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:
- Liana diversity and abundance would be higher in edge site than
non-edge sites.
- 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.
- Edge effects will cause shifts in topological roles of liana and tree
species due to changes in the distribution and abundance of the
species.
- 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