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

[摘要] ENGLISH ABSTRACT: Livestock farming affect plant growth and soil condition around artificial water points in arid and semiaridenvironments. This study investigated the following, in the northern Kalahari, Namibia: 1) theimpact oflivestock on soil condition, vegetation structure and composition with reference to artificialwater points; 2) cattle browsing preference on pre-rain flush during the hot-dry season; 3) theindependent and interactive effects of plant damage, soil nutrients and moisture content on toleranceand 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 wascarried 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 alsocontrolled. 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 woodyplants was assessed. The key results of the study are summarised in the sections below:Impacts of livestock around artificial water points in the northern KalahariLivestock 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 increasedwith proximity to the water points, but species diversity of both herbaceous and woody plants was 55and 29 % respectively of the diversity measured from 600-4000 m from the water points. Soil organiccarbon and nitrogen content was generally higher at 10 - 20 cm depth near the water points than atlower depths, while soil moisture changed little across the soil profile and farther away from waterpoints. Available phosphorus in the 10 - 20 cm soil layer was significantly higher at 200 m from waterpoints than at any other depth and distance from water points. This may have resulted from livestockdung 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 thecommencement of sedentary settlements around the anificial permanent water points some 32 to 51years ago.Cattle diet selection during the hot-dry seasonCattle consumed more browse than grass during the hot-dry season because oflow grass availabilityduring that period. This confllTiled pastoralists' impressions that cattle browse more than they grazeduring the hot-dry season in the northern Kalahari. Cattle fed on a variety of browse during the hot-dryseason, but three species, Bauhinia petersiana, Philenoptera nelsii and Terminalia sericea, accountedfor three-quarters of dry matter intake. Browse quality and availability influence the consumptionpattern of herbivores. In this regard, cattle avoided Ochna pulchra, which had consistently high levelsof fibre and condensed tannins, while they browsed heavily on the preferredP. nelsii, which containedlow concentrations of fibre and condensed tannins, but a high crude protein content. We found that dietselection 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 availabilitygradientThe field and the controlled (seedling) experiments both supported the Resource AvailabilityHypothesis. 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 andunder high soil-moisture. The strategy of plants in the low resource environments to deter rather than totolerate herbivores was not supported by this study because tolerance and resistance traits occuffedconcurrently, 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 constitutivedefence levels in low resource environments appeared to result from constrained plant growth ratherthan as a response to herbivory. Leaf production was resource-dependent. Tenninalia sericea producedmore leaves per cluster under nutrient-poor soils than under fertilizer-enriched soils. Heavy browsingand complete defoliation reduced cluster size, but improved soil nutrient status negated the negativeeffects of complete defoliation and high browsing pressure on cluster size.Compensatory responses of resistance and tolerance traits to (herbivore) damageThe effects of fertilizer and defoliation on condensed tannin were complex and dependent on thetemporal scale. Foliar damage under natural soil conditions promoted overcompensation of resistanceand tolerance traits, while fertilizer addition reduced condensed tannin concentrations. However, theinteraction effect of foliar damage and fertilizer treatments promoted full compensation. The resultssuggested that compensatory responses of tolerance and resistance traits upon damage result from thehigh accumulation of photosynthetic products in excess of constrained growth requirement under lowresource conditions. In this study, a 31.3 % lower above:below-ground biomass ratio was found inseedlings 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 resourceenvironments was constrained and, subsequently, the amount of photosynthetic products allocated tostorage and secondary metabolites. Furthermore, the compensatory ability of T. sericea revealed theadaptability of the plant to environmental stress in its struggle to maintain fitness.Responses of resistance and tolerance traits over timeThere was no evidence for increased condensed tannin (resistance trait) as a result of cattle herbivoryearly in the growing season across a soil nutrient gradient. However, the concentration of condensedtannins 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 abilityof T. sericea to restore its internal resource balance over the growing season, was influenced by theinteractive 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 entiregrowing season.Pastoralists of the Otjinene constituencyHistorically, the OvaHerero kept cattle for consumptive uses, religious rituals and as a symbol of one'sstatus in society. In this study, the respondents clearly raised cattle primarily for consumption of milkand milk by-products (93 % of respondents) and equally so to generate cash income (82 % ofrespondents). The high offtake of male weaners (calves) and young adults in particular was anindication that the pastoralists have commercialized their production system. An estimated annual cattleofftake of about 15.3 -16.5 % in the study area was above the national offtake (12.6 %) for all herds in2004. 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 toensure that the younger generation continue with pastoralism (31 % ofrespondents). Goats and sheepwere 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 thelivelihoods of pastoralists, because encroaching woody vegetation presented opportunities for browseconsumption, firewood and timber for construction of houses and livestock pens. Pastoralists regardedT. sericea, P. nelsii and Combretum collinum as the key browse species during the hot-dry season. Thepastoralists' knowledge of cattle browse consumption compared well with the actual browseconsumption based on observations of foraging cows ( r = 0. 70, P < 0. 0 5). Integration of indigenous andscientific 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, traditionalecological knowledge, Kalahari

[发布机构] Stellenbosch University