Does road-edge affect liana community structure and liana-host interactions in evergreen rainforests in Ghana?

Acta Oecologica 101 (2019) 103476

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Does road-edge affect liana community structure and liana-host interactions in evergreen rainforests in Ghana?

T

Bismark Ofosu-Bamfoa, Patrick Addo-Fordjourb,∗, Ebenezer J.D. Belfordb a b

Department of Basic and Applied Biology, School of Sciences, University of Energy and Natural Resources, P. O. Box 214, Sunyani, Ghana Department of Theoretical and Applied Biology, College of Science, Kwame Nkrumah University of Science and Technology (KNUST), Kumasi, Ghana

A R T I C LE I N FO

A B S T R A C T

Keywords: Edge effect Forest fragmentation Liana-tree interaction network Nestedness Species co-occurrence

Though lianas can respond to human disturbance and forest fragmentation, the effects of forest edge on liana community are poorly documented. This study therefore investigated the effects of road-edge on liana community structure and liana-host interactions in two evergreen rainforests in Ghana (Ankasa Conservation Area, Cape Three Points Forest Reserve). Lianas and their hosts were identified and counted in twenty-four 50 m × 50 m plots in each rainforest. The plots were evenly distributed in edge (0–50 m), interior (200 m) and deep-interior (400 m) sites. The edge site of Cape Three Points Forest Reserve supported significantly higher liana diversity, but there was no edge effect on liana diversity in Ankasa Conservation Area. There were no significant edge effects on liana species composition, abundance, and basal area in both evergreen rainforests. However, there was evidence of strong edge effects on the abundance of some individual liana species. In all the three sites of the two evergreen rainforests, liana species showed random species co-occurrence pattern, with no nested structure in liana-tree interaction network. Although forest edge had weak effects on liana community, some species had increased abundance, compensating the loss of individuals of other species at the edge. Overall, the idiosyncratic edge effect on liana species populations can blur the effects on liana community.

1. Introduction Lianas are woody climbing plants which are rooted in the soil but rely on external support provided by trees in order to reach forest canopy. Their importance in forest ecosystems is evidenced in their contribution to canopy closure after treefall, helping to maintain understorey microclimate (Schnitzer and Bongers, 2002). By this action they also serve as a connection between tree crowns, providing arboreal pathways for canopy vertebrates (Bongers et al., 2005). Lianas provide food for animals especially in the dry season when food becomes scarce because many trees do not produce flowers or fruits (Bongers et al., 2005). Notwithstanding their positive ecological importance, lianas multiply quickly especially in disturbed areas and compete intensely with trees, taking advantage of optimal light conditions in disturbed areas (Schnitzer and Bongers, 2002). By so doing, they detrimentally affect the normal growth and reproduction of trees (Schnitzer et al., 2000; Pérez-Salicrup, 2001; García León et al., 2017). In tropical forest ecosystems, various forms of human activities occur that tend to cause forest fragmentation (Harper et al., 2005). When fragmentation occurs, forests do not only become subdivided but they are also exposed to edges (Murcia, 1995). At forest edges, different

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changes can occur in both biotic and abiotic components of the ecosystem. For instance, increased light availability, temperature and wind, and reduced humidity often characterise forest edges (Harper et al., 2005). Following fragmentation, trees at the edge often become damaged due to edge effect, leading to reduced canopy cover and greater abundance of snags and logs (Harper et al., 2005). Thus, fragmentation can cause changes in forest structure at forest edges, which together with abiotic changes, could cause changes in liana composition, diversity and abundance (Laurance et al., 2001; Magrach et al., 2014; Addo-Fordjour and Owusu-Boadi, 2017; Jones et al., 2017; Campbell et al., 2018). Such a scenario can have adverse effects on tree growth, reproduction and regeneration (Schnitzer et al., 2000; León et al., 2017), and ultimately affect the interactions between lianas and trees (Morris, 2010), with implications for biodiversity conservation. Where such interactions are strong, changes in population density of species in particular networks may destabilise other species connected to them, thus affecting their diversity and abundance (McCann et al., 1998; Kokkoris et al., 1999). Despite the importance of species interactions knowledge to biodiversity conservation (Mills and Reynolds, 2004), most studies on liana communities have focused heavily on traditional measures of plant

Corresponding author. E-mail addresses: [email protected], [email protected] (P. Addo-Fordjour).

https://doi.org/10.1016/j.actao.2019.103476 Received 17 August 2018; Received in revised form 27 September 2019; Accepted 27 September 2019 1146-609X/ © 2019 Elsevier Masson SAS. All rights reserved.

