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Journal of Resources and Ecology >

2025 , Vol. 16 >Issue 1: 105 - 114

DOI: https://doi.org/10.5814/j.issn.1674-764x.2025.01.010

Plant Resources and Plant Ecology

Differences in Seedling and Sapling Densities and Species Composition between Canopy Gaps and Forest Understories in a Subtropical Forest in Bangladesh

  • Tarit Kumar BAUL , 1, 2, * ,
  • Anwarul Islam CHOWDHURY 2, 3, 4 ,
  • Md Jamal UDDIN 1 ,
  • Mohammad Kamrul HASAN 1 ,
  • Tapan Kumar NATH 5 ,
  • Lars Holger SCHMIDT 6
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  • 1. Institute of Forestry and Environmental Sciences, University of Chittagong, Chittagong 4331, Bangladesh
  • 2. Faculty of Science, Forestry and Technology, School of Forest Sciences, University of Eastern Finland, Joensuu 80101, Finland
  • 3. Department of Agricultural and Forest Sciences and Engineering, University of Lleida, Lleida 25001, Spain
  • 4. Natural Resources Institute Finland (Luke), Helsinki 00790, Finland
  • 5. Faculty of Science and Engineering, University of Nottingham Malaysia, Jalan Broga 43500, Malaysia
  • 6. Department of Geoscience and Natural Resource Management, University of Copenhagen, Frederiksberg C 1958, Denmark
*Tarit Kumar BAUL, E-mail:

Received date: 2024-02-05

  Accepted date: 2024-07-02

  Online published: 2025-01-21

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Abstract

Canopy openings have a vital role in forest structure, regeneration, and plant composition. In this study, we investigated and compared the species composition and densities of seedlings and saplings between canopy gaps (openings) and forest understories (in dense canopies) in a sub-tropical forest of Bangladesh. We objectively identified 42 canopy openings between transect lines and 42 main plots in dense canopies, sampled for regeneration and young tree patterns. For the regeneration study, we placed 2 m×2 m four subplots in each canopy opening and the main plot of dense canopy, thus making a total of 336 subplots. The species diversity of seedlings, saplings, and trees were significantly (P≤0.05) higher in the dense canopy than in the canopy opening. Although, most dominant and frequent regenerating species such as Diospyros pilosula, Garcinia cowa, Brownlowia elata, and Lithocarpus polystachya were common in both dense canopy and openings, the canopy openings had the highest Importance Value Index, suggesting the significance of gap dynamics in the dominance of native floral species. The dense canopy played an important role in forest compositions because 12 regenerating species, including Stereospermum suaveolens and Diospyros montana, were not found in the canopy openings, implying that rain forests must maintain a high canopy cover for regeneration.

Key words： canopy opening; disturbance; forest dynamics; native flora; seedling; sapling

Cite this article

Tarit Kumar BAUL , Anwarul Islam CHOWDHURY , Md Jamal UDDIN , Mohammad Kamrul HASAN , Tapan Kumar NATH , Lars Holger SCHMIDT . Differences in Seedling and Sapling Densities and Species Composition between Canopy Gaps and Forest Understories in a Subtropical Forest in Bangladesh[J]. Journal of Resources and Ecology, 2025 , 16(1) : 105 -114 . DOI: 10.5814/j.issn.1674-764x.2025.01.010

