Introduction
Bryophytes are integral components of tropical and temperate forest ecosystems, and they contribute substantially to the diversity and biomass of the forests (Grytnes et al. Reference Grytnes, Heegaard and Ihlen2006; Steel et al. Reference Steel, Wilson, Anderson, Lodge and Tangney2004). They have a wider distribution and greater altitudinal range compared to the vascular plants, making them ideal for the study of diversity changes (Andrew et al. Reference Andrew, Rodgerson and Dunlop2003). Epiphytic bryophytes, in particular, serve as reliable indicators of forest quality and integrity concerning forest structure and resource availability (Frego Reference Frego2007; Song et al. Reference Song, Ma, Yao, Liu, Li, Chen, Lu, Cao, Sun, Tan and Nakamura2015). They also play a vital role in maintaining water balance and promoting humus formation in conifer forests (Bahuguna et al. Reference Bahuguna, Gairola, Semwal, Uniyal, Bhatt, Gupta and Kumar2013). The epiphytes are also considered important components for forest management and conservation, as they are highly responsive to the intensity of management practices and related anthropogenic activities (Horvat et al. Reference Horvat, Heras, García-Mijangos and Biurrun2017). Moreover, despite their conspicuous size and simpler internal structure, bryophytes are known to be considerably sensitive to climate change, making them significant indicators for ecosystem health and impacts of climate change (Bates and Preston Reference Bates, Preston and Tuba2011; Désamoré et al. Reference Désamoré, Laenen, Stech, Papp, Hedenäs, Mateo and Vanderpoorten2012; Wierzcholska et al. Reference Wierzcholska, Dyderski and Jagodziński2020). Their distribution patterns are also greatly influenced by changes in climatic conditions associated with altitudinal and latitudinal gradients (Holz and Gradstein Reference Holz and Gradstein2005; Song et al. Reference Song, Ma, Yao, Liu, Li, Chen, Lu, Cao, Sun, Tan and Nakamura2015). Responses of bryophytes to shifts in temperature and precipitation are particularly critical, positioning them as key subjects in understanding the long-term impacts of global climate change.
Distribution and richness of species in a region and the factors interacting with these parameters have been emphasised in areas of ecology and biogeography. Species richness represents the fundamental measure of diversity, determined by counting the number of species occurring at a particular region (Tessler et al. Reference Tessler, Wittenberg and Greenbaum2016). The richness pattern usually differs depending on the regional species dynamics, taxonomic category and gradient levels (Rahbek Reference Rahbek2005; Zobel Reference Zobel1997). Although the importance of climate on bryophyte diversity is not entirely comprehensible, several studies have reported it to be the primary determinant of bryophyte richness (Callaghan and Ashton Reference Callaghan and Ashton2008; Pharo and Zartman Reference Pharo and Zartman2007; Zechmeister et al. Reference Zechmeister, Tribsch, Moser, Peterseil and Wrbka2003). Moreover, the factors account for variation in species richness along broad-scale geographical gradients (Hawkins et al. Reference Hawkins, Field, Cornell, Currie, Guégan, Kaufman, Kerr, Mittelbach, Oberdorff, O’Brien and Porter2003). Diversity commonly declines with decreasing temperature and rainfall (Rosenzweig Reference Rosenzweig1995). Correlation between species richness and water availability has also been reported from tropical and sub-tropical regions (Bhattarai and Vetaas Reference Bhattarai and Vetaas2003; Gentry Reference Gentry, Hecht, Wallace and Prance1982).
The study of species richness of bryophytes along the elevation gradient is particularly significant as this can be an effective tool for the assessment of climate change effects on the community composition (Ah-Peng et al. Reference Ah-Peng, Wilding, Kluge, Descamps-Julien, Bardat, Chuah-Petiot, Strasberg and Hedderson2012; Sundqvist et al. Reference Sundqvist, Sanders and Wardle2013). The climatic variables such as temperature, precipitation, solar radiation and air pressure differ remarkably along altitudes, and therefore, these factors have significant effects on assemblage and richness of species (Cui et al. Reference Cui, Bing, Fang, Wu, Yu, Shen, Jiang, Wang and Zhang2019; Khuroo et al. Reference Khuroo, Weber, Malik, Dar and Reshi2010; Körner Reference Körner2007). Moreover, effects of climatic conditions on species diversity are reflected better along the altitudinal gradient of mountains compared to the latitudinal gradient due to the lesser impact of dispersal limitations and geographical distance (Qian et al. Reference Qian, Jin, Leprieur, Wang and Deng2020). Over the past few years, a number of studies have focused on the relationship between the elevation gradients and species richness to understand the altitude-wise variation of species composition (Coelho et al. Reference Coelho, Gabriel, Hespanhol, Borges and Ah-Peng2021; Cordeiro et al. Reference Cordeiro, Klanderud, Villa and Neri2023; Dai et al. Reference Dai, Zhang, Qian, Li, Shi, Zhang, Zhang, Li and Wang2025; Fu et al. Reference Fu, Mei, Xu, Zhao and Gao2023; Marline et al. Reference Marline, Ah-Peng and Hedderson2020; Maul et al. Reference Maul, Wei, Iskandar, Chantanaorrapint, Ho, Quandt and Kessler2023; Rodríguez-Quiel et al. Reference Rodríguez-Quiel, Kluge, Mendieta-Leiva and Bader2022; Shershen et al. Reference Shershen, Stehn and Budke2024). Such studies are also required for effective planning of strategies for forest management under a climate change scenario (Lomolino Reference Lomolino2001; Umair et al. Reference Umair, Hu, Cheng, Ali and Ni2023). In several ecological studies, the elevation gradient has been identified as a critical factor that significantly influences diversity (Coelho et al. Reference Coelho, Gabriel, Hespanhol, Borges and Ah-Peng2021; Cardelús et al. Reference Cardelús, Colwell and Watkins2006). The changing pattern of richness of bryophytes along the elevation gradient is mainly characterised by four trends: a consistent decrease in species richness with increasing altitude (Tusiime et al. Reference Tusiime, Byarujali and Bates2007), an increase in species richness with altitude (Ah-Peng et al. Reference Ah-Peng, Chuah-Petiot, Descamps-Julien, Bardat, Stamenoff and Strasberg2007), a hump-shaped distribution with a richness peak at mid-elevation (Ah-Peng et al. Reference Ah-Peng, Wilding, Kluge, Descamps-Julien, Bardat, Chuah-Petiot, Strasberg and Hedderson2012; Sanger and Kirkpatrick Reference Sanger and Kirkpatrick2015; Song et al. Reference Song, Ma, Yao, Liu, Li, Chen, Lu, Cao, Sun, Tan and Nakamura2015) and multimodality (Sun et al. Reference Sun, Wu, Wang, Zhou, Yu, Bing and Luo2013). Most common among them is the hump-shaped pattern (Song et al. Reference Song, Ma, Yao, Liu, Li, Chen, Lu, Cao, Sun, Tan and Nakamura2015).
