Wood productivity of Prosopis flexuosa D.C. woodlands in the central Monte: Influence of population structure and tree-growth habit

A balance between forest production and protection is hard to achieve in arid zones due to their low potential for wood production. Prosopis flexuosa woodlands are the major woody formations in the Monte desert and are currently in a degraded state due to intense use. The main degradation factors in the study area are overgrazing and firewood extraction. We developed allometric models to estimate the aerial biomass of P. flexuosa, compared annual growth rates of one- and multi-stemmed individuals through dendrochronological methods, and estimated the productivity of four structurally different woodlands in the central Monte. Total dry weight was best estimated by power equations. Annual increments in basal area and dry weight were initially larger for multi- than one-stemmed individuals. However, whereas multi-stemmed individuals rapidly decreased their growth rates after 60 years of age, one-stemmed trees maintained steady growth rates during the first 100 years. Depending on woodland density and tree size, total woodland biomass varied between 4000 and 15 000 kg ha⁻¹. Wood productivity was similar in all four woodlands studied (121.6-173.7 kg ha⁻¹ year⁻¹). Our results reveal the importance of tree growth habit to productivity, and suggest that regulated extraction of firewood and poles from old multi-stemmed individuals could optimize wood productivity and contribute to the sustainable use and conservation of these woodlands.     

Carbon pools in an arid shrubland in Chile under natural and afforested conditions

The pattern of carbon (C) allocation among the different pools is an important ecosystem structural feature, which can be modified as a result of changes in environmental conditions that can occur gradually (e.g., climatic change) or abruptly (e.g., management practices). This study quantified the C pools of plant biomass, litter and soil in an arid shrubland in Chile, comparing the natural condition (moderately disturbed by grazing) vs. the afforested condition (two-year-old plantation with Acacia saligna (Labill.) H.L. Wendl.), each represented by a 60 ha plot. To estimate plant biomass, allometric functions were constructed for the four dominant woody species, based on the volume according to their shape, which showed high correlation (R² > 0.73). The soil was the largest C pool in both natural and afforested conditions (89% and 94%, respectively) and was significantly lower in the afforested than natural condition at all five soil depths. The natural condition had in total 36.5 ton (t) C ha⁻¹ compared to 21.1 t C ha⁻¹ in the afforested condition, mainly due to C loss during soil preparation, prior to plantation of A. saligna. These measurements serve as an important baseline to assess long-term effects of afforestation on ecosystem C pools.     

Dieback affects forest structure in a dry Afromontane forest in northern Ethiopia

Forests are highly susceptible to dieback under ongoing climate warming. In degraded forests, dead standing trees, or snags, have become such prominent features that they should be taken into account when setting management interventions. This study investigated (1) the extent and spatial pattern of standing dead stems of Juniperus procera and Olea europaea subsp. cuspidata along an elevational gradient, and (2) the effect of dieback on forest stand structure. We quantified abundance, size, and spatial pattern of tree dieback in 57 plots (50 m × 50 m) established at 100 m intervals along five transects. The snag density and basal area (mean ± SE) of the two species combined were 147 ± 23 stems ha⁻¹ and 5.35 ± 0.81 m² ha⁻¹, respectively. The percentages of snags were extremely high for both J. procera (57 ± 7%) and O. europaea subsp. cuspidata (60 ± 5%), but showed a decreasing trend with increasing elevation suggesting that restoration is even more urgent at the lower elevations. Snags of the two species accounted for 31 and 45% of total stand density and basal area, respectively. Living stems exhibited truncated inverse-J-shaped diameter and height class distributions, indicating serious regeneration problems of these foundation species in the study area. In addition to direct interventions to assist recruitment of climax tree species, sites with high dieback would probably benefit from snag reduction to prevent fire incidents in the remaining dry Afromontane forests in northern Ethiopia.     