1 Introduction

Tropical forests frequently experience both natural and human-caused disturbances (such as insect outbreaks, windthrow, and illicit logging). These may result in the death of one or more trees, openings in the canopy, or gaps (openings) in the forest. The structure of forests and their microclimate often alter due to forest openings (Clark et al., 1996; Orman et al., 2021; Zhu et al., 2021). Species composition and stand structure in forests are significantly impacted by openings depending on the degree of disturbance (Aubréville, 1938; Runkle, 1982; Silvestrini et al., 2015; Wu et al., 2016; Jaloviar et al., 2020). Forest canopy openings in tropical forests aid in balancing dynamics, plant succession, regeneration, and disturbance (Watt, 1947; Feldmann et al., 2018; Zhu et al., 2019; Reis et al., 2021). Tree regeneration has a significant influence on forest dynamics (Whitmore, 1984; Feldmann et al., 2020). Tree regrowth and the development of seedlings into saplings depend on the microcli-mate that forest openings provide (Sharma et al., 2019). The presence or absence of young trees, saplings, seedlings, or germinating seeds can have a significant impact on how a tree regenerates in response to a gap being formed (Marks, 1974; Stewart et al., 1991). Openings in forests can provide habitats, which are crucial for the natural regrowth of various dominant plant species and the maintenance of plant diversification in forests (Runkle, 1982; Lertzman, 1992; Woods, 2000).
In forests, a recruitment population exists in the understory or subcanopy layer that responds swiftly to perturbations (Gilliam, 2007; Lencinas et al., 2011; Mestre et al., 2017). As a result of their typically diversified composition, structure, and distribution patterns, understory plants can support a variety of habitats and resource specialties (Spyreas and Matthews, 2006; Márialigeti et al., 2016; Tinya and Ódor, 2016). The microenvironment, biodiversity, and functional heterogeneity of the sub-canopy are supported by gap dynamics, which are crucial for enhancing ecosystem stability and minimizing the homogenizing effects of canopy closure (Valerio et al., 2021). Information on plant composition, canopy opening structure, and plant structure is crucial for creating management and conservation plans for forests (Bhugeloo et al., 2021). According to previous studies (Zhu et al., 2014; Sharma et al., 2019; Awasthi et al., 2020; Reis et al., 2021), opening creation and canopy opening in forests have a considerable impact on the composition of plants and the dominance of regeneration. However, there are few quantitative data on the composition of regeneration in openings (Feldmann et al., 2020).
In Bangladesh, the Sitapahar Forest Reserve (SFR) is the last remaining intact sub-tropical hill forest, and it is home to a diverse array of native plants (Uddin and Hassan, 2012). Previous studies identified 332 plant species, including 248 species of dicotyledons (Uddin et al., 1998). Of the 85-98 tree species in SFR, the most common are Artocarpus chama, Dipterocarpus turbinatus, Erioglossum rubiginosum, Holigarna caustica, Mangifera sylvatica, Myristica linifolia, Syzygium spp., Melocanna baccifera, Lannea coromandelica, Artocarpus chama, Swintonia floribunda, Ficus benghalensis, Diospyros pilosula, Tetrameles nudiflora, and Stereospermum suaveolens (Nath et al., 2000; Chowdhury et al., 2017; Baul et al., 2021).
The importance of SFR conservation was highlighted by Nath et al. (1998 and 2000) because this forest still had a healthy tree population and diversity. Nevertheless, the forest has seen significant pressure from a variety of human disturbances, such as illicit tree cutting (Harun-Ur-Rashid and Chowdhury, 2013), which led to canopy opening and impacted stand dynamics. Logging activities changed the dynamics, immediately raised tree mortality rate, affected seedling recruitment, and increased the number of remaining trees (Amaral et al., 2019). As there was no silvicultural management done in the forest reserve, SFR has both dense canopies and canopy openings (Baul et al., 2021). This is because old-growth vegetation is naturally disturbed over time. However, there has been no research on gap dynamics in SFR. It may be possible to identify potentials for forest stand development using silvicultural recommendations by documenting species composition and regeneration under dense canopy and canopy openings. The aim of this study was to investigate and compare the species composition and densities of seedlings and saplings between canopy openings and forest understories in a sub-tropical forest of Bangladesh.

