Vol. 7(11), pp. 271-276, November, 2015 DOI: 10.5897/JENE2015.0538 Article Number: 6E4E54455715 ISSN 2006-9847 Copyright © 2015 Author(s) retain the copyright of this article http://www.academicjournals.org/JENE Journal of Ecology and the Natural Environment Full Length Research Paper An inventory of plant species found in gravel borrow pit around Gaborone, Botswana Israel A. Legwaila*, Gaone T. Thebe and Tebogo Selebatso Botswana College of Agriculture, Department of Crop Science and Production, Private Bag 0027, Gaborone, Botswana. Received 27 August, 2015; Accepted 28 September, 2015 This study sought to establish an inventory of plant species established at three gravel borrow pits around Gaborone, Botswana. At each, plant data were collected from randomly selected 10 x 10 m quadrats within and up to 30 m away from the borrow pits. Species of Acacia were the most numerous in the pits, while more non-woody than woody species were established within the pits. Most woody species within the pits were not found around the pits, and most of the non-woody species within the pits were found around them, with the exception of the Tlokweng pit. Key words: Re-vegetation, reclamation, pioneer species, seed bank, borrow pit, Acacia. INTRODUCTION Quarrying for gravel has a number of negative impact that affect the integrity of the environment including degradation of ecosystems. The removal of vegetation reduces the ecology of a site and exposes it to other environmental factors such as soil erosion that may exacerbate the negative impacts already caused. However, efforts have been made worldwide to reduce these negative impacts through reclamation by recontouring and re-vegetation (Cripps et al., 2004). In both cases, natural succession will occur, re-establishing native vegetation on the sites. This happens through species colonization, spreading, displacement and replacement over time until climax species are established (Davis et al., 1985). These processes happen as the quarry soils change overtime due to weathering and other physical, chemical and biological processes fed by pioneer species, (Legwaila, 2012). Under natural succession, establishment of vegetation on disturbed land is dependent on the availability of a seed bank from vegetation adjacent to a disturbed site. Landform and soil characteristics may also have an impact on establishment of vegetation (Davy, 2008). Landform in disturbed land may experience accelerated erosion and runoff, inhibited infiltration and unfavourable micro climatic conditions, all of which have a negative effect on the natural recovery processes of disturbed land (Whisenant, 2008). Where topsoil has been replaced after decommissioning of a borrow pit, the soil is expected to carry numerous seeds of plant species from the local environment. However, it generally takes longer for environmental impacts to decrease and for desired outcomes to be achieved under natural succession than under technical reclamation and there will always be left over impacts regardless of the type of reclamation interventions (Figure 1). When technical re-vegetation is undertaken, the process may occur more rapidly resulting in more rapid re-vegetation. Technical re-vegetation however, requires amelioration *Corresponding author. E-mail: ialegwaila@gmail.com. Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License J. Ecol. Nat. Environ. Environmental impacts 272 Time Figure 1. An illustration of the extent of environmental impacts over time after different reclamation interventions (Legwaila, 2012). Table 1. Geographiccoordinatesof the sites. Corners A B C D Bokaa S 24° 26` 02 E 26° 02`49.5 S 24° 26`03 E 26° 02` 58.8 S 24° 25`52 E 26° 02`57.7 S 24° 25`53 E 26° 02`15.7 Place Tlokweng S24° 40’ 20.52E 26° 02’14.8648 ’ ’ S24° 40 19.90E26° 02 25.7320 ’ ’ S 24° 40 29.39 E26° 02 24.4906 ’ ’ S 24° 40 29.13E 26° 02 13.3490 of the quarry soils to support plant life as well as proper choice of plants which can survive the local conditions and provide the functions that are intended for the quarry after reclamation. This requires knowledge and selection of native species that are common to establish in disturbed land and be productive under less than normal conditions of quarry sites to ensure sustainability of the vegetation. It has been found in other studies that at times exotic species establish in decommissioned quarry sites, and has been concluded that they may have been introduced by humans (Davis et al., 2000). It has also been concluded that use of non-native species is “the second most important threat to biodiversity after habitat loss”, (Vermeulen and Whitten, 1999). This study sought to identify and develop an inventory of plant species that established in borrow pits that were quarried for gravel around the city of Gaborone, Botswana. Mmokolodi S24° 28’ 09.0 E 26° 58’14.0 ’ S24° 28’ 07.0 E 26° 58 23.0 ’ ’ S 24° 28 17.0 E 26° 58 36.0 ’ ’ S 24° 28 25.3 E 26° 58 28.0 occurs in the city of Gaborone and the