ABSTRACT

Scattered trees in general and scattered Faidherbia albida in particular are preserved in agricultural landscapes in Ethiopia. A study on this tree species was conducted at Meskan District of Gurage Zone, Ethiopia with the objectives of assessing the status of soil properties under it and its population distribution in agricultural landscapes. Twenty four  soil samples from the surface layer (0-10 cm) were taken from under six  isolated mature F. albida trees at 3 concentric transects (1, 3 and 5 m) from the tree trunk  and compared with soils taken from the adjacent open areas, 12 m far from the trees. In addition, 24 undisturbed soil samples were also collected from same points using core samplers for bulk density and moisture content determinations. The collected soil samples were analyzed following standard laboratory procedures. Assessment of the population distribution of the species was carried out at three different altitudinal zones, that is,1800-2000 m above sea level (asl), 2000-2200 m asl and 2200-2400 m asl and from four land use types (homestead, farmlands, woodlots and grazing land). A complete count and Y frame transect sampling methods were used to estimate number of stem per ha. The results revealed that organic matter (OM%), organic carbon (OC%) and total nitrogen (TN%) significantly differ between radial distances. The three parameters, were higher at 1 m distance compared with the values at 5 and 12 m distances. Though the difference is statistically not significant, available P and moisture content (MC) showed a decreasing trend, and bulk density (BD) and carbon nitrogen ratio (C/N) showed an increasing trend with increasing distance from the trees trunk. Population status of F. albida in the farms is dominated by higher diameter classes, with only 2% share of stems in the younger DBH-class of less than 5 cm. This may not indicate a threat to the species as it appear  as the community do not cut and harvest the whole tree but only lop which eventually may prolong the life span of the trees. Thus, this finding suggests that scattered F. albida trees are important agro-forestry resource to sustain soil fertility and subsequently crop yields. They are also important sources of multiple products such as fodder and shade for livestock during dry period, fuel and fencing material for households.

Key words: Faidherbia albida, soil property, population status, sustainability, Ethiopia.

Geographic location  

 

This study was conducted at Meskan District located between 38?15'10.7''- 38?33'510.9'' E  and 8?1' 598.8''  - 8?16' 3029.6'' N in Gurage Zone of Southern Ethiopia. This District was selected based on the existence of abundant F. albida trees under in agricultural landscapes. The population status of F. albida was assessed in three localities:  Goyban, Mekicho and Ile while soil status was assessed only at Ile (Figure 1).

 

 

Biophysical environment of the study area

 

According to climatic record near the study area, the mean annual rainfall in the District (Figure 2)  is 1058 mm and  the mean daily maximum and minimum temperatures are 27.7 °C and 6.5°C, respectively (Figure 2) National Meteorology Services Agency, 2005. The main growing season begins towards the end of June and continues up to the end of October. The District has an altitude ranging from 1800-3500 m above sea level (asl). The dominant soil types of the District included eutric Cambisols, chromic Luvisols, chromic Vertisols,  eutric  Fluvisols , Leptosols and pellic Vertisols. The soil in the specific study site (Ile) is pellic Vertisols. The major land use and land cover in the area include farmlands, grasslands, plantation forest of mainly eucalyptus species, natural forest including area exclosure and degraded hillside. Growing indigenous on- farm trees such as Faidherbia albida,  Acaica abyssinica, Acacia seyal, Cordia africana and Croton macrostachyus together with annual crop is a common practice in the area. The dominant annual crops associated with the on-farm trees includes teff (Eragrostis tef (Zucc.)), wheat (Triticum sativum L), maize (Zea mays L.), sorghum (Sorghum bicolor L) and green paper (Capsicum frutescens L.).