Eastern Himalaya is a biodiversity hotspot with a long stretch of elevation gradient. The Darjeeling hills, an inherent part of this biodiversity zone, exhibit high biological diversity due to variation in topography, altitude and climatic conditions (Das Reference Das and Pandey1995). The ideal cold and humid climate of this region supports an exceptionally rich and compositionally diverse assemblage of bryophytes (Asthana and Gupta Reference Asthana and Gupta2021). Some significant contributions on bryo-floristics of Eastern Himalayas have been made, but the majority have primarily focused on new records and documentation of taxa (Asthana and Gupta Reference Asthana and Gupta2021; Dandotiya et al. Reference Dandotiya, Govindapyari, Suman and Uniyal2011; Dey and Singh Reference Dey and Singh2012; Gangulee Reference Gangulee1969–1980; Omar et al. Reference Omar, Sahu and Asthana2020; Srivastava et al. Reference Srivastava, Srivastava and Dixit1994). Only recently, phyto-sociological studies were conducted to understand the association of epiphytic bryophytes with Cryptomeria japonica (Halder et al. Reference Halder, Chakraborti, Lama and Mitra2024; Mukhia et al. Reference Mukhia, Mandal, Singh and Singh2019).
Cryptomeria japonica, a member of the family Cupressaceae, is a dominant conifer in the plantation forests of the Darjeeling hills and serves as the substratum for a substantial portion of bryophyte diversity. It is undeniable that forest management practices affect the quality of the substrate for cryptogams, especially the lower trunks of phorophytes. Therefore, the cryptogamic epiphytes in managed forests, especially those dwelling in the basal zone of any tree, are considered threatened and must be considered for conservation efforts (Paillet et al. Reference Paillet, Bergès, Hjältén, Ódor, Avon, Bernhardt-Römermann, Lundin, Luque, Magura, Matesanz, Meszaros, Sebastia, Schmidt, Standovar, Tothmeresz, Uotila, Valladares, Vellak and Virtanen2010). Limited studies on bryophytes have created a gap between the knowledge regarding the distribution and diversity of the bryophytes along the elevation gradient and the underlying factors affecting their distribution. Therefore, the study was framed to explore the diversity of epiphytic bryophytes along an elevational gradient in the Darjeeling hills, marking the first comprehensive survey of its kind in this region. The primary objectives were to investigate the variation of composition of epiphytic bryophytes along the altitudinal gradient in the Darjeeling hills and to compare the species richness and distribution patterns of these epiphytes across different elevations. The study also aimed to assess the implications of environmental variables on the observed elevational patterns and determine their significance in explaining bryophyte species richness in this ecosystem.
Material and methods
Study area
The study was conducted in Darjeeling Himalaya, comprising the hills of Darjeeling and Kalimpong Districts of West Bengal, India, extending from 27°13ʹ10ʺ to 26°27ʹ05ʺ N latitude and 88°53ʹ to 87°30ʺ E longitude, covering an altitudinal range between 130 and 3700 m above sea level (asl) (Figure 1). The hills ascend abruptly from the North Bengal plain, where the elevation rapidly shifts from 100–130 m asl to 2000–3000 m asl within a short distance. Numerous narrow ridges separated by closely spaced V-shaped valleys become prevalent upon entering the hilly terrains. Predominant convex and straight slopes with inclines ranging from 15º to 40º can be seen. The hills experience sub-tropical to temperate highland climatic conditions, with the annual precipitation varying between 2000 and 4500 mm. The rainy season begins at the end of May and continues up to early September. The average maximum daily rainfall is about 172 mm, but it can reach up to 500–600 mm during June and July. The average annual temperature of the study area is about 16.7 °C. January is the coldest month, while July and August are the hottest (Cajee Reference Cajee2018).
Study area represented by the colour matrix of altitude range showing the study locations.
