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Accueil du site → Doctorat → Afrique du Sud → 2006 → Effects of cattle browsing on Terminalia sericea-dominated woodlands in a semi-arid region of the Kalahari, Namibia

University of Stellenbosch (2006)

Effects of cattle browsing on Terminalia sericea-dominated woodlands in a semi-arid region of the Kalahari, Namibia

Mutjinde Katjiua

Titre : Effects of cattle browsing on Terminalia sericea-dominated woodlands in a semi-arid region of the Kalahari, Namibia

Auteur : Mutjinde Katjiua

Université de soutenance : University of Stellenbosch

Grade : Doctor of Philosophy Ph.D. 2006

Résumé
Livestock farming affect plant growth and soil condition around artificial water points in arid and semiarid environments. This study investigated the following, in the northern Kalahari, Namibia : 1) the impact oflivestock on soil condition, vegetation structure and composition with reference to artificial water points ; 2) cattle browsing preference on pre-rain flush during the hot-dry season ; 3) the independent and interactive effects of plant damage, soil nutrients and moisture content on tolerance and resistance traits of Terminalia sericea. Condensed tannin was used as a measure of resistance trait, while growth parameters were used to measure tolerance of (herbivore) damage. This experiment was carried out in small camps in the field where plant damage and soil nutrients were controlled, and (4) under nursery conditions where in addition to plant damage and soil nutrients, soil moisture was also controlled. The nursery experiment was carried out on 19 month-old seedlings of Terminalia sericea ; and 5) the perception and knowledge oflocal pastoralists on vegetation change and utilization of woody plants was assessed. The key results of the study are summarised in the sections below : Impacts of livestock around artificial water points in the northern Kalahari Livestock activities near artificial water points had significant impacts on vegetation and soils. Fewer, but tall trees with broader canopy areas grew near the water points. Herbaceous abundance increased with proximity to the water points, but species diversity of both herbaceous and woody plants was 55 and 29 % respectively of the diversity measured from 600-4000 m from the water points. Soil organic carbon and nitrogen content was generally higher at 10 - 20 cm depth near the water points than at lower depths, while soil moisture changed little across the soil profile and farther away from water points. Available phosphorus in the 10 - 20 cm soil layer was significantly higher at 200 m from water points than at any other depth and distance from water points. This may have resulted from livestock dung deposition. Long-term observations by pastoralists revealed that the density of woody vegetation and composition of plant community have changed substantially all over the rangelands since the commencement of sedentary settlements around the anificial permanent water points some 32 to 51 years ago. Cattle diet selection during the hot-dry season Cattle consumed more browse than grass during the hot-dry season because oflow grass availability during that period. This confllTiled pastoralists’ impressions that cattle browse more than they graze during the hot-dry season in the northern Kalahari. Cattle fed on a variety of browse during the hot-dry season, but three species, Bauhinia petersiana, Philenoptera nelsii and Terminalia sericea, accounted for three-quarters of dry matter intake. Browse quality and availability influence the consumption pattern of herbivores. In this regard, cattle avoided Ochna pulchra, which had consistently high levels of fibre and condensed tannins, while they browsed heavily on the preferredP. nelsii, which contained low concentrations of fibre and condensed tannins, but a high crude protein content. We found that diet selection was best explained by a multiple linear regression model (Forage ratio= 0.522+1.55 x CP- 1.6 xP- 0.60 x ADL ; R2 = 0.9385, P = 0.0019). Plant resource-partitioning between resistance and tolerance traits along a resource availability gradient The field and the controlled (seedling) experiments both supported the Resource Availability Hypothesis. Fertilizer addition increased the tolerance traits of seedlings, i.e. leaf area, leaf dry mass, height, number of branches, number ofbranches:height ratio and above : below ground biomass ratio. Seedlings in the nutrient-poor environments and under limited soil-moisture had 22.5% and 21.7% higher condensed tannin concentrations than those seedlings grown in nutrient-rich environments and under high soil-moisture. The strategy of plants in the low resource environments to deter rather than to tolerate herbivores was not supported by this study because tolerance and resistance traits occuffed concurrently, and with no