – – – – – – – 10 –

– – 40 60 – 20 80 – – – – – – – 20 – 60 260

– – – 750 – – 500 – – – – – – – – – – 125

Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle Middle

– – – 0.83 0.83 – 3.33 3.33 – 0.83 – 0.83 3.33 – 0.83 63.33 – 0.83 – 0.83 2.5 0.83 0.83 – – – –

21.65 10.82 5.41 75.76 16.24 – 173.16 32.47 21.64 5.41 5.41 5.41 81.17 5.41 32.47 146.12 5.41 32.47 – 10.82 43.29 16.24 37.88 10.83 – 27.06 –

135.12 – – 67.56 – – 135.13 – – – – – 270.25 – 33.78 641.84 – – – 135.13 – – 67.56 – 33.78 – 67.56

– –

–

–

– – – – – – – – – – – – 30 – – – – – – – –

20 40 – 20 – – – – – – – 20 100 – – – – – – – –

125 – – 125 – – – – – – – 125 250 – – – – – – – –

– –

–

–

– –

–

–

Middle Middle

2.5 13.33

16.24 194.81

– 776.96

– –

– –

– –

– –

– –

– –

Middle Middle Middle Middle Middle Middle Lower Lower

– 17.5 – – – – – –

– 97.41 10.82 5.41 – 21.64 10.82 –

– 67.57 – 67.56 – – – –

8 6 – – – – 6 –

180 20 – – – – 20 –

500 – – – – – – –

– 1.67 – – – – – 0.84

– – – – 12.31 – – 30.77

– – – – 192 – – –

16.23 43.29 281.39 75.76 – 5.41 10.83 – 5.41 5.41 5.41 –

337.81 – 202.69 – – – – 33.78 – – 33.78 –

– – – 26 – – 2 – – – – –

– 20 40 120 – – – – – – – –

– – 1500 250 – – – – – – – –

– – – – – – – – – – – 8.65

– – – – 6.15 – – – – – – 36.92

– – – – – – – – – – – 77

4 –

20 –

– –

Lower Lower Lower Lower Lower Lower Lower Lower Lower Lower Lower Lower

– 2.5 – 5.83 – – – – – – 0.83 –

Lower Lower

– –

32.47 5.41

– –

– – – – 0.84 – – – – 0.84 – – – – – – – – – – – – –

– –

– – – 6.15 – – – – 12.31 – – – – – – – 6.15 – – – – –

– –

– – – – – – – – – – – – – – – – – –

– – – – – – – – – – – – – – – – – – – – – –

– –

P. Deb, R.C. Sundriyal / Forest Ecology and Management 255 (2008) 3995–4006

4005

Appendix A (Continued ) Species

Lasianthus longicauda Hk. f. Leea indica (Burn. f and) Merr Micromelum sp. Milliusa roxburghiana (Wall.) Hk. f. f. and Th. Morus indica Thunb. Saprosma ternatum Hk. f. Saurauia cerea Griff. Sauropus sp. Unidentified Total

Canopy status

Altingia-mixed species stand

Shorea-Dipterocarp stand

Albizia stand

Trees

Trees

Saplings

Seedlings

Tree

Saplings

Seedlings

Lower Lower Lower Lower

1.67 9.17 4.17 3.33

10.82 21.65 10.83 48.70

– – – 33.78

– 18 – –

– 60 – 40

– 125 – 125

– – – –

Lower Lower Lower Lower

– 8.33 0.83 – 89.17

– 882.10 5.41 5.41 211.05

– 371.59 – – 439.14

– 12 – – 66

– 1020 40 – 640

– 125 – – –

– – – – 1.67

318

3860

12,250

83.05

362.5

4464

13,184

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