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Fig. 1. Map of Ankasa Conservation Area, Ghana, showing the forest sites.

characteristics such as size, bark architecture and tree flexibility, as well as canopy size and illumination, all of which could differ from one ecosystem to another. Also, species abundance and light availability can together influence network structure of interaction (Sfair et al., 2018). Forest fragmentation can alter these vegetation features and thus influence species interactions at forest edges (Fagan et al., 1999; Harper et al., 2005). Furthermore, edge-related changes in microclimatic conditions have been found to affect plant species survival and mortality (Laurance et al., 1998, 2000, 2002). This can ultimately alter plantplant species interactions due to loss of partners and ultimate changes in the number of links between species in a network (Fagan et al., 1999; Tylianakis et al., 2010). The present study determined road-edge effects on liana community structure and liana-tree interactions in two evergreen rainforests in Ghana. Information generated by the present study would be useful in managing biodiversity of forest edges in fragmented tropical forests. It would also contribute to the development of a robust edge theory. The two rainforests are characterized by narrow edges (width < 10 m) which are dominated by tall, broad-leaved evergreen trees. This provides much shade and facilitates a humid environment in the forest edge. Furthermore, in tropical forests, narrow linear edges (< 20 m width) are less vulnerable to edge-related wind disturbance and desiccation stress than wider edges (Laurance et al., 2009; Eldegard et al., 2015). Based on this type of forest edge, we tested the following hypotheses:

assemblages such as diversity, composition and abundance to the neglect of measures of liana-tree interactions. Two popular measures of assemblage structure, species co-occurrence and nestedness have been widely used to study plant-plant interactions in tropical and temperate forests. The use of these tools makes it possible to evaluate patterns of community assembly of organisms (Veech, 2014) and generate hypothesis about the mechanisms that structure communities (Delalandre and Montesinos-Navarro, 2018). They are recommended for use in fragmented forests because they can detect species sensitivity to fragmentation (Martínez-Morales, 2005). Though these tools are very useful in studying community assembly, only a few studies have analysed liana species co-occurrence and nestedness to characterise liana community assembly. What is more, the patterns reported so far suggest that there is no common pattern of liana-tree interactions. In a New Zealand forest, liana species showed negative co-occurrence on trees (Blick and Burns, 2009), while in a Malaysian forest, liana species were randomly distributed on their hosts (Addo-Fordjour et al., 2016). Similarly, whereas Sfair et al. (2010) observed nested structure in networks of liana-tree interactions in three distinct neotropical vegetation formations, Blick and Burns (2009) and Addo-Fordjour et al. (2016) did not find nested structure in their liana-tree interaction networks. From these findings, it appears that the nature of liana-tree interactions depends on the nature of ecosystem studied. For example, Sfair et al. (2010) suggested that the type of structure formed in liana-tree interaction network will depend on system complexity and tree 2

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Fig. 2. Map of Cape Three Points Forest Reserve, Ghana, showing the forest sites.

(Figs. 1 and 2). Ankasa Conservation Area which spans an area of 509 km2 is located in the area with the highest rainfall in Ghana. Mean annual rainfall in the area is 1700–2000 mm with peaks in April to July and September to November. Mean monthly temperatures range between 24 and 28 °C, and relative humidity in the day and night is 75 and 90%, respectively. Ankasa Conservation Area which is the richest forest, in terms of biodiversity, in Ghana habours about 800 vascular plant species. In addition, over 300 species of plants have been found in a single hectare of the forest (Wildlife Division, 2000; Ntiamoah-Baidu et al., 2001). Its rich plant biodiversity includes the endemic ground herb species Psychotria ankasensis (Rubiaceae) (Hall and Swaine, 1981). Illegal logging and mining activities have been reported in the forest reserve in the past. Forest edge in Ankasa Conservation Area was selected along an old national highway created in 1973. The road which passes through the forest reserve connects Axim to Elubo via Nkwanta. The road was abandoned in 1989 as a highway, but currently it is used

1. Liana community structure does not differ between edge and nonedge sites in the evergreen rainforests. 2. Population abundance of liana species does not differ between edge and non-edge sites in the evergreen rainforests. 3. Co-occurrence patterns in liana-tree interaction are similar between edge and non-edge sites in the evergreen rainforests. 4. Patterns of liana-tree network structure are similar between edge and non-edge sites in the evergreen rainforests.

2. Materials and methods 2.1. Study areas Lianas and trees were surveyed in two tropical evergreen rainforests in Ghana: Ankasa Conservation Area (050 15′00″N and 020 36′00″W) and Cape Three Points Forest Reserve (40 50′00″N and 020 30′00″W) 3

