Urbanization in developing countries has accelerated in the past twenty years and nearly half of world’s population are urbanized and projected to increase (K’Akumu et al., 2007). As urban  areas  expand  existing forests may be destroyed, fragmented, or included as part of urban landscapes, exposing them to a number of threats including destruction of native species, invasion by exotic species, fires caused by human activities, pest and pathogen outbreaks, and unmanaged outdoor recreation activities (Chavez, 2005). Additionally, the process of urbanization leads to destruction and removal of vast area of the forests with serious impacts on the indigenous tree species (Ouinsavi and Sokpon, 2010). The replacement of indigenous species with exotic results in biotic homogenization and reduction in the biological uniqueness of local ecosystems (Blair, 1999). Nairobi’s large and growing population is one of the main forces driving the city’s overwhelming environmental challenges. Ongoing rural to urban migration, high natural birth rates, and poor or inappropriate city planning conspire to continue degrading the city’s water and air quality. In turn, environmental degradation has impacts on human health and the economy (Tibaijuka, 2007).

 

The physical expansion of Nairobi has come at the expense of the natural environment. The urban sprawl, construction of roads and other city infrastructure has led to the loss of forests and other natural areas, such as mixed rangeland and bush lands (Tibaijuka, 2007). As a result, the forest cover receded and was replaced by coffee plantations. Later, the demand for food for the growing population led to the transformation of the city’s outskirts to other agricultural uses, which in turn were threatened by further urban growth. The rise of unplanned settlements poses a threat to the protected areas in terms of depletion, pollution, plant diversity and destruction of habitats.

 

From the aforementioned challenges, this study aimed to determine the tree variation and distribution in the major green spaces in Nairobi city, Kenya.

Description of study area

 

The city is located at the South-eastern end of Kenya’s agricultural heartland, between approximately longitude 1° 9’S, 1° 28’S and latitude 36° 4’E, 37° 10’E (Figure 1). It occupies an area of about 696 km² and the altitude varies between 1,500 and 1,850 m above sea level (Tibaijuka, 2007).

 

 

City Park is located between Limuru road and forest road. It was curved from Karura forest and maintained as a recreation facility.

 

 

Type of tree species and distribution

 

A total of 1,850 trees were sampled in 128 quadrats. Of these, 69.3% of the trees were native species, while the rest were exotic species. A total of 84 tree species were encountered which were distributed across 37 families; 69 out of 84 were native while 15 species were exotic. Plants in the Mrytaceae and Euphorbiaceae families were most common each with seven and six species, respectively. Four families had 5 species, two families had 4 species, five families had 3, and three families had 2 species each (Table 2). Overall, Karura forest recorded the highest abundance with 916 trees followed by City Park and Ngong’ forest which had 491 and 445 trees, respectively. Overall, the most dominant species was Eucalyptus paniculata followed by Drypetes gerrardii and Teclea trichocarpa. Among the ten dominant species, eight were indigenous and only two were exotic.

 

 

Karura forest contributed seven of the top ten most abundant tree species, followed by City Park with two and Ngong’ forest with one. In City Park, D. abysinnica was the most abundant tree species with a count of 54 while the least abundant trees were Ficus lutea, Elaeodendron buchananii, Pandanus utilis and Mystroxylon ethiopicum with a count of 1. At Karura forest, E. paniculata was the most abundant tree species with a count of 214 with trees like Croton aleinus, Combretum mole, Grewia similis and Vangueria madagascariensis having a count of 1. In Ngong’ forest, E. paniculata was the most abundant tree species with a count of 118 and with trees like Acocanthera oppositifolia, Cordia africana, and Maytenus undata having a count of 1 (Figure 2a to c).

 

The abundance of the identified species when compared with incidence indicates that at city park D. abysinnica was more evenly distributed, occurring in 18 of the 36 sampled quadrats; unlike E. paniculata which was unevenly distributed as it appeared only in 12 and 8 out of the sampled 51 and 41 quadrats in Karura and Ngong’ forests respectively. In Karura and Ngong’ forests, the plantations done at specific areas  were  the contributing factor of the unevenness of E. paniculata (Figure 2a to c).

 

 

 

 

Disturbance of the green spaces leads to increased unused resources which gives way to the invasive species that takes over the gaps created due to the disturbance (Davies et al., 2008). The boundaries of unprotected green space especially City Park, therefore had more invasive species dominating unlike the interior parts which were less disturbed. The study showed that Nairobi green spaces had high tree species richness; with 84 species recorded from128 quadrats sampled containing a total of 1850 trees. The tree distribution was however dominated by a few species with the top five species accounting for almost 50% of all tree species. There was a clear variation in species richness between City Park, Karura forest and Ngong forest. City Park leads in mean species richness followed by Karura forest and Ngong’ forest respectively. This could be due to higher disturbance which opens space for other species to develop as the space regenerates. City Park is more open to visitation and the bushes regularly cleared by the management thus creating some level of disturbance that allows other plant species to establish. Karura and Ngong forests though enclosed, they have experienced various levels of uncontrolled disturbance and encroachment from the surrounding communities in informal settlements. In order to protect the many indigenous plant species that create these invaluable habitats, efforts are needed to continually manage them in sustainable manner by engaging the key stakeholders. Recent initiatives such as by the ‘Friends of Karura Forest’ to fence the forest, conduct tree plantings, and initiate guided nature walks for visitors, through the Kenya Forest Service and by involving the neighboring residents of Huruma community will contribute greatly in the conservation of Karura forest. In City Park, Nature Kenya, through  ‘The  Friends  of  City’  contributes  in   gathering useful ecological information and on many occasions has helped to protect the habitat from encroachment and elevate its value as conservation and natural recreation site for City residents. Similar efforts at Ngong Forest will broaden the understanding about green spaces to a wider population, change perception and help to inform on best management practices to sustain their value. Stakeholders such as the Kenya Forest Services (KFS), the Department of Environment of Nairobi County, researchers, planners and the civil society should continue engaging the surrounding communities in the protection of urban green spaces through training and initiating environmentally friendly alternative sources of income rather than those deemed destructive. As indicated by the Shannon’s index of diversity in the various transects for the three sites the value is low mainly due to over dominance of few species, some exotic. These green spaces must be continually managed to protect and enhance species composition and distribution as they are essential habitats for biodiversity and serve other key ecological functions.

 

The authors have not declared any conflict of interests.

 

 

The authors wish to thank the International Foundation for Science (IFS) for providing funds to carry out this research, the Kenya Forest Services for the permit to access the sites and personnel from National Museums of Kenya for support in field survey.