Determining spatial and temporal variability in quantity and quality of vegetation for estimating the predictable sustainable stocking rate in the semi-arid savanna

This study assessed the importance of spatial and temporal variation in plant quality and quantity for determining sustainable stocking rates in game, commercial and communal ranches in semi-arid savanna of the Northern Cape Province, South Africa, in wet and dry seasons over a two-year period. We focussed on variation in plant biomass, phosphorus (P), crude protein (CP) concentrations and dry matter digestibility as parameters most likely to affect sustainable stocking rates. Habitat type had greater effects on plant quality, plant biomass and species composition than management type. The commercially-managed area had the highest tree density in the rocky habitat and lower plant quality than other management types. All of these features indicate that land degradation is occurring on commercial ranches in spite of rotational grazing and lower stocking density than on communal ranches. We recommend that commercial ranchers should introduce a greater variety of stock and/or game to reduce selective grazing of certain plant species. Quality measures (CP and P) gave more conservative predictions of stocking density than biomass. In this region of the Northern Cape, seasonally-inundated pan habitats are particularly valuable in spite of low-standing crop because they have the highest year-round quality. Contrastingly, ranchers should only lightly stock open savanna habitats, in spite of high standing biomass, because they have low vegetation quality and may be particularly susceptible to degradation and invasion by poisonous and unpalatable plants.     

Heritability of shoot die-back and root biomass in sixteen Pterocarpus angolensis (Fabaceae) half-sib families from Malawi,Namibia and Zambia

The experiment was conducted to ascertain narrow-sense heritability (h ²) and the family-within-provenance variation in shoot die-back, taproot length and root biomass of half-sib Pterocarpus angolensis families from Malawi, Namibia and Zambia. Provenances differed significantly in shoot die-back for both shoot die-back seasons. A within provenance family effect was not significant in the first shoot die-back season but was significant in the second shoot die-back season. An increase in proportion of seedlings dying back, to 91%, for a Malawi family was observed in the second shoot die-back season. Provenance and within provenance family effects may be due to latitudinal differences between the provenances. The h ² of shoot die-back was 0.07 (SE = 0.07) in the first shoot die-back season and 0.42 (SE = 0.27) in the second shoot die-back season. Narrow-sense heritability for root biomass was 0.81 (SE = 0.45). The h ² for the second die-back season and root biomass suggest the traits are heritable. Random selection of individual families for assessing shoot die-back is important since this trait is only restricted to seedlings and saplings and not mature trees. Within provenance family effects for root biomass were significant, indicating differences between families. Two families from the most northerly provenances of Phalombe and Skull Rock in Malawi were significantly different from other southern families. Non-significant phenotypic correlation between shoot die-back and root biomass shows that shoot die-back is not likely to be determined by root size nor taproot depth. Although a non-significant negative phenotypic correlation (–0.15) between the second shoot die-back season and root biomass was obtained, a family from Malawi that had the highest mean shoot die-back in the second shoot die-back season had the smallest root biomass. Half-sib families with post shoot die-back growth potential, in the first shoot die-back season, are likely to have a better chance of post shoot die-back growth later.     

Recovery of photosynthetic capacity in Scots pine: a model analysis of forest plots with contrasting soil temperature

Both aboveground and belowground climate affects net primary production (NNP) and forest growth. Little is known about how above and belowground factors interact. The BIOMASS-model was tested to simulate photosynthetic recovery over a wide range of soil temperatures created by snow cover manipulations on tree-scale plots in a 20-year-old Scots pine stand in northern Sweden. The differences in timing of soil warming between the plots covered a span of two months. Carbon assimilation in needles, sap flow, needle water potential and climatic parameters were measured in the field. The simulations revealed that an early start of soil warming gave a relatively early photosynthetic recovery and a 7.5% increase of NPP. Late soil warming delayed the photosynthetic recovery and reduced the NPP by 13.7%. This indicated that soil temperature needed to be accounted for, as well as air temperature, when analysing photosynthetic recovery and NPP in boreal environment. The effects of differences in soil temperature were reflected in the simulated photosynthetic recovery. The model did not fully capture the delay of photosynthetic recovery caused by a late soil warming. It was possible to integrate the complexity of the soil climate effects into a threshold date for soil thaw, using sapflow measurements together with information about air temperature and a day degree sum, as long as water availability was not limiting water uptake by roots. Although a more realistic mechanism than that currently in BIOMASS is desirable as climate change shifts the typical patterns of interplay between air and soil temperature dynamics.     

Nitrogen and phosphorus concentration in faeces: an indicator of range quality as a practical adjunct to existing range evaluation methods

Vegetation monitoring, currently complemented by climatic data in particular rainfall areas, does not always supply sufficient information to provide reliable guidelines for appropriate stocking densities on ranch or reserve scale. This paper considers how the nutrient status of animals may be used to improve the setting of management guidelines by reflecting how wild herbivores perform under differing environmental and management conditions. Faecal nitrogen concentrations of between 13 and 16 g kg^?1 are above a threshold level that indicates a dietary deficiency which may precipitate nutritional stress in animals (such as happened during the 1991/1992 drought). Faecal phosphorous concentrations of between 1.9 and 2.0 g kg^?1 over a long period of time would indicate a deficiency that may lead to low reproductive rates. Faecal analysis could therefore be used to assess whether applied stocking densities will allow grazers, and probably mixed feeders such as impala too, to select for a diet of sufficient quality for maintenance.     

