- Email: [email protected]
Response of plant communities to fire in an Acacia woodland and a dry Afromontane forest, southern Ethiopia
Forest Ecology and Management 177 (2003) 39±50
Response of plant communities to ®re in an Acacia woodland and a dry Afromontane forest, southern Ethiopia Ingalill Erikssona, Demel Teketayb,*, Anders GranstroÈmc a
Department of Silviculture, Swedish University of Agricultural Sciences, S-901 83 UmeaÊ, Sweden b Ethiopian Agricultural Research Organization, P.O. Box 2003, Addis Abeba, Ethiopia c Department of Forest Vegetation Ecology, Swedish University of Agricultural Sciences, S-901 83 UmeaÊ, Sweden Received 7 August 2001; received in revised form 2 April 2002; accepted 13 May 2002
Abstract To understand the role of ®re in degrading tropical dry forest ecosystems of north-east Africa, ®re potential was studied within two vegetation types: Acacia woodland and dry Afromontane forest. Fire behaviour was analysed in experimental ®res late in the dry season (February) and the potential response of the soil seed bank was studied on samples taken before and after ®re treatment. Bark thickness was also measured, as an indication of ®re-resistance, on a suit of tree/shrub species representing the two vegetation types and an ecotone between them. The total fuel biomass differed only marginally between the two vegetation types. But biomass of the litter fuel differed signi®cantly, and the biomass of litter fuel was two times greater in the Acacia woodland (673 g m 2) than in the dry Afromontane forest (308 g m 2). In contrast, the biomass of woody ®ne fuels was four times greater in the dry Afromontane forest (300 g m 2) than in the Acacia woodland (76 g m 2) and the biomass of live ®ne fuels was also several times greater in the dry Afromontane forest (110 g m 2), compared with the Acacia woodland (15 g m 2). During experimental ®res in the Acacia woodland, fuel consumption was 100% for all burned plots, but in the dry Afromontane forest fuel consumption ranged between 10 and 40% of the available fuel within the plots. The results from the soil seed bank study indicated little if any impact from ®re. There was a gradual increase in bark thickness of trees when going from the dry Afromontane forest over the ecotone towards the Acacia woodland site. Considering all species analysed, bark thickness of trees 15 cm in diameter at 1 m height ranged from 2.4 to 15.4 mm. Therefore, based on the differences in microclimate, fuels, ®re behaviour and bark thickness the Acacia woodland was determined to be the most ®re prone as well as ®re resilient ecosystem. In contrast, sustained combustion was unlikely at the studied dry Afromontane forest site. Spread of ®re was not possible due to the high fuel moisture content and poor fuel bed. Forest fragmentation and intentional burning of grasslands, followed by an altered microclimate, should however increase the probability of ®res penetrating the dry Afromontane forest. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Bark thickness; Fire behaviour; Fire spread; Flame length; Fuel biomass; Fuel moisture; Soil seed banks
1. Introduction
*
Corresponding author. Tel.: 251-1-454-452; fax: 251-1-461-251. E-mail address: [email protected] (D. Teketay).
A major factor favouring wide-scale burning in tropical forests is the cumulative effect of vegetation changes in recent decades. Logged-over forests are in general more vulnerable to burning due to accumulation
0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 3 2 5 - 0
40
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
of logging slash and dry ground conditions created by opening up of the canopy. In steep montane terrain, updrafts increase the intensity of a ®re and in Sabah, Indonesia, even primary forest burned extensively on steep, well-drained slopes where ®res could spread easily (Beaman et al., 1985). Plants have evolved different strategies to cope with such highly variable, competitive or stressful environments. Long-lived species, like trees, may have low-¯ammability litter to avoid, or thick bark to withstand, heat damage (Mutch, 1970; Uhl and Kauffman, 1990; Loth, 1999). The insulating capacity of bark protects cambial tissues from heat-induced mortality. Thermal bark-characteristics differ as a function of tree size, i.e. diameter, and tree species (Uhl and Kauffman, 1990). Fire mortality is, therefore, greater in small trees, whilst drought alone kills more large trees (Swaine, 1992). In dry forests, grazing will reduce the frequency of ®re and thereby promote the establishment of species such as Brachystegia. These species require ®re protection during the sapling stage, whilst becoming insensitive to ®re as adults (BackeÂus et al., 1994). This pattern may also be true for other tree species in East Africa. However, in areas with frequent ®res, the establishment of tree species could be effectively prevented by ®re (Hoffman, 1996). Vegetation responses to ®re will depend on several factors. Ideal conditions for tree establishment in dense grass tuft require an initial ®re, followed by rain and subsequent ®re protection (Lawton, 1978; Kikula, 1986; BackeÂus et al., 1994). Canopy openings, caused by ®re intrusion in gallery forests, induce a secondary succession where most recruits come from the residual soil seed bank (Kellman and Meave, 1997). The formation of soil seed banks is also a common survival strategy of Acacia species, which accumulate large quantities of viable but dormant seeds in the soil (Sabiiti and Wein, 1987). Germination of Acacia species is in general enhanced when subjected to the heat of ®re (Teketay, 1996a,b, 1997a). Fire intensity and its sub-components, fuel consumption and rate of spread, in¯uence soil temperatures. Maximum temperatures are greatly affected by the amount of ®ne fuel particles consumed. Fuel loads from 0.6 to 2.0 kg m 2 burnt on the ground will break seed dormancy and stimulate germination down to 1 and 3 cm depth, respectively (Bradstock and Auld,
1995). Acacia tortilis seeds with low moisture content will absorb water and germinate after exposure to dry heat of about 150 8C (Loth, 1999). On a local scale the change from forest to savannah is complex and fragmentary, with a mosaic of the two vegetation types apparently determined by a variety of factors. Locally the forest-savannah boundary is typically abrupt, accentuated by annual ®res, which are normally extinguished at the boundary (Swaine, 1992; Cavelier et al., 1998). Fire has for long been believed being negatively correlated with forest vegetation and positively related to grasslands, due to the ¯ammable nature of the grass during the dry season. But in West Africa, dry forests near the forest-savannah boundary form a distinctive sub-type as a result of occasional encroachment by ®res. Here, past ®res were likely to have a profound in¯uence on the composition of the forest canopy (Swaine, 1992). Although it is evident that dry forest systems, such as the dry Afromontane forest, experience contact with ®re due to the proximity to grasslands, little is known about the potential for ®re spread and the ®retolerance of the tree species. The present study was, therefore, undertaken to determine the ®re potential for two of the vegetation cover types in the Ethiopian Rift Valley: Acacia woodland and dry Afromontane forest as well as an ecotone between them, and assess the potential responses of tree and ground ¯ora and the soil seed bank after ®re. 2. Materials and methods 2.1. Study sites The study was conducted within three vegetation cover types in the Great Rift Valley of southern Ethiopia, namely the Acacia woodland in AbijataShalla Lakes National Park (78320 N and 388400 E), the ecotone (transition zone) located southeast of Lake Langano (78260 N and 388470 E) and the dry Afromontane forest at the Abaro Mountain, Wondo Genet (7860 N and 388380 E) (Fig. 1). 2.1.1. The Acacia woodland The Acacia woodland site is within the area of Abijata-Shalla Lakes National Park, which is a fenced Ostrich Farm protected since the 1970s. Topographic
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
41
Fig. 1. Location of the study sites.
relief is level at this site, with altitudes around 1640 m above sea level. The soils are predominantly andosols, characterised by weakly structured topsoil and low in organic matter (Makin et al., 1975; Argaw et al., 1999). Average annual rainfall ranges from 500 to 835 mm. There is a small rainy season in April and the main rainy season extends from July to October. A short dry period is experienced at the end of May and early June. Later, there is a notable dry season that occurs between November and February. Mean monthly temperature ranges from 13.8 to 28.0 8C (Makin et al., 1975).
