Observed variation in species richness on ecological gradients and between regions has attracted several different explanations. Climatic factors, such as energy availability, precipitation and mean annual temperature, are frequently cited to explain differences in species richness (Wright et al. 1993). The relative amount of variation in these variables may be of importance as well as absolute values (Stevens 1989, 1992). Alternatively, the history of colonization and extirpation have been used to explain regional variation in diversity (Guo et al. 1998; Latham & Ricklefs 1993a, b). Area is an important factor, for example on an elevational gradient the tops of mountains are not only cooler than the bottoms, but they are also much smaller (Rahbek 1997)

Lovett, J.C.

English

Observed variation in species richness on ecological gradients and between
regions has attracted several different explanations. Climatic factors, such as
energy availability, precipitation and mean annual temperature, are frequently
cited to explain differences in species richness (Wright et al. 1993). The relative
amount of variation in these variables may be of importance as well as absolute
values (Stevens 1989, 1992). Alternatively, the history of colonization and
extirpation have been used to explain regional variation in diversity (Guo et al. 1998; Latham &amp; Ricklefs 1993a, b). Area is an important factor, for example
on an elevational gradient the tops of mountains are not only cooler than the
bottoms, but they are also much smaller (Rahbek 1997).</jats:p

Jon C. Lovett

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Journal of Tropical Ecology

Tanzanian forest tree plot diversity and elevation

White Rose Research Online

This is a repository copy of Tanzanian forest tree plot diversity and elevation.White Rose Research Online URL for this paper:https://eprints.whiterose.ac.uk/1548/Article:Lovett, J.C. (1999) Tanzanian forest tree plot diversity and elevation. Journal of Tropical Ecology. pp. 689-694. ISSN 1469-7831 https://doi.org/10.1017/S0266467499001108eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. Journal of Tropical Ecology (1999) 15:689–694. With 1 figureCopyright  1999 Cambridge University PressSHORT COMMUNICATIONTanzanian forest tree plot diversity andelevationJON C. LOVETTEnvironment Department, University of York, Heslington, York YO1 5DD, UKKEY WORDS: Diversity, elevation, Tanzania, forest, treesObserved variation in species richness on ecological gradients and betweenregions has attracted several different explanations. Climatic factors, such asenergy availability, precipitation and mean annual temperature, are frequentlycited to explain differences in species richness (Wright et al. 1993). The relativeamount of variation in these variables may be of importance as well as absolutevalues (Stevens 1989, 1992). Alternatively, the history of colonization andextirpation have been used to explain regional variation in diversity (Guo etal. 1998; Latham & Ricklefs 1993a, b). Area is an important factor, for exampleon an elevational gradient the tops of mountains are not only cooler than thebottoms, but they are also much smaller (Rahbek 1997).In tropical Africa declines in forest tree species richness with elevation havebeen reported in both fixed area plots and elevational bands (reviewed in Ham-ilton 1989). However, a study in the Eastern Arc forests of Tanzania usingplots in which a fixed number of stems were enumerated (200 plots each of20 trees of M 20 cm dbh) did not show a decline in species richness of the plotswith elevation (Lovett 1996). In this paper additional data (a further 163 plots)are used to confirm this observation and to demonstrate that the number ofspecies found in a plot is independent of the potential pool of species. Examina-tion of some of the plots that are known to be from old secondary forests showsthat disturbance may be a factor that leads to low diversity.A total of 363 plots from the Eastern Arc forests were available for analysisfrom published and unpublished sources for the following mountains (Table1): West Usambara (Lovett 1996, Hall 1991); East Usambara (Hamilton etal. 1989); Southern Nguru (Lovett 1996); Malundwe (Lovett & Norton 1989);Northern Udzungwa (Lovett 1996); and Southern Udzungwa (Lovett et al. 1997,689J . C . LOVETT690Table 1. Eastern Arc mountains (with approximate latitude and longitude) where plots were enumeratedwith number of plots, mean number of species per plot with standard deviation and elevation range of theplots.Mountain Number of plots Mean species per plot Plot elevation range (m)West Usambara (4–5°S 38°E) 166 8.9 ± 2.2 1000–2180East Usambara (4–5°S 38°E) 70 9.5 ± 2.4 280–1220Southern Nguru (6°S 37°E) 29 10.5 ± 2.2 880–1560Malundwe (7°S 37°E) 5 11.4 ± 1.7 1120–1155Northern Udzungwa (7°S 36°E) 78 9.3 ± 2.6 410–1670Southern Udzungwa (8°S 35°E) 15 7.7 ± 3.5 550–1870J. Lovett unpubl. data). The Eastern Arc mountains of eastern Tanzania andsoutheastern Kenya are defined as those mountains on ancient crystallinegneissic rocks under the direct climatic influence of the Indian Ocean fromthe Makambako Gap (8°51′S 34°49′E) to the Taita Hills (3°25′S 38°20′E)(Lovett 1990). The Eastern Arc forests are unusual in that they contain ahigh proportion of restricted range species, leading them to be regarded as animportant biodiversity ‘hotspot’ (Mittermeier et al. 1998, Myers 1990). Thisfeature of the forests has been attributed to an exceptionally long period ofenvironmental stability in the area (Lovett & Friis 1996).The plots were assessed using the twenty-tree variable-area plotless tech-nique (Hall 1991) in which the nearest 20 trees of M 20 cm diameter at breastheight (dbh) to an objectively chosen point were measured and identified. Mostof the plots were enumerated in moist forest on the eastern slopes with amean annual rainfall in excess of 1000 mm, the exception being plots fromthe distinctive dry montane Juniperus forests with a rainfall of 600–900 mm y−1from the western West Usambara mountains. There are few meteorologicalstations in the forested area so it is not possible to directly correlate plot divers-ity with rainfall. Data from the CIAT/FAO climate database show that rainfallfor stations in eastern (M 35°E) Tanzania receiving M 1000 mm y−1 is not cor-related with elevation (n = 167, elevation range 0–1921 m, r2 = 0.015, P = 0.11).Temperature is correlated with elevation. Eastern Tanzanian meteorologicalstations receiving M 1000 mm y−1 show a lapse rate of 0.5–0.6 °C per 100 melevation (mean monthly maximum temperature = −0.50x + 28.55, r2 = 0.96;mean monthly minimum temperature = −0.57x + 24.32, r2 = 0.96; wherex = (m)/100 elevation).The potential pool of species that might occur in the plots varies with eleva-tion and rainfall. The Eastern Arc contains a total of 272 forest tree species(67 of which are endemic) that reach M 20 cm dbh (Lovett 1993 gives a figureof 274 species M 20 cm dbh or 10 m tall, but this includes two tall but slenderbamboos which would not be enumerated in the 20-tree plots). Because thereis a rapid turnover of species with change in elevation (Lovett 1996, 1998),then the number of species that might be included in a plot at any givenaltitude is substantially less than the total. In Eastern Arc dry lowland forest(0–800 m elevation) there are 52 species, in lowland forest (0–800 m) thereTanzanian forest tree plot diversity and elevation 691Figure 1. Relationship of number of species in each 20-tree plot and elevation (n = 363).are 91 species, in submontane forest (800–1400 m) there are 114 species, inmontane forest (1200–1800) there are 120 species, in dry montane forest(1500–2400 m) there are 57 species and in upper montane forest (1800–2400 m) there are 42 species (forest definitions from Lovett 1993).For the aggregated plot data from the 363 plots the lowest number of speciesfound in a plot was two, and the highest was 16. The mean number of species/plot is 9.2 ± 2.4, the mode 10 and the median 9. The species richness data isnormally distributed with a kurtosis of −0.21 (Kolmogorov–Smirnov normalitytest D = 0.026, P > 0.15). The distribution is slightly skewed towards lowerspecies numbers (skewness −0.16). There is a slight negative correlationbetween the number of species in each plot and elevation (Figure 1, r2 = 0.011,P = 0.046). Richness appears to be independent of the potential pool of species.An average plot with nine species contains at least 7.5% of the total speciespool at any elevation. Although the species pool is larger at mid-elevations,plot diversity remains constant throughout the elevation range. For example,a species rich upper montane forest plot with 15 species contains 36% of thetotal