Spatial precipitation and evapotranspiration in the typical steppe of Inner Mongolia,China – A model based approach using MODIS data

We present a simple method to derive spatial precipitation (P) and evapotranspiration (ET) for the typical steppe of the Xilin river catchment at 1 km and 8-day resolution during the main vegetation period (23 April to 28 August) of 2006. The hydrological model BROOK90 was parameterised from eddy covariance measurements. The daily model input data, precipitation, minimum (Ta_min) and maximum air temperature (Ta_max), were derived by manipulating MODIS leaf area index (LAI) and surface temperature data. P was estimated based on a linear regression of P measured at several sites against the mean gain of the MODIS LAI of surrounding 3 × 3 pixels areas (R² = 0.76). Ta_min and Ta_max were derived using a relationship between measured Ta_min and Ta_max and MODIS surface temperatures (R² = 0.92 and R² = 0.88, respectively). The mean precipitation was 145 mm; it varied between 52 mm in the north-western region and 239 mm in the eastern region. In spring, the modelled ET was low (<0.8 mm d⁻¹); evaporation dominated over transpiration and spatial differences were small. At the end of June, the mean ET reached its maximum (2 mm d⁻¹) and spatial differences were pronounced. From July on, transpiration dominated over declining evaporation, and spatial differences decreased in August.     

Mechanical shaking and soil water affect the growth of Psammochloa villosa in the Mu Us Sandland

The Mu Us Sandland is basically characterized by water shortage and high wind. Thus, wind-induced mechanical perturbation (MP) and soil water availability are likely to interact to affect plant growth. Since high water availability and MP can induce responses that are in the opposite direction, we hypothesized that MP effects on perennial grasses might be mitigated by increased soil water availability in the Mu Us Sandland. We counducted an experiment in which seedlings of Psammochloa villosa were subjected to two levels of MP (non-MP vs. MP 1 min d⁻¹) and two levels of water availability (200 ml d⁻¹vs. 400 ml d⁻¹) and measured three plant traits. MP significantly decreased plant height, total biomass, and root/shoot ratio. There were significant interactions between MP and soil water availability on plant height and root/shoot ratio. These findings imply that MP alone is a stressful factor for P. villosa and MP effects on its growth can be partially mitigated by increased soil water availability, and also suggest that P. villosa may respond to MP in a way that allows plants to survive in the windy semiarid environments.     

Nitrogen deposition effects on tissue chemistry and phosphatase activity in Cladonia foliacea (Huds.) Willd., a common terricolous lichen of semi-arid Mediterranean shrublands

In this article we evaluate the potential use of Cladonia foliacea tissue N content, C:N ratio, and phosphomonoesterase (PME) activity as biomarkers of N deposition by means of a field experiment. In order to do this, we continuously added NH₄NO₃ to a semi-arid shrubland at four rates: 0, 10, 20 and 50 kg N ha⁻¹ yr⁻¹ starting in October 2007. Tissue N content and C:N ratios, considered as N stress indicators, significantly increased and decreased, respectively, after 1.5 years. The response found suggests N saturation above 20 kg N ha⁻¹ yr⁻¹. After 2.5 years, extracellular PME activity increased with 20 kg N ha⁻¹ yr⁻¹ and this was attributed to an induced nutritional (N to P) imbalance. Above this threshold, PME significantly decreased as a consequence of the physiological stress caused by extra N. Effects on PME were dependent on the soil properties (pH and Ca and Mg availability) experienced by C. foliacea. PME response suggests a critical load of ∼26.4 kg N ha⁻¹ yr⁻¹ (20 kg N ha⁻¹ yr⁻¹ + background) for this lichen. Further tissue chemistry and PME evaluations in C. foliacea and soil surveys conducted along wide N deposition gradients will confirm the potential use of this species as a biomonitor of N pollution and the importance of soil properties on its ability to respond to atmospheric reactive N.     