The area is classi®ed as semiarid eco-climatic vegetation zone (Makin et al., 1975; Argaw et al., 1999). Natural vegetation is characterised by scattered mature trees of Acacia species and Balanites aegyptiaca, various shrubs and climbers as well as by about 1 m tall grass stratum. Cattle grazing has been restricted since the Ostrich Farm was established, but the short leaves at the base of the grasses are grazed by two races of Ostrich, namely the Somali race (Struthio camelus molholophanes) and the North African race (Struthio camelus camelus) (Getenet Wondimu, pers. commun.). Large mammals, such
42
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
as Grant's gazelle, oribi, grey duikers and mountain reedbucks, also have access to the Ostrich Farm (EWCO, 1988). 2.1.2. The dry Afromontane forest The site in the dry Afromontane forest stretches along the eastern escarpment of the Great Rift Valley. Elevation spans from 1900 to 2000 m above sea level in the study area, climbing uphill the steep western slope of the Abaro Mountain. The soils have not yet been classi®ed, but earthquake activity and the steepness of the slopes have transported colluvial material from higher altitudes. Average annual rainfall is 1200 mm, but varies between 700 and 1400 mm (Eriksson and Stern, 1987). The area has a bimodal rainfall distribution, with two rainy seasons. A minor rainy period occurs from February to April and a more extensive from July to September. There is also a characteristic long dry season from October to February. The mean temperature throughout the year is about 19 8C. The natural vegetation is dominated by Cordia africana, Ficus sur, Croton macrostachyus, Millettia ferruginea, Albizia gummifera, Olea capensis and Syzygium guineense. Shrubs or very small trees are Allophylus abyssinicus and Vernonia amygdalina. The lower strata of the forest are densely mixed with woody climbers, some of which are thorny. Scrambling species of Rubus are common, and the forest ¯oor is partly covered by herbaceous dicotyledons; plant nomenclature in this paper follows that of Friis (1992). In the end of the dry season, when ground vegetation becomes scarce or downed in the adjacent grasslands, cattle are directed into the high forest area. 2.1.3. The ecotone In the ecotone, the terrain is gently rolling, with altitudes ranging from 1840 to 1900 m above sea level within the study area. The soils are predominantly andosols with relatively high content of organic matter (Murphy, 1968; Makin et al., 1975; Argaw et al., 1999). The area has a two-peak annual rainfall between 600 and 800 mm, with short and unreliable rains during April±May and the main rainy season from June to August, sometimes up to September. The dry months are from November to February (Senbeta, 1998). Mean monthly temperature ranges from 13.8 to 28.0 8C (Makin et al., 1975). The natural vegetation
ranges from Acacia species in the lower parts to open deciduous forest or woodland in higher elevations with tall trees of Celtis africana, Olea europaea subsp. cuspidata, A. gummifera, C. africana, F. sur and C. macrostachyus (Friis, 1992). Cultivation and grazing have reduced the natural vegetation into fragments and the density of the tree layer is sparse. 3. Methods 3.1. Fuel characteristics The behaviour and spread of ®re is governed by a number of fuel characteristics. In this study, fuels are de®ned as live ®ne materials (i.e. live grasses and herbs), litter (i.e. leaf litter, dry grasses and herbs), and woody ®ne materials (i.e. all dead and downed woody debris). The total fuel biomass, fuel depth, types of fuels and moisture content of the fuels are all important factors in predicting potential ®re behaviour. The potential for ®re was studied by analysing these characteristics of the fuel bed within the Acacia woodland and dry Afromontane forest. In the ecotone, cultivation and grazing made investigations on fuel characteristics, experimental burning and soil seed banks impossible. Therefore, the study there was restricted only to bark-characteristics of trees and shrubs. To investigate the various fuel characteristics, 20 plots measuring 10 m 10 m (100 m2) and 100 m away from each other were established in each of the Acacia woodland and the dry Afromontane forest sites along four and seven line transects (ranging in length between 300±500 and 200 m apart) running in east-west and north-south directions, respectively. To eliminate any potential edge effects, plots were placed at least 30 m from boundaries of the sites. The total biomass of fuels down to the fermentation layer was sampled from two representative 30 cm 30 cm (900 cm2) subplots within the main plots. Live ®ne fuels were clipped at ground level, whereas litter and all woody ®ne fuel particles were carefully lifted by hand. Each of the three fractions was put separately in a marked plastic bag and was immediately weighed in the ®eld on a digital balance in order to quantify the moisture content. Then, all the fuel samples were airdried and stored indoors. The fractions were then
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
oven-dried at 100 8C to constant weight for moisture content and dry-weight estimations. 3.2. Fire behaviour Fire behaviour was studied during a series of experimental burns, simulating ®re late in the dry season. In the dry Afromontane forest, the 20 plots were divided into two groups, namely those found in open and closed canopies. From each group, four 1 m 1 m (1 m2) plots found in open canopy and four 1 m 1 m plots found in closed canopy were randomly selected. Then, ®ve of the plots were burned on 11 February 1999, and the remaining three on 12 February 1999. In the Acacia woodland where the canopy is homogenous, only four 1 m 1 m plots were randomly selected. These plots were burned on 15 February, 1999. All experimental burns were conducted between 13.00 and 17.30 h, when humidity was lowest. Just prior to ignition, a small fuel sample was collected for moisture determination as described in Section 3.1. Then the plots n 12 were burnt by head-®res, i.e. the ®res were allowed to spread with the wind (Bradstock and Auld, 1995). Accordingly, a 1 m hemp rope was soaked in kerosene, and two persons ignited the rope simultaneously, to assure an even ®re front. The rate of movement of the ®re front was measured with a digital stopwatch and the ¯ame length was estimated with a stick marked with a scale. 3.3. Weather At each plot, air temperature and relative humidity were measured during the fuel sampling, which was carried out between 13.00 and 17.30 h when humidity declines to its lowest levels (Holdsworth and Uhl, 1997), using a thermometer and a hygrometer, respectively. Both instruments were set 1 m above ground in shelter to avoid direct solar radiation. Similarly, during each experimental burning, air temperature, relative humidity as well as wind direction and speed were monitored. Wind direction and speed were visually monitored. 3.4. Bark thickness Tree diameter and bark thickness were measured 1 m above the ground level for all trees greater than
43
30 mm in diameter. Bark thickness was measured twice, on opposite sides of the trunk. All woody plants in the plots in the Acacia woodland and the dry Afromontane forest were identi®ed to species. In the ecotone, trees and woody plants were randomly selected and identi®ed to species. Altogether 356 trees were sampled for measurements of diameter and bark thickness from the three sites. 3.5. Soil seed banks The soil seed banks in the Acacia woodland and the dry Afromontane forest were also studied by taking soil samples from the centre of each of the 40 plots that had earlier been used for sampling fuel. At the centre of each plot, a subplot measuring 10 cm 10 cm (100 cm2) was marked and four separate soil layers were cut. The samples included a litter layer and three mineral soil layers, each 3 cm thick. To investigate the response of soil seed banks to ®re, a second soil sampling was carried out, following the same procedure as above, adjacent to the location of the ®rst sample 1 day after the experimental burning. For this purpose, soil samples were taken from only the 12 subplots (each 100 cm2) inside the 12 of the plots that were experimentally burnt as described in Section 3.2 in the Acacia woodland as well as in the open and closed canopies of the dry Afromontane forest. At both sites, the soil samples were taken in January 1999. All soil samples n 40 subplots four layers 12 burnt subplots four layers 208 were transported to the Forest Research Centre (FRC) Headquarters in Addis Abeba for processing. At FRC, each sample was sieved separately to recover bigger seeds using sieves with mesh sizes from 5.0 to 0.5 mm. Extracted seeds were collected for identi®cation and viability tests. Seeds were identi®ed to species genus or family levels. Those dif®cult to identify were given numbers and recorded as unidenti®ed species. Viability of seeds was determined by a cutting test, and a seed was considered viable when its content was white and ®rm (Teketay and GranstroÈm, 1995). Just after the sieving, soil samples were incubated in the glasshouse at FRC for germination of seeds 6 weeks after the ®rst soil sampling. The sieved soil samples were spread on cotton cloth in plastic trays and placed on tables in the glasshouse. The samples
44
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
were then kept continuously moist and were incubated for 6 months as of the beginning of March 1999. The emergence of seedlings started within the ®rst week, and seedlings were regularly recorded and discarded after identi®cation. Specimens dif®cult to identify at the seedling stage were transplanted and grown separately. To stimulate the germination of remaining seeds, the soil samples were stirred after every time seedlings had been discarded or transplanted. Identi®cation of emerging seedlings and transplanted specimens continued until October 1999, when the incubation was terminated. 3.6. Data analyses Percent moisture content of fuels was expressed on dry-weight basis (Uhl et al., 1988), where: Percent moisture field mass oven dry mass 100 oven dry mass The data collected were subjected to One-Way Analyses of Variance (ANOVA) using SYSTAT Computer Software (SPSS, 1997) where appropriate. Tukey test (Zar, 1996) was used to determine the signi®cant difference P < 0:05 among means. 4. Results 4.1. Fuel characteristics In the dry Afromontane forest, leaf litter dominated the litter fraction, which was arranged in a thin layer of about 5 cm depth on the soil surface. In contrast, the dominant proportion of the fuels in the Acacia woodland was dry grasses and herbs, elevated above the ground surface. Average fuel depth in this community was estimated at about 100 cm, providing good conditions for fuel drying and spread of ®re. The total fuel biomass in the two vegetation cover types did not differ signi®cantly, but slightly greater values were observed in the Acacia woodland compared with the dry Afromontane forest (One-Way ANOVA: F1;38 0:011, P 0:916). Biomass of the litter fuel (i.e. leaf litter, dry grasses and herbs), however, differed signi®cantly P < 0:0001. It was
two times greater in the Acacia woodland (673 g m 2) than in the dry Afromontane forest (308 g m 2). In contrast, the biomass of woody ®ne fuels was four times greater P 0:001 in the dry Afromontane forest (300 g m 2) than in the Acacia woodland (76 g m 2). The biomass of live ®ne fuels was also several times greater P < 0:0001 in the dry Afromontane forest (110 g m 2), than in the Acacia woodland (15 g m 2) (Fig. 2). The point of fuel moisture content below which the ignition and spread of ®re is possible (Holdsworth and Uhl, 1997) is around 12% (on a dry mass basis) depending on the fuel type. In the Acacia woodland 40% of the fuel samples were under this threshold. The mean moisture content of fuel components differed signi®cantly both in the Acacia woodland (One-Way ANOVA: F2;34 8:852, P 0:001) and dry Afromontane forest (One-Way ANOVA: F2;34 88:603, P < 0:0001). Mean fuel moisture in the Acacia woodland was 17% in litter, 32% in woody ®ne fuels and 63% in live ®ne fuels. In the dry Afromontane forest, only 10% of the fuel samples had moisture content below the threshold. At the time of ignition, mean fuel moisture contents under open canopy cover was 30% in litter, 32% in woody ®ne fuels, and 265% in live ®ne fuels. In areas with closed canopy cover, mean fuel moisture content was 21% in litter, 25% in woody ®ne fuels, and 356% in live ®ne fuels (Fig. 3). 4.2. Fire behaviour The greater biomass in the ®eld layer had accumulated combustible vegetation at the end of the dry season in the Acacia woodland. All ignitions were successful and the ®res spread vigorously in this community, with signi®cantly higher mean rate of spread (One-Way ANOVA: F2;9 28:427, P < 0:0001) of 0:79 0:22 (S.E.) m min 1 and mean ¯ame length (One-Way ANOVA: F2;9 5:469, P 0:028) of 108 63 cm (Table 1). The experimental surface ®res in the dry Afromontane forest did not propagate successfully. After ignition, the ground fuels alone could not sustain the spread of ®re. When the ignition source ceased, the ®re fronts only spread between 10 and 40 cm into the fuel beds. The mean rate of spread in this community n 8 was 0:11 0:09 m min 1 and the mean ¯ame length was
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
45
Fig. 2. Mean dry-weight of live ®ne fuels, woody ®ne fuels and litter in the Acacia woodland and the dry Afromontane forest (bar 1 S.E. of the mean).