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pooled for the diversity analysis. The number of species identified and counted in the forest sites was taken as the observed species richness, while Shannon diversity index was quantified using PAST statistical package (version 2.17c; Hammer et al., 2001). These indices were compared among the forest sites using permutation tests in PAST statistical package. The statistical analyses described below were conducted using RStudio 1.2.1335 (RStudio Team, 2015). To assess sampling efforts in the forest sites, liana species richness was rarefied and extrapolated using a technique that combines rarefaction and extrapolation analyses to estimate species richness based on a standardised number of individuals (Colwell et al., 2012; Chao et al., 2014). Furthermore, we generated coverage-based and sample completeness rarefaction/extrapolation curves as a way to determine sampling effort (sample coverage) and completeness (function of sample completeness with sample size), respectively in the forest sites (Hsieh et al., 2016). Rarefaction and extrapolated curves were generated with the iNEXT package in R (Hsieh et al., 2016). We performed non-metric multidimensional scaling (NMDS) analysis to visualise liana species composition patterns in the forest sites, and used nested permutational multivariate analysis of variance (PERMANOVA) (sampling area nested within forest site) to test for differences in liana species composition among the forest sites and sampling areas. Moreover, analysis of homogeneity of multivariate dispersion (PERMDISP) was run to test for the significance of differences in dispersion among the groups. These analyses which were based on Bray-Curtis dissimilarity, were conducted using metaMDS (NMDS), adonis (PERMANOVA) and betadisper (PERMDISP) functions in the vegan package of R. A three-dimensional solution was used in the NMDS analysis as it provided a lower stress and a better interpretability of species composition in each forest. The ordihull function in the vegan package was used to draw convex hulls around the points of each forest site in the NMDS. The aov function in the stats package was employed to conduct nested ANOVA (sampling area nested within forest site) in order to test for differences in mean liana species richness, abundance and basal area among the forest sites and sampling areas. Therefore, with regards to species richness, statistical comparison was made at both the plot and forest site levels with nested ANOVA and permutation test, respectively. A chi-square test was used to compare species abundance among the forest sites using chisq.test function in the stats package. Magnitude of edge influence on total liana abundance (total and individual species) was computed using the formula of Harper et al. (2005, 2015):

by tourists and management of Ankasa Conservation Area (Wildlife Division, 2000). It is maintained periodically at a width of approximately 7 m. The edge of Ankasa Conservation Area is characterised by large, tall mature trees, giving the edge vegetation an appearance similar to that of the interior and deep-interior sites. The Cape Three Points Forest Reserve is the only coastal forest remaining along the coast of West Africa. It covers an area of 51.12 km2 with a mean annual rainfall of 1400–2000 mm (Ntiamoah-Baidu et al., 2001; Poorter et al., 2004; Bird Life International, 2015). The forest also records high relative humidity that ranges from 70 to 90%. Mean monthly temperatures of the area is in the range of 21–32 °C. The reserve is rich in globally rare species including an endangered canopy tree species, Synsepalum ntimii (Sapotaceae) (Hawthorne, 2014). In Cape Three Points Forest Reserve, the edge site occurs along a 5 m width road that is used by both farmers and private rubber plantation workers. Local sources suggest the road could date back to the establishment of the reserve. This road receives regular maintenance from the users. Unlike Ankasa Conservation Area, the forest edge of Cape Three Points Forest Reserve is characterised by relatively smaller, young growing trees. 2.2. Sampling Three forest sites were identified for sampling: edge, interior, deepinterior sites. The edge site was defined by a distance of 0–50 m from the forest edge whereas the interior and deep-interior sites were defined by distances of 200 m and 400 m from the forest edge, respectively. The penetration distance of edge into forest interior differs. Whereas many studies have shown that edge effects can be detected up to 100 m (Gascon et al., 2000; Santo et al., 2008; Thier and Wesenberg, 2016), others revealed that edge effects can penetrate up to 300 m (Gascon et al., 2000; Flaspohler et al., 2001; Liu and Taylor, 2002; Laurance et al., 2018). Based on this, we set the interior parts of the forest at 100 m beyond the above limits (i.e., 200 and 400 m from forest edge). There are still a few studies that detected edge effects on forest canopies (Briant et al., 2010) and invertebrate communities (Ewers and Didham, 2008) at least 1 km from the forest edge. However, the edges in those studies were surrounded by different and wider vegetation types. Since the edge in our study is characterised by narrow roads, with the same vegetation type on each side of the roads, we did not expect edge effects to penetrate that deep into the forest. In each site, two independent sampling areas were identified and sampled. Four 50 m × 50 m sampling plots were randomly laid out in each sampling area of a forest site, with a minimum plot distance of 100 m. Consequently, a total of eight plots were inventoried in each forest site, thus making a total of 24 plots in each forest reserve. Liana individuals with a diameter of ≥1 cm measured at 1.3 m from rooting point were mostly identified to the species level (with a few to the genus level) and counted. Supporting trees (diameter at breast height ≥ 5 cm) used by lianas were also identified and recorded. Field identification was done through field guides (Hawthorne, 1990; Hawthorne and Jongkind, 2006) and the services of a plant taxonomist. Voucher specimens were collected, pressed and depoaread in the KNUST herbarium located at the Department of Theoretical and Applied Biology, KNUST, Kumasi, Ghana.