2 Materials and methods

2.1 Brief description of studied SFR

In the southeast of Bangladesh, SFR is located between latitudes 22°26ʹN-22°38ʹN and longitudes 92°08ʹE-92°17ʹE (Figure 1). This 373 ha of forest comprised of evergreen and semievergreen vegetation is administered by the Kaptai Forest Range, which is overseen by the Chittagong Hill Tract (CHT) South Forest Division (Uddin et al., 1998). This site possesses a subtropical climate with a long dry season (October-May), mean annual temperature ranging 19.9-28.3 ℃, relative humidity ranging 65%-85%, and mean annual rainfall of about 2900 mm. About 90% of the annual rainfall occurs in June-September (Uddin et al., 1998; Uddin and Hassan, 2012). The terrain is hilly, with valleys and both steep and mild slopes.
Figure 1 Sitapahar Forest Reserve (SFR) with sample plots shown (c), under Kaptai Union (b) in Bangladesh (a)
However, natural and anthropogenic disturbances, such as fire in shifting cultivation and illegal logging, are putting this natural forest under severe and ongoing threat. As a result, many economically significant native species with germplasm stocks are in danger of going extinct (Harun-Ur-Rashid and Chowdhury, 2013). For example, the only causes of dead trees in the open and dense canopies were human-induced fire and fungus, respectively (Baul et al., 2021). The bulk density of soil is higher in the dense canopy than in the canopy opening due to higher amount litter and organic matter. The soils are acidic, with not strong variation in dense canopy and canopy opening with changes in vegetation. Soil carbon (C), nitrogen (N), and C:N ratio are higher in the dense canopy than in the canopy opening but with no significant difference in K+ concentration between them. But, the concentration of P in the open canopy is highest in the canopy opening (Baul et al., 2023).

2.2 Field methods

A reconnaissance survey was conducted to have an idea of topographical view of SFR, canopy openings and undergrowth vegetation. We noticed a distinct difference between canopy coverage throughout the forest, ranging from dense canopies to canopy openings.

2.2.1 Measurement of canopy openings and dense canopies in SFR

By following Runkle (1982), we established transects in N-S direction, and registered and sampled all canopy openings intersecting the transects. In accordance with the degree of stand structural homogeneity, the parallel line transects were spaced 50-80 m apart, with a range of 2-4 transects. Openings are places where the primary forest canopy is visibly open, there are no tree branches higher than three meters, and a gap maker is present (Taylor and Lorimer, 2003). There was not a minimum opening size criteria that would have guaranteed a complete portrayal of all openings (Runkle, 1982; Hart and Grissino-Mayer, 2009; Richards and Hart, 2011). We computed the area by determining the diameter along the long axis and across the short axis for small openings that had an essentially elliptical form. Measurement of the lengths of the long and short axes allowed us to compute the area of rectangular openings (Dobrowolska and Veblen, 2008; Weber et al., 2014). The distance between the tree trunk bases from one opening edge to the other opening edge was used to measure canopy openings (Runkle, 1982). Additionally, the locations and ostensible reasons for opening creation were noted. From the ground, standing in the main plot, we used a densitometer to estimate the canopy density (Stumpf, 1993)①. Dense canopies are defined as trees with a canopy cover of >70%.

2.2.2 Tree sampling strategy in canopy openings and dense canopies

We identified, enumerated, and measured tree species (i.e., ≥5 cm diameter at breast height, dbh) for overall height and dbh in 42 openings because we believed these trees would be the future gap recovery candidates (Runkle, 1982). The dense canopy forest sections were divided into 42 primary plots, each measuring 20 m by 20 m (Figure 2). All live tree species (with dbh ≥5 cm) in each main plot had their height (m) and dbh (cm) measured, recorded, and identified. To measure tree height and dbh, we utilized diameter tape and a rangefinder (Laser Inc Technology, Trupulse 200B, USA).
Figure 2 Layout of the sampling of trees and regenerations in dense canopy and canopy openings of SFR, Bangladesh

