Flora (2002) 197, 461–474 http://www.urbanfischer.de/journals/flora
Regeneration of fourteen tree species in Harenna forest, southeastern Ethiopia Getachew Tesfaye1, Demel Teketay2* & Masresha Fetene3 1
Institute of Biodiversity Conservation and Research, P. O. Box 30726, Addis Abeba, Ethiopia Ethiopian Agricultural Research Organization, P. O. Box 2003, Addis Abeba, Ethiopia 3 Addis Abeba University, P. O. Box 1176, Addis Abeba, Ethiopia * e-mail corresponding author: [email protected]
; [email protected]
Received: Jan 8, 2002 · Accepted: Jul 20, 2002
Summary Regeneration of fourteen tree species was investigated at Harenna forest, on the southern slopes of the Bale Mountains, Ethiopia. Tree seedling densities and their spatial distribution along gradients of altitude and light were investigated using quadrats of 10 m × 5 m along line transects. Seedling mortality and herbivore damage were examined in eight permanent plots of 10 m × 5 m laid systematically at different places. Population structures of the species were investigated using quadrats of 20 m × 20 m along line transects. Species showed variation in densities of their seedlings ranging from 1065 to only 58 seedlings per hectare. Frequency of distribution of species along altitudinal gradients varied between 15 and 69 percent, indicating differences in habitat preferences among the species. In their spatial distribution to the canopy light gradient, eight tree species were found growing under canopy shade (1–5% light level) while six grew exclusively in the open (100%). More than 42% of the populations of seedlings investigated during the dry season of the year were damaged by herbivores and 27% were lost as uprooted, died and standing or were missing. Examination of the population structure of the species showed that only few species had good representation of individuals at all size classes implying healthy or normal regeneration. Many of the species showed strong peak at lower size classes followed by missing of individuals at one or more of the medium and upper height classes, which indicated that regeneration was hampered. Tree species with healthy regeneration or reverse “J” distribution included Teclea nobilis, Ocotea kenyensis and Syzygium guineense subsp guineense, whereas Podocarpus falcatus, Aningeria adolfi-friederici, Olea capensis, Croton macrostachyus and Prunus africana showed hampered regeneration. Management alternatives to enhance regeneration status of the species are discussed. Key words: mortality, herbivory, light gradient, population structure, hampered regeneration, Afromontane forest
1. Introduction Regeneration of major canopy tree species has been studied in many forest ecosystems (Denslow 1987; Yamamoto 1996). Tropical forests revealed variation in patterns of regeneration both through differences in their constituent species and the environmental variables in which they grow (Denslow 1987; Garwood 1989 ; Whitmore 1996; Demel Teketay 1997a; Kyereth et al. 1999). Such works have shown that studies on natural regeneration and seedling ecology can provide options to forest development through improvement in recruitment, establishment and growth of the desired seedlings. Also studies on tree seedlings density, their rate of mortality and damage help in the understanding of the status of species and natural rege0367-2530/02/197/06-461 $ 15.00/0
neration (Augspurger 1984; Hubbel & Foster 1986). Seedling pools in forest understories are dynamic and rates of mortality may vary both among species and also within species in gaps and shade (Bazzaz 1991; Demel Teketay 1996). Potential causes of seedling mortality could include abiotic stresses such as shade, drought and trampling, and biotic influences, such as herbivory, disease or root competition (Janzen 1971; Augspurger 1984; Demel Teketay 1997b). Information on these in general and factors that hamper natural regeneration in particular can have significant implications on the management, sustainable utilization and conservation of forests. Information on seed ecology and regeneration in dry Afromontane forests of Ethiopia has been accumulating FLORA (2002) 197
over the last decade (Tamrat Bekele 1993; Demel Teketay & Granström 1995; Demel Teketay 1992, 1997a, 1997b; Mekuria Argaw et al. 1999). Nevertheless, there is not much information on seedling ecology and natural regeneration of native trees of these forests in general (Tamrat Bekele 1993; Demel Teketay 1997b ; MekuriaArgaw et al. 1999) and the population status of economically important species in particular. The Harenna forest in the Bale Mountains, one of the few areas in Ethiopia where there is still a considerable natural forest cover (Lisanework Nigatu 1987; Mesfin Tadesse & Lisanework Nigatu 1996), is one of such forests where there is a need for information on seedling ecology and tree population structure. Over the years, the Harenna forest has been subjected to commercial and illegal logging, grazing, agriculture, settlement and fire (Getachew Tesfaye 2001). Because of the considerable damage of this mountain forest, concern has arisen about natural regeneration of the forest and the impact of human induced disturbances on selected tree species in the forest. The present study stems from this understanding. It was conducted to determine seedling densities of the dominant tree species and their spatial distribution along altitudinal gradients, seedling mortality and herbivory as well as population structure of the dominant tree species of Harenna forest.
