The Arabuko-Sokoke Forest lies between 39°50′E and, 39°40′E longitude and 3°10′S and 3°30′S latitude, along the eastern Kenyan coast (110 km north of Mombasa and 18 km south of Malindi). The topography rises 60–135 m above sea level, and the mean annual rainfall ranges from 600 mm in the northwest part to 1100 mm at the Gedi station in the northeast, with the short rains falling from November to December (Muchiri et al., 2001).
The forest has been described in detail by Moomaw, 1960; Kelsey & Langton, 1983; Thomas, 1988; Awimbo & Wairungu, 1990; Mutangah & Mwaura, 1992; and Robertson & Luke, 1993;. Arabuko-Sokoke Forest is one of the few remaining indigenous forests in Kenya, and one of the largest extant fragments of a coastal forest that once covered much of the East African coast (Burgess et al., 2003). European timber merchants harvested trees from the forest in the early 1900s and removed most of the commercially valuable timber. The most harvested tree species included Sterculia appendiculata K.Schum. ex Engl., Manilkara sansibarensis Engl., Afzelia quanzensis Welw., Brachylaena huillensis O.Hoffm. and B. spiciformis (Robertson & Luke, 1993; Wright, 1999). Robertson & Luke (1993) note how literature on Arabuko-Sokoke Forest frequently lists S. appendiculata as logged for timber even though this species is absent within the forest boundaries. Robertson & Luke (1993) suggest that much written about the Arabuko-Sokoke Forest in fact refers to a much wider area, now under permanent cultivation. These former forests include areas closer to the shore towards Kilifi (now on the east side of the main road that separates the protected forest area from areas of cultivation), and towards the west into the Mangea hills.
Arabuko-Sokoke Forest was proclaimed a crown forest in 1932 and gazetted as a forest reserve in 1943. Under the Forestry Department management, commercial exploitation was supposed to stop. However, five sawmills legally operated in Arabuko-Sokoke Forest until the 1950s, and one of these sawmills operated in the Brachystegia habitat and retained a license for the removal of B. spiciformis until the 1990s (Davies, 1993; Robertson & Luke, 1993; Blackett, 1994). In 1977, a special nature reserve was created within the forest where all extractive activities were declared illegal. The area was to be monitored through survey and permanent sample plots. Sporadic vegetation monitoring has taken place with surveys by the Kenya Forestry Research Institute (Wairungu et al., 1993; Muchiri et al., 2001), Kenya Indigenous Forest Conservation Program (Davies, 1993; Blackett, 1994) and International Council for Bird Preservation (Kelsey and Langton, 1983). In 1991, a memorandum of understanding was signed between the Kenya Forestry Department and Kenya Wildlife Service that gave both parties a management role. Each service ostensibly manages about half of the remaining Brachystegia woodland. Since 1991, illegal-logging activities have subsided but not totally ceased in the forest (M. Mwavita personal communication).
This study took place within the 7636 ha Brachystegia woodland which runs in a central strip through the Arabuko-Sokoke Forest. This is relatively open habitat dominated by B. spiciformis growing on soil of white sands (Kelsey & Langton, 1983). The decades of logging activities and current human activities have created matrices of relatively more and less disturbed patches. Hereafter, we refer to these differences as disturbed and undisturbed. Disturbed areas have conspicuously more foot paths, cut stems and old logging tracks than the relatively undisturbed ones. We counted nineteen versus eight foot paths; five versus two cut stems and two versus zero old logging tracks in the disturbed and undisturbed patches respectively.
In Arabuko-Sokoke, B. spiciformis (Leguminosae) grows up to 25 m (Chudnoff, 1984). Little is known about its regeneration requirements, annual seed production, or seed consumers within the Arabuko-Sokoke Forest. However, its phenology, flowering and fruiting and dispersal, seed germination and regeneration, growth and mortality are known for the Miombo Woodland of Zambia and Zimbambwe (Chidumayo & Frost, 1996).
The Arabuko-Sokoke Forest has two other distinct forest types (Arabuko-Sokoke Strategic Forest Management Plan 2002–2027, 2002); (1) mixed forest with a diversity of relatively dense, tall and undifferentiated trees covering an area of about 7000 ha; (2) Cynomnenta forest and thicket, which covers about 23,500 ha, occurs to the west on red Magarini sands (Kelsey & Langton, 1983), and is dominated by Cynometra webberi Baker f., Manilkara sulcata Dubard, Oldfieldia somalensis (Chiov.) Milne-Redh. and formerly B. huillensis. This latter species, much in demand for the wood-carving trade, has been logged or poached from most of the forest.