2.2.3 Assessment of natural regeneration in canopy openings and dense canopy areas

Regeneration was divided into seedlings (≤100 cm tall) and saplings (those over 100 cm tall and 1 to 5 cm dbh). We separated each canopy gap into four zones for seedlings and saplings (Figure 2). One 2 m×2 m subplot was placed in each of the four zones, resulting in four subplots in a gap.
Four nested subplots of 2 m×2 m each were put within 42 main plots in dense canopies, creating a total of 168 subplots (Figure 2). In each subplot, we identified every type of regenerated tree and tallied the seedlings and saplings present. With the help of slide callipers and measuring tape, we calculated the collar diameter (cm) and height (cm) of seedlings and saplings.
The identification of species of regenerating trees was mostly assisted by research assistants (local guides and forest guards), who were knowledgeable about the species. The presence of large or pole trees in the plots and opening contributed to the identification of the regenerated species. Additionally, for verification, digital pictures of every species were obtained. We examined digital images of plants from the following sources: Angiospermic Flora of Sitapahar at Kaptai in Bangladesh, Vascular flora of Chittagong and Chittagong Hill Tracts, Trees of Chittagong Hill Tracts, and Trees of Bangladesh (Troup, 1986a, 1986b, 1986c; Uddin et al., 1998; Das and Alam, 2001; Bangladesh National Herbarium, 2018).

2.3 Data analyses

We conducted Kolmogorov-Smirnov (K-S) test and found that data were not normally distributed. We applied nonparametric Mann-Whitney Test to identify any significant differences (P<0.05) in mean number, density, Shannon-Wiener diversity index (number of species per subplot), diameter, and height of seedlings, saplings, and trees between canopy openings and dense canopies. Equations 1-2 (Table 1) were used to calculate the densities (no. ha-1) and Shannon-Wiener diversity index of seedlings, saplings, and trees (Michael, 1990; Shukla and Chandel, 2000). For seedlings and saplings, the number of regenerating species and individuals (no. subplot-1) was computed. Then, the mean number, density, diversity, diameter, and height of seedlings, saplings, and trees between canopy openings and dense canopies were compared. Moreover, the importance value index (IVI, %), relative abundance (RA, %), relative frequency (RF%), and relative density (RD%) for regeneration were all calculated (Equations 3-8; Table 1).
Table 1 Equations used in analyses of data
No. Equations for parameters Definitions Reference
1 $~~~d=\frac{\Sigma n}{\Sigma {{A}_{g}}}\times 10000$ d: density of regenerations or trees (no. ha-1), ΣAg: total area of all openings or dense canopies, Σn: total number of individuals or regenerations of all species in each gap or dense canopies Shukla and Chandel, 2000
2 $H=\mathop{\sum }^{}{{p}_{i}}\times \ln \left( {{p}_{i}} \right)$ H: Shannon-Wiener diversity index or number of species per subplot, pi: S/Σn, where S is the number of regenerations of each species and Σn is the total number of regenerations of all species Michael, 1990
3 $RD~=\frac{S}{\Sigma n}\times 100%$ RD: relative density of a species (%), S: the number of regenerations of each species, Σn: total number of regenerations of all species Misra, 1968
4 $F=\frac{a}{b}$ F: frequency of occurrence of a species, a: number of subplots in which regenerating species occurs, b: total number of subplots studied Shukla and Chandel, 2000
5 $\text{ }\!\!~\!\!\text{ }RF~=\frac{{{F}_{i}}}{\Sigma {{F}_{i}}}\times 100%$ RF: relative frequency of a species (%), Fi: frequency of a species, ΣFi: frequencies of all species Mori et al., 1983
6 $A=\frac{S}{a}$ A: abundance of a species, S: the number of regenerations of each species, a: number of subplots in which regenerating species occurs Shukla and Chandel, 2000
7 $RA~=\frac{{{A}_{i}}}{\Sigma {{A}_{i}}}\times 100%$ RA: relative abundance of a species, Ai: abundance of a species, ΣAi: abundance of all species. Misra, 1968
8 IVI = (RD+RF+RA)×100% IVI: Importance value index

3 Results

3.1 Opening areas and their formation

A total of 42 canopy openings totaling 2807.05 m2 were noted. The canopy opening averaged 70.1 m2, with a range of 14.2 to 363.2 m2. The majority of the openings (58%) were created by stumps from illegal logging. The remaining openings were brought on by fungus and windthrow, which manifested as snags (23%), uprooting (13%), and snapped (8%).