31 12 cm. Plots under open canopy cover had a mean rate of spread of 0:09 0:03 m min 1 and a mean ¯ame length of 35 13 cm. While plots with closed canopy cover had a mean rate of spread of 0:14 0:13 m min 1 n 4 and a mean ¯ame length of 28 13 cm (Table 1). Highest ®re intensities in the experiment were achieved when rates of spread and ¯ame lengths were in¯uenced by strong winds, resulting in a high rate of spread. In the Acacia woodland, fuel consumption was 100% for all burned plots.
4.3. Bark thickness A total of 20 species of trees/shrubs were encountered during the investigation on bark thickness in all the 3±4 sites in the Acacia woodland, nine in the ecotone and 13 in the dry Afromontane forest. Of these, Acacia seyal occurred in both the Acacia woodland and the ecotone while A. gummifera, Calpurnia aurea, Celtis africana, C. africana, C. macrostachyus and F. sur were recorded both at the ecotone and the dry Afromontane forest (Table 2).
46
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
Fig. 3. Moisture content of different fuel fractions: live ®ne fuels, woody ®ne fuels and litter. 1: Acacia woodland; 2: open dry Afromontane forest; 3: closed canopy dry Afromontane; 4: all plots in the dry Afromontane forest. Note the different scale for live ®ne fuels.
Dry Afromontane forest trees had in general thin barks, ranging from 2.5 mm for A. abyssinicus to 9.2 mm for C. macrostachyus (Table 2). Most species from the dry Afromontane forest site showed a very small increase in bark thickness with increasing diameter. In this study only C. macrostachyus, M. ferruginea and C. africana had a clear relation between bark thickness and diameter.
Trees in the Acacia woodland had thick bark, ranging from A. seyal having 11.8 mm to B. aegyptiaca with 15.4 mm. Bark thickness varied greatly for samples taken at the same diameter for both Acacia senegal and A. seyal while A. tortilis and B. aegyptiaca had a smaller variation at the same diameter. In the ecotone, trees had medium bark thickness. Here, Celtis africana was the tree with the thinnest
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
47
Table 1 Microclimate, fuel characteristics and ®re behaviour during experimental ®res in the Acacia woodland and the dry Afromontane forest Relative humidity (%)
Wind speed
Fuel biomass (g m 2)
Fuel moisture (%)
31 31 29 28
Weak Weak Weak Steady
1933 417 778 1322
23 5 14 13
± ± ± ±
± ± ± ±
25 28 27 29
39 35 37 32
No wind Weak Weak Steady
1078 494 839 728
23 25 29 12
± ± ± ±
± ± ± ±
40 36 32 32
27 27 26 24
Steady Weak Strong Strong
961 839 922 844
26 13 13 6
80 60 90 200
Air temperature (8C) Dry Afromontane forest A. Open canopy 29 n 4 31 28 29 B. Closed canopy n 4
Acacia woodland n 4
bark, 4.4 mm, and the thickest was Acacia etbaica with 13.4 mm. Most species showed a clear increase in bark thickness with increasing diameter, except Celtis africana, which did not generate any trend due to the low variation in tree diameters.
Flame length (cm)
Rate of spread (m min 1)
0.77 0.49 0.99 0.91
For the species that occurred in more than one habitat, there was a tendency for bark thickness to be greater in the more open habitats. A. seyal had 5% greater bark thickness in the Acacia woodland than in the ecotone (Table 2). The same relation was found
Table 2 Mean bark thickness of species in the Acacia woodland, ecotone and dry Afromontane forest Species
Bark thickness (mm)
a
A. etbaica A. senegal A. seyal A. tortilis A. gummifera A. abyssinicus Aningeria adolfi-friederici B. aegyptiaca C. aurea Celtis africana C. africana C. macrostachyus Dracaena steudneri F. sur M. ferruginea Olea capensis subsp. Hochstetteri O. europaea subsp. cuspidata S. guineense Vepris dainellii V. amygdalina a b
Nomenclature follows Friis (1992). Number of individual trees measured.
Acacia woodland
Ecotone
Dry Afromontane forest
± 14.9 11.8 14.4 ± ± ± 15.4 ± ± ± ± ± ± ± ± ± ± ± ±
13.4 ± 11.2 ± 09.1 ± ± ± 09.9 04.4 09.9 11.5 ± 10.6 ± ± 08.3 ± ±
± ± ± ± 05.9 02.5 04.2 ± ± 03.5 06.3 09.2 05.2 08.0 07.1 04.8 ± 05.4 04.5 07.9
(20)b (20) (24)
(20)
(20) (28) (26)
(04) (04) (04) (47) (04) (31)
(10) (03) (06) (14) (05) (16) (04) (02) (08) (16) (05) (10) (05)
48
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
between the ecotone and the dry Afromontane forest, for the ®ve species that occurred at both sites. Mean difference between these sites was greater than 38%, and ranged from Celtis africana with 24% greater bark thickness in the ecotone than in the dry Afromontane forest to C. africana with 57% greater bark thickness in the ecotone than in the dry Afromontane forest. 4.4. Densities of soil seed banks In the samples from the Acacia woodland, the number of viable seeds recorded from both sieving and germination trials amounted to a mean seed density of 3365 seeds/m2 (n 20, depth 9 cm). When comparing the four plots sampled both before and after burning, mean seed density before burning was 1100 and 2200 seeds/m2 after burning (n 4, depth 9 cm). Most seeds belonged to non-woody species (either grasses or herbs). Only one tree species, A. tortilis was represented in the soil seed bank. In the dry Afromontane forest, mean seed density was 4755 seeds/m2 (n 20, depth 9 cm). Plots with open canopy cover had a mean seed density of 6125 seeds/m2 before burning and 4400 seeds/m2 after burning n 4. Plots under the closed canopy cover had a mean seed density of 3425 seeds/m2 before burning and 1325 seeds/m2 after burning n 4. From trees, Celtis africana dominated in the soil seed bank, followed by F. sur, Albizia schimperiana, C. macrostachyus, C. africana and Podocarpus falcatus. 4.5. Spatial distribution and effects of heat on seeds in the soil Vertical distribution of seeds was highest at the top 3 cm soil layers but declined both at the litter and deeper soil layers in both sites. However, a notable decline of viable seeds both in the litter layer and the upper mineral soil layer was recorded after burning in the dry Afromontane forest. On the other hand, a small decline of viable seeds in the litter layer and an increase of viable seeds in the upper mineral soil layer were recorded after burning in the Acacia woodland. 5. Discussion Sustained combustion was not possible in the dry Afromontane forest, due to the packed arrangement of
the leaf litter and its direct contact with the forest ¯oor, which retards drying (Kauffman et al., 1988). Live ®ne fuels also retarded the spread of ®re and increased the average moisture content within the fuel bed. Woody ®ne fuels, dry grasses and herbs burned readily, but as leaf litter formed a thin, relatively compact layer, the present fuels did not sustain a successful ®re spread. When dry forests are not accidentally (in exceptionally dry years) or deliberately burned, ®re may creep in and consume the thin leaf litter layer on the ground, without noticeable effect on the vegetation (Menaut et al., 1995). During the survey in the dry Afromontane forest, no evidences of past ®res were found, although burning is frequent at higher altitudes dominated by grasslands (personal observation, 1997 and 1999). On the forest ¯oor, litter and woody ®ne fuels dominated fuel composition in an almost even distribution. The amount of woody ®ne fuels may have been unnaturally low in this study, as the local communities collect most fuel wood in the forest. In the Acacia woodland, the typical fuel bed proved very favourable for ®re spread. This site has been protected for more than 20 years from anthropogenic disturbances such as tree cutting, cattle grazing, and intentional burning, which has resulted in increasing biomass in the ®eld layer and a large fuel load in the dry seasons. The dry microclimate coupled with abundant distribution of dry grasses and herbs increase its susceptibility to ®res. In the natural condition, this environment should be associated with a high ®re frequency, but as the surrounding landscape is heavily overgrazed, spread of ®re from outside is prevented. Destructive grazing was observed adjacent to the protected study area and had, most likely, reduced the frequency of past ®re events. There was a considerable overlap in tree species composition between the three vegetation types. Several of the forest species and some of the woodland species were also present in the ecotone and these showed a site-related difference in bark thickness. Bark tissues protect cambial tissues from ®re injury, and there is a strong relationship between bark thickness and peak cambial temperatures during ®res (Pinard and Huffman, 1997; Uhl and Kauffman, 1990). All ®ve species that occurred both in the dry Afromontane forest and the ecotone had greater bark thickness in the latter environment, and as expected, the tree species with the thickest bark were found in
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