2. Materials and methods 2.1. Study site The Bale Mountains massif, which is found on the eastern part of the Great Rift Valley, constitutes the largest proportion of the southeastern highlands of Ethiopia and northeast tropical Africa (Miehe & Miehe 1994). The chain of these mountains holds extended areas of closed canopy tropical forests, and harbours also major water sources of the country. The northern slopes of the mountains are occupied by dry Afromontane forests (Weinert & Mazurek 1984; Friis 1992). Relatively humid forests, which have been disturbed mainly for economic reasons, occupy the southern slopes. The forests at the southern slopes (also called the Harenna forest) represent one of the largest remaining tracts of natural forests in Ethiopia (Miehe & Miehe 1994). The forests cover an area of nearly 7000 km2, and the Bale Mountains National Park is composed of 14% of this forested land (Bzadd 1999). The Harenna forest, which forms a part of the vegetation on the southern slopes of Bale Mountains is found at about 480 km southeast of Addis Abeba. It is located between 6°40 to 7°10 N and 39°30 to 40° E. The area is known for its floral and faunal diversity as well as endemicity (Friis 1986; Hedberg 1986; Hillman 1988; Lisanework Nigatu & Mesfin Tadesse 1989). The Bale Mountains National Park (BMNP) houses among others, 50 mammals, 180 birds, 14 amphibian species, out of which 20% of the mammals, 8% of the birds, and 78% of the amphibians are endemic (Hillman 1988). 462
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The soils in Bale Mountains are of volcanic origin resulting from the oligocene erruptions of the trappean lava (Mohr 1971). The rock is trachytes with basalts and rhyolites. The basalt and trachytes weather to, mainly, red and red brown to black silty loam (Weinert & Mazurek 1984). There is no meteorological information available particular to Harenna forest. However, rainfall and temperature records from DelloMenna station, which is about ten kilometers away from the foot of the escarpment and the closest location to the lowest portion of the forest boundary, indicate that the area is characterized by eight rainy months from March to October and four dry months from November to February, with an average annual rainfall of 987 mm (Anonymous 1999). The area experiences mean monthly minimum and maximum temperatures of 19.9°C and 25.4°C, respectively. The escarpment forms a unique topographic feature, dropping very sharply in altitudes from 3800 to 2800 m and then gradually to 1500 m. The vegetation on the southern slopes was described by several authors (Mooney 1963; Chaffey 1979; Friis 1986; Mesfin Tadesse 1986; Lisanework Nigatu 1987; Uhlig 1988; Bussman 1997). Tree species exhibit distinct habitat preferences and the forest shows marked transition in species composition, structure and morphology of plants with change in altitude. The majority of trees are quality timber species (e.g., species of Podocarpus, Aningeria, Olea, Warburgia, Croton and Syzygium) forming the main components in the upperstorey. The forests contain cosiderable areas of wild coffee populations (Coffee arabica L.) and endemic tree species such as Filicium decipiens (Lisanework Nigatu & Mesfin Tadesse 1989). Thus, Harenna forest has an extraordinary importance as sources of quality timber species, non-timber forest products (NTFP) and genetic diversity. The present study was conducted in the altitudes ranging between 1500–2700 m where a relatively humid Afromontane forest is found.
2.2. Study species In the present study, 14 species of trees known to have considerable socio-economic and ecological importance have been included. A list of these species, plant families, growthhabit, size of propagules (seed or fruit), habitat or ecology and geographical distribution are presented in Table 1. The nomenclature of plant species in this paper follows Hedberg & Edwards (1989), Friis (1992) and Edwards et al. (1995). Hereafter, the species names appear in full when they are referred to for the first time but only their generic names will be used thereafter.
2.3. Methods 2.3.1. Seedling density and distribution along the altitudinal gradient The seedling survey work was conducted between January and April 2000. For the purposes of this investigation, the term ‘seedling’ refers to plants with heights up to 150 cm. To examine seedling densities and their distribution along the alti-
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Growth Habit Tree, upto 30 m
Tree, upto 20 m
Tree, upto 20 m
Tree, upto 35 m
Tree, upto 40 m
Tree, upto 15 m
Tree, upto 45 m
Tree, upto 30 m Tree, upto 50 m
Tree, upto 40 m
Tree, upto 35 m
Croton macrostachyus (Euphorbiaceae)
Dombeya torrida (Sterculiaceae)
Filicium decipiens** (Sapindaceae)
Ocotea kenyensis** (Lauraceae)
Olea capensis subsp hochstetteri (Oleaceae)
Olea europaea subsp cuspidata (Oleaceae)
Podocarpus falcatus** (Podocarpaceae)
Polyscias fulva (Araliaceae)
Aningeria adolfi-friederici** (Sapotaceae)
Prunus africana** (Rosaceae)
Syzygium guineense subsp afromontanum** (Myrtaceae)
Fruit, 17×14 mm
Drupe, 1×0.7 cm
Fruit, 4×1.5 mm
Fruit, 4.5×4 mm
Fruit, 13–15 mm
Seeds, 7×5 mm
Drupe, 1.5×1 cm
Drupe, 2cm long
Fruit, 5–8 mm
Seeds, 3–4 mm
Seeds, 7×4 mm
Size of propagule
Upland rainforest, forest edge or secondary growth; 1400–2600 m
Montane and riverine forest; 1700–2500 m
Afromontane rainforest upper canopy species; 1350–1450 m
Upland and lowland rainforest, mainly secondary up to 2450 m
Afromontane forest, pure Podocarpus or Juniperus-Podocarpus forest, in relic forest patches in farmland; 1550–2800 m
Afromontane forest, particularly drier highland forests where Juniperus is common, secondary scrub, riverine forest, as relic tree on farmland
Afromontane rainforest; 1550–2200 m
Afromontane rainforest; 1500–2100 m
Dense primary forest, riverine, thicket clumps and termite mounds in Combretum woodland; 1500–1800 m
Montane Aningeria-Albizia-Croton and Juniperus forest, montane scrub; 1600–3100 m
Montane and evergreen bush land, edges of forest and thickets, savanna, cultivated fields, waste ground, along rivers, 700–3400 m
Ethiopia, South Sudan, Zaire, Rwanda, south to Angola, Zambia, Malawi and Zimbabwe
Ethiopia, Cote d’ Ivoire, Cameron, Congo, Uganda, Kenya, Tanzania, Angola, Zambia, Malawi, South Africa, Zimbabwe, Madagascar and Comores
Ethiopia, Zaire, Rwanda, Sudan, Uganda, Kenya, Tanzania, Zambia, Malawi and Zimbabwe
Ethiopia, West Africa to Guinea, south to Mozambique and Angola
Ethiopia, Zaire, Rwanda, Burundi, Uganda, Kenya, Tanzania, Malawi, Mozambique and South Africa
Ethiopia, Cameron, Central Africa, Zaire, Sudan, Uganda, Kenya, Tanzania, Zambia, Rwanda, Malawi and Zimbabwe
Ethiopia, Cameron, Central Africa, Zaire, Sudan, Uganda, Kenya, Tanzania, Zambia, Rwanda, Malawi and Zimbabwe
Ethiopia, Zaire, South Sudan, Uganda, Kenya Tanzania, Malawi, Mozambique, South Africa
Ethiopia, Kenya, Malawi, Mozambique, Zimbabwe, and Madagascar
Ethiopia, south Sudan, Djibouti, Uganda, West Kenya, North Tanzania, East Zaire, Rwanda and Burundi
Ethiopia, west to Guinea, south to Angola, Zambia, Malawi and Mozambique
Geographic distribution within Africa
Table 1. List of studied species with information of their habit, size of propagule, habitat or ecology and geographical distribution within Africa*.
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* Sources: Bamps (1989), Friis (1992, 1995 a, b), Gilbert (1989, 1995), Hedberg (1989), Vollesen (1989, 1995); ** species used in mortality and herbivory investigation
Ethiopia, northeast Zaire, Uganda, Kenya and Tanzania Transitional montane forest, adjacent woodland often on termite mounds; 1400–1600 m Fruit, 25×15 m Warburgia ugandensis** (Canellaceae)
Tree, upto 30 m
Ethiopia, Sudan, Somalia, Kenya, Uganda and Tanzania Dry evergreen bushland, woodland, moist montane forest; 900–2800 m Fruit, 13–15 mm Teclea nobilis** (Rutaceae)
Shrub or tree, upto 16 m
from Senegal to Eritrea, Ethiopia, Somalia through most tropical Africa, south to South Africa Riverine forest, riparian woodland, occasionally transgressing into humid evergreen woodland; 1200–2500 m Fruit, 1×3.5 mm Syzygium guineense subsp guineense (Myrtaceae)
Tree, upto 35 m
Geographic distribution within Africa Habitat/Ecology Size of propagule Growth Habit Species
Table 1. (continued) 464
tudinal gradient, four line transects were established following north to south orientation and with 300 m distance between them. At every 100 m drop in altitude, quadrats having sizes of 10 × 5 m (50 m2) were laid down along the line transects. A total of 52 quadrats were investigated for the seedling survey work. Four pegs were used to mark the corners of the quadrats and strings were used around the pegs to demarcate the sampling area. In each quadrat the number and height of all seedlings of the study species were recorded. Seedling densities and frequencies of distribution of the study species were analyzed following Müller-Dombois & Ellenberg (1974).
2.3.2. Light environment The light environment in the understorey was measured using battery-operated light meter having a probe of flexible tips ending in small solar cells. Readings were taken at 150 cm above the ground in each quadrat. Measurements were made in the shade and in the open so that irradiance could be expressed in percentage of the open.
2.3.3. Seedling mortality To monitor the mortality rate of seedlings and investigate the causes of mortality, eight permanent plots of 10 × 5 m (50 m2) were laid down systematically at different places in the forest in the first week of January 2000. Each plot was divided into grids to facilitate mapping of the location of seedlings. Then, the location of healthy seedlings of eight of the study species that could be found in the quadrats was mapped, and each seedling was tagged to facilitate subsequent identification and monitoring. A total of 376 seedlings of the eight species were found and tagged (Table 1). In the third week of June 2000, the quadrats were re-visited and status of the seedlings was monitored. During the revisit, tree seedlings that were tagged and marked were found in various conditions. Some of them were found dead but normally rooted, others were uprooted from their original spot but found within the vicinity of the established plots and several others were removed and could not be found within the vicinity. Accordingly, the seedlings were categorized as healthy, defoliated, uprooted, dead and standing, or missing.
2.3.4. Herbivore damage to seedlings Damage of seedlings by herbivores was investigated by using the same quadrats and tagged seedlings. Massive or partial defoliation of seedlings together with removal of meristematic tips was considered as damage caused by herbivore.
2.3.5. Population structure of the study species ‘Population structure’ refers to the distribution of individuals in arbitrarily defined height classes, to provide the regeneration profile of the species, which is used to determine the status of their regeneration. To examine the population structure of
each species four line transects were established as in section 2.3.1. At every 100 m altitudinal drop a quadrat measuring 20 × 20 m (400 m2) was laid down. The size of quadrats was increased since not only seedlings but also saplings and mature trees were investigated. Accordingly, 52 quadrats (2.08 ha) were studied. In each quadrat, the number and height of all individuals of the study species encountered were recorded. All individuals of each species were, then, categorized into arbitrary equal height classes, i. e. 0–2, 2–4, 4–6, 6–8, 8–10, … 28–30 and > 30 m.
2.4. Data analysis The data from the seedling surveys were subjected to Canonical Correspondence Analysis using the program CANOCO (Ter Braak 1995) to get overview of the relationship between seedling distribution of the study species, altitude and light gradients. The percentage data obtained from the study of mortality and herbivore damage were arcsine transformed and subjected to One-Way ANOVA (Zar 1996).
3. Results 3.1. Seedling distribution and density along altitudinal gradient The species composition of tree seedlings showed marked restriction and distinct habitat preferences along the altitudinal gradient. Species were found in abundance and more frequently in a limited range of altitudes, and no species occurred throughout the entire gradient (Fig. 1). The species formed a peak at one altitude within their ranges of distribution or habitat. Only Aningeria adolfi-friederici formed two peaks of
equal size at two different altitudes, i.e. 1800 and 2200 m. The highest total number of seedlings was recorded at 2100 m consisting of Olea capensis, Teclea nobilis and Syzygium guineense subsp afromontanum. Similarly, Prunus africana showed its peak at 2000 m altitude and seedlings of Ocotea kenyensis, Podocarpus falcatus and Aningeria had their peaks at 1800 m. Olea europaea subsp. cuspidata showed the highest record of number of seedlings at 1600 m altitude and other species such as Filicium decipiens, Syzygium guineense subsp guineense, Warburgia ugandensis and Croton macrostachyus had their peaks at 1500 m. Seedlings of most species were distributed continuously across the altitudes within their range of occurrence. Olea capensis subsp. hochstetteri was the only species that showed either missing or quite low number of seedlings within its range of habitat at 1800 and 1900 m altitudes. Density, defined here as the number of seedlings of the study species per ha, showed strong variation among species ranging between 58 and 1,065 (Table 2). Podocarpus had the highest density followed by Teclea, Ocotea, Olea capensis and Syzygium guineense subsp afromontanum. Trees with seedling densities between 500 and 200 were, in accordance of their decreasing orders, Croton, Prunus, Warburgia, Aningeria, S. guineense subsp guineense, Olea europaea and Filicium. Polyscias fulva and Dombeya torrida had the lowest seedling densities. The difference in the density of seedlings recorded (for Podocarpus), was eighteen times greater than the lowest density (for Dombeya). Frequency of distribution (percentage of the quadrats in which the species were recorded) along the entire altitudinal gradient varied between 15 and 69% underlining the restricted occurrence of tree seedlings along the gra-
Table 2. Density of tree seedlings and their frequency of distribution along the altitudinal gradient at Harenna forest. Species
Podocarpus falcatus Teclea nobilis Ocotea kenyensis Olea capensis subsp hochstetteri Syzygium guineense subsp afromontanum Croton macrostachyus Prunus africana Warburgia ugandensis Aningeria adolfi-friederici Syzygium guineense subsp guineense Olea europaea subsp cuspidata Filicium decipiens Polyscias fulva Dombeya torrida
Podocarpaceae Rutaceae Lauraceae Oleaceae Myrtaceae Euphorbiaceae Rosaceae Canellaceae Sapotaceae Myrtaceae Oleaceae Sapindaceae Araliaceae Sterculiaceae
1065 1854 1738 1726 1719 1496 1373 1342 1284 1250 1227 1219 1150 1 58
53.8 38.4 30.7 23.0 23.0 69.2 46.1 23.0 30.7 15.3 15.3 15.3 30.7 23.0
* No. of seedlings/hectare; ** Percentage of quadrats occupied FLORA (2002) 197
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Fig. 1. Number of seedlings (height of individuals <1.5 m) of the fourteen study species recorded per 200 m2 along the altitudinal gradient at Harenna forest; PF = Podocarpus falcatus; AA = Aningeria adolfi-friederici ; PA = Prunus africana; SGA = Syzygium guineense subsp afromontanum; OC = Olea capensis ; TN = Teclea nobilis; FD = Filicium decipiens; SGG = Syzygium guineense subsp guineense; WU = Warburgia ugandensis; OK = Ocotea kenyensis ; OE = Olea europaea; CM = Croton macrostachyus ; POF = Polyscias fulva; and DT = Dombeya torrida.
the lower elevation. Of the total 52 quadrats surveyed for seedling populations, 70% were found under canopy shade and the remaining 30% in the open. Similarly, about 79% of the investigated seedling populations were found under shade and 21% in the open. Results from ordination of the data on distribution of seedlings in the altitude and light gradients showed that seedlings of eight of the study species were associated with canopy shade (Podocarpus, Aningeria, Prunus, S. guineense subsp afromontanum, O. capensis, Ocotea, Teclea and Filicium) while the remaining six species had their seedlings in open areas. Species that were associated with open areas included Croton, Polyscias, Warburgia, Syzygium guineense subsp guineense, O. europaea and Dombeya.
3.3. Seedling mortality Fig. 2. Total mean density (number of seedlings/ha) of the fourteen study species along the altitudinal gradient at Harenna forest.
dient (Table 2). Some species had comparatively wider ranges of occurrence, evidenced from their frequency of distribution. Total mean seedling density showed a peak at 2000 m altitude along the altitudinal gradient (Fig. 2). Most of the seedlings, i.e. 13,700 seedlings, at this peak were from seven tree species, namely Podocarpus, Olea capensis, Teclea, Syzygium guineense subsp afromontanum, Aningeria, Prunus and Polyscias. The second highest seedling density record, i. e. 12,850 seedlings, was found at 2100 m altitude (Fig. 2). The same species were recorded in this altitude as above, with the exception of Polyscias. On average, the medium altitudes (2000–2200 m) had nearly 12,200 seedlings per ha, which was higher than those recorded both at the upper and lower elevations. The lower elevations (1500–1900 m) had nearly 8,500 seedlings per ha on the average, contributed mainly by nine tree species, namely Podocarpus, Croton, O. europaea, Filicium, Warburgia, Ocotea, S. guineense subsp guineense, Aningeria and Polyscias. The upper elevations (2300–2700 m) exhibited the lowest seedling populations, about 850 individuals per ha. Tree seedlings recorded at the upper elevation forest zone include Teclea, Prunus, Dombeya and, rarely, Croton.
3.2. Light environment In general, light levels received under canopy shade ranged between 1 and 5% of the open areas. The forest zone at the upper elevation was more open than the forest zone at
The three categories of seedlings, i.e. uprooted, dead and standing and missing accounted for 12, 33, and 55% of the total mortality investigated. Mortality varied among the different species, ranging from 11 to 50% of their seedling populations (Fig. 3). Seedlings of S. guineense subsp afromontanum had the highest mortality rate, accounting for 50% of its seedling popula-
Fig. 3. Seedling mortality of the eight species studied during the dry season in Harenna forest; 1 = Aningeria adolfi-friederici; 2 = Prunus africana; 3 = Ocotea kenyensis ; 4 = Podocarpus falcatus; 5 = Teclea nobilis; 6 = Filicium decipiens; 7 = Olea europaea; and 8 = Syzygium guineense subsp afromontanum. FLORA (2002) 197
ed together with their leaves in most cases. The study species can be arranged in the order of decreasing damage due to herbivory as: Podocarpus < S. guineense subsp guineense < Aningeria < Filicium < Teclea < O. europaea < Ocotea < Prunus.
3.5. Population structure
Fig. 4. Damage to seedlings due to herbivory of the eight species studied during the dry season in Harenna forest; 1 = Podocarpus falcatus; 2 = Syzygium guineense subsp afromontanum; 3 = Aningeria adolfi-friederici ; 4 = Filicium decipiens; 5 = Teclea nobilis ; 6 = Olea europaea; 7 = Ocotea kenyensis; and 8 = Prunus africana.
tions. O. europaea, Filicium, Podocarpus and Teclea lost 37 to 21% of their seedling populations while the remaining species, Ocotea, Prunus and Aningeria showed relatively low rate of mortality, 16 to 11% of their seedling population. Pooled data from the eight species, Filicium, Ocotea, O. europaea, Podocarpus, Aningeria, Prunus, S. guineense subsp afromontanum and Teclea, have shown that 27% of the seedlings were dead, 42% damaged by herbivores and only 31% remained unaffected. Mortality of seedlings of the species can be ranked in decreasing order as: Aningeria < Prunus < Ocotea < Teclea < Podocarpus < Filicium < O. europaea < S. guineense subsp afromontanum.
