Silvicultural assessment of enrichment planting with commercial tree species after selective logging

The integrity of forest stands in logging concessions depends on the logging method. Selective logging is the most commonly used method in the tropics, disturbing a considerable proportion of soil and canopy cover creating distinct sites for plant establishment. The objective of this study was to evaluate the silvicultural requirements in terms of light and moisture of seedlings of some commercial tree species used for enrichment planting. This study was carried out in two Forest Management Units in the East Region and a shade house at the University of Buea campus in the South West Region of Cameroon. 15 of the 20 most exploited species were selected for the assessment of their seedling functional performance. Nineteen log yards with their corresponding skid trails were selected randomly for enrichment planting. Monthly height measurements of the seedlings were recorded for 34 months. The shade house experiment had an unbalanced factorial experiment incorporating light and moisture. The growth rate in height was significantly higher in log yards (3.8 cm/month) and least under the forest canopy (1.2 cm/month). The growth rate in height was highest under high light and high moisture in Pterocarpus soyauxii (13.3 cm/month) and least under low light and high moisture in Entandrophragma cylindricum (0.7 cm/month). Mortality was highest under the forest canopy (11.1%) and least in the skid trails (0%). The results indicated that plant species should be planted according to their light and moisture requirements during enrichment planting at the seedling stage and for a sustainable forest management.


INTRODUCTION
Tropical forests are very diverse in terms of composition, structure and functioning and serve as habitat for heterogeneous animal and plant species (Rhett., 2019). They control the climate, provide hydrological services, controls erosion in the area where it is located and provides many timber and non-timber products, that are indispensable to man's livelihood (Dkamela et al., 2009;Blanco and Yueh-Hsin, 2012;Megevand et al., 2013). Forests are thus very essential for life on earth and therefore deserve wise use and conservation for the benefit of rural people in particular who are directly dependent on the forest and for the benefit of humanity in general (Biswas, 1993). However, there has been increasing awareness and concern about the rapid rate of deforestation and forest degradation in developing countries caused by anthropogenic activities like logging, shifting cultivation and urban development.
The integrity of forest stands in logging concessions depends on the method applied in logging. Selective logging is one of the most common forms of forest use in the tropics and in Cameroon in particular with only six logged species accounting for 75% of total timber volume (Ruiz Pérez et al., 2005). This over dependence on some particular species will eventually lead to the extinction of these species in the forest ecosystem altering the natural regeneration of the forest ecosystem dynamics making the ecological and biological complexity of the forest to be profoundly disrupted (Panayotou and Ashton, 1992). This depletion of logged species is of growing concern making some of these species to be listed on the International Union for Conservation of Nature (IUCN) red list (Bourland et al., 2012). Putz et al. (2012) emphasized in their meta-analysis that although 85-100% of species of mammals, birds, invertebrates, and plants remain after selective logging, the timber volumes decline by about 65% after the first harvest if the same species are again harvested (Deckker and de Graaf, 2003). Thus though logging concessions could be used as a conservation intervention to protect forests (Gaveau, 2014), the timber volume and biodiversity may decline after selective logging To conserve the biodiversity of the forest after selective logging, an enrichment of the forest with exploited species is of outmost importance.