MEI =

e − i e + i

where e = value of the parameter at the edge site, i = value of the parameter at nonedged site (interior and deep-interior sites). The nonedge value (i.e., i) was obtained as the average of liana abundance in the interior and deep-interior sites. MEI was also quantified for individual species with total abundance ≥10 individuals (across the three sites). MEI ranges from −1 to +1, where negative values indicate negative edge influence, positive values represent positive edge influence, and zero indicates no edge influence. MEI and correlation coefficient have the same strength range (i.e., −1 to +1), so we adapted and slightly modified the guideline on correlation coefficient strength provided by Evans (1996) to describe the MEI strength on liana abundance in our study: 0 (no edge influence), ≤0.19 (very weak), 0.20–0.39 (weak), 0.40–0.59 (moderate), 0.60–0.79 (strong), 0.80–1.0 (very strong). Patterns of liana species co-occurrence and nestedness were determined using the cooc_null_model function from EcoSimR package (Gotelli et al., 2015) and oecosimu function in vegan package (Oksanen et al., 2015), respectively using RStudio. Liana-tree species matrix made up of rows (liana species) and columns (tree species) was developed for each forest site and used for the analyses. These analyses were also run

2.3. Data analysis Plant diversity was quantified and compared among the forest sites using (1) species richness (observed and extrapolated), and (2) Shannon diversity index which incorporates both species richness and evenness to provide more information about a biological community (Kwak and Peterson, 2007). These indices are based on the equations described in Magurran (1988). Preliminary analyses revealed that there was no significant difference in liana species diversity among the sampling areas. Therefore, data from two sampling areas in each forest site were 4

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separately for each sampling area in each forest site, but it was realized that the patterns of species co-occurrence and nestedness in all the sampling areas were similar to those obtained for the forest sites when their sampling area data were pooled. Consequently, only the results of the forest sites are reported in this study. The presence of a liana species on a tree species was denoted by 1 whereas 0 was used to indicate the absence of liana on a particular tree species. Liana species co-occurrence was quantified using the C-score metric, which is the average number of checkerboards for two species i and j (Stone and Roberts, 1990). The metric was calculated using the following equation (Almeida-Neto and Ulrich, 2011):

Table 1 Comparison of liana community characteristics among the forest sites in two evergreen rainforests of Ghana (i.e. Ankasa Conservation Area and Cape Three Points Forest Reserve) as determined by permutation tests (diversity indices) and nested ANOVA (liana species richness at plot level, abundance and basal area). Abundance and basal area values are means ( ± SE). For each forest, values with different letters in the same column are significantly different from each other (P < 0.05). Liana characteristic

Shannon diversity index in the sites Edge 3.47a Interior 3.36a Deep-interior 3.46a Observed species richness in the sites Edge 56 a Interior 51a Deep-interior 54a Extrapolated species richness in the sites Edge 80 Interior 64 Deep-interior 68 Mean species richness/plot Edge 16a ± 1.93 Interior 17a ± 1.22 Deep-interior 18a ± 0.95 Abundance Edge 40a ± 4.97 Interior 41a ± 4.08 Deep-interior 43a ± 3.84 Basal area Edge 0.26a ± 0.02 Interior 0.28a ± 0.04 Deep-interior 0.31a ± 0.03

∑ (Ni − Nij )(Nj − Nij) C − score =

i, j

S (S − 1)

where Ni and Nj are the rows totals (number of occurrences) of species i and j, Nij is the number of co-occurrences of both species, and S is the number of species in the matrix. NODF metric based on the following equation (see Almeida-Neto et al., 2008) was calculated to estimate liana nestedness in each the sites in the two forests.

∑ Npaired

NODF = ⎡ ⎣

n (n − 1) 2

⎤+⎡ ⎦ ⎣

m (m − 1) 2

Ankasa Conservation Area

⎤ ⎦

where Npaired is paired nestedness values of columns and rows, and n and m are the number of columns and rows, respectively. Observed NODF values were statistically compared with those obtained from 10,000 randomisations of quasiswap algorithm in order to determine nestedness patterns.

Cape Three Points Forest Reserve

3.08a 2.69b 2.97a 41a 33b 35ab 45 35 36 18a ± 0.82 12b ± 0.65 12b ± 0.87 46a ± 2.56 49a ± 6.49 41a ± 2.33 0.13a ± 0.01 0.18a ± 0.01 0.15a ± 0.03

