Geology and climate
The EAMs consist of 13 disjunct ranges (‘blocs’), stretching from the Taita Hills in southeast Kenya to the Udzungwa Mountains in south-central Tanzania (Fig. 1.4). The substrate is Precambrian crystalline basement rock, uplifted from the African plateau during the Miocene c. 30 MY BP (Schlüter, 1997), although initial faulting may have begun over 100 MY earlier (Griffiths, 1993). Elevations range from 121-2636 m above mean sea level (the summit is Kimhandu Peak in the Uluguru range).
Mean annual temperatures across the study region range from 12.4-24.1 ˚C (mean, 20.7 ˚C;
WorldClim interpolated climatology; Hijmans et al., 2005). The warmest months are
November through March, with mean daily maxima exceeding 34 ˚C on lower slopes. The
coolest months are June through August, when mean daily minima drop below 5 ˚C at high
altitudes (frosts are not uncommon in the upper-montane zone). Slopes close to the Indian Ocean are several degrees cooler than equivalent altitudes elsewhere. Air temperatures in the understory vary according to distance from the forest edge and the degree of solar radiation penetrating the canopy (Newmark, 2001; Newmark, 2005).
Annual rainfall is typically in the range 500-2000 mm.y-1, but in some blocs exceeds 3000
mm.y-1 (precipitation-radar measurements, 1997-2006; Mulligan, 2006). Windward eastern
aspects are wettest due to orographic rainfall and mist driven by Indian Ocean currents (Marchant et al., 2007) and sustain continuous broadleaved forest cover where land use allows. Drier western flanks tend to support deciduous woodland rather than moist forest assemblages (Newmark, 1998), whilst grassland and heathland are common on the uppermost montane plateaus (Finch and Marchant, 2011). The north of the study region is generally recognised to have two distinct peaks in rainfall: short rains from March to May and heavier rains from October to December. In the south, one rainy season prevails from December to April. Analysis of recent precipitation-radar measurements suggests a rather more complex and variable distribution of seasonal rains. A consistent pattern is that East Usambara and Uluguru experience the most benign dry seasons, with some eastern aspects perhumid (> 100 mm.m-1; Pócs, 1976). Drier spells are most prolonged and intense on
western margins of Ukaguru, Rubeho and Udzungwa, which receive little rain for three or more consecutive months.
Biological importance
The EAMs support around 3300-5700 km2 of moist tropical forest (Newmark, 2002;
Burgess et al., 2007a), including some of the world’s most important and fragile sites for biodiversity conservation (Brooks et al., 2002; Mittermeier et al., 2004). The mountains contain over 14% of the vascular plant species indigenous to tropical Africa in less than 0.25% of the land area1. Of these species, 471 are strictly endemic (Chapter 6), including
some now common in western households (Fig. 1.5).
Figure 1.5. Flora and fauna of the EAMs. Well known houseplants: (a) Busy Lizzie (Impatiens sp.), (b) African Violet (Saintpaulia sp.). Recently discovered: (c) critically endangered Kipunji genus, (d) grey-faced sengi / ‘elephant shrew’ (Rhyncocyon udzungwensis) – both in the ‘top ten’ new species of the decade (Conservation International and BBC Natural History Unit, 2010). Photo credits: a, Michele Menegon; b, Nobby Cordeiro; c, Trevor Jones; d, Francesco Rovero.
1Excluding Madagascar and inland water bodies, tropical Africa covers c. 22 million km2 and contains 26,848
indigenous vascular plant species (AFPD, 2010, corrected to remove introduced species; R.E. Gereau and H. Beentje, pers. comm.). The EAMs, as here defined in Chapter 2, cover c. 52,000 km2 (including plateaus) and
A number of forest-dependent birds and mammals are endemic, such as the Usambara eagle owl (Bubo vosseleri) and the recently discovered grey-faced sengi (Rhyncocyon udzungwensis; Fig. 1.5). Concentrations of range-restricted herpetofauna and invertebrates are especially impressive – single site endemism within the latter is as high as 80% (ICBP, 1992; Lovett and Wasser, 1993; Stattersfield et al., 1998; Burgess et al., 2007a; Poynton et al., 2007). Faced with ongoing habitat loss and degradation, the EAMs are considered a ‘hyper hot’ priority for conservation (Myers et al., 2000; Brooks et al., 2002).
Delineation and classification
The term ‘Eastern Arc’ was first established in the 1980s, when botanical explorations in the region were documenting large numbers of forest species of highly restricted distribution (Lovett, 1985). Some mountains in the chain were already known for their biological importance, notably in the Usambara and Uluguru ranges (Polhill, 1968; Pocs, 1976). Then in the late 1970’s, research in the Udzungwa bloc revealed that a number of the Usambara endemics also occurred farther south, indicating that the centre of endemism may be more extensive. As scientific exploration continued, rapid and widespread exploitation was taking hold, particularly mechanised logging in the Usambaras and eastern Udzungwa (Bjorndalen, 1992). In the absence of detailed scientific surveys for many of the blocs, it became imperative to formulate a predictive definition for the area of endemism (J.C. Lovett, pers. comm.). In 1988, a geological and climatological description of the Eastern Arc was established, defining the forests as “those occurring on crystalline mountains in south-east Kenya and eastern Tanzania under the direct climatic influence of the Indian Ocean” (AETFAT conference, Hamburg; see Lovett, 1990). The term Eastern Arc was subsequently adopted by the Tanzania Forestry Action Plan to delimit an area of high conservation importance (Bensted-Smith and Msangi, 1989). Soon after the mountains were recognised globally for their biodiversity value (Myers, 1990).
