17 Years Successional Enrichment Plantation of Tree Recruitment and Restoration in Tropical Forestland


 BackgroundWe examine the recruitment drive of overstorey and undergrowth of tropical forested reserve by assessing the tree diversity, species composition, species richness, and growth form of the undergrowth on 17 years of post-disturbance. ResultThe result show an increase in the diversity and even distribution of the species of the overstorey, compared to the undergrowth. Conversely, the overstorey has a lower in species richness as compared to the level of undergrowth. One way ANOVA results shows that there is no significant difference (P = 0.341). However, undergrowth density (individual/ha) is found to be significantly (P = 0.000) higher than the overstorey trees.Conclusions﻿It is evident that the forest trees and saplings are dissimilar in species abundance, which implies that sapling recruitment is a key determinant of the tree species composition of the forest. It is then concluded that the method adopted for restoration encouraged species diversity in this successional forest.

It is evident that the forest trees and saplings are dissimilar in species abundance, which implies that sapling recruitment is a key determinant of the tree species composition of the forest. It is then concluded that the method adopted for restoration encouraged species diversity in this successional forest.

Background
Forest diversity is a signi cant role player in the heart of every ecosystem with various degrees of unmanaged challenges facing ecosystem integrity ( ora and fauna) which are growing global demand of ecosystem services viz. food, timber, fuel, water, charcoal, rewood (Ho mann and Baumung 2013). The challenges are however a global struggle against forest disturbance or degradation which may be alleviated via forest recruitment and restoration. Forest disturbance is tentatively associated with the removal of vegetation cover of an ecosystem leading to a change in ecological imbalance. Technically, many countries have developed management that is acceptable in managing forest restoration of degraded or disturbed forest and this management comes in various methods which can either be endemic (temperate or tropical) to a particular geographical location or suitable to the general locality. Among this forest management, Julia Raquel (2013) mentioned a good number of research studies to have tested method (Gardner et al. 2009;Brancalion et al 2013, Bertacchi et al 2015, Montagnini et al 1997, of forest management viz enrichment plantation. Other research carried out speaks on tested endangered seed enrichment plantations in the tropical forest of Tan Phu, Vietnam (Millet, et al 2013). The natural regeneration of trees was widely used in Zinder and Maradi, Niger. Reji et al (2009) reported varied remarkable data bene ting and transforming the region. Therefore, this present study investigated the level of recruitment and restoration of trees and undergrowth in a tropical forestland 17 years post-disturbance, by assessing the changes in species composition and growth form of the undergrowth. Finally, we evaluate whether species richness increase following succession. The forest reserve is a popular forest surrounded by two distinct communities formerly known to have provided various ora and fauna biodiversity but for some reason and for a very long time had faced human disturbance following exploitation of ora and fauna with no adequate regeneration strategies for restoration. As part of strategies to conserve the animal and plant biodiversity of this forest, in 1948 (see details in Ola-Adam. 1978), a strict nature reserve (SNR) as a natural high forest, was rst demarcated from the forest reserve with the aims of protecting and conserving the indigenous species from human exploitation. In furtherance to conserving the biodiversity of this ecosystem, this forest reserve was further subjected to three monitory reserves. The rst reserve, strict nature reserve, covered the 32 hectares of natural high forest called strict nature reserve (Queen plot), the buffer zone with 15 hectares and in recent time, 2004, a regeneration method called enrichment plantation ( gure 1) was adopted to cover an additional 5 hectares of the forest reserve where selected overstorey removal had occurred. Regenerating trees were manually planted to replace what was removed from the ecosystem. A recent accompany paper (Omomoh et al 2019) was used in comparing the current status of this study to a related undisturbed tropical forest as the only existing adjacent natural forest where an unperturbed species composition still exist