Present forest composition may be largely due to natural selection operating in the past but we can expect major changes to occur in the edaphically extreme sites and those already with limited resources after logging thus hindering regeneration of these species. The main constraint in tropical forests over plant growth and regeneration is the low light intensity at the canopy understory allowing only about 2-3% of the Photosynthetically Active Radiation (PAR) to reach the understorey (Gastellu-Etchegorry et al., 1999). In selectively logged tropical forest, the creation of skid trails and log yards can disturb a considerable proportion of soil and canopy cover during timber harvesting creating distinct sites for plant establishment. Forest recovery in these sites may be substantially retarded due to substrate compaction by heavy machinery use and lack Mokake et al. 29 of onsite plant propagules after topsoil disturbance creating a more heterogeneous structure with patches of felling gaps, skid trails, and log yards (Fimbel et al., 2001;Putz et al., 2001). Large openings are subject to invasion by understory shrubs, lianas and herbs that can be an obstacle to tree regeneration by competing with the seedlings of slow growing economically important tree species (Mokake et al., 2018). The logged forest might therefore gradually be replaced by relatively species-poor forests dominated by pioneer species and over time by more diverse mixtures of later successional species, and ultimately the climax forest will re-establish (Chazdon, 2014); thereby affecting its biodiversity. One approach to recover the original forest is simply to protect these forests and allow nature to take its course; but results from the natural regeneration assessments indicate competition of the seedlings of timber species with the dense undergrowth of lianas, strubs and herbs for nutrients, moisture and light (Vieira and Scariot, 2008;Mokake et al., 2018). Silvicultural interventions including planting of high-value species are therefore necessary to overcome the relative depletion of commercial tree species, to compensate for the slow growth rate, and to ensure a future commercial timber value of the forest (ITTO, 2002) thereby reestablishing or conserving the timber biodiversity of forest stands. Through these interventions, it may be possible to enhance the rate of recovery and, moreover, deliberately manipulate forest composition to meet particular management objectives (Lamb, 2011).
Enrichment planting is a method of silvicultural management process of supplementing natural regeneration with seedlings of commercial and indigenous species (Martinez-Garza and Howe, 2003;Paquette et al., 2006a). Unfortunately, baseline information about ecological processes and the specific growth requirements of many of the commercial species that are available for planting is still very limited (Thomas and Chuyong, 2006;Paquette et al., 2006b). Also many enrichment planting trials in Africa were based on a few or single species (e.g., Khaya spp., Tarrietia utilis, Dupuy and Koua, 1993) sometimes planted on huge areas (e.g., Aucoumea klaineana in Gabon, Brunck et al., 1990). During enrichment planting, species mixtures are desirable for both improving biodiversity and the range of goods and services as well as limiting pest-induced damages (Piotto et al., 2004;Potvin and Gotelli, 2008). Different kinds of mixtures exist ranging from simple mixtures consisting of two or more species planted in single-species blocks or rows (Stanturf et al., 2014). Such mixtures are useful on sites with distinct gradients in environmental factors such as drainage or light. However, it is necessary that these species be planted in specific sites according to their growth requirements (Stanturf et al., 2014). Avon et al. (2013) indicated that, openings created along skid trails can lead to competition sequences between shade-tolerant and shade-intolerant species which in turn affect natural regeneration. Therefore care needs to be taken on the type of species planted on the different sites created during logging. However, there is little data on site tolerance and growth of timber species used for enrichment planting after selective logging in Africa (Dupuy and Mille, 1993;Piotto, 2008;Doucet et al., 2016). Most enrichment plantings carried out are not reported and are not species and site specific to edaphically extreme sites created by logging. Therefore knowledge of the growth requirements of the seedlings of commercial species are necessary if they have to be incorporated into any enrichment planting scheme. The objective of this study was to evaluate the silvicultural requirements in terms of light and moisture of seedlings of some commercial tree species using functional traits to predict species performance under uncontrolled environment of different stand conditions (e.g., log yards, skid trail and under forest canopy) and a controlled environment in a shade house experiment.

