Background

The importance of trees is underscored by their economic and environmental roles. Some tree populations have been known to control the overall plant community likely because of their biomass (Thakur and Eisenhauer, 2015; Jia et al., 2018). By so doing, they create a microenvironment through the diminished air and soil temperatures and reduce wind speeds enabling the formation of smaller niches within the forest ecosystem. Jose et al. (2008); Rao et al. (1998) posited that these events culminate in ecophysiological changes such as decreased evaporation with enhanced humidity in forest areas. Moreover, the availability of water in the forest to plants, particularly those within close proximity to tree roots has been reported (Burgess et al., 1998; Ong et al., 1999). Through the processes of hydraulic lift, these trees help to transport water from deep soil layers, which were hitherto impossible for most of the understory herbaceous plants, to drier surfaces, which were bedevilled with competition for water and other nutrients. In turn, plant abundance within tree canopies may be attributed to protection from extreme irradiation and heat effects, which invariably increases the rate of water loss by these plants (Lopez-Pintor et al. 2000). From the foregoing, it is evident that the understory regions of trees are possible microhabitats for these sets of plants.

Another consideration is many plant populations never exist in isolation of other plants. Rather, as outlined in Riginos (2009), their coexistence and interaction ultimately give rise to codominance. However, such associations may become hampered in the event that the associated tree exhibits some level of allelopathy, or the capacity to alter the availability of soil nutrients, light and other limiting resources. On the other hand, the growth performance of any tree may be affected by a number of factors including cultural practices, planting density, as well as the tree’s interaction with understory plants that also significant from an ecological viewpoint (Cantarelli et al. 2006, Leopold and Salazar 2008, Silva et al. 2010).

The ability of natural forests to meet the demand for timber requirements was evidently in doubt in Nigeria; hence, the intensification in afforestation programmes in Nigeria has been performed by the Department of Forestry and the Federal Ministry of Environment. One of the tree species adopted for afforestation programme was teak (Tectona grandis Linn. f. Verbenaceae). Actually, the usefulness and popularity of teak have been known for many centuries, which contributed to the relatively widespread distribution and cultivation throughout the tropics. Generally, the agroforestry deliberately combines tree cultivation with crops and pasture production. Accordingly, the success of the agroforestry system depends on the choice of both tree and associated crop or pasture. In either case, guaranteeing the total development of the tree is paramount; hence, the distribution of the understory plants cannot be overlooked. The question, therefore, is whether the single-tree influence on the distribution of the associated weeds counts one way or the other.

Inderjit and Callaway (2003) reported that it is important to study spatial patterns of the weeds in the field as it relates to silviculture as well as growth inhibition zones as this point to the allelopathic disposition of the trees in question. Consequently, the deliberate investigation of plant species beneath the canopy of the tree would help to pinpoint possible beneficial plant species that may be useful in weed control via allelopathy. In a bid to guarantee sustainable practice in agricultural development, allelochemicals are being viewed as possible alternatives to synthetic agrochemicals (Scrivanti et al. 2003, Maraschin-Silva and Aqüila 2006). Some of these allelochemicals, otherwise known as functional allelochemicals (Aldrich, 1984) are transformed by soil microorganisms, and as such has influence in the activity and distribution of soil microorganisms.

The capacity, however, for T. grandis to exhibit a negative influence on plant development has been previously reported (Kole et al. 2011, Manimegalai 2013). Kole et al. (2011) investigated allelopathic effects of teak leaf extract on junglerice (Echinochloa colona) and sedge (Cyperus difformis) in a rice farm. They reported no significant effects on rice germination, but inhibitory activity on the germination of the two weeds. Similarly, Evangeline et al. (2012) and Manimegalai (2013) reported allelopathic effects of Tectona grandis on the germination and seedling growth of Vigna mungo and Vigna radiata respectively.

Given the huge economic benefits of T. grandis, which has made it a largely sought after species of wood across the world including Nigeria, the possibility, therefore, exists for overexploitation of this forest resource. As such, many timber farmers may popularize their plantations with Teak. One of the major advantages of relying on the tree for agroforestry interventions over a wide area or climate is because T. grandis will survive and grow under a wide range of climatic and edaphic conditions. The question, therefore, is whether teak plantations would impact negatively on the distribution and diversity of other plants as well as the soil characteristics of the area. Although studies related to T. grandis have been carried out across other countries of the world including Nigeria (Akindele, 1989, Aborisade and Aweto 1990, Izekor and Fuwape 2011, Oyebade and Anaba 2018), not much is known about the single-tree influence of teak on plant diversity. The aim of this study, therefore, was to investigate the effects of teak plantation on plant species diversity within and around the tree, as well as the impacts on soil physicochemical characteristics.

