Bismark Ofosu-Bamfo1, Patrick Addo-Fordjour2*, E.J.D. Belford2
1Department of Basic and Applied Biology, School of Sciences, University of Energy and Natural Resources, Sunyani, Ghana
2Department of Theoretical and Applied Biology, Faculty of Biosciences, College of Science, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana
*Corresponding author: paddykay77@yahoo.com; paddofordjour.cos@knust.edu.gh
ABSTRACT
Edge disturbance can drive liana community changes and alter liana-tree interaction networks, with ramifications for forest functioning. Understanding edge effects on liana community structure and liana-tree interactions is therefore essential for forest management and conservation. We evaluated the response patterns of liana community structure and liana-tree interaction structure to forest edge in two moist semi-deciduous forests in Ghana (Asenanyo and Suhuma Forest Reserves: AFR and SFR, respectively). Liana community structure and liana-tree interactions were assessed in 24 50 × 50 m randomly located plots in three forest sites (edge, interior and deep-interior) established at 0–50 m, 200 m and 400 m from edge. Edge effects positively and negatively influenced liana diversity in forest edges of AFR and SFR, respectively. There was a positive influence of edge disturbance on liana abundance in both forests. We observed anti-nested structure in all the liana-tree networks in AFR, while no nestedness was observed in the networks in SFR. The networks in both forests were less connected, and thus more modular and specialised than their null models. Many liana and tree species were specialised, with specialisation tending to be symmetrical. The plant species played different roles in relation to modularity. Most of the species acted as peripherals (specialists), with only a few species having structural importance to the networks. The latter species group consisted of connectors (generalists) and hubs (highly connected generalists). Some of the species showed consistency in their roles across the sites, while the roles of other species changed. Generally, liana species co-occurred randomly on tree species in all the forest sites, except edge site in AFR where lianas showed positive co-occurrence. Our findings deepen our understanding of the response of liana communities and liana-tree interactions to forest edge disturbance, which are useful for managing forest edge..
Keywords: co-occurrence patterns, ecological networks, edge influence, liana diversity and abundance, modularity, nestedness, specialisation
INTRODUCTION
In tropical forests, particularly in developing economies, human interactions with forest ecosystems remain an important source of change in their structure (FAO & UNEP, 2011) and functioning (Pedro et al., 2015). A common outcome of human interface with forest ecosystems is fragmentation which results in the creation of edges (Harper et al., 2005). Edge mediated microclimatic changes may favour disturbance-adapted, light-loving species such as lianas (see Hawthorne, 1996; Laurance et al., 2001), but generally be disadvantageous to others such as trees (Laurance et al., 2006). Previous studies reported that edge effects enhanced liana diversity and abundance in some forests (Laurance et al., 2001; Campbell et al., 2018; Ofosu-Bamfo et al., 2019), but others did not detect changes in liana diversity in response to edge (Mohandass et al., 2014; Ofofu-Bamfo et al., 2019). Several properties of forest edge such as edge size, edge type, and surrounding matrix type can mediate edge effects on plant community structure (Martino, 2015), and be responsible for the varied responses of community structure to edges in different forests. As liana community assemblages respond to edge disturbance, the relationship between lianas and trees may also be altered. Fagan et al. (1999) stated that habitat edges can modify species interactions. Similarly, Porensky (2011) reported that species interactions show strong responses to forest edge. Nonetheless, there is scarcity of information on the response of liana-tree interaction network patterns to forest edge.
The knowledge of liana-tree interactions and the factors that shape them are key to fully understanding plant community composition and structure. Although different patterns of liana-tree interactions have been reported in literature, there is no consensus yet. For example, nestedness, a network pattern in which the interactions of less connected species form proper subsets of the interactions of more connected species (Bascompte et al., 2003; Landi et al., 2018; Ponisio, et al. 2019), has been used to characterise the structure of liana-tree networks. Different patterns of nestedness are reported in literature including nested (Sfair et al., 2010) and non-nested (Addo-Fordjour & Afram, 2021; Addo-Fordjour et al., 2016; Blick & Burns, 2009; Magrach et al., 2015; Ofosu-Bamfo et al., 2019) structures. Among the studies that did not find nested structure in liana-tree netowrks, some reported anti-nested structure which depicts non-random assembly (Addo-Fordjour & Afram, 2021; Blick & Burns, 2009; Magrach et al., 2015), whiles others observed non-significant nestedness that shows random assembly (Addo-Fordjour et al., 2016; Ofosu-Bamfo et al., 2019). Ecological networks can also be compartmentalised into modules whose members interact more among themselves (Carstensen et al., 2016). This phenomenon referred to as modularity, is predicted to stabilise ecological networks (Massol et al., 2017; Thébault & Fontaine, 2010). For this reason, modularity analysis of liana-tree networks can have implications for forest management and conservation. Nonetheless, only a few studies assessed the patterns of modularity in liana-tree networks. Sfair et al. (2015) did not find modular structure in their networks, but Addo-Fordjour & Afram (2021) recorded significant modular structure in liana-tree networks.