the Acacia woodland, where ®re has been a natural ecological factor in the past. Although bark thickness is the most important variable for protection, cambial temperatures during forest ®res are also in¯uenced by external bark structure (Uhl and Kauffman, 1990). The coarse-barked Acacia species and Balanites aeggptiaca have a bark structure that probably affects heat ¯ow to their cambial tissues to reduce the impact of ®re. Tree species in the dry Afromontane forest had all relatively thin bark and may be vulnerable also to low-intensity surface ®res (Holdsworth and Uhl, 1997). It has been shown that dry Afromontane (Teketay and GranstroÈm, 1995; Teketay, 1997b, 1998; Tekle and Bekele, 2000) and Acacia woodland (Skoglund, 1992; Argaw et al., 1999) ecosystems exhibit low densities of soil seed banks of woody species. The study of soil seed banks in the Acacia woodland also reveals that it contains very few seeds from the tree strata. From all samples taken from the soil seed bank, only two seeds were from trees, indicating that very few tree seeds are added to the soil seed bank in this environment. Seed density in the dry Afromontane forest was slightly higher than in the woodland, but still quite low, compared with earlier studies (Teketay and GranstroÈm, 1995; Teketay, 1997b, 1998). Wind speed, rates of ®re spread, ¯ame lengths and fuel consumption are all sub-components of ®re intensity, which in¯uence soil temperatures. With the ®ne fuel loads of about 700±800 g m 2 that burnt on the ground at both sites, soil temperatures exceeding 60 8C would reach a soil depth greater than 1 cm. The intensity of ®re and soil temperatures (> 60 8C) during the experimental ®res were suf®cient to break seed dormancy in legume species (Bradstock and Auld, 1995). The dominant part of the seeds was vertically distributed in the upper 0±3 cm of the soil. Heat from the experimental ®re would either kill or stimulate germination of these buried seeds to a depth of more than 1 cm. This study suggests that the dry Afromontane forest has not developed with frequent ®res and that species composition is likely to be substantially affected if ®re ever enters into the forest. Here, ®re could have great effects on biodiversity as well as the structure and composition of regenerating forest (Holdsworth and Uhl, 1997). Dry forests are highly sensitive to clearing which results in the loss of atmospheric humidity and
49
soil, and to burning, with the loss of the dominant non®re-resistant species (Menaut et al., 1995). Under natural climatic conditions, the dry Afromontane forest has a low potential for ®re spread. Hence, it has a natural protection against the effects of ®re, although the dry Afromontane forest contains many ®re-sensitive tree species. A future scenario can, therefore, be a decline in canopy coverage from cutting and browsing, resulting in a dryer microclimate, and increased amounts of grasses and herbs in the litter fraction. This could foster a ®re spread potential that would alter the present species composition towards more ®re-tolerant forest or woodland tree species. Acknowledgements We thank Feyera Senbeta, Mulugeta Lemenih, Deribe Gurmu, members of FRC and the National Tree Seed Project, Wondo Genet College of Forestry, Oromia Bureau of Agriculture, Shashemene Forest Industry Enterprise and members of the Abijata-Shalla National Park for their assistance during the study. We are also grateful to Demeke Negussie for his assistance in the preparation of the map. References Argaw, M., Teketay, D., Olsson, M., 1999. Soil seed ¯ora, germination and regeneration pattern of woody species in an Acacia woodland of the Rift Valley in Ethiopia. J. Arid Environ. 43, 411±435. BackeÂus, I., Rulangaranga, Z.K., Skoglund, J., 1994. Vegetation changes on formerly overgrazed hill slopes in semi-arid central Tanzania. J. Veg. Sci. 5, 327±336. Beaman, R.S., Beaman, J.H., Marsh, C.W., Woods, P.V., 1985. Drought and forest ®res in Sabah in 1983. Sabah Soc. J. 8, 10±30. Bradstock, R.A., Auld, T.D., 1995. Soil temperatures during experimental bush®res in relation to ®re intensity: consequences for legume germination and ®re management in southeastern Australia. J. Appl. Ecol. 32, 76±84. Cavelier, J., Aide, T.M., Santos, C., Eusse, A.M., Dupuy, J.M., 1998. The savannization of moist forests in the Sierra Nevada de Santa Marta, Colombia. J. Biogeogr. 25, 901±912. Eriksson, H., Stern, M., 1987. A soil study at Wondo Genet Forestry Resources Institute, Ethiopia. M.Sc. Thesis. Swedish University of Agricultural Sciences, Uppsala, UmeaÊ, Sweden. EWCO, 1988. Tourist information: Abijata-Shalla Lakes National Park. Ethiopian Wildlife Conservation Organisation (EWCO), Addis Ababa, Ethiopia.
50
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
Friis, I., 1992. Forests and forest trees of northeast tropical Africa. Kew Bull. Add. Ser. XV, 1±396. Hoffman, W.A., 1996. The effects of ®re and cover on seedling establishment in a neo-tropical savannah. J. Ecol. 84, 383± 393. Holdsworth, A.R., Uhl, C., 1997. Fire in Amazonian selectively logged rain forest and the potential for ®re reduction. Ecol. Appl. 7, 713±725. Kauffman, J.B., Uhl, C., Cummings, D.L., 1988. Fire in the Venezuelan Amazon: 1. Fuel biomass and ®re chemistry in the evergreen rainforest of Venezuela. OIKOS 53, 167±175. Kellman, M., Meave, J., 1997. Fire in the tropical gallery forests of Belize. J. Biogeogr. 24, 23±34. Kikula, I.S., 1986. The in¯uence of ®re on the composition of Miombo woodland of SW Tanzania. OIKOS 46, 317±324. Lawton, R.M., 1978. A study of the dynamic ecology of Zambian vegetation. J. Ecol. 66, 175±198. Loth, P.E., 1999. Germination strategy of Acacia tortilis in a heterogeneous East African savanna environment. The vegetation of Manyara. Scale-dependent states and transitions in the African Rift Valley. Doctoral Thesis. Wageningen University and Research Centre, The Netherlands. Makin, M.J., Kingham, J.J., Waddams, A.E., Birchall, C.J., Tefera, T., 1975. Development Prospects in the Southern Rift Valley, Ethiopia, Vol. II. Land Resource Division, Ministry of Overseas Development, England KTB/DY. Menaut, J.-C., Lepage, M., Abbadie, L., 1995. Savannas, woodlands and dry forests in Africa. In: Bullock, S.H., Mooney, H.A., Medina, E. (Eds.), Seasonally Dry Tropical Forests, Cambridge University Press, Cambridge, pp. 62±92. Murphy, H.F., 1968. A report on the fertility status and other data on some soils of Ethiopia. Experimental Bulletin 44, 294±297. Haile Sellassie I University, Dire Dawa, Ethiopia. Mutch, R.W., 1970. Wildland ®res and ecosystemsÐa hypothesis. Ecology 51, 1046±1051. Pinard, M.A., Huffman, J., 1997. Fire resistance and bark properties of trees in a seasonally dry forest in eastern Bolivia. J. Trop. Ecol. 13, 727±740.
Sabiiti, E.N., Wein, R.W., 1987. Fire and Acacia seeds: a hypothesis of colonization success. J. Ecol. 74, 937±946. Senbeta, F., 1998. Native woody species regeneration under the canopies of tree plantations at Munessa-Shashemene Forest Project Area, southern Oromia, Ethiopia. M.Sc. Thesis. Swedish University of Agricultural Sciences, Skinnskatteberg, Sweden. Skoglund, J., 1992. The role of seed banks in vegetation dynamics and restoration of dry tropical ecosystems. J. Veg. Sci. 3, 357±360. SPSS, 1997. SYSTAT 7.0.1 for Windows. SPSS Inc., USA. Swaine, M.D., 1992. Characteristics of dry forest in West Africa and the in¯uence of ®re. J. Veg. Sci. 3, 365±374. Teketay, D., 1996a. Germination ecology of twelve indigenous and eight exotic multipurpose leguminous species from Ethiopia. For. Ecol. Manage. 80, 209±223. Teketay, D., 1996b. Seed ecology and regeneration in dry Afromontane forests of Ethiopia. Doctoral Thesis. Swedish University of Agricultural Sciences, UmeaÊ, Sweden. Teketay, D., 1997a. Germination ecology of Acacia negrii, an endemic multipurpose tree from Ethiopia. Trop. Ecol. 38, 39±46. Teketay, D., 1997b. The impact of clearing and conversion of dry Afromontane forests into arable land on the composition and density of soil seed banks. Acta Ecol. 18, 557±573. Teketay, D., 1998. Soil seed bank at an abandoned Afromontane arable site. Feddes Repertorium 109, 161±174. Teketay, D., GranstroÈm, A., 1995. Soil seed banks in dry Afromontane forests of Ethiopia. J. Veg. Sci. 6, 777±786. Tekle, K., Bekele, T., 2000. The role of soil seed banks in the rehabilitation of degraded hill-slopes in Southern Wollo. Biotropica 32, 23±32. Uhl, C., Kauffman, J.B., 1990. Deforestation, ®re susceptibility, and potential tree responses to ®re in the eastern Amazon. Ecology 71, 437±449. Uhl, C., Kauffman, J.B., Cummings, D.L., 1988. Fire in the Venezuelan Amazon 2: environmental conditions necessary for forest ®re in the evergreen rainforests of Venezuela. OIKOS 53, 176±184. Zar, J., 1996. Biostatistical Analysis, third ed. Prentice-Hall, Englewood Cliffs, NJ, 718 pp.