 

 

Experimental design, soil sampling and analysis

 

For the soil study, six isolated mature trees of F. albida were selected at six adjacent farmlands under similar soil types, climate and landscape. Twenty four  soil samples were taken from the surface layer of 0–10 cm at three concentric transects of (1, 3 and 5 m) and compared with soils taken from adjacent open areas located at 12 m from the trees trunk. Approximately, 1 kg of soil sample was collected from each sample site and analyzed at Holetta soil laboratory. The samples were air-dried at room temperature, crushed, homogenized and passed through a 2mm sieve before analysis for pH, C and available P. They were also further sieved through 0.5 mm size for analysis for TN. For bulk density and moisture content determination undisturbed soil samples were also collected with a manual core sampler of 5 cm height and * 3 cm in diameter at soil depth (0-10cm).

 

Available Phosphorus was determined by Olsen’s method of bicarbonate extraction (Olsen and Summer, 1982), total nitrogen was analyzed by Kjeldahl procedure (Jackson, 1958), and organic carbon was determined by Walkley-Black dichromate method (Walkley and Black, 1934), pH was measured using (1:2.5 soils to water ratio) using digital pH meter. Bulk density was determined using core method after oven drying wet undisturbed soil samples at temperature of 105°C for 24 h. Soil-water content was determined by standard procedures described for the gravimeter method after oven drying to a constant weight at 105°C (Anderson and Ingram, 1993) for 30 h. Bulk density was calculated by dividing the weight of oven-dried soil with the volume of the core. OM was estimated by multiplying OC by 1.724. All the chemical and physical analysis were carried out in a soil-testing laboratory of Holetta Soil Laboratory, Addis Ababa Ethiopia.

 

Assessment of population status of F. albida

 

Inventory of F. albida was carried out in three Peasant Associations namely Goyban, which is a high altitude area (2200-2400 m asl); Mekicho a mid altitude area (2000-2200 m asl), and Ile a low altitude area (1800-2000 m asl). Three GPS points were selected randomly one in each locality, then a Y-frame transect sampling scheme applied for selecting 39 farms for detailed study. That means 13 farms were selected from each locality along the three transects line approximately at 100m, 300m, 600m and 900m from central farm (Figure 1). The selected farms were further categorized into four land uses (homestead, farmlands, grazinglands and woodlots). The size of each farm/plot was determined through interview of the owners and field survey. Total count method was used to estimate the number of stems per plot. In addition, Diamter at Breast Height (DBH), total height and canopy width and length were measured using caliper, clinometers and measuring tape, respectively. In addition, semi-structured interview, field observation and informal discussion was held to capture qualitative data.

 

Statistical analysis

 

The data of soil properties in response to radial distance were subjected to GLM test with one-way-ANOVA (SPSS Inc, 2006). Then, the means for treatments that showed significant differences by F-test were separated by Tukey’s honestly significant  difference test (Tukey-HSD test) and significance were declared at 0.01 and 0.05 significant levels, which is the most widely used multiple comparison procedure (Zar, 1996). Correlation test was conducted to assess the relationships between the different soil properties and radial distances. The soil properties analyzed and compared were bulk density(gcm^-3), moisture content(%), pH(H₂O), OC(%), SOM(%), total N(%), available P(ppm) and C/N. Similar GLM test with one-way-ANOVA was used to evaluate  the effect of land use and altitude on stem number of F. albida. In addition, simple descriptive statistics was used to describe distribution and characteristics of trees and qualitative data.

Soil physical properties

 