3.2 Diversity, density, and height, and collar diameter of regeneration in canopy openings and dense canopies

Under contrast to canopy openings, the mean number of regenerations per subplot and their densities were significantly (P<0.05) higher in the dense canopy (Table 2). Additionally, there were more species and greater Shannon-Wiener diversity (number of species per subplot) of seedlings and saplings in subplots under dense canopies than in canopy openings (Table 2). Furthermore, we found that for saplings, the mean height and collar diameter in the dense canopy were significantly (P<0.05) higher than those in canopy openings (Table 2).
Table 2 Abundance, density, diversity (number of species per subplot), and mean height and collar diameter of regenerations in canopy openings and dense canopy of SFR, Bangladesh
Variables Seedling Sapling
Canopy openings
(n = 168)		 Dense canopy
(n = 168)		 Canopy openings
(n = 168)		 Dense canopy
(n = 168)
Abundance of regenerating species 54 65 39 50
Regenerating species 2.63 ± 0.10 b 4.54 ± 0.19 a 0.95 ± 0.07 b 2.21 ± 0.20 a
Regenerating individuals (density subplot-1) 3.46 ± 0.13 b 5.73 ± 0.25 a 1.16 ± 0.10 b 2.56 ± 0.23 a
Regeneration density (no. ha-1) 8640.63± 333.86 b 14312.5 ± 627.33 a 2867.23 ± 246.38 b 6406.25 ± 574.45 a
Mean height (cm) 50.13 ± 1.61 a 52.09 ± 1.68 a 82.48 ± 5.44 b 127.51 ± 6.51 a
Mean collar diameter (cm) 0.69± 0.03 a 0.76 ± 0.04 a 0.97± 0.68 b 1.42 ± 0.07 a
Number of species per subplot 0.84 ± 0.04 b 1.37 ± 0.05 a 0.19 ± 0.03 b 0.64 ± 0.07 a

Note: Values followed by ± are standard error of mean. The same suffix letter indicates no significant difference (P>0.05) between canopy openings and dense canopies for the variables of seedlings and for saplings.