Methods

Study area

The study was carried out at the Moist Forest Research Station, Benin City located along Utagban road, Off Ekehuan Road (6° 34″ 0′ N, 5° 34″ 34′ E). It is a reserve measuring 1 mi2 (about 258.999 ha) jotting towards the Ogba river behind Airport road. The landmass is a reserve that was endowed with various exotic and indigenous forest tree species such as Khaya sp., Lovoa trichilioides, Nauclea diderrichii, Allanblackia floribunda, just to mention a few, as well as a wide array of animals, including reptiles, birds and mammals species before the forest was clear-felled; which lead to rigorous replanting/reforestation by successive administrations of which a Tectona grandis plantation was established measuring about 45.72 m by 91.44 m from which our study was carried. The forest was planted in the year 2011. Routine clearing of undergrowth in the forest occurs annually, usually during the dry season in other to forestall any outbreak of fire.

Sampling method and procedure

For the purpose of this study, 36.57 m by 60.96 m was marked out of the teak plantation using a measuring tape. The marked out area was divided into three columns with five rows making 15 equal sized subplots measuring 12.19 m by 12.19 m each. From the 15 subplots, five subplots were randomly selected from each of the rows. Each subplot contained an average of ten trees per plot. In each of the randomly selected subplot, only one of the trees within each of the subplot was used for the experiment. The five selected plots were pegged using small pegs not more than 0.91 m, labelled with Mon Ami black permanent markers and demarcated into 0–0.5 m, 0.5–1.0 m and 1.0–1.5 m, respectively from the base of the tree using white twines. The trees used in this study were thereafter measured. The subplots were demarcated using ranging poles, pegs, and twines. Soil samples were taken using a soil auger.

Data collection

Measurements of heights, girths and canopy heights were taken. The height of the tree and the canopy height were measured using Haga altimeter, while the girth was measured using a metre tape. A 1 m by 1 m quadrat was thrown on the subplots to identify species diversity and population count. A stem count of the flora available within the study area was used in identifying and counting the species. Soil samples were collected using soil auger within and within and beyond the canopy fringes of the trees in the study area at a depth of 10 cm from the soil surface and taken to the laboratory in a clean black polythene bag for analysis.

Laboratory analysis and identification of flora species

The soil physicochemical parameters were analysed at a laboratory following standard procedures (Bray and Kurtz 1945a,b, SSSA 1971, Haluschak 2006, ICARDA 2013, Nasir et al. 2015). The flora species collected were identified with the assistance of the Plant Taxonomists at both the Moist Forest Research Station, Benin City (Forestry Research Institute of Nigeria), and the Department of Plant Biology and Biotechnology, University of Benin, Nigeria. A plant identification text was also used where necessary (Akobundu and Agyakwa, 1998).

Data analysis

Plant abundance within and outside canopy demarcations was analysed using the IBM Statistical Package for Social Sciences version 20.0 for Windows (SPSS v.20). Correlation, mean, standard deviation and variances were the analytical parameters considered. SPSS was also used to compare soil physicochemical parameters and species abundance within and around the tree canopy. To analyse the flora species collected, diversity indices (Taxa, Dominance, Simpson, Shannon-Winner, Evenness, Brillouin, Menhinick, Margalef, Equitability, Beger-Parker and Chao-1) were used. These were analysed using the statistical software called PAST® version 2.17c. Mean, range and standard deviation were the descriptive tools considered.

Results

The morphological characteristics of T. grandis have been presented in Table 1. Plant height averaged 11.8 m whereas canopy length averaged 2.41 m. The highest level of variability amongst the trees sampled occurred with stem girth (CV = 11.99) compared to the other tree parameters measured.