The vegetation of Darjeeling Himalaya spans a range of plant communities from tropical plains to the subalpine formations at higher elevations. The diverse forest zones of this region have been classified based on the wide altitudinal range (Bhujel Reference Bhujel1996; Champion and Seth Reference Champion and Seth1968). The lower parts of the elevation belt (<1800 m) are more diverse with respect to tree species composition and are characterised by the tropical moist deciduous forest dominated by Shorea robusta C.F.Gaertn., Terminalia myriocarpa Van Heurck & Müll. Arg., Dillenia pentagyna Roxb., followed by sub-tropical mixed vegetation of Castanopsis hystrix A.DC., Machilus edulis King ex Hook.f. and Schima wallichii (DC.) Korth. (Rai and Das Reference Rai and Das2008; Spur and Burnes Reference Spur and Barnes1980). The immediate higher elevation zone is an amalgamation of both the sub-tropical and temperate vegetation, with the intermediate type of forest composition represented by plantation tree species such as Cryptomeria japonica (Thunb. ex L.) D. Don, Alnus nepalensis D. Don. Temperate vegetation (1800–2700 m) exhibits the occurrence of the tree species such as Acer campbellii Hook. f. & Thomson ex Hiern, Acer sterculiaceum Wall., Alnus nepalensis, Betula alnoides Buch.-Ham. ex D. Don, Magnolia campbellii Hook. f. & Thomson, Michelia doltsopa (Buch.-Ham. ex DC.) Figlar and Betula utilis D. Don. The exotic Cryptomeria japonica dominates in several forests in this zone (Mallick Reference Mallick2020). The subalpine vegetation above 2700 m is characterised by dwarf shrubs and trees such as Rhododendron arboreum Sm., Betula utilis D. Don and Berberis aristata DC. (Mallick Reference Mallick2020). Among the vascular epiphytes in the study area, taxa belonging to Orchidaceae and Polypodiaceae are predominant (Rai and Moktan Reference Rai and Moktan2022).
Sampling design
Field survey and sampling were performed during the pre-monsoon period (May to early June) of 2022 and 2023. The surveys were conducted at nine altitude zones between 1450 and 2250 m asl at intervals of 100 m. At each of these elevation zones, a minimum of two locations were selected as study sites having sufficient occurrence of Cryptomeria japonica without any visible anthropogenic disturbances, accounting for a total of 21 study sites across the investigated elevation gradient. The location coordinates were tracked using a GPS (Garmin, USA) and were plotted in Figure 1. At each site, sampling was performed at three randomly selected plots measuring 50 m × 50 m, separated from each other by at least 50 m. In each plot, a minimum of three trees were randomly chosen for sampling purposes. Microplots of 400 cm2 (20 cm × 20 cm) were placed on each tree at six height zones within 150 cm of height from the base. Therefore, 18 microplots were examined at each replica plot, accounting for a total of 54 microplots at each site. Coverage of bryophytes occurring within the microplots was recorded by counting the number of grid cells (1 cm2) occupied by each species. All bryophyte specimens were initially sorted in the field based on their distinct morphological features, and small clumps of each visibly distinguishable taxon were collected separately from each microplot. Various physical parameters of the phorophyte such as bark roughness, texture and diameter at breast height, as well as other relevant environmental parameters such as field vegetation type, were also documented.
Identification of specimens
The collected bryophyte specimens were air-dried and examined using microscopy for identification to the lowest possible taxonomic rank. All the morphological and anatomical attributes of the gametophytic and sporophytic plant parts were critically examined in detail. Identification was performed by consulting the keys and standard manuals outlined in the existing monographs and literature (Chopra Reference Chopra1975; Dey and Singh Reference Dey and Singh2012; Gangulee Reference Gangulee1969–1980). Identities were verified by comparing with authenticated herbarium specimens. The specimens were deposited at the herbarium of Lloyd Botanic Garden (LB), Darjeeling.
Data analysis
The abundance of a taxon was assessed by calculating the mean percentage coverage of the species at the microplot level. Species accumulation curves were prepared to evaluate the effectiveness of sampling effort using microplots with 95% confidence intervals at each elevation zone (Gotelli and Colwell Reference Gotelli and Colwell2001). The curves depicted the cumulative number of detected species at each elevation zone as a function of the number of analysed sampling plots. The species accumulation curve was constructed using the R statistical software, specaccum function from the vegan package (Oksanen et al. Reference Oksanen, Blanchet, Kindt, Legendre, Minchi, O’Hara and Wagner2015).
Relative coverage (RC), relative frequency (RF) and importance value (IV) of each taxon in a study site were determined using the equations:
$$\rm {RC = C_{\it i} / \sum C \times 100,}$$
where Ci is the percentage coverage of the i-th taxon in a microplot and C is the percentage coverage of all taxa;
$$\rm {RF = F_{\it i} / \sum F \times 100,}$$
where Fi is the number of plots having of i-th taxon and F is the total occurrence of all taxa; IV = RC + RF (Jiang et al. Reference Jiang, Yang, Zhong, Tang, Liu and Su2018).