species pool in a 20-tree sample.J . C . LOVETT692If the five plots from the driest and structurally different forest type, themontane Juniperus forest of the West Usambara mountains (mean species perplot = 5 ± 1.6, elevation 1840–1880 m, Lovett 1996), are removed from the poolto leave only moist forest plots, then there is no significant correlation betweenplot species richness and elevation (n = 357, r2 = 0.005, P = 0.17). The meanrises to 9.3 ± 2.4, the mode stays at 10 and the median rises to 10. The kurtosisalso falls to −0.19 and the skewness to −0.15. The Juniperus forest plots aresignificantly less diverse than the other plots (two-tailed t-test P < 0.0001).Anthropic disturbance for cultivation is associated with low species richness.Three low diversity plots from the Southern Udzungwa mountains at around750–1025 m elevation in the Kihansi Gorge are dominated by Filicium decipiens(Wight & Arn.) Thw. (Sapindaceae), a tree that occurs throughout easternAfrica and in Asia. The Filicium forest is adjacent to species rich forest at thesame elevation (Lovett et al. 1997). The Filicium forest plots are significantlyless diverse than the other plots (mean species per plot = 3 ± 1.5, two-tailedt-test, P < 0.0001). Although the area is now not cultivated, parts of the Filic-ium forest are on places that have been cultivated in living memory. Otherparts of the Filicium dominated forest are not so recently disturbed, with someFilicium trees in the plots up to 95 cm dbh. There are no Eastern Arc endemictree species in the Filicium plots.In another Southern Udzungwa forest at Mufindi, three plots in montaneforest (1860–1870 m elevation) growing over old cultivation as evidenced bythe remains of an undulating ridge and furrow system (Greenway 1973), alsohave significantly fewer species than other Eastern Arc plots (mean numberof species in Mufindi plots = 5.3 ± 0.56, two-tailed t-test, P = 0.005). The ageof forest regeneration on cultivation at Mufindi has not been dated, but isbeyond local memory. A marked qualitative difference between the Mufindiforests and other Eastern Arc montane forests can be demonstrated by poolingthe three Mufindi plots to create a 60 tree sample and comparing them withthree pooled plots from similar elevations elsewhere in the Eastern Arc. Datafor the West Usambara, Southern Nguru and Northern Udzungwa wereobtained by taking a random sample of three montane forest plots obtainedfrom the same forest location on each mountain at as near an elevation aspossible to the Mufindi plots. Plot elevations (m asl) are: West Usambara 1890,1850, 1850; Southern Nguru 1480, 1500, 1560; Northern Udzungwa 1650, 1670,1670; and Southern Udzungwa 1860, 1860, 1870. There are no Eastern Arcendemic tree species in the Mufindi plots compared to 20–30% of species fromthree other areas being Eastern Arc endemics (Table 2). With the Juniperus,Filicium and Mufindi plots excluded from the data pool the mean number ofspecies per plot rises to 9.4 ± 2.3 (n = 352, mode = 10, median = 10, kurtosis =−0.28, skewness = −0.07) and there is no correlation between plot species rich-ness and elevation (r2 = 0.005, P = 0.18).In summary, the additional data included here support the observation thatTanzanian forest tree plot diversity and elevation 693Table 2. Comparison of numbers of species and endemism in samples of 60 trees from four montane forestsin the Eastern Arc mountains of Tanzania. The 60-tree sample was obtained by combining three 20-treeplots.Species EndemicsWest Usambara 26 5Southern Nguru 21 4Northern Udzungwa 20 6Southern Udzungwa 9 0species richness in the Eastern Arc forests of Tanzania, measured as numberof species in a sample of 20 treesM 20 cm dbh, does not decline with elevation.Neither does the number of species per plot appear to be related to the richnessof the potential pool of species at different elevations. As elevation is closelycorrelated with temperature, it can be assumed that plot species richness isindependent of temperature. The driest forest type is less diverse than mostof the other moist forests. This may be a function of precipitation or anotherfactor, such as fire, which is associated with forests growing in low, seasonalrainfall. Forest regenerating on formerly cultivated ground has a comparativelylow diversity and no endemic species.ACKNOWLEDGEMENTSI gratefully acknowledge permission