摘要:

The degradation gradient technique was used to quantify rangeland, condition in areas ranging from poor, trampled areas close to watering points to well managed areas in a semi-arid, sweet grassland of southern Africa. Ecological values were linked to species by means of their individual positions on the degradation gradient. These species were further classified according to ecological index values. Identification of indicator species for purposes of rangeland condition assessment was done by using the correlations between the relative composition and the rangeland condition value of a species. Index values and Gaussian distribution curves were used in calculations to determine rangeland condition. The accuracy of these methods was tested and the best results were obtained by using the species curves ofThemeda triandraand other species which dominate at other positions along the gradient. The advantage of this technique lies in its simplicity and ease of use. Another advantage is the fact that only the most important indicator and dominant species in the study area are used to determine rangeland condition.

ISSN:1022-0119    

DOI:10.2989/10220119909485719

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The long-term (>30 years) impacts of mowing (for hay-making) and ungulate exclusion, on natural, unfertilised rangelands were investigated. Changes in grass species composition and soil nutrient status were measured The study site, situated on the Athi-Kapiti plains of central Kenya, comprised of an ungulate exclusion camp, subjected to an annual mow, and an adjacent control plot, exposed to herbivory from cattle and game. Significant changes were noted in four of the seventeen grass species. Relative percentages ofPennisetum mezianumandP. stramineumincreased significantly within the mowed/exclusion plot, while those ofMicrochloa kunthiiandSporobolus iocladusshowed a significant drop. No significant differences were noted in relative frequency of grass species designated as 'decreasers', or in basal cover index. Analysis of soils inside the mowed/exclusion plot showed significantly higher soil pH, and soil Ca levels, but significantly lower plant available soil P and inorganic N, when compared to the control plot. The long-term treatment of mowing/herbivore exclusion, has resulted in severe depletion of plant available soil P and inorganic soil N. Results suggest that differences in species composition can be primarily attributed to the lower defoliation levels, and shading effects inside the exclusion plot, while differences in soil nutrient status can be attributed primarily to the export of nutrients with bailed hay, and to the absence of the stimulating effect that large herbivores have on nutrient recycling. Results are consistent with the hypothesis that herbivory plays an important role in maintaining P recycling in plant available forms, particularly for soils where P may be limiting.

ISSN:1022-0119    

DOI:10.2989/10220119909485720

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The utility of panoramic photographs for studying the extent and pattern of vegetation change, in particular of bush encroachment, was investigated by revisiting thirteen sites in the Weenen and Muden areas of KwaZulu-Natal that had been photographed in 1955. The ubiquitous feature of all sites was the marked increase in woody cover, from a mean of 21% woody cover for all topographic units (plains, hill slopes, river bank and river terrace) in 1955, to 59% woody cover in 1998. Although sample sizes were small, the increase in woody cover was greater on hill slopes (45% mean increase in woody cover) than on the plains (26%), and on north-facing slopes (56%) rather than on south-facing slopes (32%). The increase in woody plant cover was accompanied by a change in composition from open or mixed woodland, to one dominated byAcaciaspecies, in particular byA. tortilisat lower altitudes and byA. karrooat higher altitudes. In some cases, scrubAcaciaindividuals were already evident in the photographs in 1955, and had grown into mature trees by 1998. Recruitment of bothAcaciaand broad-leafed species was also observed in 1998 and there was a tendency for bush clumps to develop. Broad-leafed species have dominated on moist south-facing slopes and at higher altitudes. Increases of the arborescent leaf succulents.Aloe spectabilisandOpuntia ficus-indicaand of the dwarf, stem succulent shrub,Euphorbia pseudocactus, have occurred. Fire is no longer a component of this landscape which, together with the open condition in 1955, resulting from overgrazing. may account for the vegetation changes that have taken place. Panoramic photographs proved an effective tool for monitoring vegetation change because changes in composition and structure were revealed, and because each photograph samples a large area of ground at an adequate resolution.

ISSN:1022-0119    

DOI:10.2989/10220119909485721

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Analysis of a Landsat TM image from a rangeland near Peddie, Eastern Cape, revealed differences in two vegetation indices (normalised difference vegetation index, NDVI, and moving standard deviation index, MSDI) between communal and commercial rangeland. It was suggested that the difference in the MSDI reflected differences in rangeland condition. To assess whether or not any differences could be detected in the field, vegetation parameters were recorded (cover, species composition) along ten, paired 20-m transects. Based on species forage factors for commercial live-stock production, the commercial grassland was in a significantly better condition than the communal land. The commercial farmland had a higher occurrence of palatable species while the communal land was richer in non-palatable species. A TWINSPAN classification and the NDVI and MSDI values confirmed the marked difference between the communal and the commercial land. Both the vegetative field survey data and the satellite imagery showed that the communal land was transformed in comparison to the commercial land, and this difference can be attributed to differences in land-use.