3.4. Herbivores damage to seedlings Herbivory was recognized in all the eight species investigated except Podocarpus. Damage of seedlings from herbivory was proportionally higher than uprooting, dead and standing or missing, and was highly variable among species ranging from nil (in Podocarpus) to 83% (in Prunus) of their seedling populations (Fig. 4). In addition to defoliation of the individual plants, the growing tips of seedlings were also consumed or remov468
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Two major patterns of height class distribution were observed among the fourteen species (Figs. 5 and 6). The first group showed high number of individuals at the lowest height classes, mainly at seedling stages, and a gradual decline towards the middle and upper height classes (Fig. 5). This shows continuous representation of individuals in all height classes. To this group belong Teclea, Ocotea and S. guineense subsp guineense. The second group showed high number of individuals at the lowest height class but followed by missing of individuals at any one or more of the medium or upper size classes (Fig. 6). Members of this group, i.e. Podocarpus, Aningeria, S. guineense subsp afromontanum, Warburgia, Croton, Dombeya, Polyscias, Filicium, O. capensis, O. europaea and Prunus did not show continuous representation of individuals in all height classes. Although the general trend appears similar in the second group, there exist some variations among species in representation of individuals within the height classes. For example, Filicium had shown high number of seedlings at the lowest height class and it lacked individuals in the consecutive height classes. Only few mature individuals were encountered at the upper height classes. Warburgia and Dombeya lacked young established seedlings below 25 cm height within the lowest height group. But the presence of more seedlings above 25 cm (within the lowest height class) concealed the situation (Fig. 6). The two species also lacked individuals in the intermediate height. S. guineense subsp afromontanum, Prunus, Croton and Warburgia had relatively more mature trees than the intermediate size groups (Fig. 6).
4. Discussion Seedling populations and species composition along the altitudinal gradient revealed that species had variations both in their abundance and distribution and all show a limited range of occurrence across the gradient. The restricted distribution and range of occurrence of the species can be explained by presence of steep ecological gradients in terms of, for example, soil moisture, organic matter, exchangeable cations, pH, humidity, temperature, etc. along the altitudinal gradient (Hillman 1988; Lisanework Nigatu & Mesfin Tadesse 1989; Feoli et al. 1991).
Fig. 5. Population structure of Teclea nobilis (TN), Syzygium guineense subsp guineense (SGG) and Ocotea kenyensis (OK). Height Classes: 1 = 0–2; 2 = 2–4; 3 = 4–6; 4 = 6–8; 5 = 8–10; 6 = 10–12; 7 = 12–14; 8 = 14–16; 9 = 16–18; 10 = 18–20; 11 = 20–22; 12 = 22–24; 13 = 24–26; 14 = 26–28; 15 = 28–30; 16 = >30 m.
The medium elevation had the relatively highest seedling population compared with the lower and upper elevation forest zones. This might be attributed to the closed and continuous canopy cover (Friis 1986; Lisanework Nigatu 1987) at the middle altitudes with
minor human disturbances in the form of livestock grazing and tree removal. On the other hand, the lower elevation had lower seedling density than the medium elevation, though the number of recorded species was greater at the lower elevation. The forest at the lower FLORA (2002) 197
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Fig. 6. Population structure of Podocarpus falcatus (PF); Aningeria adolfi-friederici (AA); Syzygium guineense subsp afromontanum (SGA); Prunus africana (PA); Olea capensis (OC); Filicium decipiens (FD); Olea europaea (OE); Warburgia ugandensis (WU); Croton macrostachyus (CM); Dombeya torrida (DT); and Polyscias fulva (POF). Height classes same as in Fig. 5.
