Diversity and composition of the epiphytic flora in an urban agglomeration: The case of city of Douala, Cameroon

Epiphytes constitute an element of climate regulation in the tropical zone. In Cameroon, the decline in forest area of about 220,000 ha, partly due to urbanization, has been observed. The research aim was to characterize the epiphytic flora of the urban ecosystem of Douala. Four zones represented by nine neighbourhoods were chosen for the surveys. Four transects of a maximum distance of 500 m were scanned on each neighbourhood. Epiphytes were checked by direct visual observation on the host trees. Epiphytes’s life forms were determined, and the fixation zone was established using the Johanson method. A total of 72 species of epiphytes were identified, dominated by Polypodiaceae. Casual epiphytes were the most abundant life-form, and true epiphytes were the least, divided into Polypodiaceae, Dryopteridaceae, and Orchidaceae within the genus Calyptrochillium . Twenty-two species of host trees were inventoried in the study area. Rutaceae was the most represented family with six species. True epiphytes were abundant in seaside and peripheral areas, while Casual and Hemiepiphytes were in the central sectors. The domestication of epiphytes must be initiated in order to preserve their diversity in urban ecosystem.


INTRODUCTION
Epiphytes are plants that attach themselves to and grow on other plants occurring from the forest understorey to the periphery of tree crowns (Benzing, 2020).These organisms face difficulties of obtaining adequate water, as well as mineral nutrients; and thus develop adaptive structures such as pseudo-bulbs or even rosette leaves for the accumulation of water and humus (Bola, 2002).They are the only living plants within a special ecological niche and contribute significantly, sometimes as much as 50%, to the biodiversity of tropical rainforests (Bola, 2002).The diversity of epiphytes is estimate at 28,000 species, representing about 10 to 20% of the diversity of vascular plants in the forests in the world, and approximately 30% for neo-tropical forests (Miranda et al., 2020).In tropical vegetation vascular epiphytes make up approximately 25% of vascular plant species and have important roles in maintaining forest ecosystem functions, such as nitrogen fixation, water and nutrient cycles (Nieder et al., 2001;Song et al., 2012;Stanton et al., 2014).They also regulate the biogeochemical cycle of various mineral elements by their retention during the dry season and their progressive dropping during the rainy season as well as their retention by trees (Noumi et al., 2010).They participate in the general improvement of the balance of mineral elements in the forests where they grow.Epiphytes therefore appears in the forest domain as the adaptive response of small-sized non-sciaphilous plants to the very pronounced photic deficit in the undergrowth.
Information on the diversity and abundance of epiphytes is only partially established in tropical regions because of the difficult accessibility due to their location in the canopy (Noumi et al., 2010).Indeed, African tropical forests, especially Biafreena forest, are considered as an important area of biodiversity, very few investigations have been carried out there (Noumi et al., 2010).The main ecological factor that governs the distribution of epiphytes in various ecosystems is the selective accessibility to light (Bola, 2002).This therefore suggests that this plant form would be the congruent response of small plants with high photic demand in the forest environment.However, in disturbed biotopes (substitute forests, fallows, orchards and urban agglomerations), this limiting factor being absent, another mode of ecological development of vascular epiphytes could be observed.Thus, one might think that the composition of the epiphytic flora; its pattern of colonization of trees as well as ecological conditions differ according to the environment (Jiagho et al., 2016).Urbanization is one of the major factors contributing to ecosystem degradation.Its consequences are global warming which includes changes in functioning of ecosystems, in particular due to increase in temperature and greenhouse gases and consequent rise in temperature of the atmosphere (GIEC, 2007).In metropolis of tropical region like Douala, urbanization have an effect on the vegetation and landscape (Nganmo and Priso, 2022).This then can affect the diversity of epiphytes, by the modification of some of their key life factors such as number of host plants or the level of humidity.Some research studies have been carried out on the epiphytic flora of a peri-urban forest (Kimpouni et al., 2017), but no study of epiphytes carried out in an agglomeration has been noted so far.The aim of this study, therefore, was to characterize epiphytic flora of the city of Douala, a metropolis of Central Africa's sub-region.

