CHANGES IN VEGETATION AND SOIL PROPERTIES ALONG A SLOPE


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CHANGES IN VEGETATION AND SOIL PROPERTIES ALONG A SLOPE

Taşkın Öztaş ¹ , Ali Koç ² , Binali Çomaklı ²

¹ Atatürk University, Faculty of Agriculture, Department of Soil Science, Erzurum-Türkiye
² Atatürk University, Faculty of Agriculture, Department of Field Crops, Erzurum-Türkiye

ABSTRACT

Differences in soil formation along a hillslope result in significant differences in soil properties, plant production, and vegetation. The objective of this study was to determine changes in vegetation and soil properties along a slope in rangelands. Four study sites with three landscape positions; summit, backslope and footslope, were selected. In each landscape position, botanical composition, the rate of canopy coverage, the degree of rangeland quality, and some important surface soil properties were determined. The rate of grasses in botanical composition varied from 39.5 % to 84.0 %, and on the average, it was lowest at summit and highest at footslope positions. However, the rates of legumes at summit positions were lower than those of the other positions in two range sites. The rate of canopy coverage changed between 19.6 % and 45.2 %. The highest rates were obtained in footslope positions, but the lowest rates in summit. The degree of range quality varied between 3.2 and 5.5. Although, there was no significant differences among the positions regarding range quality degree, it was generally lowest at the summit positions. Clay content of surface samples was lowest at the backslope in all sites. But, there was no significant differences in clay contents of the summit and footslope positions. Changes in bulk density did not show significant trend along the slopes. It was generally higher in backslope and footslope positions than that of the summit position. Soil moisture content at soil sampling, organic matter content, and available P were generally higher at the footslope positions than those of backslope and summit. The CaCO3 content of soil was lowest at footslopes in all sites.

INTRODUCTION

Soil erosion, which is one of the most important causes leading to desertification, is a serious problem in many countries. Today, over 500 million tons of productive soil and large amounts of plant nutrients are lost mostly by water erosion every year in Türkiye. Since the factors determining runoff and erosion are a consequence of complex interactions of vegetation and soil characteristics (Thurow et. al., 1986), soil erosion occurs at widely varying rates over the landscape, and even along a typical landscape profile within a small area (Foster, 1988).

Differences in soil formation along a hillslope result in significant differences in soil properties (Brubaker, et al. 1993), plant production (Jones et al., 1989) and vegetation (Bragg, 1978). Soil properties and vegetation can also be altered over time under different land uses and management systems. In rangelands, grazing and more particularly overgrazing and its attendant effect of depletion of plant cover and litter and trampling of the soil is the most important factor contributing to erosion (Branson et al., 1981). The grazing lands of Türkiye are subject to quite heavy, uncontrolled grazing pressure and the forage production capacities of these lands are gradually decreasing, therefore, ultimately reflecting typical examples of land degredation all over Türkiye (Tükel and Hatipoğlu, 1996). This study was conducted to determine botanical composition and changes in soil properties with topographical positions in selected rangelands.

MATERIAL AND METHODS

Four study sites representing typic rangeland conditions of Erzurum province were selected. All sites, except Palandöken range site, have been subjected to heavy and nearly 9 months grazing pressure by livestock. Sheep fescue was the dominant species in all sites. The study sites have never been cultivated and located on gentle to steep slopes. Slope gradient varied from 2 to 4 % at summit, 30 to 45 % at backslope and 4 to 6 % at footslope. Each site had different direction, and elevation at the range sites changed between 1770 and 2610 m (Table 1). All sites had similar topographical positions; summit, backslope and footslope.

Three soil samples were collected from surface layer (0 - 10 cm) of each position, mixed and analyzed for some physical and chemical properties. Particle size distribution was determined by the hydrometer method (Gee and Bauder, 1986), bulk density by the core method (Blake and Hartge, 1986), soil moisture content at sampling by the gravimetric method, organic matter by the Walkey Black method (Schnitzer, 1982), CaCO3 content using the Scheibler calcimeter (McLean, 1982) and plant available-P by the Olsen method (Olsen and Sammers, 1982). Botanical composition of the range sites was determined by the line intercept method developed by Canfield (1941). Measurements were taken over a 60 m long transect and the basal area was considered in the measurements. Using the botanical composition values (de Vries et. al., 1951), the range quality degree was determined for each range site. Range condition classification was performed based on the range quality degrees as; 0-2 is very poor, 2.1-4.0 is poor; 4.1-6.0 is moderate; 6.1-8.0 is good, and 8.1-10 is very good (Gökkuş et.al., 1995). The LSD test was used for multiple comparison prosedures (Smith and Dowdy, 1983).