Table 1 Mensuration of the Tectona grandis stands
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The soil physicochemical parameter was determined around the region covered by the canopy as well as beyond its fringes (Table 2). The pH was significantly lower under tree canopy (pH 4.4) compared to outside the canopy demarcation (pH 5.4). However, in spite of the minimal differences in electric conductivity (EC) between the two soil areas, no significant differences were observed (p < 0.05). Similarly, no difference between soil composition of calcium (15.2–17.3 meq/100 g), potassium (1.1–1.4 meq/100 g), magnesium (13.4–18.2 meq/100 g) and Sulphate (14.9–18.6 mg/kg) were reported in the soil samples collected with the subplot, whether close or far from the tree base. Total organic carbon and total nitrogen within 1.5 m from the tree base were significantly higher than beyond (Table 2). As reported earlier, 5 subplots (Q1–Q5) within the forest were randomly selected. Each subplot contained at least 20 plants out of the 36 identified in the forest; including Eleusine indica, Cynodon dactylon, Axonopus compressus and Oplismenus burmannii. However, Aneilema beniniense, Sida garckeana, Reisantia indica, Mallotus oppositifolius, Euphorbia hirta, Alchornea laxiflora, Tridax procumbens, Chromolaena odorata, Ageratum conyzoides, Panicum laxum, Ludwigia abyssinica, Setaria barbata and Sorghum arandinaceum were absent within 0.5 m from the tress base (Table 3).

Table 2 Physicochemical parameter of soil within each designated subplot in the forest
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Table 3 Plant distribution at radial distance of 0.5 m from trunk of tree (under canopy)
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Within the distance of 0.5–1.0 m from the base of the tree, there were a total of 26 plants identified of different species totaling 179 (Table 4). As with the previous demarcations (0–0.5 m), Reisantia indica, Euphorbia hirta, Tridax procumbens, Chromolaena odorata were also absent (Table 4). Plant distribution under the canopy from within the 1.0–1.5 m radial demarcation included a total of 762 individual plants species comprising of 28 taxa (Table 5); these included Eleusine indica, Cynodon dactylon, Axonopus compressus, Anthropogon gayanus, Tridax procumbens, Snydrella nodiflora and Smilax anceps respectively. Comparing the results of total plant species counted within the 3 demarcations under the tree canopy, it was generally observed that the totality of individual plant species increased further away from the base of the tree. Within the 1.0–1.5-m space, Commelina diffusa, Aneilema beniniense and Aspilia Africana had the highest coefficient of variability amongst the plants discovered. The totality of plant species counted within the entire subplots showed an average of 398 Cynodon dactylon plant species per plot and 215 Panicum maximum species per plot (Table 5).

Table 4 Plant distribution at radial distance of 0.5–1.0 m from trunk of tree (under canopy)
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Table 5 Plant distribution at radial distance of 1.0–1.5 m from trunk of tree (under canopy)
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As shown in Table 6, Triumfetta cordifolia was the fewest plant species within the subplots and was only found in 1 of 5 subplots. However, Eleusine indica , Cynodon dactylon, Axonopus compressus , Commelina diffusa, Aneilema aequinoctiale, Sida garckeana, Schrankia leptocarpa, Anthonotha macrophylla, Reisantia indica, Brachiaria deflexa, Mallotus oppositifious, Euphorbia hirta, Alchornea laxiflora, Alchonea cordifolia, Combretum hispidum, Newbouldia laevis, Tridax procumbens, Synedrella nodiflora, Chromolaena odorata, Gomphrena celosiodes, Panicum laxum, Ludwigia abyssinica, Icacina trichantha, Oplimenus burmanii, Paspalum conjugatum, Setaria barbata, Phylanthus amarus, Sorghum arandinaceum and Smilax anceps were represented in at least 4 of 5 subplots

Table 6 Plant distribution with each quadrant, inclusive of vegetative counts about the test tree
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The percentage of plants abundance within specified distances from the tree base and under the canopy was compared with the totals obtained within the respective subplots and expressed on a percentage (Table 7). E. indica had a 7.03% relative abundance at 0.5 m from the tree base, and 9.83 % further away from the tree, and then 10.16% at the 1.0–1.5-m radial distance from the tree. This was the same for Combretum hispidum, Newbouldia laevis, Gomphrena celoiodes, Aspilia Africana, Ludwigia abyssinica, Oplismenus burmannii, Paspalum conjugatum, Stetera barbata, and Phylantus amarus. However, the relative abundance of Smilax aceps, Schrankia leptocarpa and Icacina trichantha was highest when the plants were closer to the tree base than further away; thereby suggesting possible rhizospheric influence of T. grandis. Aneilema beniniense, Sida garckeana, Reisantia indica, Mallotus oppositifolius, Euphorbia hirta, Alchornea laxiflora, Momoedceae chrantia, Tridax procumbens, Chromolaena odorata, Ageratum conyzoides, Panicum laxum, Ludwigia abyssinica, Stetera barbata and Sorghum arandinaceum were all absent within 0.5 m from the tree base; perhaps suggesting inhibitory rhizospheric influence.