Study sites
An enrichment planting was established in two Forest Management Units (FMU) (FMUs 10-052 and 10-025) in the East Region of Cameroon ( Figure 1) while the shade house experiment was carried out at the University of Buea main campus in the South West Region of Cameroon ( Figure 2). The East Region occupies the Southeastern portion of the Republic of Cameroon. It lies between latitude 3°08 to 3°21 North and longitude 14°31 to14°52 East. It is bordered to the East by the Central African Republic, to the South by the Republic of Congo, to the North by the Adamawa Region, and to the West by the Centre and South Regions. It has an area of 109,011 km² with its soil predominantly ferrallitic, rich with iron and red in colour. The East Region has a wet equatorial climate with high temperatures (24°C on average) and a lack of traditional seasons. Instead, there is a long dry season from December to May, a light wet season from May to June, a short dry season from July to October, and a heavy wet season from October to November (Fitzpatrick, 2002). Relative humidity and cloud cover are relatively high, and annual mean precipitation 1500-2000 mm except in the extreme eastern and northern portions, where it is slightly less. Relative humidity is highest in the month of June and lasts till the month of December (Fitzpatrick, 2002).
On the other hand, the shade house experiment was carried out in the South West Region of Cameroon at the University of Buea main campus located in the Fako Division ( Figure 2) and in the coastal belt of the Gulf of Guinea (Cronin et al., 2014). Buea is located at Longitude 4° 09' North to Latitude 9° 14' East, having a distance of approximately 823 km from the East region where the seedlings were harvested (Google Maps) and a humid tropical climate with a rainfall not greater than 50 mm over a 5 months span (Oates et al., 2004). The climate is strongly seasonal, with one pronounced dry season between December and February and a long wet season from march to November (Newbery et al., 2006). The site has a precipitation of 100 mm each month which peaks in August (Cronin et al., 2014). The mean annual temperature is about 25 0 C (Oates et al., 2004). The relative humidity of the area is between 75 and 85 % throughout the year on the South Western side of Mount Cameroon due to the coastal influence and the incidence of mist and orographic cloud formation (Cronin et al., 2014). The soils are mainly volcanic which are relatively fertile (CDC, 1997). The Mount Cameroon region lies within the tropical rainforest of West Africa. According to the different agro ecological zones in Cameroon, the South West and East Region both belong to the tropical rain forest. However, the South West Region has a monomodal rainy season while the East Region has a bimodal rainy season. This is the more reason why the shade house experiment could conveniently be carried out in the South West Region.

Selection of commercial species used in controlled and uncontrolled environments
15 of the 20 most exploited species in Cameroon were selected based on the seeds and seedlings found on the forest floor for the assessment of their seedling functional performance in both natural forest stands (uncontrolled environment) and shade house (controlled environment) ( Table 1).The Fabaceae had 4 species, Sterculiaceae had 3 species, and Meliaceae had 2 species; while the Sapotaceae, Combretaceae, Lecythidaceae, Irvingiaceae, Ochnaceae and Moraceae were represented by a species each (Figures 1 and 2).

Raising of seedlings for enrichment planting and shade house experiments
All the seedlings and seeds for this study was collected from the forest of the East Region. Seedlings for the enrichment planting were collected from the nursery of the National Forestry Development Agency (ANAFOR) located in the FMU 10052 (Plate 1). For the shade house experiment, wildings of Lophira alata, Pterocarpus soyauxii, Entandrophragma cylindricum, and Petersianthus macrocarpus were collected from the forest floor in the East Region of Cameroon. The seedlings were of that year's seed rain as cotyledonous scares were still very visible. These were wrapped in a mixture of top soil and sand and watered every two hours. Upon getting to the shade house, they were immediately transplanted in to black polythene bags (18 x 17 x 15 cm) filled with top soil.
The seedlings of Erythrophleum sauveolens, Klainedoxa gabonensis, Mansonia altissima and Terminalia superba were raised from the seeds of these species collected from the forest of the East Region of Cameroon. Seeds having a thick seed coat were pre-treated before sowing into the germinator. These included the seeds of Erythrophleum sauveolens, which were treated in 10% of concentrated Sulphuric acid for 10-15 minand Klainedoxa gabonensis which were soaked in water for three days. The seeds were planted at 1cm depth in a germinator of 1 m × 50 cm and watered daily with the use of an 11 L watering can, in order to maintain the moisture of the soil (Plate 2). The germinator was sheltered at 2 m height with 70% shade cloth spread wide overhead to block direct sunlight. New germinants were censured at 2 days intervals for 30 days as hypocotyls emerged above the soil surface. However some species had no seedlings that germinated, maybe due to the fact the transportation conditions were not favorable enough for the specific species.