Specialisation at the network and species levels can cause non-nested and modular organisation of species (Addo-Fordjour & Afram, 2021; Médoc et al., 2017; Castledine et al., 2020). Thus, in liana-tree networks in which coevolution leads to specialisation (Sfair et al., 2015), the networks may tend to be non-nested and/or modular. Another important metric used to characterise network structure is species co-occurrence, which describes the frequency of pairs of liana species to co-occur on the same phorophyte species (Zulqarnain et al., 2016). Like the above-mentioned network metrics, mixed patterns of liana species co-occurrence have been reported in literature, which include positive co-occurrence (Addo-Fordjour et al., 2016; Zulqarnain et al., 2016), negative co-occurrence (Blick & Burns, 2011, 2009), and random co-occurrence (Addo-Fordjour et al., 2016). With the mixed findings on the structure of liana-tree interactions in literature, there is the need for more studies to be conducted to determine the most consistent patterns. Knowledge of co-occurrence patterns is important for increasing our understanding of species interactions and predicting community stability and maintenance, and ecosystem functioning, all of which may be useful in forest conservation (Vizentin-Bugoni et al., 2016).
The current study determined the response of liana community assemblages and the patterns of structure of liana-tree interaction networks to edge in two moist semi-deciduous forests in Ghana. The forest edges we studied were surrounded by large matrices of crop farmlands, thus making the edges much exposed. The nature and size of land matrix bordering forest edges play a key role in determining the intensity of edge effects on plant community structure (Aragόn et al., 2015). To this end, edges bordered by wide land matrices are expected to exert stronger effects on plant communities than edges surrounded by narrow area of land (Addo-Fordjour & Owusu-Boadi, 2016). Furthermore, because the nature of the land matrix surrounding our forest edges is physiognomically dissimilar to the forest vegetation, the microclimate variation between the forest edge and interior may be enhanced (Aragόn et al., 2015). Thus, edge effects on lianas and liana-tree interaction patterns in the two moist semi-deciduous forests may be apparent. Edge disturbance permits greater penetration of sunlight into forest edges, and also increases forest edge dryness (Thier & Wesenberg, 2016), both of which can favour liana proliferation. On the basis of the above, we tested the following hypotheses:
  1. Liana diversity and abundance would be higher in edge site than non-edge sites.
  2. We expected that as edge disturbance enhances liana abundance at the forest edge, network connectance will increase, resulting in less specialised, nested, and non-modular network structures in edge site, while the networks in the non-edge sites will be less connected, more specialised, non-nested, and modular.
  3. Edge effects will cause shifts in topological roles of liana and tree species due to changes in the distribution and abundance of the species.
  4. As sunlight and dry conditions are elevated at edge sites relative to the non-edge sites, competition of lianas for the resources in edge site may be lower. Moreover, as edge effects tend to cause tree mortality at forest edges (Murcia 1995), the number of available host species may reduce, increasing liana infestation per host. Thus, we expected that liana species in edge sites would show positive co-occurrence on host trees, while the species in non-edge sites will randomly co-occur on their hosts.
The findings of our study would be useful in the management of forest edges and conservation of edge species. Our study seeks to add valuable information to literature, thus helping to obtain general patterns of liana assemblages and structure of liana-tree interactions in relation to edge effects. The findings would also contribute to the development of a robust edge theory in view of the fact that there is dearth of information on the role of edge disturbance in shaping the patterns of liana-tree network structure in forests.
METHODOLOGY