RELIABILITY OF THE CURRENT RANGE ASSESSMENT TECHNIQUES IN SEMI ARID RANGELANDS EL BAJA AREA WHITE NILE STATE

Abstract

The annual and biennial herbaceous forage plants and some selected shrubs from (El Baja) area (13° 36΄ and 14° 10΄ N ; 31° 45΄ and 32° 23΄ E), White Nile State, were assessed between 15th and 30th October 2002. The most effective rainfall period occurred between June and July 2002. The principal objective of the study was to investigate the reliability of vegetation assessment techniques that help attaining suitable management indicators. In the course of this study, numerous prevailing techniques were used to assess ground cover and bare soil, herbage mass and herbage mass composition of herbaceous plants, and available browse of selected shrubs. These included: a. The use of the traditional Loop method and the introduced Wheel-point method, to determine ground cover and bare soil. b. The use of the Double sampling procedure and the Dry Weight Rank method (DWR), to determine herbage mass and composition. c. The use of in-direct methods of Le Houerou and Hoste (1977) and Breman (1975), to determine herbage mass in relation to the annual rainfall. d. The use of Adelaide technique, to determine available browse of some selected shrubs. Ground cover, consisting of living vegetation and litter, was found to be high (93%), as assessed by the Loop method. This value was low (38%), as assessed by the Wheel-point method. The most dominant forage plants, as assessed by the Loop method were: Aristida adscenionis, Dactyloctenium aegyptium, Eragrostis tremula and Fimberstylis dichotoma. They were: Aristida adscensionis, Fimberstylis dichotoma, Eragrostis tremula and Indigofera sp , as determined by the Wheel-point method. Regardless of these variations in results, both methods were found to be useful and they have identified some management indicators. The methods were considered to be ecologically based. Herbage mass was assessed by three methods. The first, was the quadrate method by which herbage mass was assessed to be 314 kg/ha 394 kg/ha when quadrates of areas 1m2 and 0.5 m2 were used respectively. This method was found to be laborious, destructive and time consuming. The second, was the Double sampling procedure. Herbage mass was calculated by a linear regression in the form of: Y = a + b X. The results indicated that, the co-efficient of determination (R2) of the three single operators were found to be 0.97, 0.98 and 0.94 with 1m2 quadrate and they were 0.98, 0.97 and 0.99 with 0.5m2 quadrate. Herbage mass was calculated to be 308 kg/ha for (1m2) and 318 kg/ha for (0.5m2). The results do not show any significant differences between the three single operators and the two quadrate sizes. On the other hand, the Dry Weight Rank (DWR) method was used in conjunction with the Double sampling procedure to assess herbage mass composition. The results show that , these values fall within the range of 0.2% and 37% with 1m2 quadrate size and on the range of 0.1% and 55% with 0.5m2 quadrate size. The combined methods were not laborious, non-destructive and not time consuming. They were also considered to be productivity based and they are highly recommended for application in this study area and other similar areas. Herbage mass was lastly assessed by an indirect method that relates herbage mass to annual rainfall. Results show that herbage mass was calculated to be 702 kg/ha by Le Houerou and Hoste (1977) equation, and 928 kg/ha by Breman (1975) equation whereas the consumable herbage mass was 281 kg/ha. Both equations tend to over-estimate herbage mass as compared with the quadrate method and the Double sampling procedure. The available browse was determined by Adelaide technique through a step-wise regressions in the form of : Y=a+bx1+cx2, where x1 and x2 are the width and height respectively of the selected shrubs not exceeding 1.5 meters. Results show that the co-efficient of determination (R2) of these regressions were 0.62, 0.83, 0.60 and 0.80 for the shrubs Leptadenia pyrotechnica, Acacia nilotica, Acacia raddiana and Acacia seyal, if the estimated weight was used. Hence, the available browse was found to be 145.7, 282.4, 200.4 and 508.4 grams for the above-mentioned shrubs. On the other hand, when the calculated weight was used, R2 were 0.60, 0.87, 0.72 and 0.95 and available browse was 54.3, 123.4, 176.8 and 188.0 grams. The Adelaide technique in its simple application was found to be reliable, non-destructive and not time consuming. The overall findings of this study suggest that no one method by itself is sufficient to assess rangeland productivity and composition and that an integrated approach incorporating both ecological and production based methods, is desirable to assess rangelands condition and they will enable to identify correctly management indicators.