Millet (Pennisetum typhoides) yield and selected soil attributes as influenced by some tree types of the semi-arid tropics of Sudan

Two experiments were conducted in southern Kordofan State to determine the influence of Acacia senegal L., Balanites aegyptiaca L. and Azadirachta indica L. on millet (Pennisetum typhoides) yield, soil quality and to monitor decomposition and nutrients release from tree litters. Yield under A. indica (174.83 kg ha⁻¹) and B. aegyptiaca (173.09 kg ha⁻¹) were significantly higher than the control (121.43 kg ha⁻¹). The lowest yield (111.04 kg ha⁻¹) was recorded under A. senegal. Straw dry matter under B. aegyptiaca (1161.5 kg ha⁻¹) and A. indica (857.8 kg ha⁻¹) was significantly higher than both under A. senegal (321.8 kg ha⁻¹) and the control (454.8 kg ha⁻¹). Trees varied in their capacity to induce changes in soil properties whereas effects on soil N were not substantial. A. indica had a decomposition rate (0.6283 week⁻¹) 2.0 times higher than that of B. aegyptiaca (0.2057 week⁻¹) and A. senegal (0.267 week⁻¹). The highest rate of P and K release from A. indica and B. aegyptiaca litters has resulted in significant accumulation in the soil indicating these tree litters are potential sources for these elements. The capacity of trees to improve soil fertility could offer an alternative management system for improved cultivation of field crops.     

Role of competition in restoring resource poor arid systems dominated by invasive grasses

Understanding the role competition intensity and importance play in directing vegetation dynamics is central to developing restoration strategies, especially in resource poor environments. We hypothesized 1) competition would be intense among invasive and native species, but 2) competition would be unimportant in explaining variation in target plant biomass and survivorship relative to other factors driving these variables. We performed a two year addition series field experiment to quantify competition intensity and importance. Densities of two invasive (cheatgrass and medusahead) and two native (Sandberg’s bluegrass and bluebunch wheatgrass) species were arranged in monocultures and mixtures of two, three and four species, producing varying total densities and species proportions. Multiple linear regression models predicting individual plant biomass and survivorship were developed. Based on biomass, competition intensity coefficients ranged from −0.38 to 0.63 with R² < 0.06. All survivorship data produced poor fitting regression models (R² < 0.05). Our results suggest neither competition intensity nor importance influenced plant dominance in resource poor environments during the two years of establishment. Land managers may be more successful at restoration of resource poor ecosystems by overcoming abiotic barriers to plant establishment rather than focusing on plant-plant interactions.     

Plant water relations of thornscrub shrub species, north-eastern Mexico

As an approach to understand how diurnal and seasonal plant water potentials (Ψ) are related to soil water-content and evaporative demand components, the responses of six thornscrub shrubs species (Havardia pallens, Acacia rigidula, Eysenhardtia texana, Diospyros texana, Randia rhagocarpa, and Bernardia myricaefolia) of the north-eastern region of Mexico to drought stress were investigated during the course of 1 year. All study species showed the typical diurnal pattern of variation in Ψ. That is, Ψ decreased gradually from predawn (Ψ_pd) maximal values to reach minima at midday (Ψ_md) and began to recover in the late afternoon. On a diurnal basis and with adequate soil water-content (>0.20 kg kg⁻¹), diurnal Ψ values differed among shrub species and were negatively and significantly (p<0.001) correlated with air temperature (r=−0.741 to −0.883) and vapor pressure deficit (r=−0.750 to −0.817); in contrast, a positive and significant (p<0.001) relationship was found to exist with relative humidity (r=0.758–0.842). On a seasonal basis, during the wettest period (soil water-content>0.20 kg kg⁻¹), higher Ψ_pd (−0.10 MPa) and Ψ_md (−1.16 MPa) values were observed in R. rhagocarpa, whereas lower figures (−0.26 and −2.73 MPa, respectively) were detected in A. rigidula. On the other hand, during the driest period (soil water-content<0.15 kg kg⁻¹), Ψ_pd and Ψ_md values were below −7.3 MPa; i.e. when shrubs species faced severe water deficit. Soil water-content at different soil layers, monthly mean relative humidity and monthly precipitation were significantly correlated with both Ψ_pd (r=0.538–0.953; p<0.01) and Ψ_md (r=0.431–0.906; p<0.05). Average soil water-content in the 0–50 cm soil depth profile explained between 70% and 87% of the variation in Ψ_pd. Results have shown that when gravimetric soil water-content values were above 0.15 kg kg⁻¹, Ψ_pd values were high and constant; below this threshold value, Ψ declined gradually. Among all shrub species, A. rigidula appeared to be the most drought tolerant of the six species since during dry periods it tends to sustain significantly higher Ψ_pd in relation to B. myricaefolia. The remaining species showed an intermediate pattern. It is concluded that the ability of shrub species to cope with drought stress depends on the pattern of water uptake and the extent to control water loss through the transpirational flux.     