Study area
The regeneration method of enrichment plantation was established in 2004 (Fig 2) under the auspices of the Federal Government of Nigeria as forestry assisted project with the objective of recuperating the forest with regenerating trees. Since then the forest had been under intensive care and protection of the Forestry Research Institute of Nigeria for over a decade till date. The seedling ages at the time of planting were not known but since 17 years of post-disturbance of enrichment plantation, this forest can be categorized to what is called an older successional forest type (Perera 2005) or late successional forest.
In this context, we observed that this typical tropical older or late successional forest is absolutely devoid of mid-and early-successional driven trees e.g. Trema orientalis, Manihot glazovia, Alchornea laxi ora (Benth.) Pax & K. Hoffm etc. The transitional dynamics of early-and mid-successional forest to latesuccessional forest usually brings about the natural tree removal of mid-successional species in late successional forest. To be more speci c in this context, regenerating trees or young trees are referred to as sapling, herbaceous as forbs, climber as vine, grasses as graminoid, and plants of a early forest disturbance as pioneer plants e. g. agricultural weeds.
The forest is located in Nigeria at N07 0 .2645 1 , and E005 0 .03675 1 . Before enrichment plantation was embarked upon, a vast portion of the forest reserve was consistently known to have undergone heavy and series of anthropogenic activities such as timber logging and exploitation in the hands of illegal

Data Collection
To establish the species composition of the overstorey vegetation, trees, and shrubs enumeration of the oristic composition of each study plot were enumerated. Identi cation of the species and their nomenclature followed the ora of tropical West Africa (Hutchinson et al 1963) and Tree of Nigeria (Keay 1989) while the leguminous members (Fabaceae) followed the order of new families of legume phylogeny working group (LPWG) of Azani et al (2017). Samples of plant species whose identi cation is in doubt were collected, pressed for identi cation, and taken to FUTA herbarium for proper identi cation.
Sampling technique and selection of sample plots The laying of the plots was carried out using the systematic line transects as shown in gure 3. A 50 m transect was centrally located in the forest where three sampling plots of 50 x 50 m were laid in alternate sides with the aids of the prismatic compass and ranging poles for positioning. The forest biodiversity indices were used to investigate the diversity of trees, shrubs, and other life forms of the study site.

Forest trees and species composition structure
Four-eight forest trees were identi ed from all the three plots of this study area ( Quantitative details of other dominant trees were recorded among the mature tree measured, the species with more high relative density were Pterygota macrocarpa K. Schum., Buchholzia coriaceaEngl.,Celtis zenkeri Engl., Sterculia rhinopetala K. Schum., and Trilepisium madagascariense DC (Table 1).The diameter class was extremely high only in 11-20 girth among all the diameter class distribution recorded (Fig. 5). The two most dominant tree species in this diameter class distribution were Buchholzia coriaceaEngl., and Picralima nitida (Stapf) Th. & H. Dur. The diameter class decreased from 51-60 cm to 91-100 cm but extremely low at 0-10 cm. The analysis of variance for density of sapling varied and signi cantly higher than the tree of the forest (one way ANOVA: P <. 0.05 and P= 0.000). However, there is no signi cant difference in the relative density between the forest tree and sapling (P= 0.341). This con rmed that the forest trees and saplings are dissimilar in species abundance, which however means that sapling density is surviving the tree species composition of the forest. Figure 6 shows the graph of mean ± standard deviation of 2.41±1.55 tree species and 17.55±8.06 of tree sapling obtained in the forest structure (Fig 6). The Shannon-Wiener index (H'), Simpson's index (CD) and Evenness index (e) for the tree species were calculated as 3.59, 0.05 and 0.80, respectively.

Recruitment status of forest undergrowth
Four-ve (45) forest undergrowth species were identi ed in the forest oor of this study consisting of 6 vines, 3 forbs, and 36 tree saplings ( Table 2). One of the most notable differences in this forest undergrowth is the occurrence at which the life forms recruit and regenerate in the forest oor (Fig 4). Our results show that the sudden increase in recruitment level of tree sapling can be linked to natural disappearance or natural removal of early-and mid-successional species to later stages in succession. Our records show a remarkable impact on the regeneration life form owing to a wide increase in sapling occurrence (Fig 4). The different life forms observed in the forest undergrowth, tree sapling show the highest percentage (54%), followed by vines, 26%, mature trees 14%, and forbs 7%. The result from table 2 shows species diversity of forbs and vines in this forest to be extremely low and invariably consists of what ora of West tropical Africa described as forest forbs and vines, namely; Anchomanes difformis, Cyrtosperma senegalense, Geophila obvallata and Culcasia scandens. According to tree sapling context, Sterculiaceae is the most dominant family contributing the total of 22.9% of its individual members (8) (14), Ricinodendron heudelotii (18), Entandrophragma utile (18), Trilepisium madagascariense (28), Lecaniodiscus cupanioides (14), Cola gigantea (16), Mansonia altissima (17), Sterculia tragacantha (24), Celtis zenkeri (28).