Results of soil moisture content and bulk density are presented on Table 1. Both sSoil moisture content and bulk density did not show any significant difference (P < 0.05) difference between the radial distances (Table 1). However, relatively higher moisture content of (29.53±1.01%) was recorded at 1m distance, while similar lower moisture content of 28.75±1.13%, 28.35±1.45%, 28.32±1.49% were recorded at 3m, 5m and 12m distnaces, respectively (Table1). This result goes well with the finding of Manjur et al., [2014] who found non-significant difference in moisture content between radial distances from tree trunk under F. albida and C. macrostachyus in Southern Ethiopia. The result also conforms with the studies by Hadgu et al., (2009) and Kamara and Haque (1992) who found no significant difference in moisture content between radial distance in Ethiopia. On the other hand, the present result disagrees with the finding of Rhoades (1995) who found higher moisture content under the canopies of F. albida trees than outside the canopy. Though not significantly different, lower BD of 1.12±0.04g/cm3 was observed at 1m, and slightly higher BD of 1.13±0.03 g/cm3, 1.14±0.06 g/cm3 and 1.15±0.02 g/cm3 were measured at 3m, 5m and 12m radial distances, respectively (Table 1). This observation contradicts with that of Manjur et al., (2014) found a significant difference in bulk density between radial distances from F. albida and C. macrostachyus trees in Southern Ethiopia. Bit the result is consistent with findings of Abebe (2006) who reported lower non-significant bulk densities under F. albida, C. macrostachyus and C. africana trees in Eastern Ethiopia. It also matches with the  finding of  Belsky et  al (1989), who  reported lower  bulk density  under  trees  than  outside the canopy  in  Kenya.

 

 

Soil chemical properties 

 

Among the selected soil chemical properties, OC, SOM and TN showed significant differences with radial distances (Table 2). Soil pH did not show significant difference. A roughly similar soil pH of 6.18±0.06; 6.15±0.09; 6.11±0.07 and 6.12±0.07 were observed at 1, 3, 5 and 12 m distances from the trees trunk, respectively (Table 2). This result goes well with the finding of Manjur et al., (2014), who reported similar soil pH among radial distances from trunks of F. albida and C. macrostachyus in Southern Ethiopia. Another study by Kamara and Haque (1992), also found similar soil pH under and outside the canopy of F. albida in Ethiopia. Jiregna et al.,(2005) also reported a similar patter of soil pH under  and outside canopy of C. africana  and  C. macrostachyus in Eastern Ethiopia. OM was significantly different between radial distances (P < 0.01). Higher OM of 4.7± 0.14% was measured at 1m whereas lowest OM of 4.04±0.08% was measured at 12m. Both OC and OM content showed a continuous decline with radial distances from the tree base. This may reflect the strong influence of trees on organic inputs. Likewise, organic carbon significantly varied at (P < 0.05) among radial distances. Higher OC of 2.74±0.08 % was found at 1m while lowest OC of 2.35±0.04% were found at 12m distance (Table 2). In general soil under tree canopy contained better soil chemical attributes than outside the tree canopy. These results agree with previous reports by Abebe, (2006) and Manjur et al., (2014) who found higher OC under the canopy F. albida, C. africana and C. macrostachyus than outside canopy in Ethiopia. It is obvious that OM has a positive influence on soil fertility, plant nutrition and biological activity in the soil (Brady and Weil, 2002). 

 

 

Total nitrogen also showed a similar significant difference (P < 0.05) between radial distances. Higher TN (0.21± 0.02%) was measured at 1m distance and lowest TN (0.17±0.00%) was measured at 12m distance (Table 2). This results matches the findings of Hadgu et al., (2009) and Umar et al., (2012) who found higher N content under the canopies of the trees than far away from the influence of trees in Ethiopia and Zambia. Another study by Kamara and Haque (1992) also found a higher N content under F. albida canopies than outside the canopies in Ethiopia. Similar other studies (e.g. Abebe, 2006; Manjur et al., 2014; Sanda and Atiku, 2013; Saka et al., 1994) reported a similar pattern of difference in total N between radial distances.

 

The observed higher SOM, OC and TN under canopy compared with open area could be explained due to leaf litter fall and decomposition of dead roots from the tree and nutrient cycling and nitrogen fixation by tree species. Shedding of its leaves during rainy period when there is limited livestock movement might be a contributing factor for higher soil fertility under canopy compared with outside the canopy. It is obvious that F. albida has the ability to fix atmospheric nitrogen and subsequently convert it to plant available nitrate form (Buresh and Tian, 2004). For instance, Kamara and Haque (1992) found higher content of N (3.85%) from fresh leaves of the trees in Ethiopia. On top of this, deep rooting system of F. albida enables the tree to take up nutrients from deeper soil horizons and that are subsequently returned to the top soil through litter fall. Some of the key informants estimated that the root of F. albida system extends up to 25m down ward for extraction of nutrients and moisture. In present study the contribution of external sources such as cattle dung is could be minimal as farmers regularly pollard the tree canopy for reducing shade that can pose problem on associated food crops.