Altitudinal range of occurrence of each species was determined following the method described by Ah-Peng et al. (Reference Ah-Peng, Wilding, Kluge, Descamps-Julien, Bardat, Chuah-Petiot, Strasberg and Hedderson2012). This method estimates the range by subtracting the minimum altitude of occurrence of a species from the maximum altitude at which the species was recorded, presuming that a species can be present at all altitudinal zones between the extremes. However, the species that were recorded at only one altitudinal zone were allocated within a 100 m distribution range. ‘Local rarity’ of a species in the study area was determined from the plot data based on three criteria: narrow geographic range, low frequency and small population size (Song et al. Reference Song, Ma, Yao, Liu, Li, Chen, Lu, Cao, Sun, Tan and Nakamura2015). Diversity of the epiphytes was determined by measuring Hill numbers, which quantify the abundance-based species diversity using the effective number of species (Chao et al. Reference Chao, Gotelli, Hsieh, Sander, Ma, Colwell and Ellison2014; Hill Reference Hill1973; Jost et al. Reference Jost2010). Hill numbers are organised in four orders differing only in a single parameter q, which is sensitive to the relative abundance of species. At q = 0, diversity is indicated by species richness, which is examined at the level of presence or absence. At q = 1, diversity is based on the exponential of Shannon entropy (H’) and is dependent on the relative abundance of all species, which are weighed equally (Krebs Reference Krebs1999). Therefore, the order measures the contribution of rare species. The order q = 2 is the inverse of Simpson’s concentration (1/D), which signifies the contribution of some common and dominant species. At q = 3, diversity measures the contribution of the most dominant species, and the value is the inverse of Berger–Parker’s index (1/d) (Berger and Parker Reference Berger and Parker1970). A polynomial regression was performed to model the variation of species richness along the elevation gradient. The bias-corrected Chao estimator for species richness was used to prepare the model as it is less affected by detection probabilities and uneven species distribution, making it suitable for community data with rare species (Colwell and Coddington Reference Colwell and Coddington1994; Hortal et al. Reference Hortal, Borges and Gaspar2006).
To study the beta diversity, the Whittaker index (Whittaker Reference Whittaker1960) was calculated by using the formula:
$$\beta = \left( {{s \over \alpha }} \right) - 1$$
where α = mean number of species per altitude zone and s = total number of species recorded across the studied altitudinal zone (Mena and Vázquez-Domínguez Reference Mena and Vázquez-Domínguez2005). The diversity parameters were analysed using the PAST 4.03 software package (Hammer et al. Reference Hammer, Harper and Ryan2001).
To evaluate the overall variation in species composition along the altitudinal gradient, non-metric multidimensional scaling (NMDS) ordination and permutational multivariate analysis of variance (PERMANOVA, 999 permutations) were carried out by using R statistical software, ‘metaMDS’ function of vegan package (version 2.6–4) (Oksanen et al. Reference Oksanen, Blanchet, Kindt, Legendre, Minchin, O’hara, Simpson, Solymos, Stevens, Wagner and Oksanen2013). ‘Bray–Curtis’ distance was calculated, and two dimensions (k = 2) were used for the selection of optimum dimensions in NMDS. To visualise the group scores along the NMDS ordination plot, ‘gg_ordiplot’ functions of ggordiplots package (version 0.4.3) was used (Quensen Reference Quensen2018). A PERMANOVA analysis with 999 permutations was employed to test the significance of the difference between the groups (studied altitudes) based on the bryophyte species composition (Anderson Reference Anderson, Balakrishnan, Colton, Everitt, Piegorsch, Ruggeri and Teugels2017). Further, to evaluate the effect of environmental parameters on the assemblage of bryophytes at the elevation zones, the Canonical Correspondence Analysis (CCA) was performed as a multivariate ordination technique considering the investigated attributes as the variables. Mean annual temperature, mean diurnal range (MDR), isothermality (ITH), mean annual precipitation, mean windspeed value (WSP), mean vapour pressure and solar radiation were considered as the environmental variables. The climate data of each studied altitudinal zone were extracted from WorldClim 2.1 at a spatial resolution of 1 km2 (Fick and Hijmans Reference Fick and Hijmans2017).
Results
Species composition
A total of 37 epiphytic bryophytes from 21 families were recorded, which included 31 mosses and 6 liverworts (Table 1). Most of the mosses belonged to the Hypnales and Dicranales orders, while Jungermanniales was the predominant order of liverworts. Among the members of Dicranales, the maximum recorded taxa were of Dicranaceae and Leucobryaceae. Syrrhopodon confertus, a member of Calymperaceae, was the most frequent among the identified taxa, recorded at almost 95.5% of all studied plots, followed by the Bazzania ovistipula, a liverwort of Lepidoziaceae, found in 66.7% of the plots. Other common taxa in terms of percentage of occurrence in the studied plots include Dicranella heteromalla (52.4%), Leucobryum humillimum (38.1%), Anisothecium spirale (38.1%) and Herbertus dicranus (38.1%). Syrrhopodon was also recorded as the most abundant moss at the plot of occurrence and more or less uniformly distributed in the form of dense cushions throughout the basal trunk. Leucobryum humillimum and Dicranodontium didymodon are mostly restricted towards the extreme base of the host in the form of large cushions. Other mosses of Dicranales colonised as small cushions. The liverworts and Hypnales members occurred in the form of a smooth or rough mat.
List of identified taxa and their importance value at the nine elevation zones
Altitudinal range of distribution and species richness
The cumulative number of recorded taxa was plotted as a function of sampling numbers to generate a species accumulation curve. The curves for sampling at all nine elevation zones exhibited sufficient sampling effort, as almost all the curves reached a plateau (Figure 2). The altitudinal ranges of distribution of the recorded taxa are represented in Figure 3. Among the 37 identified taxa, Syrrhopodon confertus exhibited the highest range of occurrence as it was detected from all the elevation zones. Dicranella heteromalla and Bazzania ovistipula were recorded at all elevations starting from and above 1550 and 1650 m, respectively. Anisothecium spirale and Leucobryum humillimum also exhibited a considerable range of distribution. Sixteen taxa were found to be confined only a single elevation zone. Some taxa such as Octoblepharum albidum, Campylopus schimperi, Pylaisiadelpha tenuirostris, Thuidium sparsifolium and Calymperes erosum were only restricted to the lower altitudes (below 1650 m), whereas species like Plagiothecium denticulatum, Dicranodontium didictyon, Oreoweisia laxifolia and Trachypodopsis serrulata were found exclusively at higher altitudes (above 2150 m).