3.3 Importance value index and densities of natural regeneration

Among the most abundant species, Lithocarpus polystachya ranked third in importance values as seedlings and sixth as saplings under dense canopies but it had the highest importance values as seedlings and saplings in canopy openings (Tables 3 and 4). In contrast, under dense canopies, Garcinia cowa had the highest importance value as seedlings and the third highest importance value as saplings and, while it still ranked third in importance values as seedlings in canopy openings, it ranked lowest in importance values as saplings in canopy openings (Tables 3 and 4). The number of regenerated seedling species was 65 and 54 in the dense canopy and canopy openings, respectively (Table 2 and Supplementary Table A1). The number of sapling species was 50 and 39 in the dense canopy and canopy openings, respectively (Table 2 and Supplementary Table A2). Densities of Ficus auriculata for seedlings, saplings, and trees were substantially greater in open canopy than in dense canopy (Figure 3). However, compared to openings in the canopy, densities of Garcinia cowa seedlings and saplings in dense canopy were substantially greater. D. pilosula and L. acuminata sapling densities in dense canopy were substantially greater than those in canopy openings.
Table 3 Relative density (RD), relative frequency (RF), relative abundance (RA), and importance value index (IVI) of seedlings of dominant species in canopy openings and dense canopy of SFR, Bangladesh
Seedling species RD (%) RF (%) RA (%) IVI (%)
Canopy openings Dense
canopy		 Canopy openings Dense
canopy		 Canopy openings Dense
canopy		 Canopy Openings Dense
canopy
Brownlowia elata 4.7 2.4 4.8 2.8 1.9 1.4 11.4 6.6
Diospyros pilosula 10.3 7.3 8.3 6.9 2.4 1.7 21.0 15.8
Ficus auriculata 4.5 0.9 3.3 1.1 2.6 1.3 10.5 3.2
Garcinia cowa 8.5 9.9 7.4 7.5 2.2 2.1 18.1 19.5
Lithocarpus acuminata 5.4 3.1 5.7 3.0 1.8 1.6 13.0 7.7
Lithocarpus polystachya 10.5 6.6 11.7 5.0 1.7 2.1 23.9 13.7
Myristica linifolia 4.5 2.9 4.5 3.6 1.9 1.3 11.0 7.7
Protium serratum 1.4 4.6 1.4 5.0 1.9 1.5 4.8 11.1
Quercus gomeziana 5.1 4.2 5.0 5.0 1.9 1.3 12.0 10.5
Unknown or unidentified 4.2 0.9 4.8 0.8 1.7 1.7 10.6 3.4
Table 4 Relative density (RD), relative frequency (RF), relative abundance (RA) and importance value index (IVI) of saplings of dominant species in canopy openings and dense canopy of SFR, Bangladesh
Sapling species RD (%) RF (%) RA (%) IVI (%)
Canopy openings Dense
canopy		 Canopy openings Dense
canopy		 Canopy openings Dense
canopy		 Canopy Openings Dense
canopy
Brownlowia elata 10.3 6.8 10.5 6.2 2.7 2.2 23.5 15.2
Diospyros pilosula 7.0 12.6 8.6 10.7 2.2 2.4 17.8 25.7
Ficus auriculata 9.7 1.5 9.9 1.7 2.7 1.8 22.3 4.9
Garcinia cowa 1.6 6.8 2.0 6.7 2.2 2.1 5.8 15.6
Lithocarpus acuminata 3.8 7.3 4.6 6.2 2.2 2.4 10.6 15.9
Lithocarpus polystachya 12.4 2.9 10.5 2.8 3.2 2.1 26.2 7.8
Myristica linifolia 7.0 1.5 5.3 1.1 3.6 2.6 15.9 5.2
Protium serratum 4.3 5.3 3.9 5.6 3.0 1.9 11.3 12.9
Quercus gomeziana 5.4 3.9 3.7 13.1
Unknown or unidentified 5.9 2.9 4.6 2.2 3.5 2.6 14.1 7.8
Figure 3 Densities (no. ha-1) of seedling, sapling, and tree for dominant species in canopy openings () and dense canopy () of SFR, Bangladesh

Note: Bars represent the standard error of the mean. The same suffix letters indicate no significant difference (P>0.05) between canopy openings and dense canopies for seedlings, saplings, and trees.

3.4 Species abundance, tree density, height and dbh in canopy openings and dense canopies

We found more tree species in dense canopy than in canopy openings. The mean tree (over 5 cm dbh) individual density was significantly higher in the dense canopy than in the canopy openings (Table 5). Although there was no significant variation, the mean tree height and dbh in the dense canopy were higher than in the canopy openings (Table 5). By contrast, mean tree densities of B. elata, L. polystachya, L. acuminata, and M. linifolia were significantly greater in open canopy than in dense canopy (Figure 3).
Table 5 Abundance of tree species, and density, mean dbh and height of trees in canopy openings and dense canopy of SFR, Bangladesh
Tree variables Canopy openings
(n= 42)		 Dense canopy
(n= 42)
Abundance of species 49 88
Density of trees (no. ha-1) 262.67 ± 23.22 b 798.67 ± 140.62 a
Tree mean height (m) 6.33 ± 0.06 a 7.19 ± 0.19 a
Tree mean dbh (cm) 10.01± 0.56 a 13.52 ± 0.82 a

Note: Values followed by ± are standard error of mean. The same suffix letter indicates no significant difference (P>0.05) between canopy openings and dense canopies for the variables of trees.