Response of plant communities to ®re in an Acacia woodland and a dry Afromontane forest, southern Ethiopia Ingalill Erikssona, Demel Teketayb,*, Anders GranstroÈmc a
Department of Silviculture, Swedish University of Agricultural Sciences, S-901 83 UmeaÊ, Sweden b Ethiopian Agricultural Research Organization, P.O. Box 2003, Addis Abeba, Ethiopia c Department of Forest Vegetation Ecology, Swedish University of Agricultural Sciences, S-901 83 UmeaÊ, Sweden Received 7 August 2001; received in revised form 2 April 2002; accepted 13 May 2002
Abstract To understand the role of ®re in degrading tropical dry forest ecosystems of north-east Africa, ®re potential was studied within two vegetation types: Acacia woodland and dry Afromontane forest. Fire behaviour was analysed in experimental ®res late in the dry season (February) and the potential response of the soil seed bank was studied on samples taken before and after ®re treatment. Bark thickness was also measured, as an indication of ®re-resistance, on a suit of tree/shrub species representing the two vegetation types and an ecotone between them. The total fuel biomass differed only marginally between the two vegetation types. But biomass of the litter fuel differed signi®cantly, and the biomass of litter fuel was two times greater in the Acacia woodland (673 g m 2) than in the dry Afromontane forest (308 g m 2). In contrast, the biomass of woody ®ne fuels was four times greater in the dry Afromontane forest (300 g m 2) than in the Acacia woodland (76 g m 2) and the biomass of live ®ne fuels was also several times greater in the dry Afromontane forest (110 g m 2), compared with the Acacia woodland (15 g m 2). During experimental ®res in the Acacia woodland, fuel consumption was 100% for all burned plots, but in the dry Afromontane forest fuel consumption ranged between 10 and 40% of the available fuel within the plots. The results from the soil seed bank study indicated little if any impact from ®re. There was a gradual increase in bark thickness of trees when going from the dry Afromontane forest over the ecotone towards the Acacia woodland site. Considering all species analysed, bark thickness of trees 15 cm in diameter at 1 m height ranged from 2.4 to 15.4 mm. Therefore, based on the differences in microclimate, fuels, ®re behaviour and bark thickness the Acacia woodland was determined to be the most ®re prone as well as ®re resilient ecosystem. In contrast, sustained combustion was unlikely at the studied dry Afromontane forest site. Spread of ®re was not possible due to the high fuel moisture content and poor fuel bed. Forest fragmentation and intentional burning of grasslands, followed by an altered microclimate, should however increase the probability of ®res penetrating the dry Afromontane forest. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Bark thickness; Fire behaviour; Fire spread; Flame length; Fuel biomass; Fuel moisture; Soil seed banks
1. Introduction
*
Corresponding author. Tel.: 251-1-454-452; fax: 251-1-461-251. E-mail address: [email protected] (D. Teketay).
A major factor favouring wide-scale burning in tropical forests is the cumulative effect of vegetation changes in recent decades. Logged-over forests are in general more vulnerable to burning due to accumulation
0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 3 2 5 - 0
40
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
of logging slash and dry ground conditions created by opening up of the canopy. In steep montane terrain, updrafts increase the intensity of a ®re and in Sabah, Indonesia, even primary forest burned extensively on steep, well-drained slopes where ®res could spread easily (Beaman et al., 1985). Plants have evolved different strategies to cope with such highly variable, competitive or stressful environments. Long-lived species, like trees, may have low-¯ammability litter to avoid, or thick bark to withstand, heat damage (Mutch, 1970; Uhl and Kauffman, 1990; Loth, 1999). The insulating capacity of bark protects cambial tissues from heat-induced mortality. Thermal bark-characteristics differ as a function of tree size, i.e. diameter, and tree species (Uhl and Kauffman, 1990). Fire mortality is, therefore, greater in small trees, whilst drought alone kills more large trees (Swaine, 1992). In dry forests, grazing will reduce the frequency of ®re and thereby promote the establishment of species such as Brachystegia. These species require ®re protection during the sapling stage, whilst becoming insensitive to ®re as adults (BackeÂus et al., 1994). This pattern may also be true for other tree species in East Africa. However, in areas with frequent ®res, the establishment of tree species could be effectively prevented by ®re (Hoffman, 1996). Vegetation responses to ®re will depend on several factors. Ideal conditions for tree establishment in dense grass tuft require an initial ®re, followed by rain and subsequent ®re protection (Lawton, 1978; Kikula, 1986; BackeÂus et al., 1994). Canopy openings, caused by ®re intrusion in gallery forests, induce a secondary succession where most recruits come from the residual soil seed bank (Kellman and Meave, 1997). The formation of soil seed banks is also a common survival strategy of Acacia species, which accumulate large quantities of viable but dormant seeds in the soil (Sabiiti and Wein, 1987). Germination of Acacia species is in general enhanced when subjected to the heat of ®re (Teketay, 1996a,b, 1997a). Fire intensity and its sub-components, fuel consumption and rate of spread, in¯uence soil temperatures. Maximum temperatures are greatly affected by the amount of ®ne fuel particles consumed. Fuel loads from 0.6 to 2.0 kg m 2 burnt on the ground will break seed dormancy and stimulate germination down to 1 and 3 cm depth, respectively (Bradstock and Auld,
1995). Acacia tortilis seeds with low moisture content will absorb water and germinate after exposure to dry heat of about 150 8C (Loth, 1999). On a local scale the change from forest to savannah is complex and fragmentary, with a mosaic of the two vegetation types apparently determined by a variety of factors. Locally the forest-savannah boundary is typically abrupt, accentuated by annual ®res, which are normally extinguished at the boundary (Swaine, 1992; Cavelier et al., 1998). Fire has for long been believed being negatively correlated with forest vegetation and positively related to grasslands, due to the ¯ammable nature of the grass during the dry season. But in West Africa, dry forests near the forest-savannah boundary form a distinctive sub-type as a result of occasional encroachment by ®res. Here, past ®res were likely to have a profound in¯uence on the composition of the forest canopy (Swaine, 1992). Although it is evident that dry forest systems, such as the dry Afromontane forest, experience contact with ®re due to the proximity to grasslands, little is known about the potential for ®re spread and the ®retolerance of the tree species. The present study was, therefore, undertaken to determine the ®re potential for two of the vegetation cover types in the Ethiopian Rift Valley: Acacia woodland and dry Afromontane forest as well as an ecotone between them, and assess the potential responses of tree and ground ¯ora and the soil seed bank after ®re. 2. Materials and methods 2.1. Study sites The study was conducted within three vegetation cover types in the Great Rift Valley of southern Ethiopia, namely the Acacia woodland in AbijataShalla Lakes National Park (78320 N and 388400 E), the ecotone (transition zone) located southeast of Lake Langano (78260 N and 388470 E) and the dry Afromontane forest at the Abaro Mountain, Wondo Genet (7860 N and 388380 E) (Fig. 1). 2.1.1. The Acacia woodland The Acacia woodland site is within the area of Abijata-Shalla Lakes National Park, which is a fenced Ostrich Farm protected since the 1970s. Topographic
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
41
Fig. 1. Location of the study sites.
relief is level at this site, with altitudes around 1640 m above sea level. The soils are predominantly andosols, characterised by weakly structured topsoil and low in organic matter (Makin et al., 1975; Argaw et al., 1999). Average annual rainfall ranges from 500 to 835 mm. There is a small rainy season in April and the main rainy season extends from July to October. A short dry period is experienced at the end of May and early June. Later, there is a notable dry season that occurs between November and February. Mean monthly temperature ranges from 13.8 to 28.0 8C (Makin et al., 1975).