Species accumulation curves of all elevation zones showing the proportion of observed species accumulated in sampling plots.
Range of distribution of epiphytic bryophytes along the elevation gradient from 1450 to 2250 m.
Syrrhopodon confertus was the most frequent moss in all the study zones, as depicted by its highest relative frequency. Even the moss exhibited the highest relative coverage in most of the elevation zones (except 1650 m), ranging from 26.43% at 1650 m to 70.61% at 2050 m. These implied the high IV of this taxon at all the elevation zones (Table 1). IV of Syrrhopodon varied from 40.7 at 1650 m to 87.28 at 2050 m. The values were considerably higher compared to all other bryophytes at all the elevation zones. At 1550 m, Entodon flavescens, Heterophyllium amblyostegum and Isopterygium tenerum were co-dominant alongside Syrrhopodon, as predicted from their IVs, which were 33.33, 23.60 and 21.05, respectively. Campylopus sub-fragilis exhibited the highest relative coverage of 31.58 % at 1650 m, and it was the co-dominant moss having an IV of 38.72. Among the liverworts, Bazzania ovistipula was observed as the co-dominant epiphyte at the basal trunk above 1650 m, which can be predicted from their higher relative coverage and IV at those elevation zones. On the contrary, Isopterygium albescens, Campylopus schimperi, Thuidium sparsifolium, Taxiphyllum taxirameum, Oreoweisia laxifolia and Trachypodopsis serrulata can be considered ‘locally rare’ following the criteria of local rarity mentioned in Song et al. (Reference Song, Ma, Yao, Liu, Li, Chen, Lu, Cao, Sun, Tan and Nakamura2015) and Vanderpoorten and Engels (Reference Vanderpoorten and Engels2003). The taxa were observed only within a single altitudinal zone, occurring in <10% of the microplots with <10% of coverage.
The Hill number values at order q = 0 indicated the species richness based on the presence of the epiphytes at each elevation zone. The highest species richness was observed at 1550 m altitude zone with 10.67 ± 0.57 taxa per studied site. The bias-corrected Chao estimator predicted the lowest richness at 2050 m (5.69 ± 1.13). A pattern of variation of species richness along the elevation gradient was predicted from polynomial regression analysis. The model with the lowest Akaike information criterion (AIC) value was selected (R2 = 0.30; p < 0.05; df = 16; corrected AIC = 62.98) (Figure 4A). It depicted an initial hump-like appearance due to a richness peak at 1550 m. However, the pattern can be predicted as multimodal because the richness value is enhanced again at the highest elevation zone.
(A) Species richness pattern along elevation gradient represented by polynomial regression plot based on Chao2 estimate (R2 = 0.30; df = 16; p < 0.05), (B) Hill numbers at four orders of q based on the coverage data of bryophyte taxa along the elevational gradients, where the orders of q are interpreted as follows: q = 0: species richness; q = 1: exponential of Shannon entropy; q = 2: inverse of Simpson’s concentration; q = 3: inverse of Berger–Parker’s index. Hill Numbers are represented as mean values ± SD calculated from the coverage data of each plot of an elevation zone.
The Hill number profile curves of all the elevation zones exhibited almost similar trends, where all the values sharply declined with increasing coefficient of q (Figure 4B). The result symbolises an uneven abundance of the bryophyte taxa at all the elevation bands due to the high dominance of some species. Sharp decline of diversity curves from order q = 0 to q = 1 indicated the presence of rarely abundant species at all the elevation zones. However, any trend of such a decrease with altitude could not be predicted. A simple measure of (q = 2)/(q = 0) indicated the extent of decrease in the effective number of species (Rodríguez-Quiel et al. Reference Rodríguez-Quiel, Kluge, Mendieta-Leiva and Bader2022). However, the species-rich sites, that is, 1550 and 2250 m, showed a greater decrease in values, depicting a greater decline in the effective number of species at those zones. The extent of decrease from order q = 2 to q = 3 was lowest at 1450 m, indicating domination of a single epiphyte (Syrrhopodon confertus) over the other taxa of the community. In contrast, the curve declined most at 2250 m, predicting the presence of a number of co-dominant epiphytes alongside the most dominant one at the investigated sites.
Relationship between elevation gradient and species composition
The results of the Whittaker beta diversity index depicted the change in bryophyte species composition between the elevation zones. The species composition of 1450 and 1550 m is relatively different from the higher elevation zones as predicted from their pairwise values (>0.7) (Table 2). The highest index value was observed between 1650 and 1750 m altitude zones (0.88). Comparatively lesser values between the pairs of elevation zones above 1750 m indicated relatively homogeneous species compositions above 1750 m.