According to White’s (1983) classification of African vegetation, the Tanzanian flora consist of Afromontane, Zanzibar-Inhambane and Lake Victoria phytogeographical types. Lovett’s (1990) refinement of these spatial divisions identifies the EAMs according to their soil type and main climatic influence – proxies for long-term isolation and climatic stability, factors hypothesised to be central to the exceptional concentrations of endemism (Lovett and Wasser, 1993; Fjeldså and Lovett, 1997). Forests influenced predominantly by the climatic regimes of the Great African Lakes are not considered part of the Eastern Arc; Lake Victoria was considerably smaller during the last glacial maximum, disrupting rainfall patterns and
associated forest vegetation (Hamilton, 1982). Forests on volcanic soils are also omitted, these mountains being geologically younger (e.g., Mt. Kilimanjaro, 1-2 MY old; Schlüter, 1997) and containing fewer species of restricted distribution.
Mittermeier et al. (1998) originally combined Tanzania’s coastal forests and those of the EAMs within a single biodiversity hotspot. This classification was later revised, with the mountains now belonging to the Eastern Afromontane Biodiversity Hotspot (Mittermeier et al., 2004). From a taxonomic standpoint, the split is controversial due to submontane and coastal habitats in Tanzania sharing a large number of range-restricted taxa (279 vascular plant species; Gereau et al., in prep.). From a conservation perspective, the grouping of Afromontane habitats is arguably appropriate to address the management challenges specific to mountain regions, their ecology and people (Kreutzmann, 2001).
Scientific interest in the EAMs has grown in recent years, resulting in numerous assessments of biological and natural capital importance (e.g., CEPF, 2003; Burgess et al., 2007a; Mwakalila et al., 2009). Whilst the predictive, landscape-scale definition of Lovett (1990) has been widely applied, prior to the current work (Chapter 2) precise spatial limits for the area of endemism have been lacking – a result of the inherent subjectivity in defining mountain extent (Gerrard, 1990) and of broader uncertainties in delimiting areas of endemism (Anderson, 1994; Harold and Mooi, 1994). Thus, fundamental biogeographical questions such as “what is the area of the EAM?” and “how many (endemic) species occur there?” have been troublesome to address with rigour or consistency until now.
Forest use and protection
Humans have an exceptionally long history of forest use in East Africa (Marchant et al., 2010). Some of the earliest hominoid remains were discovered close to the study region, in Olduvai Gorge, northern Tanzania. Occupation of the EAMs by hunter gatherers and subsistence farmers at least pre-dates the Common Era (Lovett and Wasser, 1993). Historically, subsistence use by small local communities and nomadic pastoralists, such as the collection of poles, firewood, food and medicinal plants, had minimal long-term impact on forest structure. Over the past 200 years, however, population growth and associated demand for agricultural land, wood-based fuels and other forest products, have impeded forest regeneration (Newmark, 1998; Finch and Marchant, 2011). Commercial practices such as mining, clearance for plantations and extraction of valuable timber species such as Khaya anthotheca (African mahogany), Milicia excelsa, Pterocarpus and Olea spp. have
greatly impacted forest extent and condition, whilst returning little in the way of benefits to local communities (Bjorndalen, 1992; Burgess et al., 2005). In 1984, commercial timber harvesting in catchment forest reserves was banned, but illegal pit-sawing continues in many areas. Present-day forests represent less than 30% of their preclearance extent (Newmark, 2002; Burgess et al., 2007a; Hall et al., 2009). There are around 200 fragments across the 13 mountain blocs (Fig. 1.4), with a mean patch size of 21 km2 (median, 1.5 km2; range, 0.01-
526 km2). In Taita, just 2% of the original forest area remains.
Figure 1.6. Examples of forest conversion and use. (a) Amani Nature Reserve in East Usambara: submontane forest on the left, tea estates on the right. (b) Forests capture fog inputs and regulate seasonal water flows. (c) Firewood collection day in Udzungwa National Park – the primary fuel for rural communities. (d) Terraced slopes in Chome Forest Reserve, South Pare. Photo credits: a-b, Philip Platts; c, Julia Latham; d, Jemma Finch.
Effective management is important across a range of spatial and thematic scales (Mwakalila et al., 2009): globally, for biodiversity conservation and climate change mitigation; nationally, for water supplies, energy production, climate regulation, soil conservation and nutrient cycling; and locally, for communities directly reliant on forest products (Fig. 1.6).
Three quarters of the EAM forest area is gazetted, of which over half is designated for catchment protection or multi-resource use (Burgess et al., 2007b). Levels of degradation within these forest reserves vary greatly from place to place, depending on levels of staffing, proximity to roads and settlements, and cultural traditions (Green et al., 2012). Some are under Joint Forest Management, whereby local communities enter agreements with government regarding forest use. In addition, local communities manage forests on village land. Both participatory management schemes have potential to improve forest condition (Blomley et al., 2008), but ambiguous tenure rights and often long-winded official ratification risk reducing incentives for sustainable use. More stringent, state regulation applies to nature reserves (gazetted for forest conservation) and national parks, which combined cover nearly a third of the total forest area. Strong management effectively reduces degradation within these sites, but might arguably lead to leakage (displacement of resource depletion to surrounding areas) or poverty if resource needs are not otherwise fulfilled (Pfeifer et al., 2012 in Appendix I).