indication of trade-offs between investments into resistance traits (e.g. tannin synthesis) and tolerance traits (e.g. leaf area and plant height). Furthermore, the high constitutive defence levels in low resource environments appeared to result from constrained plant growth rather than as a response to herbivory. Leaf production was resource-dependent. Tenninalia sericea produced more leaves per cluster under nutrient-poor soils than under fertilizer-enriched soils. Heavy browsing and complete defoliation reduced cluster size, but improved soil nutrient status negated the negative effects of complete defoliation and high browsing pressure on cluster size. Compensatory responses of resistance and tolerance traits to (herbivore) damage The effects of fertilizer and defoliation on condensed tannin were complex and dependent on the temporal scale. Foliar damage under natural soil conditions promoted overcompensation of resistance and tolerance traits, while fertilizer addition reduced condensed tannin concentrations. However, the interaction effect of foliar damage and fertilizer treatments promoted full compensation. The results suggested that compensatory responses of tolerance and resistance traits upon damage result from the high accumulation of photosynthetic products in excess of constrained growth requirement under low resource conditions. In this study, a 31.3 % lower above:below-ground biomass ratio was found in seedlings grown on the low-resource environment than in the high-resource environment. Thus, compensatory ability of T. sericea reflected the degree to which growth in the low resource environments was constrained and, subsequently, the amount of photosynthetic products allocated to storage and secondary metabolites. Furthermore, the compensatory ability of T. sericea revealed the adaptability of the plant to environmental stress in its struggle to maintain fitness. Responses of resistance and tolerance traits over time There was no evidence for increased condensed tannin (resistance trait) as a result of cattle herbivory early in the growing season across a soil nutrient gradient. However, the concentration of condensed tannins increased as the seasons progressed from early hot-dry season (October) to mid hot-wet season (February). In contrast, repeated early-season manual defoliation induced condensed tannin by 89 .9% in nutrient-poor environments. Resource allocation to phytochemical substances and (perhaps) the ability of T. sericea to restore its internal resource balance over the growing season, was influenced by the interactive effects of soil fertility and (the degree of) plant damage. The compensatory ability of T. sericea may have ensured that the negative effects of early-season damage did not endure for the entire growing season. Pastoralists of the Otjinene constituency Historically, the OvaHerero kept cattle for consumptive uses, religious rituals and as a symbol of one’s status in society. In this study, the respondents clearly raised cattle primarily for consumption of milk and milk by-products (93 % of respondents) and equally so to generate cash income (82 % of respondents). The high offtake of male weaners (calves) and young adults in particular was an indication that the pastoralists have commercialized their production system. An estimated annual cattle offtake of about 15.3 -16.5 % in the study area was above the national offtake (12.6 %) for all herds in 2004. The second priority for raising cattle was to retain a cultural identity (35 % of respondents), achieve a good social standing by possessing a sizeable number of cattle (33 % ofrespondents) and to ensure that the younger generation continue with pastoralism (31 % ofrespondents). Goats and sheep were primarily used for meat consumption (77 % and 69 % of respondents, respectively). The current state of vegetation change in the northern Kalahari had beneficial outcomes for the livelihoods of pastoralists, because encroaching woody vegetation presented opportunities for browse consumption, firewood and timber for construction of houses and livestock pens. Pastoralists regarded T. sericea, P. nelsii and Combretum collinum as the key browse species during the hot-dry season. The pastoralists’ knowledge of cattle browse consumption compared well with the actual browse consumption based on observations of foraging cows ( r = 0. 70, P < 0. 0 5). Integration of indigenous and scientific knowledge, methods of natural resource inventorisation, evaluation ofland use impacts and management practices in pastoral systems may not only enhance local-level (community-based) adaptive range resource management, but will also enrich ecological research outputs.

Mots clés : bush encroachment, soil hydrochemistry, diet selection, plant defences, traditional ecological knowledge, Kalahari

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