Influence of understory removal, thinning and P fertilization on N2fixation in a mature mesquite (Prosopis glandulosavar.glandulosa) stand

The natural abundance¹⁵N/¹⁴N method was used to estimate the influence of silvicultural and P fertilization treatments on N accretion, N₂fixation and N partitioning among tissues in a mature mesquiteProsopis glandulosavar.glandulosastand in Texas. The silvicultural treatments consisted of understory removal, herbicide treatment of brushy resprouts, thinning trees to single stems and 100 kg ha⁻¹P fertilization. The trees had a mean basal diameter of 17·8 cm with 8 to 35 cm range. The stand was slow growing with the increase in dry matter ranging from 0·465 Mg ha⁻¹year⁻¹to 0·701 Mg ha⁻¹year⁻¹for the 8 years after the treatments were applied. N accretion after 8 years ranged from 3·1 kg ha⁻¹year⁻¹to 4·4 kg ha⁻¹year⁻¹.Due to the range in δ¹⁵N of the leaves, twigs, branches and trunk, we used the weighted (by biomass) average δ¹⁵N per tree in calculations of the percent N derived from N₂fixation (%Ndfa). There was considerable variability in δ¹⁵N of the reference plants, i.e. from 3·3 to 5·9. In contrast there was low variability in the background δ¹⁵N of nearby soils (7·0±1·0). As the total above-ground biomass δ¹⁵N of a grass grown outside the influence of mesquite (7·8±0·58) had the same δ¹⁵N as the soil (7·5±1·0), we used the grass outside the influence of mesquite and the weighted tree mean δ¹⁵N to calculate % of N derived from N₂fixation.The decrease in intraspecific competition by thinning multistemed trees to single stemmed trees was the only treatment that significantly (p= 0·0001) increased growth. Interspecific competition, i.e. understory removal, did not increase growth. There were no significant differences in total N production or N fixation among treatment means. The most striking result was the highly positive correlation between tree δ¹⁵N and total N per tree and biomass per tree (R²= 0·90,F= 164·4, df. = 18, mean square error (MSE) = 0·155,p= 0·0001). This implies that the younger trees colonizing infertile soils relied more heavily on N₂fixation than larger trees which accumulated 1200 kg ha⁻¹more N under their canopies. The percentage N derived from N₂fixation ranged from 63 to 73% in the various treatments. Despite the high percentage of N derived from N₂fixation, the N₂fixation of the stand was very low, i.e. 1·98 to 2·80 kg N ha⁻¹year⁻¹, due to the low growth of the stand. We believe that comparisons of the whole tree weighted δ¹⁵N to background soil δ¹⁵N provides a more reasonable approach to estimate % N₂fixation than comparisons of leaves of fixers and reference plants.     