Beta diversity index values based on Whittaker’s index formula between the altitude pairs
Significant change in the bryophyte community structure along the altitude gradient can be predicted from the PERMANOVA (999 permutations) based on a Bray–Curtis dissimilarity matrix for the percentage coverage of taxa (p < 0.01, R2 = 0.539). NMDS analysis for species composition at different altitudes exhibited a prominent gradient separating altitudinal plots along the first two axes, NMDS1 and NMDS2. The stress dimension analysis estimated the stress value of 0.081, which is less than 0.2 (Clarke Reference Clarke1993), and based on that, optimum dimensions were selected in the NMDS analysis. The first two axes of the ordination plot explained almost 51% and 47% of the variation, respectively. Distinct clusters of plots represent altitudinal zones with specific species composition (Figure 5). The plots of 2250 m altitudes were clustered towards the positive end of the first axis, whereas the plots of 1450 m formed a group towards the negative end of it. The plots of 1450 m exhibited higher scores along the second axis and were clustered towards the positive end. Notably, the 1550 m altitudinal plots were found to be the most distinct, forming an entirely separate cluster towards the negative end of both axes. The other plots, ranging from 1650 m to 2150 m, were grouped towards the middle part of the plot, scattered throughout the positive and negative sides of the second axis but more inclined towards the positive side of the first axis.
Non-metric multidimensional scaling (NMDS) plot using Bray–Curtis dissimilarity matrix (dimensions = 2, stress value = 0.081) showing placement of the study sites along the axes based on the bryophyte species composition. The ellipses include 95% data for each group.
Effects of climatic variables on species composition
The climatic data for the specific sampling sites were obtained from the WorldClim database (Fick and Hijmans Reference Fick and Hijmans2017), and their effects on the assemblage of taxa were evaluated by CCA. The first two axes of CCA exhibited the eigenvalues 0.40 and 0.22, respectively. The two axes explained 36.52 % and 20.36 % of the investigated variables. MDR, ITH and WSP were positively aligned towards the second axis, while high negative loading along that axis was observed for annual mean temperature (AMT), annual precipitation (APR), mean vapour pressure (VPR) and solar radiation (SR) (Figure 6). The taxa such as Octoblepharum albidum, Erythrodontium julaceum and Campylopus pyriformis, which occurred only at low altitudes, exhibited high negative loading towards the second axis, indicating the influence of AMT, APR, VPR and SR on the abundance of the mosses. Some species, which were more frequent at 1550 m, such as Heterophyllium amblyostegum, Isopterygium tenerum, Isopterygium albescens, Thuidium sparsifolium and Entodon flavescens, showed high positive loading towards the first axis, indicating positive influence of all the climatic factors on their abundance. Other taxa, such as Syrrhopodon confertus and Anisothecium spirale, which were recorded at many of the altitude zones, mostly restricted towards the negative side of the first axis. This signifies that the climatic factors do not have any positive influence on their distribution.
Canonical Correspondence Analysis (CCA) showing the effect of the climatic variables on the occurrence of taxa along the elevation gradients. Blue dots represent the identified taxa abbreviated as the first three letters of genus or the first three letters of genus and species (in case of identical genus abbreviations). Climatic variables: annual mean temperature (AMT), mean diurnal range (MDR), isothermality (ITH), annual precipitation (APR), mean windspeed value (WSP), mean vapour pressure (VPR) and solar radiation (SR).
Discussion
The study represents the first comprehensive effort to investigate the diversity and distribution patterns of epiphytic bryophytes along the altitudinal gradients in the Darjeeling hills. Although the composition pattern of bryophytes is markedly influenced by the phorophyte (Kuusinen Reference Kuusinen1996; Studlar Reference Studlar1982), maintaining consistency in host tree species across all studied altitudinal zones was not feasible in this study because each zone supports distinct vegetation types and dominant tree species. To address this disparity, we selected Cryptomeria japonica as the representative host tree as it is a predominant conifer in the region across all selected altitudinal zones (Tolangay and Moktan Reference Tolangay and Moktan2024). A notable variation in the distribution of epiphytic bryophytes along the elevational gradient was documented. Epiphytic mosses were found to be more abundant than the epiphytic liverworts in almost all altitudinal transects. Sample-based species accumulation curves for all altitudes justified the sampling effort, showing a slowdown in the species accumulation after approximately 8–9 sampling. Although it is difficult to achieve exhaustive sampling in biodiversity hotspots, the relatively flat end of the curves signified sufficient sampling effort for the study (Ah-Peng et al. Reference Ah-Peng, Wilding, Kluge, Descamps-Julien, Bardat, Chuah-Petiot, Strasberg and Hedderson2012).