3. Results based rarefaction showed high sample coverage (Ankasa: - observed species coverage: 92.1–94.5%, extrapolated species coverage: 92.7–96.8%; Cape Three Points - observed species coverage: 97.6–99.1%, extrapolated species coverage: 99.5–100%) (Fig. 4a and b). Likewise, there was high sample completeness in the three forest sites indicated by the observed sample completeness curves (Fig. 5a and b). By increasing the number of liana individuals, sample completeness only increased marginally in the three forest sites as shown by the extrapolated sample completeness curves. In each of the evergreen rainforests, the NMDS ordination of the liana communities did not reveal distinct separation among the three forest sites based on species composition of the plots (Ankasa Conservation Area: Fig. 6, stress = 0.14; Cape Three Points Forest Reserve: Fig. 7, stress = 0.15). The convex hulls associated with the different forest sites in Ankasa Conservation Area overlapped and showed that the interior site occupied more ordination space than the deepinterior site, which in turn, occupied more ordination space than the edge site. In Cape Three Points Forest Reserve, the convex hulls overlapped, with the interior and deep-interior sites occupying more ordination space than the edge site. This trend was confirmed by the PERMANOVA results which indicated that there were no significant differences in liana species composition among the forest sites (Ankasa Conservation Area: F.Model = 1.20, R2 = 0.11, P = 0.19; Cape Three Points Forest Reserve: F.Model = 0.84, R2 = 0.08, P = 0.73). Equally, liana species composition was similar among the sampling areas in each of the forest (Ankasa Conservation Area: F.Model = 0.72, R2 = 0.10, P = 0.95; Cape Three Points Forest Reserve: F.Model = 0.65, R2 = 0.09, P = 0.96). There were no significant differences in dispersion among the forest sites in Ankasa Conservation Area (PERMDISP: F2, 21 = 1.20, P = 0.32) and Cape Three Points Forest Reserve (PERMDISP: F2, 21 = 2.33, P = 0.12).

3.1. Liana community structure 3.1.1. Liana species diversity and composition A total of 109 liana species were identified in the two evergreen rainforests (Appendix 1). Mean liana species richness per plot was similar among the three forest sites (Table 1; Nested ANOVA, n = 24, F2, 18 = 0.51, P = 0.61) as well as the sampling areas (sampling area nested within site: P = 0.69) in Ankasa Conservation Area (Table 1). When diversity was compared at forest site level, there was no significant differences in the various diversity indices among the forest sites in Ankasa Conservation Area. In Cape Three Points Forest Reserve, mean liana species richness per plot differed significantly among the forest sites (Nested ANOVA, n = 24, F2, 18 = 46.5, P < 0.01). Specifically, the edge site supported higher liana species richness than the interior and deep-interior sites (P < 0.01), although there was no difference between the interior and deep-interior sites (P = 0.85). Conversely, sampling areas in the forest sites (i.e., sampling area nested within forest site) supported similar liana species richness (P = 0.95). At the forest site level, species richness observed at edge site was significantly higher than the interior site (P = 0.01), but there were no differences among the other forest site pairs. Also, Shannon diversity index was significantly higher in the edge site than the interior site (P < 0.01), although the value at the edge site did not differ from that in the deep-interior site. In the two evergreen rainforests, individualbased rarefaction-extrapolation curves indicated that liana species were reasonably well sampled in the forest sites, although the sampling was not complete to capture all species (Fig. 3a and b). The curves in Cape Three Points Forest Reserve were closer to reaching asymptote than those in Ankasa Conservation Area. The extrapolated portion of the curves estimated higher liana species richness when the reference sample size was doubled for each forest (Table 1). The analysis extrapolated higher numbers of liana species in the edge site than the interior and deep-interior sites of the two evergreen rainforests. The majority of undetected species are most likely rare since the coverage-

3.1.2. Liana abundance and basal area A total of 1,953 liana individuals were recorded in the two 5

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Fig. 3. Sample-based rarefaction (solid lines) and extrapolation (dotted lines) curves (with 95% confidence intervals: shaded areas) of liana species richness in Ankasa Conservation area (a) and Cape Three Points Forest Reserve (b). The symbols represent the reference samples of the forest sites.

Fig. 4. Coverage-based rarefaction (solid lines) and extrapolation (dotted lines) curves (with 95% confidence intervals: shaded areas) for liana species richness in Ankasa Conservation area (a) and Cape Three Points Forest Reserve (b). The symbols represent the reference samples of the forest sites.

very weak positive MEI on total liana abundance was recorded in the edge site of Cape Three Points Forest Reserve (MEI = 0.01). MEI on individual liana species abundance varied considerably among the species in the two evergreen forests, ranging from −0.81 to 1 in Ankasa Conervation Area and −1 to 0.68 in Cape Three Points Forest Reserve (Appendix 1; Figs. 8 and 9). In Ankasa Conservation Area, Combretum smeathmannii and Dichapetalum toxicarium had very strong negative MEI on their abundance. The MEI values for Tiliacora dielsiana abundance was very strong and positive. The abundance of Salacighia letestuana and Dichapetalum oblongum experienced strong positive MEI. The MEI values for the abundance of Manniophyton fulvum and Acacia pentagona was moderate and negative. Likewise, O. nitida and Grewia malacocarpa had moderate but negative MEI on their abundance. The MEI for the rest of the species was either weak or very weak. On the part of Cape Three Points Forest Reserve, only Leptoderris fasciculata recorded very strong MEI on its abundance. Six of the species namely, Strychnos asterantha, Strychnos usambarensis and Tristemonanthus nigrisilvae on one hand, and G. malacocarpa, Griffonia simplicifolia and C. adenocaulis on the other hand showed strong negative and positive MEI, respectively.

