Effects of cattle browsing on Terminalia sericea-dominated woodlands in a semi-arid region of the Kalahari, Namibia
Dissertation (PhD)--University of Stellenbosch, 2006.
ENGLISH ABSTRACT: 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. Keywords; bush encroachment, soil hydrochemistry, diet selection, plant defences, traditional ecological knowledge, Kalahari
AFRKAANSE OPSOMMING: Diere boerdery in die noordelike Kalahari het invloed op plantegroei en grond toestand, veral rondom water-boorgate wat gesink is vir water voorsiening aan mense en diere. Die volgende was ondersoek: 1) die effek van beweiding op grond minerale, plantegroei, bosverdigging en plant verskeidenheid (spesies) met afstand vanaf die water punte; 2) beweiding van borne gedurende die droe seisoen wanneer blaar-wisselende borne hul blare hergroei het. Koeie was agtervolg in die veld om hul beweidings patroon te ondersoek; 3) die onafhacl<like en interaktiewe effek van beweidingskade en grond vrugbaarheid op plantegroei (in terme van die verdraagbaarheid van beweidingskade) en weerstand van Terminalia sericea teen beweiding. Weerstand teen beweiding was in terme van tannin gemeet. Vir hierdie eksperiment was klein kampe in die veld opgerig en gekontrolleerd vir die getal beeste en grond bemisting; 4) die effek van die hoeveelheid water, grond vrugbaarheid, beweidingskade en hul interaksies op die groei, verdraagbaarheid van skade, en weerstand van T. sericea teen beweiding. Hierdie eksperiment was in 'n kwekery uitgevoer op 19 maande-oud saailinge; en 5) die persepsie en ervaring van plaaslike boere ten opsigte van die invloed van diere boerdery op plantegroei, en die benuttigmgswaarde van borne en bosse vir diere asook mense. Die studie bring navore die volgende resultate: 1) die effek van diere boerdery op grond minerale en plantegroei was beperk tot 200 men 600 m vanaf die water punte onderskeidelik. Plant spesies was verminderd onder die invloed van intensiewe diere activiteite naby die water punte. 'n Paar kruie soos Sida cord~fo/ia het hierdie area domineer; 2) Beeste hetmeeste van die tyd (71 %) aan borne en bosse gewei gedurende die droe seisoen. Termina/ia sericea, Bauhinia petersiana en Philenoptera nelsii het 74.5 % van die total inname van borne en bosse uitgemaak. Philenoptera nelsii was die mees verkiesde plant; 3) Beweidingskade en grond vrugbaarheid het groot invloed op T. sericea se groei, verdraagbaarheid van skade en weerstand teen beweiding gehad. Terminalia sericea het baie gegroei wanneer die grond se vrugbaarheid was verhoog. Hoe beweiding het groei punte verkort, aantal blare en blaargrote verminder. In hierdie verband het hoe grond vrugbaarheid die effek van beweiding geneutraliseer. Weerstand teen beweiding was hoog onder Jae grond vrugbaarheid, maar was verminderd as grond vrugbaarheid verhoog was. Grond armoede het Terminalia sericea instand gehou om beweiding te verdra en ook meer tannin te produseer na beweiding. Daar was groat ooreenkoms tussen die eksperimente in die natuurlike veld en in die kwekery. 4) Die plaaslike boere besit goeie kennis van die verandering in hul omgewing. Hulle beskou die huidige situasie van bosverdigging as voordelig vir hul bestaan, omdat dit voeding aan hul diere verskaaf; vuurhoud vir kook en verhitting, asook material vir die bou van krale en huise. Dus, restoureer werk op oorbebosde weivelde moet inagneem die bydrae van home en bosse tot die welvaart van plaaslike boere en die voeding van diere. Beweiding van borne en bosse kan tot voordeel wees vir diere as grond vrugbaarheid behou of verbeter word, omdat borne en bosse dan lae weerstand teen beweiding hied. Verder rnoet ervaring en kennis van die plaaslike ekologie inageneem word vir die daarstelling van volhoubare boerdery praktyke. Sleutelwoorde; bosverdigging, grand minerale, diet voorkeer, plant verdediging, tradisionele ekologiese kennis, Kalahari