ISSN:1022-0119    

DOI:10.2989/10220119909485722

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The objective of this work was to determine the proper sowing rate of wild oat (Avena fatua) in the depleted north-eastern semi-arid grasslands of Jordan. Wild oat seeding rates of 15, 30, 45 and 60 kg ha−lwere evaluated in field studies that were conducted over two years at Jordan University of Science and Technology Campus located 22 km to the east of Irbid, Jordan. Measurements recorded each year included plant height, number of tillers per plant, oven-dry forage mass and seed yield. Forage and seed production increased in response to increasing sowing rate. The highest production values were obtained at a sowing rate of 45 kg ha−lIncreasing sowing rate to 60 kg ha−lreduced both forage and seed yield, indicating that the optimum seeding rate for wild oat is 45 kg ha−1Forage and seed yield had a cubic relationship with sowing rate, having R2values of 0.55 and 0.78, respectively. Tiller production showed a negative linear relationship with sowing rate (R2= 0.35).

ISSN:1022-0119    

DOI:10.2989/10220119909485723

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Small-plot cutting trials are of particular importance in research relating to intensive pastures. Undetected changes in soil fertility during the course of experimentation may detract from the validity of results in trials of this kind. Information from field trials conducted in KwaZulu-Natal during the past two decades are used to illustrate the potential impact of soil fertility aspects in small-plot cutting trials. A major potential pitfall is the underestimation of potassium (K) depletion accompanying the removal of herbage at harvesting. In an Italian ryegrass trial 518 kg K ha−1was removed in harvested material in a single season. Failure to compensate (by fertilizer topdressings) for removals of K may result in growth, particularly in the more favourable treatments, being inadvertently restricted. Soil acidification during the course of experimentation is a second factor which may threaten the usefulness of data from cutting trials. Acidification is promoted by the application of relatively high rates of N fertilizers; in this context, variable acidification where N fertilization is a factor in trials warrants particular attention. Removals, immobilization and leaching of phosphorus and sulphur as well as inherent soil heterogeneity on the trial site are additional important considerations in small-plot trials. The only effective means of dealing with these problems is through the testing of soil samples drawn from individual treatments at suitably regular intervals.

ISSN:1022-0119    

DOI:10.2989/10220119909485724

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The monthly herbage production, water-use efficiency (WUE: above-ground phytomass production per unit of evapotranspiration), surface runoff and soil loss were determined on grassland in three different ecological conditions, viz. poor, moderate and good, over a four year (1995/1996 to 1998/1999) period. In addition the same measurements were also made on an undisturbed bare soil surface and a soil cultivated twice per annum, only for the last four years. Herbage production and water-use efficiency decreased (P<0.01) when plant cover was reduced or grassland and soil condition declined. The effect of rainfall on productivity is enhanced as plant composition improves from poor through moderate to good. The highest seasonal WUE of 4.76 kg ha−1mm−1occurred in grassland in good condition. The dense plant cover of grassland in good condition provides a situation in which surface runoff and soil loss are lower (P<0.01) than that from grassland in poor condition. Direct evaporation from the bare uncultivated soil surface was on average 69% of the rainfall. These monthly production and water-use efficiency results are of the few, if not the only, data available for the semi-arid climates of southern Africa and can be used with great success in future fodder flow planning.

ISSN:1022-0119    

DOI:10.2989/10220119909485725

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

A non-destructive method is required for estimating the forage yield of small stands ofLeucaena leucocephala. Non-linear regression was used to investigate the relationship between the height of leucaena trees and wood and edible dry matter (edm) yield, and the relation between the diameter (at the point of browsing) of browsed leucaena twigs (DPB) and twig mass. Exponential regression (y = a + brx) of the logged biomass data provided the most accurate and precise description of the tree height-biomass and DPB-biomass relationships. More than 75% of the variation in wood and total tree yield was accounted for by the regressions on tree height, at an accuracy of within 8% of the true mean. The regression for edm on height was poor (r2= 0.57) and was not improved substantially by including the number of basal stems as a predictor. The amount of leaf and stem material removed by browsers can be precisely (r2> 0.90) and accurately (within 11% of the mean) predicted from DPB, thus providing a simple means of estimating browse offtake from leucaena trees.

ISSN:1022-0119    

DOI:10.2989/10220119909485726

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Double normal distributions can be used to resolve many sward height frequency distributions into two components representing the 'short' (patches) and 'tall' (non∼patches) areas in the sward. The effect of sample size on the precision and accuracy of parameters of sward height distributions was examined by drawing sub-samples (n=10) of increasing sample size (50 to 1 000) from simulated height data (n=10 000) from three different, typical height distributions, viz. normal (ungrazed), bimodal (leniently grazed) and positively skewed (intensely grazed). The coefficient of variation of components of all three distributions decreased sharply with increasing sample size and CVs for all means were <15% with 200 height measures, and <10% for all means, with the exception of the 'tall' mean in the bimodal distribution, at a sample size of 100. At a given sample size, proportions in the two components were less-precisely measured than the means, especially when the components are equally represented in the population (i.e. bimodal), where 500 measurements are required for a precision of 15%. Accuracy also increased with sample size, and with 400 samples, deviations were within 10% of the true values for most parameters of the three distributions. A sample size of 200 is recommended for quantifying the mean height of 'short' and 'tall' components of the sward whereas 400-500 samples are required to precisely estimate their relative proportions.

ISSN:1022-0119    

DOI:10.2989/10220119909485727

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

ISSN:1022-0119    

DOI:10.2989/10220119909485728

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor