The forest, as Nairobi, has two wet seasons: April to June and October to December. In July and August it is cool, cloudy and dry. From August to December it is sunny and dry. January, February and early March are hot and dry months.
The average annual rainfall is 930 mm (37 inches), varying from 1250 mm (50 inches) during wet El Niño periods to 350 mm (14 inches) during drought spells. The peak average rain months are April, May and November (see chart, right).
Temperatures throughout the year vary according season, cloud and sunshine, but the range is from 8° C (45° F) on a chilly August morning to 28° C (82° F) on sunny February afternoon.
2012 appears to have been the wettest year in 67 years: nearly twice the long-term average (dotted line in chart on right below). Rainfall measurements taken near the Karura’s southern boundary since 1945 at the Muthaiga Country Club suggest that only the famous flood year 1963 came close to the 1,770 mm (70″) measured along the Gitathuru River.
The April 2012 total of 512 mm was the most ever recorded for that month, and December’s 300 mm was second only to Dec ’63. Moreover, records in recent years show an increasing annual trend (orange line in chart). Anyone still doubt the climate is changing?
The forest sits on million-year-old Late Tertiary volcanic rocks. Technically speaking, the common rock forms are volcanic tuffs with intercalated flows of basaltic sponge cake. Both types are occasionally exposed in Karura’s deeper river valleys, and the tuffs yield the familiar grey building stone of Nairobi. “Chimneys” of larva are occasionally found exposed on ridges in the western and middle sections of the Forest.
The Karura landscape rolls gently between and through shallow valleys. Drainage is generally southeasterly. Depressions throughout the forest impede drainage and cause formation of small edaphic grassy swards and swamps, some of which are under threat from thirsty Eucalyptus trees.
Five perennial tributaries of the Nairobi River pass through the forest running roughly west to east and cutting through gently undulating landscape. These are:
- the Ruaka, which runs through the Runda Estate and forms part of the northern forest boundary;
- the Karura, which cuts through a deep valley in the northern part of the main forest;
- the Gitathuru, which separates the main forest along its southern edge from the Old Muthaiga Estate;
- the Thigiri (a tributary of the Gitathuru), which forms the northern border of the Sigiria section of the forest; and
- the Mathare River, which borders the forest in Peponi.
The Karura River valley offers a precarious and stunning descent through indigenous forest to the large waterfall and the Mau-Mau caves.
Since the Late Tertiary, when Mt. Kenya and Kilimanjaro were formed, the area has suffered only moderate tectonic disturbances. Thus the parent rocks have been weathered deeply to produce even soil profiles. Under natural forest, the resulting soil is very deep, ridding brown clayey loam with slow but free profile drainage. Such soils become particularly sticky when wet, yet dry very quickly with a tendency to shrink and crack. The upper few inches of soil are usually stained dark brown with incorporated humus; no deep litter layer develops.
Beneath grassland in freely drained areas, the soil is very similar to that found under forest stands. However, in the low-lying areas a different soil is found. These low-lying areas are intermittently water-logged; the water table fluctuates considerably and a small amount of fine soil material is usually transported down the catena from neighbouring higher ground. Under such conditions, the soil is usually very heavy, dark grey clay; this is often stained black with un-decomposed humus, so-called ‘black cotton’ soil. Between 5 cm (2 in) to one metre (ca. 3 ft) below the clay layer, a red-brown laterite occurs, a product of re-cementation that is rich in iron compounds and associated with swampy areas and a shifting water table.
Generally speaking, the forest soils are eminently suited to tree growth, except in the impeded drainage of swampy sites that provide natural edaphic grassy glades characteristic of Kenya’s upland forests.
Forest plantations cover some 630 ha. Species include imports from South America, Australia and the Asian sub-continent, such as Araucaria cunninghamii, Grevillea robusta, Eucalyptus saligna, E. globule, Cupressus torulosa and Cupressus lusitanica.
Almost all the plantations in the forest have passed their economic rotation age. The Eucalyptus range from 38 to 83 years, Araucaria, 44 to 46 years, and Cupressus, 34 to 46 years. Most of these plantations have already become to succumb to age-related drying. Part of the FKF-KWS management plan includes a campaign to replace the degraded plantations with indigenous species.