Wood biomass functions for Acacia abyssinica trees and shrubs and implications for provision of ecosystem services in a community managed exclosure in Tigray,Ethiopia

In the Ethiopian highlands, remarkable recovery of vegetation has been achieved using exclosures, protecting vegetation against livestock browsing and firewood harvesting. But these emerging forest resources require tools for sustainable use, implying knowledge on biomass stocks and growth. In this study we developed biomass functions estimating total, stem and branch biomass from diameter at stump height (DSH) and tree height (H) for an 11-year old exclosure in Tigray, Ethiopia. In a systematic grid of 55 plots, DSH and H of all trees and shrubs were recorded. 40 Acacia abyssinica trees were selected for destructive sampling. Allometric relationships using a natural log–log model were established between aboveground biomass, DSH and H. Models with only DSH were found best with R² between 0.95 and 0.98. The functions were 10 fold cross-validated and R²_{_}cv ranged from 0.94 to 0.97, indicating good model performance. The models were found well in range with those of other seasonal forests in East Africa. Total aboveground biomass was estimated 25.4 ton ha⁻¹ with an annual production of 2.3 ton ha⁻¹, allowing sustainable wood fuel use for 4 persons ha⁻¹. The presented predictive functions help to harmonize between ecological and societal objectives and are as such a first step towards an integrated planning tool for exclosures.     

Effect of rhizobia inoculation on the seed (herbage) production of mungbean (Phaseolus aureusRoxb.) grown at Uzbekistan

In field trials conducted at Uzbekistan, inoculation of mungbean (Phaseolus aureusRoxb.) with commercial strain CIAM1901 ofBradyrhizobiumsp. (Phaseolus) increased (on average for two cultivars) the herbage mass by 46·6±6·0%, seed mass by 39·2 ± 3·6%, mass of 1000 seeds by 16·0 ± 0·8%, nitrogen content in seeds by 58·3 ± 8·9%, starch content in seeds by 30·0 ± 5·5% and number of nodules by 254%. Inoculation with this strain produced the same herbage (seed) mass as NH₄NO₃application (120 kg ha⁻¹of N), while the combined treatment of rhizobia and 60 kg ha⁻¹of N produced significantly higher yields than rhizobia inoculation alone or application of 120 kg ha⁻¹of N. The rhizobia strain M11 was isolated from Uzbekistan soils and significantly exceeded the commercial strain CIAM1901 in its influence on herbage mass by 3·9–10·6%. Two-factor analysis of variance demonstrated that the herbage mass and number of nodules are controlled mainly by the rhizobia strain genotypes, while seed yield, number of pods, mass of 1000 seeds and N and starch content in seeds are influenced by the plant cultivar genotypes.     

Allometric relationships and peak-season community biomass stocks of native shrubs in Senegal's Peanut Basin

Allometric equations and community biomass stocks are presented for Guiera senegalensis J.F. Gmel (Gs) and Piliostigma reticulatum (DC.) Hochst (Pr) – two native shrub species in the Sahel. These shrubs are of interest because they dominate semi-arid sub-Sahalien Africa but have been largely overlooked as a key biomass component and regulator of ecosystem composition and function in this landscape. In Year 1, best predictors of aboveground biomass were height and number of stems (Gs) and crown diameter (Pr); and for belowground biomass were height and basal diameter (Gs) and basal diameter (Pr). In Year 2, height and crown diameter were the best predictors of aboveground biomass (R² = 0.90 for Gs and 0.87 for Pr), whereas basal diameter and number of stems (Gs) and basal diameter (Pr) were best predictors of belowground biomass. Peak-season biomass estimates ranged from 0.44 to 4.58 ton ha^?1 (mean = 2.38 ton ha^?1) in the Gs sites and from 0.33 to 7.38 ton ha^?1 (mean = 3.71 ton ha^?1) in the Pr communities. Both species exhibited unusually large root:shoot ratios (4.5:1 for Gs and 10.2:1 for Pr). Although models differ between years, allometric relationships provide reasonable biomass estimates for Gs and Pr.     

The contribution of two common shrub species to aboveground and belowground carbon stock in Iberian dehesas

Shrubs play an important role in water-limited agro-silvo-pastoral systems by providing shelter and forage for livestock, for erosion control, to maintain biodiversity, diversifying the landscape, and above all, facilitating the regeneration of trees. Furthermore, the carbon sink capacity of shrubs could also help to mitigate the effects of climate change since they constitute a high proportion of total plant biomass. The contribution of two common extensive native shrub species (Cistus ladanifer L. and Retama sphaerocarpa (L.) Boiss.) to the carbon pool of Iberian dehesas (Mediterranean agro-silvo-pastoral systems) is analyzed through biomass models developed at both individual (biovolume depending) and community level (height and cover depending).The total amount of carbon stored in these shrubs, including above- and belowground biomass, ranges from 1.8 to 11.2 Mg C ha⁻¹ (mean 6.8 Mg C ha⁻¹) for communities of C. ladanifer and from 2.6 to 8.6 Mg C ha⁻¹ (mean 4.5 Mg C ha⁻¹) for R. sphaerocarpa. These quantities account for over 20–30% of the total plant biomass in the system. The potential for carbon sequestration of these shrubs in the studied system ranges 0.10–1.32 Mg C ha⁻¹ year⁻¹ and 0.25–1.25 Mg C ha⁻¹ year⁻¹ for the C. ladanifer and R. sphaerocarpa communities' respectively.     

Plant productivity, predation, and the abundance of black-tailed jackrabbits in the Chihuahuan Desert of Mexico

The abundance of black-tailed jackrabbits (Lepus californicus) can fluctuate dramatically. We used data from the Chihuahuan Desert to test the relative strength of top-down (predation) or bottom-up (food availability) limiting forces. Predictions for the top-down hypothesis were, 1) a positive relationship between coyote (Canis latrans) and jackrabbit abundance (numerical response) and 2) a positive relationship between percent occurrence of jackrabbits in coyote scats and jackrabbit abundance (functional response). Predictions for the bottom-up hypothesis were, 1) plant productivity is directly related to precipitation, 2) jackrabbit abundance is positively related to precipitation and plant productivity, and 3) changes in abundance of jackrabbits over the reproductive season will be directly related to precipitation and plant productivity. We found a limited numerical response but no functional response of coyotes to jackrabbit abundance. Forb productivity was significantly related to annual precipitation levels (r² = 0.69, p = 0.002). Grass productivity was related to annual precipitation (r² = 0.34, p = 0.028). Jackrabbit abundance (r² = 0.38, p = 0.002) and changes in abundance (r² = 0.73, p < 0.001) were significantly related to precipitation and forb and grass productivity. We conclude that precipitation levels and plant productivity affect jackrabbit abundance more than predation levels.     

Field growth comparisons of invasive alien annual and native perennial grasses in monocultures

Throughout the western United States, the invasive annual grass, medusahead (Taeniatherum caput-medusae L. Nevski), is rapidly invading grasslands once dominated by native perennial grasses, such as bluebunch wheatgrass (Pseudoroegneria spicata (Pursh) A). It is also invading grasslands dominated by less undesirable invasive annual grasses, especially cheatgrass (Bromus tectorum L.). Understanding medusahead growth dynamics relative to native perennial grasses and cheatgrass is central to predicting and managing medusahead invasion. We hypothesized that medusahead would have a higher relative growth rate (RGR), a longer period of growth, and as a consequence, more total biomass at the end of the growing season than the native perennial grass and cheatgrass. In 2008 (dry conditions), 250 seeds and in 2009 (wet conditions), 250 and 100 seeds of each species were sown in 1 m² plots with 5 replicates. Shoots were harvested on 3-25 day intervals throughout the growing season. The native perennial grass had more biomass and higher RGR than medusahead in the dry year, but the relationship was reversed in the wet year. Precipitation in 2008 was well-below average and this level of drought is very infrequent based on historical weather data. Medusahead had a longer period of growth and more total biomass than cheatgrass for both years. We expect that medusahead will continue to invade both native perennial and less undesirable invasive annual grasslands because of its higher RGR and extended period of growth.     