neighbouring areas. The land in these areas was previously used for communal grazing. The borrow pits were selected based on their close proximity to Gaborone. Their coordinates were taken at four corners of the sites (Table 1). The soils around these sites were slightly variable but generally supported a shrub savanna and savanna vegetation structure. The soils around Bokaa and Tlokweng pits were haplic lixisols which are common in the tropics with predominant dry seasons. They form subsequent to leaching of clay. They have very low levels of plant nutrients and are highly erodible. Mmokolodi on the other hand had eutric regosols which are weakly developed soils very common in unstable landforms. They also have low levels of nutrients and nutrient holding capacity. Experimental design Within each borrow pit site, four (4) quadrants of 10 m by 10 m were randomly selected. Around each borrow pit, eight quadrants of 10 m by 10 m from different sides of the pits were randomly selected to conduct an inventory of plant species around the pits. This procedure was performed to enable comparison of plant species within the borrow pits and those aroundthem. METHODOLOGY Study sites Collection and identification of plant species This study was conducted at three decommissioned borrow pits around Gaborone. The pits were located in the villages of Bokaa, Mmokolodi and Tlokweng. These borrow pits came about as a result of the high infrastructure construction developments that All plants seen in the quadrats were recorded to species level. For those that could not be reliably identified in the field, specimens were collected for later identification in the herbarium at the Botswana College of Agriculture. The same procedure was Legwaila et al. 273 Table 2. Plant species found within the three borrow pits. Bokaa Plants within pit Plant outside pit Acacia erubescens* Acacia tortilis* Acacia gerrardii* Acacia mellifera* Acacia mellifera* Achyranthesaspera Acacia nigrescens* Acrotome inflata Acacia tortilis* Aristida congesta subsp barbicollis Acrotome hispida Aristida conjesta Aristida congesta Asparagus bechuaniscus Aristida congesta subsp. barbicollis Combretum imberbe* Asparagus bechuanicus Carissa bispinosa* Ceratotheca triloba Chloris virgata Chloris virgata Combretum apiculatum* Corchorus olitorius Combretum hereroense* Cucumis myriocarpus Cucumis myriocarpus Dicerocaryum eriocarpum Cyperus turrillii Dicoma tomentosa Ehretia rigida* Dodonae aviscosa* Euclea undulate* Ehretia rigida* Evolvulus alsinoides Euclea undulata* Gomphocarpus fruticosus Evolvulus alsinoides Gomphrera celosioides Fimbristylis hispidula Grewia flava* Gomphocarpus fruticosus Grewia flavescens* conducted outside the borrow pits from a distance of 30 m to the edge of the pits. RESULTS AND DISCUSSION Overall there were 44 species (11 woody and 33 non-woody) found at all borrow pits (Table 2). The woody species made a total of 25% of all the species found. The majority of the woody species were Acacia species making 46% (five different species). All of the other six species contributed 9% each. A total of 5 woody species, four Acacia Tlokweng Plants within pit Plant outside pit Acacia erubescens* Acacia erubenscens* Acacia grrrardii* Acacia gerrardii* Acacia mellifera* Acacia mellifera* Acacia tortilis* Aristida congesta Achyranthesaspera Aristida congesta subsp barbicollis Aristida congesta subsp barbicollis Asparagus bechuaniscus Asparagus bechuaniscus Combretum imberbe* Ceratotheca triloba Carissa bispinosa* Corchorus olitorius Ceratotheca triloba Dicerocaryum eriocarpum Cucumis myriocarpus Dicoma tomentosa Cyperus turrillii Echinochloa holubii Dichrostachys cinerea* Ehretia rigida* Gomphrera celosioiides Euclea undulate* Grewia flava* Gomphrena celosioides Grewia flavescens* Guillemineadensa Kalanchoe lanceolata Hermbstaedtiafleckii Lantana rugosa Kyphocarpa angustifolia Ocimum canum Melinis repens Pappea capensis Monsonia angustifolia Peltophorum africanum* Sesbania bispinosa Pogonarthria squarrosa species and Euclea undulate were present at each of the pits, (Table 3). The predominance of the Acacia species might be an indication of their greater tolerance to low levels of resources within the pits (Smith and Smith, 2014; Donfack et al., 1995). This enables them to efficiently exploit the little resources in the soil, better than other species. There was also an exotic species (Dodonea viscosa) at Bokaa pit commonly used as an ornamental plant in Botswana. Its existence in the pit could be attributed to the anthropogenic activities that took place at the pit during operation (Davis et al., 2000). Of the 33 non-woody species Mmokolodi Plants within pit Plant outside pit Acacia tortilis* Acacia tortilis* Acacia erubescens* Asparagus bechuanicus Acacia gerrardii* Cambretum imbrebe* Acacia mellifera* Carissa bispinosa* Acacia nigrescens* Combretum apiculatum* Achyranthes aspera Combretum hereroense* Acrotome hispida Dichrostachys cinerea Aristida congesta Subsp