elevations had a closed canopy but with several irregular openings caused by human disturbances. This might have created favorable conditions for gap preferring species, such as Croton, to germinate and establish. Naturally occuring coffee was the major component of the shrub layer at the lower elevation, and human management practices such as weeding, ground clearance and canopy thinning have affected both the structure and composition of the forest. Weeding and ground clearances, which are traditional management practices of coffee in the forest, might have reduced the number of regenerating seedlings in the understorey. Forest burning and thinning activities have lead to the removal of all previously existing mature trees creating gaps in the canopy, which in turn affect the microenvironment in the understorey, and hence natural regeneration. Unlike the lower and medium elevations, which are occupied by forest trees, herbaceous plant communities colonize the upper altitudes, and mature trees are sparsely distributed. Similarly, the number of tree seedlings of the study species recorded at this zone was relatively low, only few hundreds per hectare. Here, a thick herb and ground vegetation was observed suppressing regeneration of seedlings through competition for space. Both sexual (seeds) and asexual (coppicing) means of regeneration have been observed during the investigation. These phenomena were observed for Aningeria, Croton, Dombeya, Ocotea, Podocarpus, Prunus, S. guineense subsp guineense and Teclea. Overall, most of the forest tree species investigated had several hundred seedlings per hectare in understorey implying that the “seedling bank” (Whitmore 1990) is their major route of regeneration (Garwood 1989). Similarly, other major tree species of the dry Afromontane forests of Ethiopia so far studied exhibited such accumulation of suppressed seedlings that would contribute to forest regeneration (Demel Teketay 1997 b). The study species in Harenna forest also exhibited differences in their light requirements. Based on the spatial distribution of their seedlings in the canopy light gradient on the forest floor, the fourteen study species have been categorized in two functional groups, shadetolerant and light-demanding ones. The shade tolerant species include Aningeria, Filicium, Ocotea, O. capensis, Podocarpus, Prunus, S. guineense subsp afromonatanum and Teclea. The light demanding species include Croton, Dombeya, O. europaea, Polyscias, S. guineense subsp guineense and Warburgia. This is in agreement with similar previous studies in dry Afromontane species (Demel Teketay & Granström 1995; Demel Teketay 1997b; Gemedo Dalle 1999) with the exception of O. europaea. The seedlings of O. europaea were found in open canopy sites at 1600 and 1500 m altitu-
des in Harenna forest implying a light-demanding behavior of the species. Seedling mortality was highly variable among the study species ranging from 11 to 50% of the populations. Although causes of seedling mortality have not been investigated in the study, diverse possible factors responsible for seedling mortality are effective, including trampling or damage by larger animals, drought, nutrient deficiencies, diseases or a combination of these factors. Several dead seedlings of S. guineense subsp afromontanum (with the highest mortality) have shown unique signs of chlorosis, whereas the seedlings of other species had a typical green color before their death. Insufficient soil moisture was also assumed as one of the probable causes that contributed to death of the seedlings since dead but standing and normally rooted seedlings could be found. Seedlings are shallow rooted and seedling mortality rate is usually higher in dry seasons (Peterken 1966). Several studies elsewhere have indicated that seedling mortality with values up to 90% occurs when the environment becomes drier and severe (Silvertown 1982). Although the rate varied among species, herbivory was found to play a significant role in seedling population dynamics and regeneration of the species. Massive defoliation occurred and, more commonly, parts of the leaves together with the growing tips of the plant were consumed. By this way serious reduction of both photosynthetic and meristematic tissues occurred. The effect of herbivory on a seedling population could be highly variable. In some seasons and places the whole cohorts or a significant proportion of the population was lost by herbivory while in others the effects seemed to be less important (see also Adams 1975). The impact of herbivory on forest regeneration, thus, may vary from location to location and season to season. Selective herbivory influences forest regeneration by differentially affecting species in a plant community. Selectively affected plants are generally assumed to have evolved defenses against herbivore damage (Janzen 1971; Crawley 1983). In our field observations, several previously damaged seedlings of Prunus were found resprouting from damaged stumps even when the above ground parts did not exist. Seedlings of Ocotea and Olea europaea developed coppices from damaged above ground parts. The extent of coppice resprouts following herbivory are usually proportional to the amount of tissue lost, and for many species there is a threshold level below which regrowth can be adversely hampered (Hendrix 1988). The coppicing characteristic of species after herbivore damage suggests that these plants have developed the ability to compensate, at least partially, for the selection pressure by animal feeding. FLORA (2002) 197
Species with seedlings that suffered greatest herbivores damage, namely Prunus and Ocotea, had a relatively lower mortality from other causes as compared with the other species. The accumulated evidence from previous studies elsewhere has shown that herbivory is a major factor influencing growth, reproductive success and the dynamics of plant population as well as the community (Silvertown 1982; Hendrix 1988). The patterns of population structures indicated that only few species had a good representation of individuals at all size classes implying that a healthy regeneration is taking place in these cases. For instance, Ocotea, Teclea and S. guineense subsp guineense had high number of individuals at the seedling stage and a gradual decrease towards sapling and mature trees indicating continous or good regeneration. Such patterns, commonly referred to as reverse “J” distribution, represent stable population structures (Silvertown 1982). Among the three species, Ocotea and Teclea are represented as sub-canopy trees at the medium elevations, where disturbance is relatively low. However, most of the species investigated showed absence of individuals at any one or more of the height classes. Major canopy and economically important species, namely Aningeria, Croton, O. capensis, Podocarpus, Prunus, S. guineense subsp afromontanum and Warburgia exhibited peaks