The germinated seedlings were then transplanted into 18 x 17x 15 cm black polythene bags filled with top soil.

Enrichment planting of seedlings under different stand conditions in the East Region of Cameroon
Each FMU (10-052 and 10-025) has an average of 50-60 small log yards with an average area of 1,050m 2 with many skid trails. Nineteen log yards (LYs) with their corresponding skid trails (STs) were selected randomly for enrichment planting with seedlings of the selected timber species. Seedlings of 10 of the 15 selected species for the study were used for the enrichment planting (Table 1). The Plate 1. Seedlings in the ANAFOR nursery before transplanting in the log yards and skid trails in the East region of Cameroon. a: Entandrophragma cylindricum seeds in the germinator, b: Pterocarpus soyauxii seedlings in the germinator, c: pretransplanted Pterocarpus soyauxii seedlings from the germinator, d: Entandrophragma cylindricum seedlings, e: Triplochyton scleroxylon seedlings, f: partial view of nursery site.
leaves of the seedlings were prunned and planted using the line method with a spacing of 5 × 5 m in log yards, skid trails and under the forest canopy. These seedlings were tagged in order to differentiate them from already established and germinating seedlings on the forest floor. A total of 666 seedlings were planted in 19 Log yards (Lys=550) with their corresponding skid trails (STs=116) and 200 under the forest canopy (Table 1). Monthly height measurements of the seedlings were recorded using a metric tape over a period of 34 months (March 2012-December 2014).

Shade house experiment in the South West Region of Cameroon
The shade house was constructed with a Tildnet netting material (Tildnet, UK). A Photosyntheically Active Radaition (PAR) quantum sensor (Skye, USA) was used to measure the amount of light intensity available for the plants. An amount of 65% of sunlight was available to the plants. The three light levels were obtained by varying the layers of the netting material as follows: (i) High: no netting of approximately 100% Photosyntheically Active Radaition (PAR); (ii) Medium: single layer of Tildnet netting of approximately 65% PAR; (iii) Low: double layers of Tildnet netting approximately 20% of PAR. The moisture levels were as described by Hall et al. (2003); (i) High: watering with 200 ml of water twice a week (ii) Low: watering with 200 ml of water once a week.
The seedlings of 8 of the 15 selected exploited species were used for the shade house experiment (Table 1). The unbalanced factorial experiment incorporated two factors: light with three levels (high, medium and low) and moisture with two levels (high and low) giving a six treatment combinations ( Table 2). The layout was in three blocks with three replicates. Rain was prevented from adding to the moisture level by placing a transparent plastic sheet atleast one meter above the seedlings. A total of 103 seedlings were used for the shade house experiment and monthly height measurements were recorded over a duration of six months from June 2014 -December 2014.

Data analyses
Growth rates in height of seedlings were determined as described by Hunt (1982); (i) Absolute growth rates of seedlings Absolute growth rate = Log H2 −Log (H1) t2−t1 Plate 2. Germinating seedlings of some commercial timber species in the germinator for the shade house experiment. a=Mansonia altissima, B= seedlings of Mansonia altissima, C=germinating seeds of Alstonia boonei, D= seedlings of Alstonia boonei, E=germinating seeds of Erythrophleum sauveolens, F=seedling of Erythrophleum sauveolens.    Wallis and ANOVA tests were used to determine a significance difference between means for the enrichment planting and shade house experiments, respectively. All analyses were done using MINITAB version 17 (MINITAB 2010) and Excel 2013 (Microsoft Corporation 2013) with statistical significance fixed at P < 0.05.