evergreen rainforests (Appendix 1; Ankasa Conservation Area: 908 individuals and Cape Three Points Forest Reserve: 1,045 individuals). Liana abundance did not differ significantly among the three forest sites in Ankasa Conservation Area (Table 1; Nested ANOVA, n = 24, F2, 18 = 0.17, P = 0.84) and Cape Three Points Forest Reserve (n = 24, F2, 18 = 1.11, P = 0.35). Correspondingly, all the three sites in the two forests supported similar liana basal area (Nested ANOVA, n = 24, F3, 18 = 2.91, P = 0.08 for Ankasa Conservation Area; n = 24, F3, 18 = 1.83, P = 0.19 for Cape Three Points Forest Reserve). Generally, there was no significant effect of sampling area (sampling area nested within forest site) on the two liana structural attributes (P > 0.05) in both forests. Chi-square tests revealed that population abundance of some liana species, for example, Agelaea pentagyna, Landolphia owariensis, Oncinotis nitida (Ankasa Conservation Area) and Calycobolus africanus, Cissus adenocaulis, Combretum sordidum (Cape Three Points Forest Reserve) differed significantly among the forest sites (Appendix 1; P < 0.05). There was a very weak negative MEI on total liana abundance in the edge site of Ankasa Conservation Area (MEI = −0.02). However, a 6

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Fig. 5. Sample completeness curves for rarefied (solid lines) and extrapolated (dotted lines) samples (with 95% confidence intervals: shaded areas) that link the curves in Figs. 3 and 4. The symbols represent the reference samples of the forest sites.

Three Points Forest Reserve was 63, 61 and 62% in the edge, interior and deep-interior sites, respectively. Liana species co-occurrence recorded in the forest sites of Ankasa Conservation Area and Cape Three Points Forest Reserve followed a similar pattern (Table 2). The observed C-score values in the edge, interior and deep-interior sites of the two evergreen rainforests were consistent with their corresponding simulated C-score values (P > 0.05). Likewise, In Ankasa Conservation Area, the observed NODF value recorded in each of the forest sites was not significantly different from that expected by chance (Table 3; P > 0.05). In the same vein, there were no significant differences in the observed and expected values in the three forest sites in Cape Three Points Forest Reserve (P > 0.05).

Leptoderris sassandrensis, S. letestuana, Leptoderris miegei and C. sordidum experienced moderate MEI on their abundance. There was weak MEI on the abundance of the remaining liana species in the forest. Two species each in Ankasa Conservation Area (Cissus silvestris, Asclepia sp.) and Cape Three Points Forest Reserve (Artabotrys oliganthus, M. fulvum) did not experience edge influence on their abundance. 3.2. Liana-tree interactions 3.2.1. Liana species co-occurrence and nestedness Liana-host networks in the three forest sites within Ankasa Conservation Area were 32 liana and 36 tree species (edge site), 20 liana and 24 tree species (interior site), and 32 liana and 36 tree species (deep-interior site). In this forest, 59.4, 58.4 and 56% of trees supported lianas in the edge, interior and deep-interior sites, respectively. In Cape Three Points Forest Reserve, liana-host networks were made up of 40 liana and 33 tree species in the edge site, 32 liana and 30 tree species in the interior site, and 31 liana and 19 tree species in the deep-interior site. Percentage of trees that hosted lianas in the three sites of Cape

4. Discussion 4.1. Liana community structure The findings of the study revealed that edge distance did not have considerable effects on liana species diversity and composition in

Fig. 6. Non-metric multidimensional scaling (NMDS) ordination of species composition of liana communities showing convex hull for each forest site in Ankasa Conservation Area, Ghana. 7

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Fig. 7. Non-metric multidimensional scaling (NMDS) ordination of species composition of liana communities showing convex hull for each forest site in Cape Three Points Forest Reserve, Ghana.

road edges could reduce microclimate differences between edge and interior parts of the forests, making edge effects on liana species diversity and composition negligible. It must be noted that absence of edge effects in the forest presently does not necessarily mean it was not present in the past. As a dynamic phenomenon, edge effects can diminish over time (Harper and Macdonald, 2002; Larrivée et al., 2008). Unlike Anksasa Conservation Area, edge effects on liana species diversity were evident in Cape Three Points Forest Reserve as the edge supported higher liana diversity. A number of edge related variables such as edge width and age, microclimate, and tree density and

Ankasa Conservation Area. This observation is supported by the work of Addo-Fordjour and Owusu-Boadi (2017) which also examined the effects of linear edge on liana community assemblages in an upland evergreen forest and found no edge-related changes in liana species diversity. The absence of edge effects on liana species diversity and composition may be due to low vulnerability of the narrow forest edges to edge-related wind disturbance and desiccation stress (Laurance et al., 2009; Eldegard et al., 2015). Arroyo-Rodríguez et al. (2017) showed that changes in air temperatures from forest edges bordered by roads to forest interior was not significant. Therefore, the narrow nature of the