at the seedling stage and missing of individuals at the sapling or pole size stages. The absence or decline in number of individuals in the middle and upper height classes indicates that there is selective removal of these usable-sized individuals by local people for the purposes of construction, fuelwood or any other domestic uses. Semi-pastoral communities normally pass through this forest for a period of three to four months. These pastoral communities construct small huts in the forest making a sort of temporary village until they move to the next grazing sites. Podocarpus, which is also among those trees selectively removed locally, is the most commercially exploited timber species from this forest. It accounted for more than 80% of the total timber harvest from the area, and was also the leading exploited timber species in the country in the past (Demel Teketay 1992; Tamrat Bekele 1993). Analysis of population structure of this particular species has shown that there are few remaining mature trees in the population as a result of heavy logging. Selective logging in this forest involves cutting of the best individuals from a stand without due consideration for their replacement or future regeneration. Such a highly selective logging operation can place extremely negative pressure on the harvested species, reducing their quality (genetic make-up) and abundance thereby resulting in the local disappearance of merchantable trees. Filicium showed a high number of individuals at the lowest height class, particularly at the seedling estab472
FLORA (2002) 197
lishment stage, and no individuals at the next height classes. A similar pattern of population structure was observed before for Maesa lanceolata and Prunus in other dry Afromontane forests where human interference is extremely high (Demel Teketay 1997b). Filicium occupies only the lower altitudes of Harenna forest where coffee grows naturally (Friis 1986). The absence of individuals of Filicium in other height classes than the seedling stage can be partially attributed to the management activities of the people such as weeding, ground clearing, thinning, including burning. Both Warburgia and Dombeya lack young seedlings below 25 cm height. Warburgia might have been affected by disease pathogens, which can attack both the fruits and the seeds. Forestry extension workers emphasized that the fruits and seeds of this species were found highly infected by disease and seed collection has become difficult in recent years. When compared with the status of their regeneration in other forests, the studied species showed some variation. For instance, from results of a previous study (Demel Teketay 1997b) it became obvious that populations of Podocarpus, O. capensis and O. europaea in Menagesha and Gara-Ades forests had a good regeneration, i.e. continuous representation of individuals in all size classes. In Harrena forest, due to selective removal of trees for timber and construction the species could not exhibit their potential of regeneration as it was observed in other forests. In contrast, Teclea had shown hampered regeneration in other dry Afromontane forests where it lacked individuals at the lowest height classes (Demel Teketay 1997b). In contrast, it had a higher seedling density and exhibited a good regeneration status in the present study. Therefore, populations of a given tree species can exhibit different regeneration status under different forest ecosystems, which may be attributed to previous and current disturbances as well as the history of the particular forest. Generally, many of the investigated species had population structures that showed peaks at the seedling stage with few or no individuals at sapling, pole size or mature stages. This suggests the need to develop and implement appropriate forest management activities in the area in order to fill the existing regeneration gaps and facilitate the healthy / continuous regeneration of not only the study species but also all other woody species in the forest. Ultimately, such activities would ensure a sustainable development of the forest. The Harenna forest is currently under strong human pressure as a result of expansion of agriculture and settlement (usually involving fire), extraction of timber and non-timber forest products as well as grazing. Present logging operations focus on restricted timber species with little thought for their future regeneration. Selective logging in the forest has already lead to the depletion of
mature trees of Podocarpus. Moreover, the felling operations cause enormous damage to the young plants, and the standing trees become injured. Over-exploitation of seed bearing mature trees of Podocarpus and other species can reduce their genetic make-up as well as abundance, and result in their local disappearance due to loss of reproductive individuals in the population. Based on results of the study, the following recommendations are forwarded as an implication for management and conservation options: (1) reduce the intensity of forest grazing to minimize the existing pressure on tree seedling populations; (2) reduce the intensity of selective removal of individuals of Podocarpus for timber, particularly mature trees, so that they remain in the population for seed production; (3) introduce exploitation of a broader range of species guided by properly prepared management plans that would permit the development of continuous structures and stable population of the trees; (4) ensure that weeding, ground clearance and thinning activities in the coffee-zone of the forest will not affect individuals of trees known to have problems of regeneration; (5) plan and implement practices that would enhance the regeneration of trees, particularly those that are already exhibiting a poor regeneration status in the forest ; and (6) initiate further studies on other environmental variables, including edaphic factors, and causes of mortality of the seedlings in order to get a clear picture about their influence on tree regeneration in Harenna.
Acknowledgements Financial support for the study was obtained from Swedish Agency for Research Cooperation in Developing Countries through the Department of Biology, Addis Abeba University. The first Author would like also to thank Prof. T. C. Whitmore, Dr. M. D. Swaine, and Dr. Micheal Fenner, all from UK, for provision of materials. Prof. Sebsebe Demissew, Dr. Zerihun Woldu and Dr. Tamrat Bekele from Addis Abeba University are acknowledged for their comments on the draft manuscript. Ato Yossef Assefa is greatfully acknowledged for assisting in the field and Herbarium. Members of the Bale Zone Agriculture Development Department (BZADD), the Forestry Research Centre (FRC) and The National Herbarium (ETH) are also highly acknowledged. We thank the two anonymous reviewers for their valuable comments on the earlier version of the manuscript.
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