The area is classi®ed as semiarid eco-climatic vegetation zone (Makin et al., 1975; Argaw et al., 1999). Natural vegetation is characterised by scattered mature trees of Acacia species and Balanites aegyptiaca, various shrubs and climbers as well as by about 1 m tall grass stratum. Cattle grazing has been restricted since the Ostrich Farm was established, but the short leaves at the base of the grasses are grazed by two races of Ostrich, namely the Somali race (Struthio camelus molholophanes) and the North African race (Struthio camelus camelus) (Getenet Wondimu, pers. commun.). Large mammals, such
42
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
as Grant's gazelle, oribi, grey duikers and mountain reedbucks, also have access to the Ostrich Farm (EWCO, 1988). 2.1.2. The dry Afromontane forest The site in the dry Afromontane forest stretches along the eastern escarpment of the Great Rift Valley. Elevation spans from 1900 to 2000 m above sea level in the study area, climbing uphill the steep western slope of the Abaro Mountain. The soils have not yet been classi®ed, but earthquake activity and the steepness of the slopes have transported colluvial material from higher altitudes. Average annual rainfall is 1200 mm, but varies between 700 and 1400 mm (Eriksson and Stern, 1987). The area has a bimodal rainfall distribution, with two rainy seasons. A minor rainy period occurs from February to April and a more extensive from July to September. There is also a characteristic long dry season from October to February. The mean temperature throughout the year is about 19 8C. The natural vegetation is dominated by Cordia africana, Ficus sur, Croton macrostachyus, Millettia ferruginea, Albizia gummifera, Olea capensis and Syzygium guineense. Shrubs or very small trees are Allophylus abyssinicus and Vernonia amygdalina. The lower strata of the forest are densely mixed with woody climbers, some of which are thorny. Scrambling species of Rubus are common, and the forest ¯oor is partly covered by herbaceous dicotyledons; plant nomenclature in this paper follows that of Friis (1992). In the end of the dry season, when ground vegetation becomes scarce or downed in the adjacent grasslands, cattle are directed into the high forest area. 2.1.3. The ecotone In the ecotone, the terrain is gently rolling, with altitudes ranging from 1840 to 1900 m above sea level within the study area. The soils are predominantly andosols with relatively high content of organic matter (Murphy, 1968; Makin et al., 1975; Argaw et al., 1999). The area has a two-peak annual rainfall between 600 and 800 mm, with short and unreliable rains during April±May and the main rainy season from June to August, sometimes up to September. The dry months are from November to February (Senbeta, 1998). Mean monthly temperature ranges from 13.8 to 28.0 8C (Makin et al., 1975). The natural vegetation
ranges from Acacia species in the lower parts to open deciduous forest or woodland in higher elevations with tall trees of Celtis africana, Olea europaea subsp. cuspidata, A. gummifera, C. africana, F. sur and C. macrostachyus (Friis, 1992). Cultivation and grazing have reduced the natural vegetation into fragments and the density of the tree layer is sparse. 3. Methods 3.1. Fuel characteristics The behaviour and spread of ®re is governed by a number of fuel characteristics. In this study, fuels are de®ned as live ®ne materials (i.e. live grasses and herbs), litter (i.e. leaf litter, dry grasses and herbs), and woody ®ne materials (i.e. all dead and downed woody debris). The total fuel biomass, fuel depth, types of fuels and moisture content of the fuels are all important factors in predicting potential ®re behaviour. The potential for ®re was studied by analysing these characteristics of the fuel bed within the Acacia woodland and dry Afromontane forest. In the ecotone, cultivation and grazing made investigations on fuel characteristics, experimental burning and soil seed banks impossible. Therefore, the study there was restricted only to bark-characteristics of trees and shrubs. To investigate the various fuel characteristics, 20 plots measuring 10 m 10 m (100 m2) and 100 m away from each other were established in each of the Acacia woodland and the dry Afromontane forest sites along four and seven line transects (ranging in length between 300±500 and 200 m apart) running in east-west and north-south directions, respectively. To eliminate any potential edge effects, plots were placed at least 30 m from boundaries of the sites. The total biomass of fuels down to the fermentation layer was sampled from two representative 30 cm 30 cm (900 cm2) subplots within the main plots. Live ®ne fuels were clipped at ground level, whereas litter and all woody ®ne fuel particles were carefully lifted by hand. Each of the three fractions was put separately in a marked plastic bag and was immediately weighed in the ®eld on a digital balance in order to quantify the moisture content. Then, all the fuel samples were airdried and stored indoors. The fractions were then
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
oven-dried at 100 8C to constant weight for moisture content and dry-weight estimations. 3.2. Fire behaviour Fire behaviour was studied during a series of experimental burns, simulating ®re late in the dry season. In the dry Afromontane forest, the 20 plots were divided into two groups, namely those found in open and closed canopies. From each group, four 1 m 1 m (1 m2) plots found in open canopy and four 1 m 1 m plots found in closed canopy were randomly selected. Then, ®ve of the plots were burned on 11 February 1999, and the remaining three on 12 February 1999. In the Acacia woodland where the canopy is homogenous, only four 1 m 1 m plots were randomly selected. These plots were burned on 15 February, 1999. All experimental burns were conducted between 13.00 and 17.30 h, when humidity was lowest. Just prior to ignition, a small fuel sample was collected for moisture determination as described in Section 3.1. Then the plots n 12 were burnt by head-®res, i.e. the ®res were allowed to spread with the wind (Bradstock and Auld, 1995). Accordingly, a 1 m hemp rope was soaked in kerosene, and two persons ignited the rope simultaneously, to assure an even ®re front. The rate of movement of the ®re front was measured with a digital stopwatch and the ¯ame length was estimated with a stick marked with a scale. 3.3. Weather At each plot, air temperature and relative humidity were measured during the fuel sampling, which was carried out between 13.00 and 17.30 h when humidity declines to its lowest levels (Holdsworth and Uhl, 1997), using a thermometer and a hygrometer, respectively. Both instruments were set 1 m above ground in shelter to avoid direct solar radiation. Similarly, during each experimental burning, air temperature, relative humidity as well as wind direction and speed were monitored. Wind direction and speed were visually monitored. 3.4. Bark thickness Tree diameter and bark thickness were measured 1 m above the ground level for all trees greater than
43
30 mm in diameter. Bark thickness was measured twice, on opposite sides of the trunk. All woody plants in the plots in the Acacia woodland and the dry Afromontane forest were identi®ed to species. In the ecotone, trees and woody plants were randomly selected and identi®ed to species. Altogether 356 trees were sampled for measurements of diameter and bark thickness from the three sites. 3.5. Soil seed banks The soil seed banks in the Acacia woodland and the dry Afromontane forest were also studied by taking soil samples from the centre of each of the 40 plots that had earlier been used for sampling fuel. At the centre of each plot, a subplot measuring 10 cm 10 cm (100 cm2) was marked and four separate soil layers were cut. The samples included a litter layer and three mineral soil layers, each 3 cm thick. To investigate the response of soil seed banks to ®re, a second soil sampling was carried out, following the same procedure as above, adjacent to the location of the ®rst sample 1 day after the experimental burning. For this purpose, soil samples were taken from only the 12 subplots (each 100 cm2) inside the 12 of the plots that were experimentally burnt as described in Section 3.2 in the Acacia woodland as well as in the open and closed canopies of the dry Afromontane forest. At both sites, the soil samples were taken in January 1999. All soil samples n 40 subplots four layers 12 burnt subplots four layers 208 were transported to the Forest Research Centre (FRC) Headquarters in Addis Abeba for processing. At FRC, each sample was sieved separately to recover bigger seeds using sieves with mesh sizes from 5.0 to 0.5 mm. Extracted seeds were collected for identi®cation and viability tests. Seeds were identi®ed to species genus or family levels. Those dif®cult to identify were given numbers and recorded as unidenti®ed species. Viability of seeds was determined by a cutting test, and a seed was considered viable when its content was white and ®rm (Teketay and GranstroÈm, 1995). Just after the sieving, soil samples were incubated in the glasshouse at FRC for germination of seeds 6 weeks after the ®rst soil sampling. The sieved soil samples were spread on cotton cloth in plastic trays and placed on tables in the glasshouse. The samples
44
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
were then kept continuously moist and were incubated for 6 months as of the beginning of March 1999. The emergence of seedlings started within the ®rst week, and seedlings were regularly recorded and discarded after identi®cation. Specimens dif®cult to identify at the seedling stage were transplanted and grown separately. To stimulate the germination of remaining seeds, the soil samples were stirred after every time seedlings had been discarded or transplanted. Identi®cation of emerging seedlings and transplanted specimens continued until October 1999, when the incubation was terminated. 3.6. Data analyses Percent moisture content of fuels was expressed on dry-weight basis (Uhl et al., 1988), where: Percent moisture field mass oven dry mass 100 oven dry mass The data collected were subjected to One-Way Analyses of Variance (ANOVA) using SYSTAT Computer Software (SPSS, 1997) where appropriate. Tukey test (Zar, 1996) was used to determine the signi®cant difference P < 0:05 among means. 4. Results 4.1. Fuel characteristics In the dry Afromontane forest, leaf litter dominated the litter fraction, which was arranged in a thin layer of about 5 cm depth on the soil surface. In contrast, the dominant proportion of the fuels in the Acacia woodland was dry grasses and herbs, elevated above the ground surface. Average fuel depth in this community was estimated at about 100 cm, providing good conditions for fuel drying and spread of ®re. The total fuel biomass in the two vegetation cover types did not differ signi®cantly, but slightly greater values were observed in the Acacia woodland compared with the dry Afromontane forest (One-Way ANOVA: F1;38 0:011, P 0:916). Biomass of the litter fuel (i.e. leaf litter, dry grasses and herbs), however, differed signi®cantly P < 0:0001. It was
two times greater in the Acacia woodland (673 g m 2) than in the dry Afromontane forest (308 g m 2). In contrast, the biomass of woody ®ne fuels was four times greater P 0:001 in the dry Afromontane forest (300 g m 2) than in the Acacia woodland (76 g m 2). The biomass of live ®ne fuels was also several times greater P < 0:0001 in the dry Afromontane forest (110 g m 2), than in the Acacia woodland (15 g m 2) (Fig. 2). The point of fuel moisture content below which the ignition and spread of ®re is possible (Holdsworth and Uhl, 1997) is around 12% (on a dry mass basis) depending on the fuel type. In the Acacia woodland 40% of the fuel samples were under this threshold. The mean moisture content of fuel components differed signi®cantly both in the Acacia woodland (One-Way ANOVA: F2;34 8:852, P 0:001) and dry Afromontane forest (One-Way ANOVA: F2;34 88:603, P < 0:0001). Mean fuel moisture in the Acacia woodland was 17% in litter, 32% in woody ®ne fuels and 63% in live ®ne fuels. In the dry Afromontane forest, only 10% of the fuel samples had moisture content below the threshold. At the time of ignition, mean fuel moisture contents under open canopy cover was 30% in litter, 32% in woody ®ne fuels, and 265% in live ®ne fuels. In areas with closed canopy cover, mean fuel moisture content was 21% in litter, 25% in woody ®ne fuels, and 356% in live ®ne fuels (Fig. 3). 4.2. Fire behaviour The greater biomass in the ®eld layer had accumulated combustible vegetation at the end of the dry season in the Acacia woodland. All ignitions were successful and the ®res spread vigorously in this community, with signi®cantly higher mean rate of spread (One-Way ANOVA: F2;9 28:427, P < 0:0001) of 0:79 0:22 (S.E.) m min 1 and mean ¯ame length (One-Way ANOVA: F2;9 5:469, P 0:028) of 108 63 cm (Table 1). The experimental surface ®res in the dry Afromontane forest did not propagate successfully. After ignition, the ground fuels alone could not sustain the spread of ®re. When the ignition source ceased, the ®re fronts only spread between 10 and 40 cm into the fuel beds. The mean rate of spread in this community n 8 was 0:11 0:09 m min 1 and the mean ¯ame length was
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
45
Fig. 2. Mean dry-weight of live ®ne fuels, woody ®ne fuels and litter in the Acacia woodland and the dry Afromontane forest (bar 1 S.E. of the mean).