The overall diversity of bryophytes on basal trunks of Cryptomeria japonica was not significantly high due to the occurrence of a limited number of taxa and the dominance of a few species such as Syrrhopodon confertus. The pattern aligns with the previous reports, which demonstrated that conifers are colonised by a limited number of epiphytes (Gustafsson and Eriksson Reference Gustafsson and Eriksson1995; Kuusinen and Penttinen Reference Kuusinen and Penttinen1999). Acidic and exfoliating barks of conifers generally restrict the growth of diverse species (Barkman Reference Barkman1958). Interestingly, the altitudinal pattern of species richness in our study diverges from the commonly reported unimodal or a proper hump-shaped relationship, where maximum richness typically occurs at somewhere near mid-elevation (Ah-Peng et al. Reference Ah-Peng, Wilding, Kluge, Descamps-Julien, Bardat, Chuah-Petiot, Strasberg and Hedderson2012; Frahm and Ohlemüller Reference Frahm and Ohlemüller2001; Song et al. Reference Song, Ma, Yao, Liu, Li, Chen, Lu, Cao, Sun, Tan and Nakamura2015). In contrast, the present study demonstrated a nearly humped-shaped pattern in the first half of the curve between 1450 and 2050 m altitudinal range, followed by an increasing trend above 2150 m (Figure 4A). The trend aligns with the multimodal species richness pattern reported by Sun et al. (Reference Sun, Wu, Wang, Zhou, Yu, Bing and Luo2013). Most of the previous records demonstrated that richness peaks somewhere near the mid-elevation zones (Ah-Peng et al. Reference Ah-Peng, Wilding, Kluge, Descamps-Julien, Bardat, Chuah-Petiot, Strasberg and Hedderson2012; Sanger and Kirkpatrick Reference Sanger and Kirkpatrick2015), and the trend coincides with the optimum local environmental conditions (Ah-Peng et al. Reference Ah-Peng, Chuah-Petiot, Descamps-Julien, Bardat, Stamenoff and Strasberg2007). However, the initial richness peak in our study was observed at the lower half of the altitude gradient due to the abundance of some locally dominant species, and the initial elevation of the hump-shaped pattern was not clearly defined. Wolf (Reference Wolf1993) suggested that the richness peak may result from an overlap of species from respective higher and lower elevation zones. Extending the survey to include tropical forests at lower altitudes could have provided a more comprehensive understanding. But the sampling below the investigated range was not feasible as the host plant does not frequently occur below 1400 m asl in the Darjeeling hills. The second trend of increase in species richness above 2150 m is due to the exclusive occurrence of a number of taxa such as Plagiothecium denticulatum, Oreoweisia laxifolia and Trachypodopsis serrulata. However, as a consequence of the limited distribution of the host tree above 2250 m, it could not be predicted whether the trend reflects a broader ecological trend or a result of ecotonal transition.
The slopes of the Hill number diversity curves reflect the evenness of the communities (Gotelli and Chao Reference Gotelli, Chao and Levin2013). In the present study, the values sharply declined along higher orders of q, indicating low evenness at the investigated sites of all the elevation zones (Figure 4B). The low effective number of species was due to the dominance of some taxa, particularly Syrrhopodon confertus and Bazzania ovistipula. Here, a number of less abundant species contributed primarily to species richness rather than to community evenness. While the previous reports suggest a trend of change in evenness along altitude (Coelho et al. Reference Coelho, Gabriel, Hespanhol, Borges and Ah-Peng2021), our study could not predict any such consistent pattern. The reduced effective number of species, especially at richness peaks such as 1550 and 2250 m, highlighted the role of ‘locally rare’ species in inflating richness without significantly altering dominance structures. At 1550 m, rare taxa such as Isopterygium albescens, Campylopus schimperi, Thuidium sparsifolium and Calymperes erosum contributed to richness, while at 2250 m, Oreoweisia laxifolia and Trachypodopsis serrulata played a similar role. However, the strong decline in Hill numbers from order q = 2 to q = 3 at 2250 m pointed to disproportionately high codominance by a few species, including Dicranella heteromalla and Bazzania ovistipula, along with the ubiquitous Syrrhopodon confertus. The higher value of calculated beta diversity along the altitudinal gradient, especially across the 1650 m threshold, indicates a highly varied species composition. This may increase the chances of encountering more occasional epiphytic bryophytes. Association of overall beta diversity pattern with altitudes has also been depicted in previous studies (Araújo et al. Reference Araújo, Costa, Souza, Batista and Silva2022).
The highest frequency of Syrrhopodon confertus across all altitudinal transects is consistent with the previous findings from this region, which also identified it as the dominant moss on the basal trunk of Cryptomeria japonica (Halder et al. Reference Halder, Chakraborti, Lama and Mitra2024). The distribution of Syrrhopodon confertus is not likely influenced by altitudinal gradient or environmental heterogeneity. This is contradictory to a previous report concluding the influence of altitude on the distribution of Calymperaceae members based on a study on the tropical rainforest of Brazil (Farias et al. Reference Farias, Silva, Maciel-Silva and Pôrto2017). While the acidic bark of conifers limits the assemblage of cryptogams, the high abundance of Syrrhopodon likely reflects its capacity to tolerate substrates with low pH. Moreover, this genus is among the most prolific producers of gemma, which facilitates their rapid local colonisation through the formation of dense cushions (Reese Reference Reese2001). Such traits confer a competitive advantage over the other bryophytes in terms of space and resources, leading to reduced overall species diversity. A shift in species composition at higher elevations (2250 m) and codominance of a few other taxa along with Syrrhopodon are likely driven by local climatic conditions. In general, Calymperaceae members such as Syrrhopodon are generalists that do not limit themselves to any specific substrate and thereby predominantly occur homogeneously in any particular site (Reese Reference Reese2001). Among the liverworts, Bazzania was observed as predominant in our study. Abundance of this leafy liverwort was also reported by Mukhia et al. (Reference Mukhia, Mandal, Singh and Singh2019) on trunks of Cryptomeria based on a survey at Senchal Wildlife Sanctuary of Darjeeling hills, within the elevation range between 1500 and 2600 m. No occurrence of this plant below 1550 m indicates its specific distribution range within temperate cloud forests.