Growth rates in height of seedlings in different treatments/experiment sites
Growth rates in height of seedlings were significantly different (p=0.01) between forest stand conditions with the log yards having the highest (3.8 cm/month) and the least under the forest canopy (1.2 cm/month) (Figure 3). Growth rates in height were also significantly different between species across the stand conditions (p=0.01) with the highest found in Aningeria altissima (7.1 cm/month) in the log yards while the least was in Entandrophragma cylindricum (0.2 cm/month) under the forest canopy (Figure 4). This is further confirmed by the growth rates in height of each species between the various stands found in Table 3.
Similarly in the shade house, absolute growth in height (cm/month) was significantly different between treatments and species (p=0.01). This was highest in Pterocarpus soyauxii (13.3 cm/month) under high light and high moisture condition while E. cylindricum had the least absolute growth rate (0.7 cm/month) under low light and high moisture condition. The relative growth in height on the other hand was highest in A. altissima (100 cm/month) under medium light and high moisture condition and least in E. cylindricum (1.9 cm/month) under low light and high moisture condition (Table 4) indicating that a combined effect of light and moisture has a significant effect on the growth rate in height of the species.
Apparently, Terminalia superba and E. cylindricum   found in all treatments in the shade house had a linear increment in mean height with time in all treatments from the medium and high light conditions (Figure 5a and b, respectively). A confirmation that light was the main determining factor of growth.

Species seedling mortality
Mortality was not significantly different between species in the different stand conditions and between stand conditions (p =0.1). Mortality was generally very low for all the seedlings planted with no death recorded along the skid trails (0 %). Mortality was recorded for just 4 species; Terminalia superba, and Baillonella toxisperma (9.1%) in log yards, and E. cylindricum and Eribroma oblongum (11.1%) under the forest canopy. Thus mortality was highest under the forest canopy and least in the skid trails. Similarly, mortality was not significantly different between treatments (p=0.4) and species under controlled conditions (p=0.5). Most species had a zero mortality. However the highest mortality occurred in T. superba (11.1%) under low light and high moisture condition and   (Table 5). A factorial analysis indicated that light and a combined effect of light and moisture, had significant effects on the growth rates in height of these species in the shade house as shown in Table 6. Specifically, the research findings revealed that light had a significant influence on all species for growth rate in height except for A. altissima (p = 0.9) and  (Table  7). Taking into consideration the growth rates and factorial analysis, the growth requirements of these species at the seedling stage can be proposed; with the treatment of low light low moisture not having any species (Table 8).

DISCUSSION
Climatic factors like light, temperature, moisture, and nutrients affects the growth and developmemt of plant species (Yang and Tian, 2004). In the tropical regions, different climatic conditions may have developed under the canopies of great rain forest trees (Forseth, 2010). Because of this, plants are adapted to different types of light and moisture levels adapting easily to their environment. Changes in the light intensity, quality and duration and moisture content are therefore important features of tropical forests. These changes are related to openings in the forest canopy caused by selective logging, branch and tree fall (Girona et al., 2017). Various intensities of light and moisture content affect plant growth, reproduction, primary production and thus indirectly the structure of the forest. Tree growth is thus affected by many factors, such as silvicultural treatment, competition from neighbouring trees, and microclimate (Girona et al., 2017). Separating the effect of these factors can be difficult thus the main objective of this study; to determine the growth requirements of some commercial timber species used for enrichment planting.
The main impact of these results would therefore relate to the canopy structure and light penetration, with secondary effects on temperature and moisture content of vegetation and soils (Kammesheidt, 2002) as light limitation in tropical forests takes many years to re-establish in log yards following forest canopy opening. The results of this study indicated a significantly higher growth rate in height of commercial species in log yards and least under forest canopy; as well as a higher absolute growth rate in height under high light and low moisture and least in low light and high moisture. This might be due to the light availability caused by the opening of the forest canopy after selective logging. Similar results were obtained by Sist and Nguyen-The' (2002), Brenes et al. (2004) andNe Win et al. (2012). In Bolivia, Fredericksen and Mostacedo (2000) found the greatest growth rates of tree regeneration on areas with the greatest amount of light transmittance, including log yards and log roads.