31 12 cm. Plots under open canopy cover had a mean rate of spread of 0:09 0:03 m min 1 and a mean ¯ame length of 35 13 cm. While plots with closed canopy cover had a mean rate of spread of 0:14 0:13 m min 1 n 4 and a mean ¯ame length of 28 13 cm (Table 1). Highest ®re intensities in the experiment were achieved when rates of spread and ¯ame lengths were in¯uenced by strong winds, resulting in a high rate of spread. In the Acacia woodland, fuel consumption was 100% for all burned plots.
4.3. Bark thickness A total of 20 species of trees/shrubs were encountered during the investigation on bark thickness in all the 3±4 sites in the Acacia woodland, nine in the ecotone and 13 in the dry Afromontane forest. Of these, Acacia seyal occurred in both the Acacia woodland and the ecotone while A. gummifera, Calpurnia aurea, Celtis africana, C. africana, C. macrostachyus and F. sur were recorded both at the ecotone and the dry Afromontane forest (Table 2).
46
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
Fig. 3. Moisture content of different fuel fractions: live ®ne fuels, woody ®ne fuels and litter. 1: Acacia woodland; 2: open dry Afromontane forest; 3: closed canopy dry Afromontane; 4: all plots in the dry Afromontane forest. Note the different scale for live ®ne fuels.
Dry Afromontane forest trees had in general thin barks, ranging from 2.5 mm for A. abyssinicus to 9.2 mm for C. macrostachyus (Table 2). Most species from the dry Afromontane forest site showed a very small increase in bark thickness with increasing diameter. In this study only C. macrostachyus, M. ferruginea and C. africana had a clear relation between bark thickness and diameter.
Trees in the Acacia woodland had thick bark, ranging from A. seyal having 11.8 mm to B. aegyptiaca with 15.4 mm. Bark thickness varied greatly for samples taken at the same diameter for both Acacia senegal and A. seyal while A. tortilis and B. aegyptiaca had a smaller variation at the same diameter. In the ecotone, trees had medium bark thickness. Here, Celtis africana was the tree with the thinnest
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
47
Table 1 Microclimate, fuel characteristics and ®re behaviour during experimental ®res in the Acacia woodland and the dry Afromontane forest Relative humidity (%)
Wind speed
Fuel biomass (g m 2)
Fuel moisture (%)
31 31 29 28
Weak Weak Weak Steady
1933 417 778 1322
23 5 14 13
± ± ± ±
± ± ± ±
25 28 27 29
39 35 37 32
No wind Weak Weak Steady
1078 494 839 728
23 25 29 12
± ± ± ±
± ± ± ±
40 36 32 32
27 27 26 24
Steady Weak Strong Strong
961 839 922 844
26 13 13 6
80 60 90 200
Air temperature (8C) Dry Afromontane forest A. Open canopy 29 n 4 31 28 29 B. Closed canopy n 4
Acacia woodland n 4
bark, 4.4 mm, and the thickest was Acacia etbaica with 13.4 mm. Most species showed a clear increase in bark thickness with increasing diameter, except Celtis africana, which did not generate any trend due to the low variation in tree diameters.
Flame length (cm)
Rate of spread (m min 1)
0.77 0.49 0.99 0.91
For the species that occurred in more than one habitat, there was a tendency for bark thickness to be greater in the more open habitats. A. seyal had 5% greater bark thickness in the Acacia woodland than in the ecotone (Table 2). The same relation was found
Table 2 Mean bark thickness of species in the Acacia woodland, ecotone and dry Afromontane forest Species
Bark thickness (mm)
a
A. etbaica A. senegal A. seyal A. tortilis A. gummifera A. abyssinicus Aningeria adolfi-friederici B. aegyptiaca C. aurea Celtis africana C. africana C. macrostachyus Dracaena steudneri F. sur M. ferruginea Olea capensis subsp. Hochstetteri O. europaea subsp. cuspidata S. guineense Vepris dainellii V. amygdalina a b
Nomenclature follows Friis (1992). Number of individual trees measured.
Acacia woodland
Ecotone
Dry Afromontane forest
± 14.9 11.8 14.4 ± ± ± 15.4 ± ± ± ± ± ± ± ± ± ± ± ±
13.4 ± 11.2 ± 09.1 ± ± ± 09.9 04.4 09.9 11.5 ± 10.6 ± ± 08.3 ± ±
± ± ± ± 05.9 02.5 04.2 ± ± 03.5 06.3 09.2 05.2 08.0 07.1 04.8 ± 05.4 04.5 07.9
(20)b (20) (24)
(20)
(20) (28) (26)
(04) (04) (04) (47) (04) (31)
(10) (03) (06) (14) (05) (16) (04) (02) (08) (16) (05) (10) (05)
48
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
between the ecotone and the dry Afromontane forest, for the ®ve species that occurred at both sites. Mean difference between these sites was greater than 38%, and ranged from Celtis africana with 24% greater bark thickness in the ecotone than in the dry Afromontane forest to C. africana with 57% greater bark thickness in the ecotone than in the dry Afromontane forest. 4.4. Densities of soil seed banks In the samples from the Acacia woodland, the number of viable seeds recorded from both sieving and germination trials amounted to a mean seed density of 3365 seeds/m2 (n 20, depth 9 cm). When comparing the four plots sampled both before and after burning, mean seed density before burning was 1100 and 2200 seeds/m2 after burning (n 4, depth 9 cm). Most seeds belonged to non-woody species (either grasses or herbs). Only one tree species, A. tortilis was represented in the soil seed bank. In the dry Afromontane forest, mean seed density was 4755 seeds/m2 (n 20, depth 9 cm). Plots with open canopy cover had a mean seed density of 6125 seeds/m2 before burning and 4400 seeds/m2 after burning n 4. Plots under the closed canopy cover had a mean seed density of 3425 seeds/m2 before burning and 1325 seeds/m2 after burning n 4. From trees, Celtis africana dominated in the soil seed bank, followed by F. sur, Albizia schimperiana, C. macrostachyus, C. africana and Podocarpus falcatus. 4.5. Spatial distribution and effects of heat on seeds in the soil Vertical distribution of seeds was highest at the top 3 cm soil layers but declined both at the litter and deeper soil layers in both sites. However, a notable decline of viable seeds both in the litter layer and the upper mineral soil layer was recorded after burning in the dry Afromontane forest. On the other hand, a small decline of viable seeds in the litter layer and an increase of viable seeds in the upper mineral soil layer were recorded after burning in the Acacia woodland. 5. Discussion Sustained combustion was not possible in the dry Afromontane forest, due to the packed arrangement of
the leaf litter and its direct contact with the forest ¯oor, which retards drying (Kauffman et al., 1988). Live ®ne fuels also retarded the spread of ®re and increased the average moisture content within the fuel bed. Woody ®ne fuels, dry grasses and herbs burned readily, but as leaf litter formed a thin, relatively compact layer, the present fuels did not sustain a successful ®re spread. When dry forests are not accidentally (in exceptionally dry years) or deliberately burned, ®re may creep in and consume the thin leaf litter layer on the ground, without noticeable effect on the vegetation (Menaut et al., 1995). During the survey in the dry Afromontane forest, no evidences of past ®res were found, although burning is frequent at higher altitudes dominated by grasslands (personal observation, 1997 and 1999). On the forest ¯oor, litter and woody ®ne fuels dominated fuel composition in an almost even distribution. The amount of woody ®ne fuels may have been unnaturally low in this study, as the local communities collect most fuel wood in the forest. In the Acacia woodland, the typical fuel bed proved very favourable for ®re spread. This site has been protected for more than 20 years from anthropogenic disturbances such as tree cutting, cattle grazing, and intentional burning, which has resulted in increasing biomass in the ®eld layer and a large fuel load in the dry seasons. The dry microclimate coupled with abundant distribution of dry grasses and herbs increase its susceptibility to ®res. In the natural condition, this environment should be associated with a high ®re frequency, but as the surrounding landscape is heavily overgrazed, spread of ®re from outside is prevented. Destructive grazing was observed adjacent to the protected study area and had, most likely, reduced the frequency of past ®re events. There was a considerable overlap in tree species composition between the three vegetation types. Several of the forest species and some of the woodland species were also present in the ecotone and these showed a site-related difference in bark thickness. Bark tissues protect cambial tissues from ®re injury, and there is a strong relationship between bark thickness and peak cambial temperatures during ®res (Pinard and Huffman, 1997; Uhl and Kauffman, 1990). All ®ve species that occurred both in the dry Afromontane forest and the ecotone had greater bark thickness in the latter environment, and as expected, the tree species with the thickest bark were found in
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