METHODOLOGY
Douala, the economic capital of Cameroon, is located in the Littoral region between 03° 40' -04° 11' N and 09° 16' -09° 52' E. It has six subdivisions of about 150 neighbourhoods (Priso et al., 2011).The landscape is on the whole characterized by its flatness with many swamps which make urbanization difficult.Altitude ranges between 0 in lowlands, to 13 m at the peaks.The climate is coastal equatorial type, with two annual seasons, the dry season did not exceed three months.Average temperatures vary little throughout the year, the maxima are reached in February (27.6°C) and the minima in July (24.8°C).Relative humidity and water vapour pressure are always very high (Din et al., 2002).Precipitation can reach 783 mm in August, 749 mm in July and 649 mm in September.The vegetation is dominated by mangroves in the low hydromorphic areas, and the plateaus offer the presence of degraded littoral forests, thickets or shrubby savannahs.
The study sites were distributed in the centre towards the periphery of the town according to four sectors: (1) Seaside (SS) which included the neighbourhood Bonanjo with administrative buildings bounded on the west by the Wouri river; (2) Industrial sector (IS) which neighbourhoods Espoir and Ndogpassi characterize by food and metallurgical industries; (3) Urban sector (US) included indigenous villages Ndogbong and Ndoghem characterized by popular areas; (4) Peri-urban sector (PU), which neighbourhoods Pk15, Pk16, and Pk17 that constitute urban extension areas characterize by deforestation and high level of construction of buildings (Figure 1).
In the centre of each neighbourhood surveyed, a 4-way crossroads was chosen, and transect with a length of 500 m was measured on each axis.Five plots of 250 m² (or 250 m²) were delimited along these transects using a double tape measure, spaced by a distance of 20 m.For each site, 20 plots were thus delimited, that is, an area of 5000 m² and a total area of 4 ha.Inside each plot, every tree with a minimal diameter of 10 cm was examined.The presence or absence of epiphytes was noted for each tree prospected, and the species were identified on site, using an identification key of reference manuals such as "Flore du Cameroun" and "Arbres des forêts denses d'Afrique Centrale" (Vivien and Faure, 2011).Unidentified specimens were taken to the Laboratory of Plant Biology of the University of Douala, or at the National Herbarium of Cameroon, for detailed analysis and taxonomic identification.The epiphytes life-forms were determined according to Addo-Fordjour et al. (2009): (1) True or strict epiphytes are species that normally spend their entire lifespan as epiphytes; (2) Hemi-epiphytes are species without contact with the ground at the beginning of their life but which they establish later (primary hemi-epiphyte) or which break contact with the ground at the end of their life (secondary hemi-epiphyte); (3) Casual epiphytes, which are normally terrestrial plants but which can grow on trees.
For the determination of the distribution of epiphytes on the host plant, each phorophyte was divided into four zones according to the Johansson method (Wang et al., 2016) (Figure 2), Trunk Zone (TZ), Inner Crown Zone (ICZ), Middle Crown Zone (MCZ), and Outer Crown Zone (OCZ).The TZ refers to the host trunk areas below the first branch; the ICZ covers the area from the first branch to the second branch; the MCZ covers the area from the second branch to the third branch; and the OCZ refers to the remaining areas above the third branch.

Diversity of vascular epiphytes
An inventory of epiphytic flora showed 72 species  belonging to 57 genera grouped into 32 families.The families with the highest number of species were Polypodiaceae (6 species), Asteraceae (5 species) and Cucurbitaceae (5 species).The life-forms were divided into 08 species (11.11%) of True epiphytes, 16 species (22.22%) of Hemi-epiphytes, and 48 species (66.67 %) of Casual epiphytes.True epiphytes belonged to two groups, Pteridophytes (7 species) and Angiosperms with the Orchidaceae (1 species).The specific richness of lifeforms in the studied sectors revealed a higher number of True epiphytes in US (6 species) and SS (5 species), Hemi-epiphytes were more abundant in PU (10 species) and US (9 species), while Casual epiphytes showed higher specific richness in PU (30 species) and IS (28 species) (Table 1).