Table 7 Relative abundance of plant species within the radial distances within the tree canopy
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Statistical differences between plant abundance within and outside canopy demarcations have been presented (Table 8). For Eleusine indica, plant abundance under the canopy and outside canopy demarcations were statistically similar; implying that the tree may not have significantly affected plant distribution. Species abundance of Cynodon dactylon, Axonopus compressus, Anthropogon gayanus, Commelina diffusa, Aneilema beniniense, Aneilema aequinoctiale, Sida garckeana, Anthonotha macrophylla, Reisantia indica and Euphorbia hirta were generally suppressed.

Table 8 Statistical differences between plant abundance within and outside tree canopy
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Diversity indices of plant species within and outside canopy demarcations were compared (Table 9). Generally, there were fewer species within 1.5 m from the tree than beyond this radial demarcation, thus indicating inhibitory effects of tree presence. The implication of this suppressed species abundance within close proximity to the tree is the possibility for a number of dominant species to spring up around the tree canopy. With a Brillouin index of 2.941 beyond the canopy demarcation and 2.601 within the canopy, it was suggested that the group diversity of plant species outside the 1.5-m demarcation was slightly higher than within. However, going by the Berger-parkerindex value of 0.258 under tree canopy (UC) compared to 0.190 beyond the demarcation (BC), the dominant species within 1.5 m from the tree were more abundant than those in beyond (Table 9). There was a highly significant negative correlation between species abundance and total organic carbon of the soil outside the tree canopy (R = − 0.880, p < 0.05) (Table 10). Similarly, species index also negatively correlated with soil sulphates (R = − 0.906) at spaces beyond 1.5 m from the tree. Species abundance outside the 1.5-m radial demarcation may have been positively influenced by the soil’s organic carbon from soils in close proximity with the tree (R = 0.916, p < 0.05). The implication of the correlation is that species abundance outside the tree canopy could be enhanced by positively influencing total organic carbon within the canopy or reducing organic carbon outside canopy demarcation.

Table 9 Comparing diversity indices of plant species within or outside canopy demarcation of T. grandis
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Table 10 Bivariate correlation between soil physicochemical parameters and species abundance within and around tree canopy
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Discussion

The results of this study showed that some selected physicochemical characteristics of the soil were influenced by the proximity of the tree to the point of soil collection for analysis.

The pH of the topsoil (0 – 15 cm) obtained randomly within 1.5 m from the base of the tree was higher than somewhere within and beyond the canopy fringes. This supports the earlier findings of Rhoades (1997), who described that soil pH under the single-tree influence was lower under canopy than the outside. Another possible explanation for reduced pH may be in the exudation of organic acids which plants used most times as phytochelators to enhance absorption of nutrients or impede the accumulation of pollutants, as the case may be (Salt et al. 1999). In a similar study by Imoro et al. (2012) in the Afrensu Brohuma Forest Reserve in Ashanti region, Ghana, the authors reported that soil pH was directly influenced by T. grandis (pH = 7.04), when compared with the control plot (pH = 7.53). Watanabe et al. (2009) documented lower pH values (pH = 7.14).

Kanazawa et al. (1994), Pellet et al. (1995) reported that plants in most iron-contaminated soils usually have need of organic acids that enhance bio-availability of soil-bound iron. The survival of most of these plants in acidic soils also depends on their ability to exude citric and malic acids, amongst other organic acids to chelate the highly phytotoxic rhizospheric Al3+ to form a less toxic complex, a phenomenon which is also common in many oxisols and ultisols; particularly the most predominant soil type in Benin City, Nigeria. Apart from the fact that Al3+ enhances soil acidity (Merino-Gergichevich et al. 2010), the release of organic acids within root zones of the tree to chelate the metal further reduces the soil pH around this rejoin; perhaps the justification for the reduced pH reported compared to outside the canopy demarcation.

Although no single mechanism is responsible for changes in soil chemistry, we observed single-tree influence of T. grandis was also observed in the soil composition of total organic carbon, total nitrogen and soluble phosphorus. The concentrations of these soil characteristics under the canopy were higher than beyond; also confirming earlier reports (Rhoades 1997, Zinke 1962). The possibility exists therefore that the forest environment probably affect soil nutrients dynamics as earlier suggested by Lal (2005). Imoro et al. (2012) reported that soil nitrogen under the T. grandis plantation minimally surpassed that outside the tree plantation.