Fig. 8. Magnitude of edge influence (MEI) on liana species abundance in Ankasa Conservation Area. Each species name has been abbreviated to the first letter and first four letters of the generic name and specific epithet, respectively (See Appendix 1 for full names). Species with no MEI (i.e. MEI = 0) are not shown in the figure. 8

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Fig. 9. Magnitude of edge influence (MEI) on liana species abundance in Cape Three Points Forest Reserve. Each species name has been abbreviated to the first letter and first four letters of the generic name and specific epithet, respectively (See Appendix 1 for full names). Species with no MEI (i.e. MEI = 0) are not shown in the figure.

mortality can influence species diversity at edge sites (Wekesa et al., 2019). Though these features were not measured in the current study, they could contribute to the above mentioned trend. For instance, the edge site of Cape Three Points Forest Reserve is characterised by much more active roads compared with Ankasa Conservation Area. This feature could provide more exposure of forest edges to microclimatic changes which may influence liana species diversity. Previous studies revealed that forest edges support elevated liana abundance in relation to forest interiors (Laurance et al., 2001; Magrach et al., 2014; Jones et al., 2017; Campbell et al., 2018). However, edge effect on liana abundance was not apparent in the two evergreen rainforests, since the three forest sites supported comparable liana numbers. The presence of low MEI on liana abundance in the evergreen rainforests gives further credence to the above mentioned trend. Similarly, forest edge within the evergreen rainforests did not enhance liana basal area. The difference in the response of liana abundance to edge between the current and previous studies could be related to differences in forest edge size. Whereas the current study examined lianas along narrow road edges (width of 5 and 7 m), the previous studies were conducted in wider fragmented edges isolated from surrounding forests (by distances up to 1000 m) due to pastures and agricultural land use matrix. Since narrow linear edges (< 20 m width) such as road edges are less vulnerable to edge-related wind disturbance and desiccation stress than wider clearings (Laurance et al., 2009; Eldegard et al., 2015), it is possible that forest edges in the current study were less effective in influencing liana communities compared to fragmented edges (with larger width) in previous studies. In evergreen rainforests, evergreen trees with large crown and leaves often reduce sunlight penetration to the forest floor (Bonan, 2008). This feature together with high, frequent rainfall and associated high humidity could offer moist conditions at the edges of the two evergreen

Table 2 Co-occurrence patterns of liana species in the forest sites of two evergreen rainforests in Ghana (i.e. Ankasa Conservation Area and Cape Three Points Forest Reserve) as determined by C-score statistic in EcoSimR package in R programming. Forest reserve

Observed C-score

Ankasa Conservation Area Edge 4.83 Interior 6.89 Deep-interior 6.25 Cape Three Points Edge 10.04 Interior 9.07 Deep-interior 5.83

Simulated C-score

Z-value

P-value

4.81 6.76 6.32

0.37 0.87 −0.76

0.36 0.20 0.26

10.16 9.39 6.02

−0.64 −1.47 −1.19

0.29 0.06 0.10

Table 3 Summary of nestedness analysis of liana species in the forest sites in two evergreen rainforests in Ghana (i.e. Anakasa Conservation Area and Cape Three Points Forest Reserve) as determined by NODF statistic in vegan package in R programming. Forest reserve

Observed NODF metric

Ankasa Conservation Area Edge 5.19 Interior 13.05 Deep-interior 5.69 Cape Three Points Edge 13.45 Interior 16.32 Deep-interior 18.74

Mean simulated NODF metric

Z-value

P-value

6.18 12.90 6.66

−1.66 0.14 −1.50

0.10 0.91 0.14

13.88 16.76 19.10

−0.56 −0.44 −0.33

0.57 0.66 0.73

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distribution (with no nested structure) along their host trees, suggesting that the structure of interaction between lianas and trees was robust to edge effects. Although edge effects on lianas at the liana community level was weak, some species had increased abundance, compensating the loss of individuals at the edge. Overall, our findings show that the idiosyncratic edge effects on liana species populations can blur the effects on liana communities. Our findings largely deviate from previous studies that showed that liana assemblages increase at forest edges (Laurance et al., 2001; Magrach et al., 2014; Jones et al., 2017; Campbell et al., 2018). The difference between our study and the earlier ones may be related to differences in the characteristics of the edges studied. Narrow edges bordered by similar vegetation types appear to have weaker effects on liana communities than wide open edges surrounded by different vegetation types. Thus, the current study provides new findings that have important implications for the development of a robust edge theory that goes beyond the fact that edge disturbance increases liana assemblages.