Diversity of host trees
The inventory of phorophytes revealed 347 individuals belonging to 22 species, 17 genera and 13 families.Anacardiaceae were most represented with four species, followed by Rutaceae (3 species), Arecaceae (2 species) and Combretaceae (2 species).Apocynaceae and Burseraceae were the paucispecific families with only one species, respectively Voacanga africana and Dacryodes edulis.Elaeis guineensis, was phorophyte with most epiphyte species (55), followed by Cocos nucifera and Mangifera indica (27 species each).Albizia adianthifolia and Citrus maxima recorded the lower number of species (Figure 3).The presence of epiphytes in considerable abundance on E. guineensis and C. nucifera shows the importance of palm trees as host plants for the epiphytes.

Spatial pattern of vascular epiphytes
A total of 804 individuals of epiphytes was recorded in the studied area, the US showed the higher number (323 individuals), followed by SS (203 individuals), IS (179 individuals) and PUS (99 individuals.This epiphytic abundance was compared to the total number of trees of each sector in Figure 4.It reveals the absence of a real gradient between the number of trees and number of epiphytes.The breakdown occurs in IS which has a greater number of trees than AS, but a few numbers of epiphytes compare to the relative tree abundance. Figure 5 shows the composition in life-forms of studied sectors.In SS, True epiphytes were the dominate lifeform (75.37%), followed by Casual (16.26%), and Hemiepiphytes (8.37%); in IS, there was a similar proportion of Casual (50.84%) and True (49.16%);PUS recorded a proportion of True epiphytes of 50.51%,Casual (28.28%) and Hemi at 21.21%; in US, we recorded a proportion of 49.85% of Casual epiphytes, followed by True (38.08%), and Hemi epiphytes (12.07%).It seems the dominance of True epiphytes is in the peripheral sectors of the town.
The distribution of true epiphytes among the studied sectors (Table 1) revealed that Micrograma species and Platycerium stemaria were recorded in all the sectors, and were thus qualified as ubiquitous.Nephrolepis biserrata and Pyrrosia species were recorded in three sectors and were qualified as escorts; Syngonium podophyllum was encountered in IS and US, while Asplenium nidus, Calyptrochilium species and Pyrrosia mechowii have only been reported respectively in one sector.They were qualified as characteristic of the Urban sector for A. nidus, and of Sea side for both others.

Distribution of epiphytes life-forms among host trees
The analysis of the fixation zone of epiphytes life-forms from trunk to the tree top showed an ascending gradient (from the trunk to the top) for True epiphytes, and a decreasing gradient (for top to the trunk) for Casual.Upper canopy was colonized by True epiphytes with four species (80%), and Hemi epiphytes (1 species).Middle canopy recorded three life-forms, dominated by True epiphytes (5 species), followed by Hemi (2 species), and Casual epiphytes with one species.The inner canopy was also dominated by True epiphytes (7 species), but this time followed by Casual (4 species), and Hemi epiphytes (1 species).Trunk was dominated by Casual epiphytes (48 species), Hemi (16 species), and True epiphytes (8 species) (Figure 6).