4 Discussion

The dense canopy had a 20%-28% higher abundance of regenerating species than canopy openings. Similar results were observed from two national parks in Bangladesh, Khadimnagar and Lawachara (Rahman et al., 2011; Deb et al., 2015). Dobrowolska and Veblen (2008) found that the species composition within the surrounding tree canopy is correlated with the quantity of seedlings and saplings in gaps. Accordingly, in this study, densities of seedlings and saplings were 66%-123% greater in the dense canopy than in the canopy openings. This is probably due to the higher proportion of older trees in dense canopy forests, which support intense regeneration (Baul et al., 2021). Moreover, when trees grow in height and small gaps close as they expand their lateral branches, resulting in growing regenerations of shade-tolerant species. Most of the gaps that have been observed will close due to lateral branch growth before individuals in the understory can reach the canopy (Hart et al., 2009). On the other hand, through illegal logging, habitat reduction, and deforestation, anthropogenic disturbances can hinder the regeneration of species in canopy gap areas (Martínez-Ramos et al., 2016a; Zermeño-Hernández et al., 2015, 2016). However, because there were no appreciable variations in collar diameter and regeneration height in the stands with dense canopies, the size of seedlings, saplings, and trees was not adversely affected in the canopy opening areas. This could be attributed to excessive sunlight penetrating via opening. Canopy opening through gap formation boosts seed production and light availability on the ground, which generally encourage the growth and establishment of seedlings (Martínez-Ramos et al., 2016a, b; Silva et al., 2019).
The condition of biodiversity of regenerating species at various stages of their growth is indicated by diversity index. We observed that the dense canopy had a much larger diversity of regeneration, with Shannon-Weiner diversity index of up to 63% higher for seedlings than the canopy opening areas. This might be due to the presence of a wider variety of tree species and their closeness within the dense canopy. This index was 108% higher, for trees > 5 cm dbh under the dense canopy than in the canopy opening locations, indicating that species diversity was consistent from the seedling stage to trees. This suggests that stands with dense canopies are probably relatively less disturbed and nutrient stocks which favors the regeneration and growth of a variety of species. Baul et al. (2023) found a positive association of species diversity and nutrient phosphorus (P), sulphur (S), manganese (Mn), and zinc (Zn) in dense canopy in the same forest site. On the other hand, lower index in the canopy opening areas suggested less species diversity in the regenerated areas, which is likely because of numerous human disturbances like unlawful harvesting. Rahman et al. (2016) also noted a decrease in species variety as a result of human disturbances in the Kaptai National Park (KNP), which includes the research region.
According to IVI values, D. pilosula, G. cowa, Q. gomeziana, B. elata, L. polystachya, and L. acuminata were the most prevalent and frequent regenerating seedlings and saplings under both dense canopy and canopy openings. In the Chittagong South Forest Division's Dudhpukuria-dhopachori wildlife sanctuary, L. acuminata was reported to be the main regenerating species (Hossain et al., 2017). D. pilosula was reported to be predominate in SFR (Chowdhury et al., 2020), and Rampahar Natural Forest Reserve, which is next to SFR, was home to Q. gomeziana (Chowdhury et al., 2018). In this study, IVI in canopy openings was 2-3 times greater in B. elata and L. polystachya than in dense canopy. This demonstrates the significance of gap dynamics in the dominance of native floral species during regeneration. The process by which forests acquire a complex age and size structure typical of older stands is made possible by canopy gaps (Hart and Grissino-Mayer, 2009). Moreover, F. auriculata trees, saplings, and seedlings were more abundant in openings than under dense canopies. This species is an intermediate successional species and can grow until the canopy reaches a height when sunlight becomes more accessible in openings (Xiang and Chen, 2004). Besides, small seeds of tropical trees that prefer openings germinate rather than sprout (Pearson et al., 2002). The presence of adequate light in the openings may explain the increased densities of large-seeded trees of B. elata, L. polystachya, L. acuminata, and M. linifolia in the canopy openings compared to the dense canopy.
In contrast, since 12 regenerating species (including seedlings and saplings) were identified in the dense canopy plots but not in the canopy openings, dense canopy plays a key role in the dynamics of SFR stands, covering six times more land than canopy openings. For instance, species found in the dense canopy were only S. suaveolens, D. montana, and Engelhardtia spicata (Supplementary Tables A1 and A2), suggesting that rain forests maintain a high canopy cover merely to ensure there are enough seeds available. In the dense canopy as opposed to the canopy openings, there were higher densities of seedlings/saplings of G. cowa, D. pilosula, L. acuminata, and P. serratum. This is likely because these plants can tolerate shade, and the humidity may help with germination and seedling survival.
However, there were a few big trees and saplings of G. cowa and L. acuminata appeared under dense canopy. This failure of seedlings to develop into large trees is most likely caused by their late-successional trait (Tunjai and Elliott, 2012). Among the late successional species predominated in dense canopy areas were Diospyros pilosula, Garcinia cowa, Castanopsis indica, Lithocarpus polystachya, etc. In comparison to a dense canopy, microclimatic elements including air temperature, light, and moisture in canopy openings affect species composition and recovery patterns (Yamamoto, 2000; Asner et al., 2013; Bhugeloo et al., 2021). The presence of lower amount of humus, with high elevation in the open site compared to the dense canopy area of the Sitapahar forest (Baul et al., 2021) may not hold water in the soil, affecting negatively species composition and seedling establishment. Removal of trees was found to decrease litter input in the soil, resulting deficiencies of N, P, and K and decreasing forest growth in tropical forests (Alvarado, 2015). Topography and edaphic factors (e.g., bulk density, soil texture, pH, organic matters, nutrients) as well as biotic interventions and canopy cover are more important environmental and vegetation variables for restoration and conservation as well as distribution of woody species (Deák et al., 2021; Ali et al., 2022).