 

On the other hand, available P did not show significant difference among radial distances (Table 2). However, relatively higher available P of 17.10±2.02 ppm was measured at 1m while relatively lowest values of available P (14.81±0.88 ppm), was measured at 5m (Table 2). This result goes well with findings of Abebe (2006) who found better available P under F. albida, C. africana and C. macrostachyus trees in Ethiopia. These results also confirm the findings of Kamara and Haque, (1992) who detected non-significant variability of available P among radial distance under F. albida in Eastern Harargie, Ethiopia.

 

The result, however, does not match with the findings by Manjur et al., (2014) who reported significant difference in available P among radial distances from tree trunk under F. albida and C. macrostachyus trees in Southern Ethiopia.

 

Likewise, C/N ratio did not show significant difference among radial distances. Nonetheless, there is an increasing trend with increasing radial distance as shown in Table 2.

 

Correlation assessment between the different soil parameters (Table 3) revealed a positive correlation between SOM, N, pH and MC. Similarly, positive correlation was observed between TN and P. The relationship between SOM and BD was negative, which is expected. The higher the SOM, the lower the bulk density should be. Radial distances, and SOM and TN are also negatively correlated, which again is expected. This again confirms the positive effect of trees on SOM and TN. Similar inverse relationships between radial distances and SOM, N, P, K were reported from  Ethiopia by Kamara and Haque (1992) and Umar et al,.(2012) from Zambia.

 

 

Distribution and characteristic of F. albida trees in the study area

 

Tree stem distribution by DBH class indicated 74% of the stems to fall in the DBH class over 25 cm, while only a lower proportion of (2%) fall in a yunder (juvenile) DBH-class of less than 5cm (Figure 3). This shows the dominance of mature trees. This finding matches with the study of Kamara and Haque (1992) who reported higher proportion of mature trees compared with young trees in a similar farming system in Ethiopia. The present results also goes well with finding from other area by (Adamu, 2012) and Reed et al., (1992) who found higher number of mature trees compared with young individuals  in Nigeria  and West Africa, respectively. Similar lower young population was reported elsewhere by (Kirmse and Norton, 1984). 

 

 

The  mean  tree  height and  trunk diameters  were 7.3m ±0.54m  and 30cm  ±  0.03cm, respectively (n = 30). The canopy cover of the tree range between 3-5m with mean of 3.7m ±0.36m.

 

Effects of land use  and altitude on stem number of F. albida

 

Within altitudinal zones, land use did not show significant influence (P < 0.05) on number of stems. However, in terms of absolute values, farmlands had higher stem number of 10.04±2.44 per ha followed by homesteads (9.26±3.04 per ha), grazing lands (6.25±6.25) and woodlots (4.13±4.1) at lower altitude zone. Similarly, in mid altitude farmlands contained higher individual F. albida trees (4.62±2.83 per ha) followed by homestead (0.67±0.67 per ha) while grazing land and woodlots did not contain any F. albida trees.  On the high altidtude zone, overall lower number of stems (2.8±2.88 per ha) were found, and these are found on grazing land. There was no F. albida stem found in homestead, farmlands and woodlots at higher altitudin zone. This indicates the altitudinal stretch of the species does not go beyond the mid altitude range described in this study. Moreover,  the observed lower proportion of growing population in all considered land use may be attributed interest of farmers to preserve only mature trees and  to high grazing pressure, F. albida is highly palatable to all herbivores particularly for camel and goat. The tree being one of the few species with green foliage during the dry season the young seedlings and saplings may be readily eaten if not protected. Hence, the young seedlings should be protected from livestock trampling and human disturbance. Lower level awareness about the value of the F. albida may be contributing factors which is also agreed upon by farmers and development agents. On the other hand, altitudinal zone significantly affected the number of stems (P < 0.05). Higher number of F. albda stem (10.17±2.5 stem per ha) was found at lower altitudinal, the elast at  higher  altitudinal  (0.93±0.9  stem per ha) (Figure 4). The observed significant variation in stem number in response to altitudinal gradient could be explained by difference in temperature and soil fertility and moisture. Some of the interviewed farmers perceived that the growth of F. albida is slow in the upper altitude than lower altitude as results of cool temperature and poor soil fertility in upper altitude. The observed higher stem number may also be associated with associated vegetation and soil types. For example, higher number of stems was found under woodland vegetation and Vertisols at lower altitude compared to afromontane forest and cambisol at mid- and higher altitude (Table 4).