The NMDS analysis revealed that the epiphytic bryophyte communities at 1450, 1550 and 2250 m exhibited distinct species compositions compared to other elevation zones (Figure 5). Such localised distributions are likely shaped by favourable microhabitats or any specific environmental factor that contributes to enhanced habitat suitability (Sporn et al. Reference Sporn, Bos, Kessler and Gradstein2010, Gradstein Reference Gradstein, Gradstein, Homeier and Gansert2008). At the lower elevations (1450–1550 m), for instance, species confinement could be governed by higher mean annual precipitation and moderate temperatures, as suggested by the CCA. These conditions, particularly increased water availability, are well-documented as critical drivers of bryophyte diversity and performance (Sillett and Antoine Reference Sillett and Antoine2004; Song et al. Reference Song, Ma, Yao, Liu, Li, Chen, Lu, Cao, Sun, Tan and Nakamura2015). Relatively higher species richness at 2250 m is potentially associated with local climatic variables such as increased ITH. Greater temperature stability has also been previously proposed as a factor facilitating the persistence and establishment of bryophyte populations in montane environments (Santos and Costa Reference Santos and Costa2010; Frahm and Gradstein Reference Frahm and Gradstein1991; Joshi and Joshi Reference Joshi and Joshi2022). The plagiotropic mosses such as Plagiothecium denticulatum exhibited moderate to high abundance at this elevation zone. The local climatic conditions such as persistent humidity, low temperature and frequent lateral precipitation may collectively create an optimal condition for their growth. These factors are increasingly recognised as key regulators for successful colonisation of epiphytic bryophytes (Cornelissen and ter Steege Reference Cornelissen and Ter Steege1989; Richards Reference Richards and Schuster1984; Wolf Reference Wolf1993). In the Darjeeling hills, most of the conifer forests at these altitude belts are often characterised by humid and cloudy conditions. The pattern of precipitation is light with frequent lateral precipitation, which provides an optimal condition for the growth of the bryophytes (Gradstein Reference Gradstein1995). Campylopus sub-fragilis exhibited predominance at sites of 1650 m asl in spite of a narrow range of occurrence. This pattern reflects an interplay between microclimatic favourability and dispersal limitations, reinforcing observations that bryophyte distributions can be highly sensitive to localised environmental filters (Söderström and During Reference Söderström and During2005; Mota-Oliveira et al. Reference Mota de Oliveira, Ter Steege, Cornelissen and Robbert Gradstein2009).
The present study focused exclusively on the assemblage of epiphytic bryophytes on a single host species, and therefore, the findings cannot be generalised to represent overall bryophyte diversity across elevation zones. Individual tree species often support a distinct assemblage of epiphytes (Wolf Reference Wolf1994). Although any individual epiphyte is rarely host specific, the overall community composition of epiphytes is heavily influenced by species-specific traits of host trees (Schmitt and Slack Reference Schmitt and Slack1990). The high abundance of Syrrhopodon confertus and Bazzania ovistipula on the investigated host does not necessarily reflect host preference. This can only be assessed by examining their distribution across multiple hosts. The species occurring in large abundance may be considered as colonists according to life history strategy classification (Vanderpoorten et al. Reference Vanderpoorten, Engels and Sotiaux2004). Consequently, identifying these species as potential indicators requires further research on other host plants in this region to gain a broader perspective.
It can be concluded that the present investigation has offered a comprehensive overview of the diversity of epiphytic bryophytes in montane forests of the Darjeeling hills, based on a detailed survey of their composition on basal trunks of Cryptomeria japonica. Species richness varied with altitude, exhibiting a multimodal trend with an initial hump-shaped appearance due to the maximum richness occurring at 1550 m asl, followed by an increase in richness above 2150 m. Diversity profiles clearly depicted low evenness at the studied sites due to dominance of some taxa along with a number of local rare species. Species-rich sites also exhibited a greater decrease in the effective number of species. While Syrrhopodon confertus was observed as the most frequent and ubiquitous moss throughout the altitude range, many other epiphytes have a very narrow range of occurrence. NMDS analysis substantiated a distinct composition of bryophytes between the lowest and highest ranges among the studied altitude zones. Moreover, the diversity is influenced by climatic factors, suggesting that a specific set of climatic requirements determines local colonisation of bryophytes at any particular altitude. The distribution data may provide an insight into assessing the potential impact of a changing climate on bryophyte assemblages. As the bryophytes of the basal trunks are more susceptible to forest management practices, the findings may be considered while developing effective management strategies with an aim at conserving these plants in a changing climate scenario. Furthermore, broader studies incorporating multiple host species, forest types and functional bryophyte groups may provide a more comprehensive understanding of the ecological dynamics governing these unique and often overlooked communities.
Acknowledgements
The authors are deeply indebted to the Directorate of Forests, Government of West Bengal, for providing permission to conduct the survey in the forests under the Darjeeling Wildlife Division. They are also thankful to the Principal of Acharya Prafulla Chandra Roy Government College for permitting the use of infrastructural facilities and to Dr. Shuvadeep Majumdar, Assistant Professor, Scottish Church College, Kolkata, for his help to identify the liverwort specimens.
Authors’ contribution
All authors contributed to the study conception and design. KH was responsible for the fieldwork, identification of plant samples, laboratory experiments, data analyses and preparation of a draft of the manuscript. Corresponding author (SM) conceived the idea, framed the experimental procedure, identified plant samples, performed data analyses and finalised the draft of the manuscript. PCL framed some parts of the experiments and reviewed the manuscript. All authors read and approved the final manuscript.
Financial support
This research received no specific grant from any funding agency, commercial or not-for-profit sectors.
Competing interests
The authors declare that they have no known competing interests.
Availability of data and material
All required data will be made available on request.
Code availability
Not applicable.