5 Conclusions and implications

We observed that for seedlings and saplings, respectively, the dense canopy had roughly 63% and 237% higher species diversity than the canopy opening areas. This suggests that species diversity is retained from the seedling stage to trees in dense canopy. In comparison to the canopy openings, the densities of seedlings and saplings in the dense canopy were 66 and 123% greater, which was likely a result of higher stocks and densities of mature trees. The highest IVI was found in the canopy openings, indicating the significance of gap dynamics in the dominance of regeneration of native floral species. The most dominant and frequent regenerating tree species, including D. pilosula, G. cowa, B. elata, and L. polystachya, were present in both dense canopy and canopy openings. Due to the availability of sufficient light in the canopy opening areas, the greater densities of B. elata, L. polystachya, L. acuminata, F. auriculata, and M. linifolia trees was observed in canopy opening areas compared to dense canopy. Thus, species composition and recovery patterns are influenced by microclimatic variables (such as light) in canopy openings as opposed to an entire canopy.
This study has conservation implications. For example, findings suggest protecting native mother trees in the gaps for their in-situ conservation that augment natural regeneration. Enrichment planting with light tolerant regenerating species in the gaps and assisted natural regeneration in the dense canopy can assist conserving and enhancing tree diversity.
This study had limitations, including gap ages, which hinder the full understanding of gap dynamics. Moreover, regeneration might be influenced by the microclimate (e.g. light) and edaphic (e.g. soil nutrients) conditions of the stands, which we could not study in this research. This was a point-in-time study. A repeated study would provide an opportunity to understand the changes in composition and densities due to disturbance regimes such as cyclones, fires, etc.

Acknowledgements

This research was supported by a grant funded by Research and Publication cell of University of Chittagong, Bangladesh.

References

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