congesta Euclea undulata* Aristida congesta subsp barbicollis Evolvulus alsinoides Asparagus bechuanicus Gomphocarpus fruticosus g Corchorus olitorius Gomphrera celosioides Dicerocaryum eriocarpum Grewia flava* Dicoma tomentosa Grewia flavescens* Echinochloa holubii Kalanchoe lanceolata Euclea undulata* Kyphocarpa angustifolia Evolvulus alsinoides Peltophorum africanum* Fimbristylis hispidula Perotis patens Gomphrena celosioides Pogonarthria squarrosa Gomphocarpus fruticosus Grewia flava* Guilleminea densa - found at the different pits, 12 (36%) were common to all pits (Table 3). There was a high number of non-woody plants that made the understory made up of grasses and small shrubs. The trees that existed with them might have created conducive micro-climatic and soil conditions, facilitating their growth and establishment (Smith and Smith, 2014; Whisenant, 2008). It was expected that most plants that established within the borrow pits would either have been from the soil seed bank in the surrounding land or as a result of seed dispersion 274 J. Ecol. Nat. Environ. Table 2. Contd. Gomphrena celosioides Grewia flava* Guilleminea densa Hermbstaedtiafleckii Hypertelis bowkeriana Kohautia subverticillata Kyphocarpa angustifolia Melinis repens Monsonia angustifolia Ocimum canum Rhigozum brevispinosum* Schmidtia pappophoroides Sesbania bispinosa Sida cordifolia Solanum sp. Streptopetalum serratum Urochloa mosambicensis Kalanchoe lanceolata Kyphocarpa augustifolia Lantana rugosa Ocimum canum Pappea capensis Peltophorum africanum* Perotis patens Pogonarthria squarrosa Solanum sp Streptopetalum serattum Tarchonanthus camphoratus* Tricholaena monachne Urochloa mosambicensis - Streptopetalum serattum Tricholaena monachne Urochloa mosambicensis Ziziphus mucronata* - Streptopetalum serattum Tricholaena monachne Urochloa mosambicensis Zyzypus mucronata* - Hermbstaedtiafleckii Hypertelis bowkeriana Kohautia subverticilla Kyphocarpa angustifolia Melinis repens Monsonia angustifolia Pogonarthria squarrosa Sesbania bispinosa Sida cordifolia Tricholaena monachne Urochloa mosambicensis - *Woody species found within the pits. Table 3. Plant species common at all three borrow pits. Botanical Name Acacia tortilis* Acacia gerrardii* Acacia mellifera* Acacia nigrescens* Aristida congesta subsp. barbicollis Asparagus bechuanicus Dicerocaryum eriocarpum Dicoma tomentosa Enchinochloa holubii Euclea undulate* Gomphrena celosioides Guilleminea densa Hypertelis bowkeriana Kyphocarpa angustifolia Common Name Mosu (Hairy umbrella thorn) Moga, Moki (Red thorn; Grey haired acacia) Mongana (Black thorn; Hook thorn) Mokoba ( Knob thorn) Seloka; Bojang-ja-motlhaba-jo-bonnye; (Buffalo grass) Lesitwa, (Wild asparagus) Tshetlho; Legatapitse (Devil’s thorn; boot protector plant) Ombahu (Limpopo grass; Antelope grass; Kalahari water grass) Motlhakola (Thicket euclea) Mositanoka (Prostate globe amaranthas; bachelor’s button) Mohulapitse (Small mat weed) Motlhabana; Munyu-wa-pasi Mosono-wa-mmutla; Silky burweed; Hare’s tail bush - Legwaila et al. 275 Table 3. Contd. Melinis repens Monsonia angustifolia Sesbania bispinosa Lenapa; Senyane (Fairy grass) Phusana; Tsatsalopane (Crane bill) Mositanokana; Selaole (Spiny sesbina) Number of species *Woody species found within the pits. 8 7 6 5 4 3 2 1 0 Common Species Species found outside pit only Bokaa Tlokweng Mmokolodi Borrow pits Figure 2. A comparison of woody species within the pits and adjacent to the pits. from the vegetation stand surrounding the pits. The distance of the different species from the pits and their dispersal characteristics could also have affected what got established in the pits (Makhabu and Marotsi, 2012). It should be noted that vegetation surrounding the pits was only sampled to a distance of 30 m. It was found that the majority of woody species that grew around the pits were not found growing within the pits (Figure 2). With the exception of the Tlokweng pit, majority of non-woody plants that were found around the pits were also found growing inside the pits (Figure 3). Conclusion It can be concluded that Acacia species have a great tolerance for poor underdeveloped soils found in decommissioned borrow pits. This can make them suitable candidates for pioneer species in technical reclamation, especially when there is a limitation of topsoil available for revegetation. They can be used to provide conducive environmental conditions for other plant species. It can also be concluded that most non-woody species are easy to establish in disturbed lands. They are useful in the control of negative impacts such as soil erosion as well as improving the capability for water infiltration. This contributes to the success of other processes such as decomposition which can accelerate development 276 J. Ecol. Nat. Environ. Number of species 14 12 10 8 Common Species 6 Species found outside pit only 4 2 0 Bokaa Tlokweng Mmokolodi Borrow pits Figure 3. 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