 

 

 

The measured density of F. albida in the study area also falls within the ranges of previous studies such as by Kamara and Haque (1992) who reported the average density of 6.52 trees/ha, and Poschen (1986) who reported the density range of 1-10 tree /ha, both in Ethiopia.

 

Perception of farmers on effects of F. albida on crop yield germination and population status

 

Out of the interviewed farmers (n=18), 60% perceived that F. albida might decrease crop yield underneath its canopy depending on crop types grown. The majority (10%) perceived that crop yield will decrease regardless of crop type, while about 30% perceive no difference under and outside the canopy. This finding is also supported by field observation. F. albida affects uniform germination of specific crops particularly crops such as maize and teff compared to wheat and barley. Similarly, farmers indicated that crop maturity differs under and outside the canopy; with those under the canopy maturing late compared to the crop outside the canopy. This may have to do with the shading effect of the tree as well as the associated higher soil moisture under the canopy. The presence excess OC may also favors vegetative growth at the expense of reproductive growth.

 

Regarding threat to the tree species, despite the dominance of old diameter class, 100% of the respondent agreed that the tree would not face threat of disappearance in the future. This is justified by the fact that farmers cut only the branches rather than cutting if from the trunk. Multiuse nature of the tree has been emphasized the respondents for conserving it and caring for it by the interviewed farmers. All of the interviewed farmers confirmed the tree to provide diverse products such as fuel wood, fencing and livestock forage.

 

In general, based on interview, informal discussion and field observation, F. albida trees provide   different ecosystem services such as increasing soil fertility, crop yield and biodiversity conservation. The tree also provide fuel wood, fencing and livestock forage. However, some ecosystem disservice was perceived by farmers , particularly depressing uniform germination  of crop and crop yields particularly  teff and maize  if the canopy is left  un lopped for more than 2 years.

 

The results of this study showed that significant differences in OM, OC and TN were observed between radial distances from F. albida trees while P, pH, MC, BD and C/N did not show any significant difference. Higher SOM, OC and TN were found in soils under the canopy of the trees compared with soil outside the canopy. Land use did not affect the distribution of F. albida while altitudinal zone significantly affected the distribution of F. albida. Higher number of stems were measured under farmlands at low altitude. The observed higher proportion of mature individuals compared with lower proportion of young individuals signals the importance of planting new seedlings and protection of existing once from disturbance. Lower level of awareness about the ecological values of the F. albida may also be a contributing factor for decline of the population of the trees. The trees provide fodder for livestock feeding, fuel wood, fencing. It is obvious that a good stand of F. albida can be maintained only if rigorous and concerted efforts are made to plant and nurture the naturally growing trees. To keep the sustainability of this important multipurpose tree species, the following measures are recommend: the population of growing  trees should be protected from livestock pressure and planting of new seedling in farmlands and closed area and degraded hillsides in order to conserve genetic pool of the species. The branches of the trees should be removed through pollarding for growing crops such as maize and teff in order to facilitate passages of light and rainfall for uniform germination under the canopy. Further research should be conducted in order to understand and select compatible annuals crops that could better grow under the canopy of trees.

 REFERENCES