Abiotic and herbaceous vegetational characteristics of an arid rangeland in Kenya

A two-year study was conducted with an overall objective of characterising the structure and function of an arid rangeland in Kenya. A plot measuring 100 × 100m was used for this study. Data on rainfall and temperature were recorded at the plot site whereas data on evaporation rates and relative humidity were obtained from the meteorological office near the study site.

Herbaceous aboveground material was sampled at monthly intervals using a rectangular 0.25m² quadrat frame. Clipped material was separated by species and classified dead or live by physical examination. A 5cm diameter metal soil corer was used to sample belowground plant material, at monthly intervals. The sampled material was washed with running water over 2mm sieves and classified dead or live using the vital staining technique. All weights and calculations were based on organic weight.

Total aboveground standing crop ranged from 84.6g m^?2 to 295.4g m^?2, with a mean of 162.3 ± 60.6g m^?2. Mean monthly aboveground standing crop for 1992 and 1993 was 142.8 ± 53.8 and 178.5 ± 63.3g m^?2 respectively. The two values were significantly different (p<0.10). Aboveground biomass yield ranged from 17.7g m^?2 to 242.7g m^?2, with a mean of 104.3 ± 58g m^?2 and a coefficient of variation of 58%. Mean aboveground standing crop was 59 ± 24g m^?2 Monthly values ranged from 28.8g m^?2 to 120g m^?2, with a 38% coefficient of variation.

The range for total belowground standing crop was from 83.3g m^?2 to 232.7g m^?2, and a mean of 155.2 ± 46g m^?2. The values had a coefficient of variation of 30%. Mean total monthly belowground plant material yield for 1992 and 1993 was 137.6 ± 41g m^?2 and 169.9 ± 46g m^?2 The coefficients of variation were 59% and 28% respectively. The mean monthly belowground biomass yield was 51.6 ± 33g m^?2 with a coefficient of variation of 64%. Mean monthly yield for belowground dead material was 103.7 ± 32g m^?2, with a coefficient of variation of 31%. There was no significant difference (p>0.01) in the mean belowground dead material yield between 1992 and 1993.

In 1992, annual NPP was 439.2g m^?2, giving a net primary productivity of 1.22g m^?2day^?l. Monthly NPP ranged from 17.2g m^?2 to 90.1g m^?2 In 1993, annual NPP was 944.5g m^?2, equivalent to a net primary productivity of 2.62g m^?2 day^?1. Monthly NPP was between 27.4g m^?2 and 548.6g m^?2. Over the 1992–1993 period, NPP was 1 383.7g m^?2, equivalent to a productivity of 1.92g m^?2day^?1. Trends in monthly NPP closely followed the trend in rainfall. On the whole, herbaceous vegetation production and productivity were episodic in nature and closely linked to rainfall. The high primary productivity puts arid and semi-arid rangelands under sharp focus as CO₂ sinks, whose role in the amelioration of greenhouse effect could be more important than is currently appreciated.     

四川盆地慈竹生物量模型及其在丰产培育中的应用

本文采用龄级平均竹法研究了四川盆地慈竹生物量及其预测模型在各栽培区的应用。通过多元统计分析探讨了竹材产量和质量与林分结构及立地环境的关系;同时,提出了慈竹丰产培育的技术指标及林学措施。     

Effect of Balanites glabra canopy cover on grass production,organic matter and soil moisture in a southern Kenyan rangeland

A study was undertaken in Kenya's southern savanna rangelands to determine the seasonal effect of Balanites glabra canopy cover on aboveground grass biomass, grass species composition, soil organic matter and soil moisture content. The study was conducted during the period June to December 1999 in order to capture both the dry and wet season effects. The grass biomass in the sub-canopy zone (2-4m from tree trunk) was found to be significantly higher than in the mid-canopy (0-2m from tree trunk) and open grassland zones (4-6m from tree trunk) during the dry season. However, the difference between the sub-canopy and the open grassland was not significant during the wet season, implying that the role of a tree canopy in enhancing grass biomass is greater during the dry than the wet season. Variations in percent grass species composition from the mid-canopy to the adjacent open grassland were observed, indicating that while B. glabra canopy cover favours certain grass species, other species find the micro-climatic conditions under the canopy unfavourable. Soil organic matter in the mid-canopy zone was significantly higher than in the sub-canopy and adjacent open zones during both dry and wet season. Although the sub-canopy zone exhibited significantly higher soil moisture content than the mid-canopy and open grassland zones during the dry season, the difference between the sub-canopy and the adjacent open grassland during the wet season was not significant, suggesting that the tree canopy influence on soil moisture is more pronounced in the dry than the wet season.