from the Tanzania Commission forScience and Technology to conduct research in Tanzania, to Dr Alan Hamiltonfor use of data from the East Usambara mountains, and to Dr John B. Hall foruse of data from Mazumbai in the West Usambara mountains. Comments onan earlier draft of the paper were provided by Luis Daniel Llambi Cartaya andtwo reviewers.LITERATURE CITEDGREENWAY, P. J. 1973. A classification of the vegetation of East Africa. Kirkia 9:1–68.GUO, Q., RICKLEFS, R. E. & CODY, M. L. 1998. Vascular plant diversity in eastern Asia and NorthAmerica: historical and ecological explanations. Botanical Journal of the Linnean Society 128:123–136.HALL, JOHN B. 1991. Multiple-nearest-tree sampling in an ecological survey of Afromontane catchmentforest. Forest Ecology and Management 42:245–266.HAMILTON, A. C. 1989. African forests. Pp. 155–182 in Lieth, H. & Werger, M. J. A. (eds). Ecosystemsof the world 14B. Tropical rain forest ecosystems, biogeographical and ecological studies. Elsevier, Amsterdam.HAMILTON, A. C., RUFFO, C. K., MWASHA, I. V., MMARI, C. & LOVETT, J. C. 1989. A survey offorest types on the East Usambara using the variable-area tree plot method. Pp. 213–225 in Hamilton,A. C. & Bensted-Smith, R. (eds). Forest conservation in the East Usambara Mountains, Tanzania. IUCN,Gland.LATHAM, R. E. & RICKLEFS, R. E. 1993a. Global patterns of tree species richness in moist forests:energy-diversity theory does not account for variation in species richness. Oikos 67:325–333.LATHAM, R. E. & RICKLEFS, R. E. 1993b. Continental comparisons of temperate-zone tree speciesdiversity. Pp. 294–314 in Ricklefs, R. E. & Schulter, D. (eds). Species diversity in ecological communities:historical and geographical perspectives. University of Chicago Press, Chicago.LOVETT, J. C. 1990. Classification and status of the Tanzanian forests. Mitteilungen aus dem Institut fürAllgemeine Botanik in Hamburg 23a:287–300.LOVETT, J. C. 1993. Eastern Arc moist forest flora. Pp. 33–55 in Lovett, J. C. & Wasser, S. K. (eds).Biogeography and ecology of the rain forests of Eastern Africa. Cambridge University Press, Cambridge.J . C . LOVETT694LOVETT, J. C. 1996. Elevational and latitudinal changes in tree associations and diversity in the EasternArc mountains of Tanzania. Journal of Tropical Ecology 12:629–650.LOVETT, J. C. 1998. Continuous change in Tanzanian moist forest tree communities with elevation.Journal of Tropical Ecology 14:719–722.LOVETT, J. C. & FRIIS, I. 1996. Some patterns of endemism in the tropical north east and easternAfrican woody flora. Pp. 582–601 in van der Maesen, L. J. G., van der Burgt, X. M. & van Medenbachde Rooy, J. M. (eds). The biodiversity of African plants. Proceedings XIVth AETFAT Congress 22–27 August1994, Wageningen, The Netherlands. Kluwer Academic Publishers, Dordrecht.LOVETT, J. C., HATTON, J., MWASUMBI, L. B. & GERSTLE, J. H. 1997. Assessment of the impactof the lower Kihansi hydropower project on the forests of Kihansi Gorge, Tanzania. Biodiversity andConservation 6:915–933.LOVETT, J. C. & NORTON, G. W. 1989. Afromontane rain forest on Malundwe Hill, Mikumi NationalPark. Biological Conservation 48:13–19.MITTERMEIER, R. A., MYERS, N., THOMSEN, J. B., FONSECA, G. A. B. & OLIVIERI, S. 1998.Biodiversity hotspots and major tropical wilderness areas: approaches to setting conservation priorities.Conservation Biology 12:516–520.MYERS, N. 1990. The biodiversity challenge: expanded hot-spots analysis. The Environmentalist 10:243–256.RAHBEK, C. 1997. The relationship among area, elevation, and regional species richness in neotropicalbirds. American Naturalist 149:875–902.STEVENS, G. C. 1989. The latitudinal gradient in geographical range: how so many species coexist inthe tropics. American Naturalist 133:240–256.STEVENS, G. C. 1992. The elevational gradient in altitudinal range: an extension of Rapoport’slatitudinal rule to altitude. American Naturalist 140:893–911.WRIGHT, D. H., CURRIE, D. J. & MAURER, B. A. 1993. Energy supply and patterns of speciesrichness on local and regional scales. Pp. 66–74 in Ricklefs, R. E. & Schulter, D. (eds). Species diversityin ecological communities: historical and geographical perspectives. University of Chicago Press, Chicago.

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Tanzanian forest tree plot diversity and elevation

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