rainforests. These factors can further weaken edge effects on liana abundance and basal area (Addo-Fordjour and Owusu-Boadi, 2017). With respect to MEI on individual species abundance, different liana species responded differently to edge in the two rainforests. The abundance of some species (e.g., L. miegei, G. malacocarpa, C. sordidum) decreased in the interior and deep-interior sites in relation to the edge site, while other species (e.g., T. dielsiana, S. letestuana, T. nigrisilvae) had increased abundance in the interior and deep-interior sites compared to the edge site. Generally, MEI on liana species abundance showed high variability among the species, indicating that liana species exhibited a wide-range of responses to edge in the two evergreen rainforests. It appears that the forest edge environment was more favourable to some liana species than others since there was strong positive edge influence on the abundance of some liana species but other species experienced strong negative edge influence on their abundance. Though at the community level, edge may favour overall liana abundance increase as reported in some previous studies (Laurance et al., 2001; Magrach et al., 2014), the findings of the current study suggest that edge creation may not be favourable to some individual liana species. This calls for the inclusion of individual liana species in the assessment of edge effects on liana communities so as to get a better understanding of liana response to forest edge.

Author 1: BISMARK OFOSU-BAMFO I confirm that I am an author of the above mentioned manuscript, which is currently being submitted to Acta Oecologica. I have given final approval of the version being submitted to the journal. The following is my authorship contribution:

4.2. Liana-tree interactions There was no evidence of nested structure of lianas among host trees in all the forest sites of the two evergreen rainforests. The existence of non-nested structure of the liana communities suggests that perhaps liana species showed differential response to major gradients such as disturbance, resource availability etc., that are associated with species richness (see Elmendorf and Harrison, 2009). Nielsen and Bascompte (2007) provided evidence of strong influence of species richness and number of interactions on network structure. In the current study, liana species richness was similar among virtually all the sites, and therefore could be responsible for the presence of similar network structure within the three sites in the two forests. In all the forest sites liana species exhibited random co-occurrence, a pattern suggesting the absence of host-specific interactions in the forest sites. Studies have shown that disturbance can cause changes in resource distribution in the environment (Pickett and White, 1985; Luza et al., 2016), thereby altering species distribution from random to non-random (Luza et al., 2016). Thus, in the absence of disturbance plant assemblages may likely show random species co-occurrence (see García-Baquero and Crujeiras, 2015). This may explain why lianas showed random co-occurrence on host trees in the interior and deep-interior sites of the two evergreen rainforests. Campbell et al. (2018) showed that edge disturbance due to agricultural land-use alters liana-tree host interactions. Nonetheless, in the current study, lianas were still randomly distributed on host trees in the edge sites just as in the forest interiors, suggesting lack of edge effects on liana-tree interaction. Comparatively, road edge sites in the present study are less open in relation to agricultural land use edge and this may account for the apparent lack of edge effects on liana-tree interactions in the edge site. The above-mentioned pattern of liana species co-occurrence recorded in the forest sites reveal little or no competition among liana species in the two evergreen rainforests.

Contribution

Yes/No

Details

Conceived the idea Designed the study Collected the data Contributed analysis tool Performed analysis Wrote the paper Supervision Other contribution Revision of manuscript

No Yes Yes No Yes Yes No Yes Yes

Non-applicable Participated Fully collected the data Non-applicable Participated I wrote the Methodology Non-applicable Reviewed the manuscript Participated partially

Author 2: PATRICK ADDO-FORDOUR I confirm that I am the corresponding author of the above mentioned manuscript, which is currently being submitted to Acta Oecologica. I have given final approval of the version being submitted to the journal. The following is my authorship contribution:

5. Conclusion There was no evidence of edge effects on liana species diversity in Ankasa Conservation Area, though an opposite trend was recorded in Cape Three Points Forest Reserve. Furthermore, forest edge did not influence liana species composition or enhance liana structural attributes (abundance, basal area) in the two evergreen rainforests in Ghana. However, there was enough evidence indicating variable edge effects on the abundance of some individual liana species. Our findings showed that regardless of forest site, lianas exhibited random

Contribution

Yes/ No

Details

Conceived the idea Designed the study Collected the data Contributed analysis tool Performed analysis Wrote the paper

Yes Yes No Yes

I conceived the research idea Participated Non-applicable I provided some of the tools for statistical analysis

Yes Yes

Supervision Other contribution Revision of manuscript

Yes Yes Yes

Participated I wrote the Results and Discussion sections of the manuscript I was one of the supervisors of the project I reviewed and revised the draft manuscript Participated fully

Author 3: EBENEZER J.D. BELFORD I confirm that I am an author of the above mentioned revised manuscript, which is currently being submitted to Acta Oecologica. I have given final approval of the version being submitted to the journal. The following is my authorship contribution: 10

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Contribution

Yes/No

Details

Conceived the idea Designed the study Collected the data Contributed analysis tool Performed analysis Wrote the paper Supervision Other contribution Revision of manuscript

No Yes No Yes

Non-applicable Participated Non-applicable I provided some of the tools for statistical analysis Participated I wrote the Introduction I was the other supervisors of the project I reviewed the draft manuscript Non-applicable

Yes Yes Yes Yes No

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