Discussion
With the growing number of plant and tree species of overstorey and understorey in tropical forest, 17 years of enrichment plantation establishment, has revealed a lot of difference in species recruitment and composition of the successional forest. The ndings showed that the tropical forest is known to have the family Sterculiaceae member as major dominant in tree diversity ( Sterculiaceae had higher members of tree species diversity, however, aside this, our study con rmed the comparative nding of Sharkar and Devi (2014) in a semi-tropical forest where; Moraceae, Meliaceae, Apocynaceae, Euphorbiaceae, Ebenaceae, and Fabaceae were found to be family major species driver in the restoration status of a tropical semi-evergreen forest of Assam, Northeast India.
Our study further exempli ed, the seedling diversity of non-woody species of undergrowth identi ed at the soil surface level in which, Araceae, a forest non-woody family were identi ed as the second highest dominant species e.g. Anchomanes difformis, Culcasia scandens, Cyrtosperma senegalense, and Rhaphidophora africana. The impact of the dominant species except Anchomanes difformis the annual undergrowth is their natural regeneration to establish as long-lived understorey in a tropical humid rainforest. In general, when pioneer species of early forest succession (Omomoh et al 2020) decreases i.e. when agricultural weeds of early-successional forest is giving ways to forest tree seedling/sapling, hence an indication of forest transiting to stable forest. It is obvious from our studies that the dense natural regeneration of sapling's increase in this forest type is in association with abundant light availability, an important factor precluding regeneration ( The Geophila obvallata (Table 2) a forest non-woody creeping prostrate herb usually found in stable forest oor, often around the base of tropical forest tree con rms the forest becoming species-rich in plant diversity. This can be categorized as late successional forest based on the associated forest attribute described in Perera (2005). The enrichment plantation shows us that the agricultural weeds associated with and peculiar to young successional forest and forest plantation (Omomoh et al. 2020) are nearly absent in this forest type. This study is in support of Perera (2005)  percentage distribution among the smaller DBH class 2-10 cm in Wondo Genet Afromontane forest, Ethiopia which further substantiates the fact that older successional forest may have more individual trees averagely ranging at 5-20 diameter class. However, looking at the result of this study (Figure 5), the oristic composition and forest structure of this forest were consistent except the fewer or no older trees to compete with. Apparently, trees with larger canopy cover may have been selectively removed looking at the species similarity gap between the mature trees and sapling of this study as also reported in the comparative study.
Several dominant tree species which were highly valued timber trees in natural tropical forest (Omomoh et al. 2019) are apparently the natural history (Brudvig et al. 2017) of enrichment plantation during predisturbance. Our studies revealed that the timber trees were under-represented and somewhat well represented among the saplings (Nuñez et al. 2019). This may have supported the effort of enrichment plantation in this forest to restore trees diversity (Bertacchi et al. 2015). This absence of overstorey trees has created light opening and its incursion may have contributed greatly to the increase in seedling regeneration and sapling recruitment (Webb & Sah 2003) than any other life form. The regeneration of an ecosystem depends greatly on the level of vegetation disturbance. An ecosystem with a mild disturbance would regenerate easily at a shorter period of time than heavily disturbed vegetation (Ganlin et al. 2006). The diversity index results obtained from Shannon-Wiener, species richness and species evenness showed (Table 3) that the forest trees are somewhat diverse and more even than the undergrowth species except in species richness where forest undergrowth was higher than the forest trees. This diversity index showed vast dissimilarity between the forest trees and undergrowths and this was likewise reported (Ganlin et al. 2006& Palmer et al. 2000 where species richness was higher in a low disturbance.

Recommendation
A collaborative effort on ecosystem restoration and sustainable forest management can intensify the biodiversity integrity of disturbed forests for the next decade to come (Bernier et al. 2017). We recommend that a successional forest would need a collaborative forest management campaign to reduce and stop nomads from infringing into susceptible forests to uproot the wildling/seedling (sapling) for commercial garden or forest plantation production at local and international level.    Figure 1 Map showing the location of Akure forest reserve enrichment plantation. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.