Indigenous trees cover approximately 260 ha (not including some 25 ha in the largely alienated 110-ha salient east of Kiambu Road). Species include Olea europeae subsp. auspidata, Croton megalocarpus, Warburgia ugandensis (Muthiga in the vernacular), Brachyleana huillensis (Muhugu, the iconic image on the FKF logo), Uvaridendron anisatum, Markhamia lutea, Vepris nobilis, Juniperus procera (Cedar), Craebea brownii (a huge specimen sits just outside the largest Mau-Mau cave), Newtonia buchananii, Salvadora persica (Mswaki, the Toothbrush Bush), Ficus thonningii (Mugumu), Trichilia emetica, Calondendrum capense and Dombeya goetzenii.
Additionally a number of shrubs are also found which have wide local medicinal use; these include Strychnos henningsii (Muteta), Erythrococca bongensis (Muharangware), Vangueria madagascariensis (Mubiro), Rhamnus prinoides (Mukarakinga), Caesalpinia volkensii (Mubuthi), Solanum incanum (Mutongu, Sodom Apple), Elaeodendron buchananii (Mutanga) and Rhus natalensis (Muthigio).
The name ‘Muthaiga’ (the neighbourhood forming the southern boundary of the main Karura forest) derives from the Kikuyu name — ‘Muthiga’ — of the East African Greenheart tree, Warburgia ugandensis? Look out for the dark green elongated leaves and the knobbly, reddish-brown bark. You can often see where a chunk of bark has been harvested by a traditional herbalist to infuse as a remedy for chest pains, colds, malaria and toothache, or in a stew: most parts of the tree have a hot, peppery taste.
The riparian belts along the Gitathuru and Ruaka streams host groves of Arudinaria alpina, Kenya’s native bamboo species. The exotic giant bamboo Dendrocalamus giganteus is mainly found growing within the tree nursery along the Karura River. Additionally, small wetlands are found throughout the forest (occupying some 10 ha). These provide important habitats for birds and bird watchers.
Indigenous orchids — representatives of nine Kenyan species — have been re-introduced to Karura Forest thanks to generous donations of plants from members of the Kenya Orchid Society in August 2012. With the assistance of Friends of Karura Forest, Alexandra Contos and Daniel Odhiambo (striped shirt), one of Kenya’s foremost orchid experts, carefully placed the plants in suitable mid-canopy positions.
Left is one of the Aerangis already in flower.
Mammals, Birds and Reptiles
The forest is known to host a variety of animals. These include the Suni, Harvey’s Duiker, Bushbucks, Bush Pigs, Genets, Civets, Honey Badgers, Bush Babies, Porcupines, Syke’s Monkeys, Bush Squirrels, Hares and the Epauletted-Bat. A Side-striped Jackal has been recorded in Sigiria.
A young female bushbuck who took up residence in the reforestation area not far from the Limuru Road gate. She seemed quite relaxed among the protected young trees. The only problem she had was all the flies that emerged with the good October 2012 rains.
To date, some 200 bird species have been seen in the forest. These include Ayres Hawk-eagle, the African Crowned Eagle, the Silvery-cheeked Hornbill, Hartlaub’s Turaco, the Narina Trogon, the African Wood Owl, Crested Cranes, Sparrows, Doves, Weavers and Vultures. The call of the African Snipe has been heard at Lily Lake.
A pictorial list of the main bird species (compiled by Mr. Amedeo Buonajuti) is available at the entrances to the forest free of charge.
Reptiles include the rock pythons, numerous other harmless snakes, plated and monitor lizards.
A detailed inventory of the fauna is planned to be conducted with the National Museums of Kenya.
The rains give opportunities for spotting animal tracks. Here are some bushbuck tracks along the road between Junction 6 and the entrance to KFEET. It’s probably an adult male, judging from the size of the track compared to that of a kid or small woman who walked along the road as well. The bushbuck probably comes out at night to browse along the roadside bushes.
Photo tip: to get the most out of a track shot, make sure the track is between you and the sun.
Insects and other Arthropods
A detailed inventory of non-vertebrate species is planned. Preliminary collections have been undertaken by ICIPE, the International Center for Insect Physiology and Ecology, and you may catch a glimpse of white, tent-like insect traps within the forest