The growth and development of plants are affected by environmental factors (Yang and Tian, 2004) which would have effects on plant morphology, especially blade size, texture, thickness and plant morphology (Huang et al., 2009). Among the various environmental factors, light is one of the most important variables affecting photosynthesis as well as plant growth and development (Smith, 2000). Plants require light not only as an energy source but also as a clue to adjust their development to environmental conditions which might be the log yards, skid trails or under forest canopy. During photosynthesis, absorbed energy is transferred to the photosynthetic apparatus, which is comprised of Photosystem I (PSI), Photosystem II (PSII), electron transport carriers (cytochrome b6f (cytb6f), plastoquinone (PQ), plastocyanin (PC)), and ATP synthase. Both quality quantity and duration of incident light can have drastic impacts on photosynthetic activity and photosystem adaption to changing light quality (Belkov et al., 2019) which eventually has an effect on the growth rate.
The greater canopy openness and soil disturbance in log yards favours tree regeneration in log yards than in skid trails (Denslow, 1995;Dickinson and Whigham, 1999). This high degree of light scavenging and likely light limitation found in skid trails and under canopy; confirms the reason why growth of these species is retarded significantly in the other sites as compared to that of the log yards. Also Skid trails (conditions similar to medium light and medium moisture) have higher ground damage as compared to log yards which are usually restored before enrichment. Asner et al. (2004) indicated higher ground damage in skid trails (7-12%) than in log yards (1%) of the total harvest area in the Eastern Amazon of Brazil. This in turn leads to soil compaction and therefore retards seed germination and thus seedling growth after enrichment in the skid trails which is not usually restored. Studies have shown that soil compaction can delay the establishment of seedlings (Pinard et al., 2000;Fredericksen and Mostacedo, 2000). However, if the soils are dry during logging, commercial tree species can regenerate abundantly on skid trails (Dickinson and Whigham, 1999;Dickinson et al., 2000). This is however not the case in this study as the soil type here is clay loam which is easily compacted by logging machines; thus soil compaction should be limited during logging. Also taking into consideration that the level of canopy openness is reduced in skid trails and almost absent under the canopy, the reduced evaporation of water easily leads to soil compaction.
Competing fast growing vegetation like weeds and lianas at ground level can result in growth failure of the slow growing exploited seedlings in skid trails (Ne Win et al., 2012). Asner et al. (2004) indicated canopy gap closure in skid trails (0.5 years post-harvest) recovered faster than the corresponding log yards (1.5 years postharvest) from selective logging. Due to greater canopy openness and frequent use by logging trucks in the log yards, skid trails had greater competing vegetation than in log yards thus greater growth of competing vegetation in log yards. Breckage and Clark (2003) indicated similar results with seedling growth and survival higher in canopy gaps with forest types of under story removal; evident in log yards than in skid trails. Quang et al. (2020) confirmed that competition from saplings and trees that were situated outside the enriched areas emerged as a major source of competition in some plots. A significantly higher growth in skid trails than under forest canopy may be due to the reduced radiation found under the forest canopy. Similar results were obtained by Duah-Gyamfi et al. (2012) who studied the natural regeneration of timber species following selective logging in Ghana indicating that growth rates were higher in the main skid trails than in unlogged. In the skid trails, the amount of light is moderate keeping enough moisture necessary for the growth of seedlings. In this study no species died in the skid trails indicating the best growth condition for most seedlings in the forest.
Similarly in the shade house experiment, light was the main factor which had a significant influence on the growth of the seedlings of these species. These treatments mimic conditions found in the forest. Light availability in the forest depends on the level of disturbance of the forest canopy. The disturbance categories were characterized in terms of light availability, measured indirectly as canopy openness (Duah-Gyamfi et al., 2014). Log yards had the highest canopy openness, which indicates high light level than skid trails, which indicates the medium light level. Under the forest canopy, only 2-3% of PAR indicating the low light intensity.
