Spatial variation in density, species composition and nutritive value of vegetation in selected communal areas of the North West Province

Abstract

The study was designed to assess spatial variation in terms of density, species composition and nutritive value of vegetation in selected communal grazing lands located in the Ngaka Modiri Molema district municipality of the North West province. For the first study on tree species assessement, three 2.2 km transects, which served as replicates were established at each of the selected grazing areas. The three transects were placed at least 200 m from each other. Along each transect, points were marked within 500-700 m (considered as near sites), >700 m-1.4 km (middle site) and >1.4 km – 2.2 km (far sites) from the homesteads to form 9 sampling sub-transects. Three 10 m x 10 m homogenous vegetation units were marked at each sub-transect and spaced 20 m from each other. The homogenous units (HVU) were used to record density, height and canopy diameter of individual woody plants. Plant identification was carried out using a combination of scientific and indigenous local knowledge. A total of 21 browse species were found across all sites. Grewia flava and Acacia erioloba were the most dominant species in all soil types across the study areas. There was no significant effect of distance from the homesteads on density, canopy cover (CC), total tree equivalent (TTE) and plant height. There was a significant effect of soil type on density, canopy cover, total tree equivalent and plant height. The red-brown sand soil type had higher (P<0.05) total plant density (827.7 plant/ha), CC (9.6%); TTE (2886.4 TTE/ha) than in clay-loamy soil type area. Red-brown sand soil type area had higher (P<0.05) values for all height levels than clay-loamy soil type. For grass sampling and assessment of grass species composition, within each sub-transect, 10 m × 10 m homogenous vegetation unit (HVU) was marked. In each HVU, 1 m2 quadrat was randomly placed to sample soil and grass species resulting in a total of 9 samples per site. Grass samples (per each species) were collected per quadrat, oven-dried and milled through a 1 mm sieve for chemical analysis. A total of 28 grass species were identified in all study areas, of which 23 species were perennials. Twenty one percent of the total grasses were classified to be of high grazing value, 50% medium grazing value and 29% as low grazing value. Most of the highly palatable species were found at sites far from the homesteads. Cymbopogon pospischilii, Eragrostis bicolor and aristida species were the most commonly occurring grasses in many sites in the grazing area under clay-loamy (CL) and Red-brown sand (RBS) soil types. Sodium, P, K, Ca, Mg and Mn concentration was higher (P<0.05) in CL soil than in RBS soil. Iron concentration was higher in RBS soil than in CL soil. Tree and grass samples were collected and analysed for chemical and in vitro ruminal degradability. Eragrostis trichopora (100 CP g/kg) in Tsetse, Cynodon dactylon (62 and 66 CP g/kg) in Six-hundred and Makgobistadt, Melenis repens (70 CP g/kg) in Loporung communal area had the highest CP values than all other grass species in their respective areas. Cymbopogon pospischilii (540.6 g/kg DM) and E. trichopora (562.0 g/kg DM) had the highest (P<0.05) DM degradability values at 48 h at clay-loamy soil type grass species. All grass species harvested in Makgobistadt and Loporung communal areas had similar DM degradability values at 48 h. The highest crude protein content (P < 0.05) was recorded in leaves of Grewia monticola (190.4 g/kg DM) than in all other species in the study area. Although all the browse species contained lower amounts of tannins in their leaves, the highest (P<0.05) CT content was found in Dichrostachys cinerea leaves (0.993 and 1.044 AU550/200 mg) than all other browse species in the study areas. The last study was carried out to determine key characteristics of common grass species under controlled environmental conditions, including their phenological patterns, relative growth rates as well as their chemical composition, and in vitro ruminal degradability. Differences (P<0.05) were observed on morphological characteristics within grass species, growth stage and their interaction. Fingerhuthia africana had higher (P<0.05) CP content (102 g/kg) than all other grass species. Eragrostis bicolor had higher (P<0.05) number of tiller developed at reproductive stage than all other grass species. Due to different morphological characteristics and feeding value, these species could complement each other in rehabilitating the communal areas affected by heavy grazing. Changing the vegetation structure by reducing woody plant density in Makgobistadt and Loporung communal areas can create a conducive environment for open grasslands to occupy the communal area and create biodiversity within grass species.