We established two 1400 × 800 m study plots, in disturbed (3°19′S and 39°55′E) and relatively undisturbed (3°25′S and 39°52′E) areas within the Brachystegia forest. We surveyed the plots during the dry season; January–May 2002.
Within each large plot, we established five 400 m long transects each 250 m apart. Along each transect, we established five 30 × 30 m subplots at 50 m intervals (Pulido & Diaz, 1997). We identified, counted, and measured diameter at breast height (dbh) of all trees within each subplot. For trees with multiple stems, we summed total basal area and assigned that to the individual.
Tree species abundance and diversity. We encountered eleven canopy tree species. Because of the rarity of four of these species [Strychnos madagascariensis Poir., Memecylon fragrans A.Fern. & R.Fern., Newtonia paucijuga (Harms) Brenan, Rytigynia spp], we restrict formal statistical analyses to the seven most common tree species, except when calculating species diversity indices where all species were included. We set the density threshold for common tree species at ≥1 individual ha−1. Density estimates came from averaging the density of a species across the subplots of a given habitat. A Mann–Whitney U-test with subplots as replicates tested for differences in tree numbers between the disturbed and undisturbed habitats, except J. magnistipulata which occurred only in the undisturbed habitat. The importance value (IV) for each tree species within the Brachystegia woodland was calculated as (Curtis, 1959): IV = relative density + relative frequency + relative dominance (dbh).
We calculated the diversity of tree species for each subplot using the reciprocal of Simpson’s Diversity Index (Magurran, 2004):
where ni = the number of individuals in the ith species and N = the total number of individuals of all species. This measure of diversity increases with the numbers of species and the evenness with which individual trees are spread among the different species. We used a t-test to see whether tree species diversity differed between the two habitat types.
Tree species dispersion patterns. To characterize each tree species dispersion pattern quantitatively, we calculated Lloyd’s index (Vandermeer, 1981):
where s2 = variance in number of individuals among subplots, λ = mean number of trees per subplot. If a tree species is randomly distributed, the variance of the distribution should equal the mean; if trees are evenly distributed, the variance is significantly less than the mean. If tree distribution is clumped, the variance is significantly greater than the mean. To test for nonrandom dispersions (clumped or even), we used a chi-square test to compare observed counts of individual trees per subplot with counts generated using a Poisson distribution (Vandermeer, 1981).
Tree size distributions. We used MANOVA to test for effect of habitat type (disturbed and undisturbed habitats) on diameter at breast height using the seven trees species as the dependent variables. We then used a coefficient of skewness (g1) to summarize the asymmetry of tree species size distributions with respect to habitat (Bendel et al., 1989; Wright et al., 2003). The coefficient of skewness was calculated using the natural logarithm of dbh (Bendel et al., 1989) for each individual of each tree species. The assumption is that the regular interval along the scale of the natural logarithm of the stems more closely approximate the stems’ ages. Coefficient of skewness
where n represents the number of individual trees, xi represents the natural logarithm of dbh for individual tree i, represents the mean of xi, and s represents the standard deviation of the xi. Skewness is significant if skewness/standard error of skewness is greater than 2. The coefficient of skewness (g1) is positive for size distributions with abundant smaller sized trees (saplings) and few larger trees (adults), and is negative for distributions with fewer smaller trees and abundant larger trees. Poorter et al. (1996) interprets greater numbers of smaller trees relative to larger ones as indicative of a stable or a growing tree population, fewer numbers of smaller trees relative to larger ones as indicative of an unstable or declining population.
For B. spiciformis, in which significant skewness occurred in both disturbed and undisturbed habitats, we categorized the trees into four size classes based on diameter: 4–8 cm; 8.1–13.3 cm; 13.4–22 cm and >22.1 cm and tested for differences in tree size distributions between the two habitats by chi-square test.
We used SYSTAT version 10.2 (SPSS Inc., 2000) for all statistical analyses. We report values for tree numbers, densities, diversity and diameter at breast height as mean ± SE. Nomenclature follows Beentje (1994).