However a combined effect of light and moisture had a significant influence on the growth of the seedlings. Most growing plants contain about 90 percent water. Also the light-responsive photosynthetic process is driven by the released electrons through the water-splitting reaction on the PSII side, followed by Nicotinamide adenine dinucleotide phosphate (NADP + ) reduction to NADPH, and proton flow into the lumen in order to generate Adenosine Triphosphate (ATP) (Ceccarelli et al., 2004). Generated NADPH and ATP serve as an energy source for the carbon fixation process in photosynthesis (Ceccarelli et al., 2004). This was confirmed by the relative growth rate being highest under medium light and high moisture while the least was observed under low light and high moisture. Furthermore, no deaths were recorded in the skid trails indicating that too much sunlight is not necessary for the seedlings of some of these species. Although light is necessary for the germination of seedlings, the quality quantity and duration of light will determine their survival in a particular environment.
This is in corroboration with the shade house and factorial experiments where most species had a significant growth from the medium light condition and an interaction between light and moisture had a significant growth rate. A linear increment in height over time in log yards is an indication of the presence of light process of photosynthesis after gap opening in the forest structure. This gap opening is also present in the skid trails which form a canopy faster than in the log yards leading to decrease in the growth of most species in the skid trials. This is corroborated by the fact that species like E. cylindricum and T. superba had a linear increment in all treatments from medium light; results are similar to the results obtained by Doucet et al. (2016) indicating their shade tolerance. This is further confirmed by the highest mortality under the forest canopy than in skid trails and log yards. Chandrashekara and Ramakrishnan (1993) indicated that the higher irradiance in gaps results in better growth, regardless of the ecological category. According to Kobe (1999), the mortality of certain rainforest tree seedlings such as Trophis racemosa, Castilla elastica, Pourouma aspera and C. obtusifolia decreased with increase in light intensity (to 20% full sun). Chazdon et al. (1996), mentioned that light availability would be higher in the center of the gap (that is, in the logging yard) than in the gap edge (skid trails) or in the understorey of a forest. This therefore implies a higher mortality under forest canopy than in log yards.
The growth of these species under the different stand conditions also depends on the growth requirements of the specific species at the seedling stage. Some may be shade tolerant, mid tolerant or shade intolerant or better still shade bearers, non-pioneer or pioneer respectively (Hawthorne, 1995) or late or early-succession species. Restricted as seedlings to gaps, early successional species are light demanders (pioneer species) while late successional species grows under the forest canopy (shade bearers). Mid tolerant species are species which require gaps to develop beyond saplings. These are species that are obvious light-demanders, and typical trees of secondary forest and fallow, yet which exist as seedlings in the deepest shade of little-disturbed forest (Hawthorne, 1995;Antobre et al., 2021). Thus while some species grow in light, others persists grow under shade but do not grow appreciably in the shaded understory; accounting for the heterogeneous structure in tropical forest. The requirement of light for the germination of seeds of certain plant species prevents germination and thus not favourable for seedling establishment and growth (Fenner and Thompson, 2005). The light requirement of such seeds and seedlings acts as a mechanism that determines where and when germination takes place, and is important for survival of the plant species concerned, as it prevents stored seed reserves from being depleted. Some seeds germinate equally well in light and darkness, whilst others germinate better under only light or darkness (Chanyenga et al., 2012). Thus different plants have different light requirements for their growth and development.