Enhanced accumulation of organic carbon and phosphorus is most likely attributed to the enormous organic materials, which are consequences of the decay of fallen litter that gathers around the tree. In a number of isolated cases, as observed in the study, some of the foresters, when carrying out routine slashing of the weeds around the planted forest, usually gather most of the weeds and place them around the trunk as mulch. Increasing the quantity of plant material incorporated into the soil usually would further advance soil nutrient standing. Increased organic matter has been reported by Dinakaran and Krishnayya (2010) in teak forested areas.

Single-tree influence in plant association is one of several factors that affect the overall dynamics in agroforestry systems (Rhoades, 1997). Some authors reveal that such influences may be necessitated by phytotoxins in the soil that may be plant-related (Harborne 1977, Rauha et al.2000). Allelopathy is one of such plant-mediated influences that affect tree-plant interactions (Harborne 1977). Plants produce a large diversity of secondary metabolites including phenols and fatty acids which have an overall allelopathic effect on the growth and development of neighbouring plants species (Li et al., 2010). Other impeding factors may be poor availability of light necessitated by the tree canopy (Rauha et al. 2000). This means that those weeds or plants species that were located very close to the tree base would be sparsely abundant or distributed. This was the general observation about plant species abundance within 1.5 m from the base of the tree base; thus implying a negative single-tree influence of T. grandis on neighbouring plant diversity. It is, however, important to note that close pointy of the tree also enhanced the development of those plant species that were hitherto not found beyond the canopy.

In another development, the species abundance of some plants increased away from the canopy cover, whereas, for some, it decreased outside the cover than within the cover. Specifically, the growth of Sida garckeana, Reisantia indica, Momordica charantia and Tridax procumbens was completely impeded within 1.5 m from the tree. Being in close proximity to the base of the tree necessitated the development of Triumfetta cordifolia, which was not found in any other location other than under canopy within the forest. This might be due to the allopathic influence of T. grandis. Moreover, even though previous workers (Falk et al., 2008; Schnabel et al. 2017; Habashi and Waez-Mousavi, 2017) have reported a similar selective effect of single-tree on some plant species and soil microfauna, the mechanism is still unclear and may be short lived.

The possible association between soil physicochemical characteristics and plants species abundance under the single-tree influence suggested that increased sulphates in the soil might enhance plant species abundance under the influence of the tree canopy. Sulphates have been reported to enhance nutrient availability and acquisition by plants (Prade et al. 1993; Mitra et al. 2009). However, a negative association with phosphates was observed outside the tree canopy. Phosphorus is an essential macronutrient for plant growth, and it is limiting crop production in many regions of the world (Holford 1997). Increased phosphorus lead to increased plant development because phosphorus converts sunlight into usable energy, and essential to cellular growth and reproduction (Malhotra et al., 2018). The association statistics presented in Table 10 suggest both the negative and positive association between species abundance and total organic carbon under and outside canopies respectively. The negative association of this essential plant nutrient with plant species abundance within and beyond the canopy fringes calls for more scrutiny.

Species abundance outside the 1.5-m radial demarcation positively correlated with the total organic carbon of soils in close proximity with the tree (R = 0.916, p < 0.05); the implication being that enhancing soil organic carbon with the tree canopy may be an important factor in increasing species abundance beyond this demarcated area. As reported earlier, total organic carbon within the 1.5-m demarcated area to the tree was significantly higher than away from the area. Given the significant role organic carbon plays in plant species development, diversity and abundance through enhancing soil porosity, aggregate stability and water-holding capacity (Wehr et al. 2017), it is suggested that a reduction in organic matter of soil may have, amongst other biological and physicochemical factors, contributed to poor plant abundance of some plant species. Although there was generally more plant species outside the 1.5-m demarcation than within, an increase in soil organic matter may further enhance such plant species abundance.

Conclusion

The single-tree influence of T. grandis on plant species abundance as well as characteristics of topsoil in an 8-year old planted forest has been investigated. Much as increased diversity of certain species was reported in close proximity to T. grandis, most of the plant species identified were negatively impacted very close to the tree. Given the fact that plant-plant associations affect the quality of forest soils, the impact of T. grandis in forest soil quality is possibly a factor of the outcome of its association with neighbouring plant species.