RESULTS AND DISCUSSION

Changes in Soil Properties : Clay content of soils was the lowest at the backslope positions of all sites (Table 2). This may due to severity of erosion on these sites. Although, clay contents of soil at footslope positions were generally higher than those of summit, except Umudum range site, there was no significant differences among them. Changes in bulk density values did not show significant trend along the slopes. However, it was generally higher in footslope positions. This may be an effect of soil compaction as a result of higher soil moisture contents of footslope positions as compared to the others. Higher soil moisture contents at footslope positions were expected because of runoff and seepage effect from the upper slopes. CaCO3 contents of the range site soils were quite different from each other, which may be attributed to parent material. CaCO3 content of soil was the lowest at footslope positions in all sites. This indicates that footslope positions get excess water from the upper positions, which leached CaCO3 from the surface layer.

Organic matter content of soils was less than 2.1 % in the first three sites (Table 2). However, it was greater than 2.0 % in all positions of the Palandöken range site. Organic matter content was the highest at footslope positions which may directly related to higher surface cover rates in these sites, because of higher amounts of available water content for plant growth.
The range sites were not so different from each other respect to available -P content which varied from 3.2 to 6.1 kg-P2O5/da. Similar to the variation of organic matter content, plant-available-P content was the lowest at backslope positions in all sites. These results indicated that the supply of nutrients in the study sites was generally not enough for optimal plant growth.

Changes in Vegetation : Botanical compositions of the range sites given based upon basic plant groups were shown in Table 3, 4 and 5. The rate of grasses in the botanical composition varied between 39.5 % and 84.0 %. It was lowest at footslope positions of Güzelyurt, Kösemehmet and Umudum rangelands. In contrast, the rate of grasses increased from 49.7 % at summit position up to 84.0 % at footslope position of the Palandöken rangeland. On the other hand, the rates of legumes in botanical composition of summit positions were less than those of the other positions in Güzelyurt and Umudum range sites.

The rates of other species, mostly invader species, were the highest at the summit positions of the three range sites. However, it was highest at the footslope position of Kösemehmet. Since, the increase of the rate of invader species in botanical composition of a site refers overgrazing (Holechek et.al., 1995), it could be concluded that the grazing pressure on the first three range sites was extremely higher than that of Palandöken range site.

The rate of canopy coverage was lower than 45 % in all sites, and got down to 19.6 % at the backslope position of Güzelyurt range site (Table 6). The rate of canopy coverage was the highest at footslope position in all sites, except Kösemehmet. This may be a result of severety of erosion due to overgrazing.

The degree of range quality changed between 3.2 and 5.5 (Table 7). The Palandöken range site had the highest range quality degree among the study sites. This indicates that the grazing pressure was less in the Palandöken range site as compared to the other range sites.

CONCLUSION

The results of this study indicated that vegetation and soil properties along a hillslope had great differences. Clay content, bulk density, soil moisture at sampling, organic matter content and plant available P were generally higher at footslope positions as compared to the other positions. This means may that clay particles, organic matter and plant-available P were lost by erosion which occured at more severe rates on the summit and backslope landscapes. However, the reason for high bulk density values at footslope positions was that the grazing pressure on footslopes was almost higher than upper landscape positions. On the other hand, CaCO3 content of soil was lowest at footslope positions of all range sites, which was attributed to leaching effect.

The range condition classes indicated that the range sites were either in poor or moderate class. Also, the canopy coverage rates reached the critical value, which was considered as 30 % (Marshall, 1973), for rapid water erosion in most parts of the range sites, even it was went below the critical value in some positions. Therefore, grazing pressure on these sites must be reduced by introducing effective range manegement systems.

Grasses are the dominant species in the climax plant community of rangelands with good quality (Holechek et.al., 1995). However, in our research sites, except the Palandöken, the rate of grasses in botanical composition was lower than 50 %, which was a good indication of overgrazing in these sites. If the current pressure on rangelands is continued, the rate of grasses in botanical composition gets lower and lower every year, as a trend of natural plant succession. All of these changes will cause low productivity and decline in biodiversity of our rangelands, and finally desertification. All of these results showed that the rangelands around Erzurum province are under overgrazing pressure, as in all around Türkiye. Therefore, the utilizition of rangeland according to management princeples must be established, soil erosion must be controlled, and degradated rangelands must be taken into rehabilitation programme. Finally, the regulations foreseen in rangeland act which was legitimised in 1998 must be put in to practice.

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