The findings of this study indicates that the highest growth rate in A. altissima in log yards indicates that the species is a light loving early successional. This was confirmed by the shade house experiment where the species had its highest growth rate under high light and low moisture conditions. High light and low moisture are similar conditions in the log yards after gap opening. Lemmens (2007) indicated that A. altissima thrives more on the driest soils in the semi-deciduous forests of tropical forests. Specifically this species had the highest growth rate in log yards and under the forest canopy this indicates the shade tolerance of this species since it can survive in both conditions. Also this is confirmed by the fact that in the shade house experiment, light did not have a significant effect on the growth of A. altissima. This indicates its shade tolerance at the seedling stage having fast growth under openings thus a pioneer species. The highest growth rate of P. soyuaxii in skid trails indicates that the species is shade intolerant and a light demander in corroboration with Khurana and Singh (2001) and Doucet et al. (2016) who indicated that P. soyuaxii was a non-pioneer and light demander. The species prefers moist and well drained soils with an average temperature of 23°C conditions similar to the log yards. Under the forest canopy, the soil is moist and there are situations of flooding with a temperature reduction which might impede the growth of these species. Also germination and seedling growth are rather fast in this species (Jansen, 2005). This is in corroboration with the results obtained in shade house experiment where P. soyuaxii had the highest growth rate in height under high light and high moisture conditions. This is in contrast with E. cylindricum that had the least growth rate in skid trails and under the forest canopy. This may be due to the slow growth of this species with a growth rate of 20-40 cm/year (Kémuezé, 2008). This is an indication of its intolerance of shade and a nonpioneer light demander species (Doucet et al., 2016). Also this species is extremely sensitive to parasites (gallforming insectsand/or shootborers) (Bosu et al., 2006;Opuni-Frimpong et al., 2008 andDoucet et al., 2016) which are usually persistent under the forest canopy and skid trails due to their high level of moisture as compared to the log yards. Such damage in fallow forests with canopy structure very similar to young plantations has also been reported by Hall (2008). The tradeoffs among survival, growth, and herbivory present a serious challenge for these species (Goodale et al., 2014). This is confirmed by the fact that in the shade house, the species had its best growth rate under high light and low moisture conditions and the fact that it did not have the least growth in log yards. Kémeuzé (2008) also indicated that seedlings of up to 2 years old require light shade but there after they should be gradually exposed to more light.
Mortality was highest in T. superba and Baillonella toxisperma in the log yards and E. cylindricum and Eribroma oblongum under the forest canopy. According to the shade house experiment, T. superba had the best growth rate under medium light and low moisture; confirming its shade tolerance. Kimpouni (2009) indicated that T.superba are often abundant along roadsides and in medium-sized forest gaps indicating they are usually found in areas of reduced light intensity. Also Doucet et Mokake et al. 41 al., 2016 indicated that this species is a pioneer species; thus its shade tolerance. Aslo a high mortality of species like E. cylindricum under the forest canopy is due to the high mortality of this species in the natural forest with less than 1% reaching 10cm diameter (Kémuezé, 2008). Doucet et al., 2016 indicated a high mortality rate of this species 690 days after enrichment planting. Taking this into consideration it is recommended that T. superba, M. altissima and L. alata should be planted in skid trails while P. macrocarpus, E. cylindricum, Erythrophleum sauveolens, A. altissima and P. soyauxii should be planted in log yards However these species might change their growth requirements after the seedling stage. Thus it is recommended that these sites be ameliorated through treatments, to minimize the effects of competition, compaction and other limitations on the regeneration and recovery of logged over forests after the seedling stage for a sustainable development.

Conclusion
The growth rate in height was higher in log yards than in skid trails and under forest canopy. This was due to the amount of light available when a gap is formed under the forest canopy. An interaction of light and moisture influenced the growth in height of these species in the forest ecosystem depending on their growth requirements. The results of this study therefore indicate that every species has a specific growth requirement; thus should be planted where appropriate during enrichment planting at the seedling stage. However some level of management is required to maintain the growth of these species for forest regeneration after logging. Thus further research should be carried out on the survival of these species after the seedling stage inorder to ensure their survival and growth while ensuring sustainable management.

CONFLICT OF INTERESTS
The authors have not declared any conflict of interests.