the Acacia woodland, where ®re has been a natural ecological factor in the past. Although bark thickness is the most important variable for protection, cambial temperatures during forest ®res are also in¯uenced by external bark structure (Uhl and Kauffman, 1990). The coarse-barked Acacia species and Balanites aeggptiaca have a bark structure that probably affects heat ¯ow to their cambial tissues to reduce the impact of ®re. Tree species in the dry Afromontane forest had all relatively thin bark and may be vulnerable also to low-intensity surface ®res (Holdsworth and Uhl, 1997). It has been shown that dry Afromontane (Teketay and GranstroÈm, 1995; Teketay, 1997b, 1998; Tekle and Bekele, 2000) and Acacia woodland (Skoglund, 1992; Argaw et al., 1999) ecosystems exhibit low densities of soil seed banks of woody species. The study of soil seed banks in the Acacia woodland also reveals that it contains very few seeds from the tree strata. From all samples taken from the soil seed bank, only two seeds were from trees, indicating that very few tree seeds are added to the soil seed bank in this environment. Seed density in the dry Afromontane forest was slightly higher than in the woodland, but still quite low, compared with earlier studies (Teketay and GranstroÈm, 1995; Teketay, 1997b, 1998). Wind speed, rates of ®re spread, ¯ame lengths and fuel consumption are all sub-components of ®re intensity, which in¯uence soil temperatures. With the ®ne fuel loads of about 700±800 g m 2 that burnt on the ground at both sites, soil temperatures exceeding 60 8C would reach a soil depth greater than 1 cm. The intensity of ®re and soil temperatures (> 60 8C) during the experimental ®res were suf®cient to break seed dormancy in legume species (Bradstock and Auld, 1995). The dominant part of the seeds was vertically distributed in the upper 0±3 cm of the soil. Heat from the experimental ®re would either kill or stimulate germination of these buried seeds to a depth of more than 1 cm. This study suggests that the dry Afromontane forest has not developed with frequent ®res and that species composition is likely to be substantially affected if ®re ever enters into the forest. Here, ®re could have great effects on biodiversity as well as the structure and composition of regenerating forest (Holdsworth and Uhl, 1997). Dry forests are highly sensitive to clearing which results in the loss of atmospheric humidity and
49
soil, and to burning, with the loss of the dominant non®re-resistant species (Menaut et al., 1995). Under natural climatic conditions, the dry Afromontane forest has a low potential for ®re spread. Hence, it has a natural protection against the effects of ®re, although the dry Afromontane forest contains many ®re-sensitive tree species. A future scenario can, therefore, be a decline in canopy coverage from cutting and browsing, resulting in a dryer microclimate, and increased amounts of grasses and herbs in the litter fraction. This could foster a ®re spread potential that would alter the present species composition towards more ®re-tolerant forest or woodland tree species. Acknowledgements We thank Feyera Senbeta, Mulugeta Lemenih, Deribe Gurmu, members of FRC and the National Tree Seed Project, Wondo Genet College of Forestry, Oromia Bureau of Agriculture, Shashemene Forest Industry Enterprise and members of the Abijata-Shalla National Park for their assistance during the study. We are also grateful to Demeke Negussie for his assistance in the preparation of the map. References Argaw, M., Teketay, D., Olsson, M., 1999. Soil seed ¯ora, germination and regeneration pattern of woody species in an Acacia woodland of the Rift Valley in Ethiopia. J. Arid Environ. 43, 411±435. BackeÂus, I., Rulangaranga, Z.K., Skoglund, J., 1994. Vegetation changes on formerly overgrazed hill slopes in semi-arid central Tanzania. J. Veg. Sci. 5, 327±336. Beaman, R.S., Beaman, J.H., Marsh, C.W., Woods, P.V., 1985. Drought and forest ®res in Sabah in 1983. Sabah Soc. J. 8, 10±30. Bradstock, R.A., Auld, T.D., 1995. Soil temperatures during experimental bush®res in relation to ®re intensity: consequences for legume germination and ®re management in southeastern Australia. J. Appl. Ecol. 32, 76±84. Cavelier, J., Aide, T.M., Santos, C., Eusse, A.M., Dupuy, J.M., 1998. The savannization of moist forests in the Sierra Nevada de Santa Marta, Colombia. J. Biogeogr. 25, 901±912. Eriksson, H., Stern, M., 1987. A soil study at Wondo Genet Forestry Resources Institute, Ethiopia. M.Sc. Thesis. Swedish University of Agricultural Sciences, Uppsala, UmeaÊ, Sweden. EWCO, 1988. Tourist information: Abijata-Shalla Lakes National Park. Ethiopian Wildlife Conservation Organisation (EWCO), Addis Ababa, Ethiopia.
50
I. Eriksson et al. / Forest Ecology and Management 177 (2003) 39±50
Friis, I., 1992. Forests and forest trees of northeast tropical Africa. Kew Bull. Add. Ser. XV, 1±396. Hoffman, W.A., 1996. The effects of ®re and cover on seedling establishment in a neo-tropical savannah. J. Ecol. 84, 383± 393. Holdsworth, A.R., Uhl, C., 1997. Fire in Amazonian selectively logged rain forest and the potential for ®re reduction. Ecol. Appl. 7, 713±725. Kauffman, J.B., Uhl, C., Cummings, D.L., 1988. Fire in the Venezuelan Amazon: 1. Fuel biomass and ®re chemistry in the evergreen rainforest of Venezuela. OIKOS 53, 167±175. Kellman, M., Meave, J., 1997. Fire in the tropical gallery forests of Belize. J. Biogeogr. 24, 23±34. Kikula, I.S., 1986. The in¯uence of ®re on the composition of Miombo woodland of SW Tanzania. OIKOS 46, 317±324. Lawton, R.M., 1978. A study of the dynamic ecology of Zambian vegetation. J. Ecol. 66, 175±198. Loth, P.E., 1999. Germination strategy of Acacia tortilis in a heterogeneous East African savanna environment. The vegetation of Manyara. Scale-dependent states and transitions in the African Rift Valley. Doctoral Thesis. Wageningen University and Research Centre, The Netherlands. Makin, M.J., Kingham, J.J., Waddams, A.E., Birchall, C.J., Tefera, T., 1975. Development Prospects in the Southern Rift Valley, Ethiopia, Vol. II. Land Resource Division, Ministry of Overseas Development, England KTB/DY. Menaut, J.-C., Lepage, M., Abbadie, L., 1995. Savannas, woodlands and dry forests in Africa. In: Bullock, S.H., Mooney, H.A., Medina, E. (Eds.), Seasonally Dry Tropical Forests, Cambridge University Press, Cambridge, pp. 62±92. Murphy, H.F., 1968. A report on the fertility status and other data on some soils of Ethiopia. Experimental Bulletin 44, 294±297. Haile Sellassie I University, Dire Dawa, Ethiopia. Mutch, R.W., 1970. Wildland ®res and ecosystemsÐa hypothesis. Ecology 51, 1046±1051. Pinard, M.A., Huffman, J., 1997. Fire resistance and bark properties of trees in a seasonally dry forest in eastern Bolivia. J. Trop. Ecol. 13, 727±740.
Sabiiti, E.N., Wein, R.W., 1987. Fire and Acacia seeds: a hypothesis of colonization success. J. Ecol. 74, 937±946. Senbeta, F., 1998. Native woody species regeneration under the canopies of tree plantations at Munessa-Shashemene Forest Project Area, southern Oromia, Ethiopia. M.Sc. Thesis. Swedish University of Agricultural Sciences, Skinnskatteberg, Sweden. Skoglund, J., 1992. The role of seed banks in vegetation dynamics and restoration of dry tropical ecosystems. J. Veg. Sci. 3, 357±360. SPSS, 1997. SYSTAT 7.0.1 for Windows. SPSS Inc., USA. Swaine, M.D., 1992. Characteristics of dry forest in West Africa and the in¯uence of ®re. J. Veg. Sci. 3, 365±374. Teketay, D., 1996a. Germination ecology of twelve indigenous and eight exotic multipurpose leguminous species from Ethiopia. For. Ecol. Manage. 80, 209±223. Teketay, D., 1996b. Seed ecology and regeneration in dry Afromontane forests of Ethiopia. Doctoral Thesis. Swedish University of Agricultural Sciences, UmeaÊ, Sweden. Teketay, D., 1997a. Germination ecology of Acacia negrii, an endemic multipurpose tree from Ethiopia. Trop. Ecol. 38, 39±46. Teketay, D., 1997b. The impact of clearing and conversion of dry Afromontane forests into arable land on the composition and density of soil seed banks. Acta Ecol. 18, 557±573. Teketay, D., 1998. Soil seed bank at an abandoned Afromontane arable site. Feddes Repertorium 109, 161±174. Teketay, D., GranstroÈm, A., 1995. Soil seed banks in dry Afromontane forests of Ethiopia. J. Veg. Sci. 6, 777±786. Tekle, K., Bekele, T., 2000. The role of soil seed banks in the rehabilitation of degraded hill-slopes in Southern Wollo. Biotropica 32, 23±32. Uhl, C., Kauffman, J.B., 1990. Deforestation, ®re susceptibility, and potential tree responses to ®re in the eastern Amazon. Ecology 71, 437±449. Uhl, C., Kauffman, J.B., Cummings, D.L., 1988. Fire in the Venezuelan Amazon 2: environmental conditions necessary for forest ®re in the evergreen rainforests of Venezuela. OIKOS 53, 176±184. Zar, J., 1996. Biostatistical Analysis, third ed. Prentice-Hall, Englewood Cliffs, NJ, 718 pp.