DISCUSSION
A total of 72 species of epiphytes was found as a result of the study.This is more than the 28 species found in an urban forest in Brazzaville (Gabon), 29 species in a semideciduous forest in Ghana, and 61 species found in the oriental sector of Lac Kivu in DRC (Buhendwa et al., 2014;Kimpouni et al., 2017).Results showed some phanerophytes as epiphytes (Alchornea cordifolia, Cecropia peltata, Ceiba pentandra, Elaeis guineensis, Mangifera indica, Psidium guajava, Rauvolfia vomitoria, Vernonia amygdalina, Voacanga africana) with few occurrences.The diaspores of these terrestrial species were likely transported in the substrate on the other plant species by animals, gravity, or wind (Chapman et al., 1999).
True epiphytes represented 10% of vascular epiphytes encountered in the study area, and were thus the rarest life-form.Casual epiphytes were commonest life-form with 63.86%.Similar results were also found in Côte d' Ivoire and Ghana (Addo-Fordjour et al., 2009;Gnagbo et al., 2016).Nevertheless, Mucunguzi (2007) in DRC, and Kimpouni et al. (2017) in Congo found True epiphytes as the commonest life-form, whereas True epiphytes were the rarest life-form in this study.About True epiphytes, seven species belonged to Pteridophytes, and one from the Angiosperms, that is, Orchidaceae family.This is unlike Noumi et al. (2010) in the National Park of Korup, who found 102 Orchidaceae   Source: Author luminosity, level of humidity, and trunk diameter of phorophytes (Sonké et al., 2001;Kimpouni et al., 2014).
A decreasing gradient of number of epiphytes species from trunk to the upper canopy was found in this study.This is consistent with the findings of other studies (Wang et al., 2016;Noumi et al., 2010) that suggested that this Results of the present research showed that urban sector has the higher epiphyte richness, and the higher number of true epiphytes.However, the peri-urban sector recorded the lowest number of true epiphytes due to the expansion of the town, and urbanization.The common occurrence of casual epiphytes can be explained by the presence of palm trees whose foliar forks serve as substrate and diaspore receptors.Indeed, in the African semi-deciduous forests, hosts trees that have cracked bark on the trunk harbour more epiphytes than those with a less cracked bark (Zapfack, 1993).
The number of individuals of strict epiphytes was more abundant in peripheral areas of the city, and less in the central areas.The central zones would, therefore, be less favourable to the development of strict epiphytes.This is certainly due to the high level of industrial and anthropogenic activities that take place there.Indeed, epiphytes are even used as indicators of the conservation status of forest ecosystems, and are sensitive to anthropogenic activities (Gnagbo et al., 2016).The status of epiphytes would also help to identify areas of the city with higher humidity.Indeed, epiphytes are indicators of the humidity of the environment.It can therefore be said that the peripheral areas of the city of Douala are the wettest, and could constitute a site for the conservation of epiphytes.
Calyptrochilium spp., the only Orchidaceae species was found in the administrative sector.It was far from the industrial and urban sectors which provide many sources of pollution.Orchidaceae epiphytes are commonly inventoried in the montane forests and/or in the ecosystem without anthropic intervention such as protected areas, or primary forests (Noumi et al., 2010;Stevart and Sonké, 2002).The presence of Calyptrochilium spp. in the urban ecosystem implies an adaptation to survive in this environment.Thus, it is important to domesticate this species for its better conservation.

Conclusion
This study demonstrates that epiphytes are present in the urban ecosystem of Douala, and may persist in fairly stable representative numbers unless increased industrial activities and urbanization makes the urban centers less amenable for the epiphytes.Inventories made revealed 72 species of epiphytes.Casual epiphytes were the most abundant life-form.Pteridophytes were the major constituents of True epiphytes, Calyptrochilium spp. was the only Orchidaceae recorded in the study area.True epiphytes showed an ascending gradient of fixation, from trunk to crown, while Hemi and Casual epiphytes had a decreasing gradient of fixation, from crown to trunk.True epiphytes were rare in the industrial sector and urban sector where pollution was higher.Phorophytes were diversified, and the palm E. guineensis was the most colonised mainly with Casual epiphytes.Additional assessment of epiphytes should be made for their conservation in this ecosystem.

Figure 1 .
Figure 1.Map of the studied zone.Source: Author

Figure 3 .
Figure 3. Epiphytes diversity of host species.Source: Author

Figure 4 .
Figure 4. Abundance of phorophytes and epiphytes in the studied sectors.

Figure 5 .Figure 6 .
Figure 5. Abundance of epiphytes life-forms in the studied sectors.Source: Author