Soil hydrological and erosional responses in areas of woody encroachment, pasture and woodland in semi-arid Australia

In arid and semi-arid areas, woody encroachment is the increase in density, cover, extent and/or biomass of woody plants. Woody encroachment is often associated with increased runoff and soil erosion. Hydrological and erosional responses of woody encroachment and of pastures established after management of encroachment in semi-arid Australia are not well understood. This study compared the hydrological and erosional responses across vegetation states comprising woody plant encroachment (>1200 stems ha⁻¹), recently established pastures (<23 years of age), long-established pasture (50-100 years of age) and open woodland (<330 stems ha⁻¹) in semi-arid eastern Australia. Responses were measured using rainfall simulation with intensity of 35 mm h⁻¹ for 30 min applied on 1 -m² plots. Runoff and sediment production did not differ significantly between vegetation states. Average runoff in woody encroachment was 9.0 mm h⁻¹, followed by recent pasture (8.2 mm h⁻¹), long-established pasture (5.9 mm h⁻¹) and open woodland (4.2 mm h⁻¹). Total sediment production in recent pasture was 11.6 g m⁻², followed by woody encroachment (9.0 g m⁻²), long-established pasture (7.3 g m⁻²) and open woodland (4.3 g m⁻²). Runoff and sediment production were significantly lower at one pasture site (0.9 mm h⁻¹ and 1.3 g m⁻²) where rotational grazing and minimum tillage had been implemented than in the adjacent paired woody encroachment site (10.3 mm h⁻¹and 6.5 g m⁻², respectively). This example of a pasture that had been managed to increase ground cover illustrated the effect of pasture management on reducing runoff and sediment production. Across all vegetation states, small scale runoff and sediment production were minimal or zero where total ground cover was 73% or higher.     

Comparative study of seed germination and growth of Kochia prostrata and Kochia scoparia (Chenopodiaceae) under salinity

Soils of arid regions of Central Asia contain salts of different types that may differentially affect seed germination and plant development. We studied effect of NaCl, Na₂SO₄, 2NaCl + KCl + CaCl₂ and 2Na₂SO₄+K₂SO₄+MgSO₄ on germination of Kochia prostrata and Kochia scoparia seeds under a range of concentrations from 0.5 to 5% and at two constant temperature regimes +22 °C and +6 °C. The observed salt tolerance limit of germination at constant temperature +22 °C for both species was 5-6%, while at low temperature (+6 °C) this limit was 2%. The salt tolerance of young plants (before flowering) was 3% for NaCl. Low concentrations of sulfuric and mixed salts had a stimulating effect on seed germination in K. prostrata. Despite similarity of salt-tolerance limits the studied species showed a significant difference in seed recovery ability, i.e. the ability of ungerminated, salt-soaked seeds to germinate after transfer to fresh water. K. scoparia demonstrated a full germination recovery after seed transfer to distilled water while K. prostrata showed only a partial recovery.     

Physical impact of grazing by sheep on soil parameters in the Nama Karoo subshrub/grass rangeland of South Africa

The direct short-term impact of three rates of stocking (4, 8 and 16 small-stock units [SSU] ha^?1) was quantified in terms of soil characteristics of arid Nama Karoo vegetation (subshrub/grass). Mature Merino wethers grazed in the experimental plots throughout May in 1995 and 1996 (the plots were not subjected to grazing at any other time). Stocking rate proved inversely related to initial infiltration rate. Light trampling (4 SSU ha^?1) loosened the topsoil sufficiently to increase the initial infiltration rate: infiltration capacity of soil in fields stocked at 4 SSU ha^?1 and 16 SSU ha^?1 was 17% higher and 14% lower respectively than that of soil of ungrazed rangeland over the two grazing periods. Increased soil compaction and greater bulk density due to higher stocking rates significant decreased the infiltration rate. Compared to ungrazed rangeland stocking rates of 4, 8 and 16 SSU ha^?1 over the two grazing periods increased bulk density respectively by 2.73%, 6.67% and 8.945% and compaction by 10.90%, 16.78% and 20.90%. No grazing also increased bulk density and soil compaction and decreased infiltration rate. Light stocking (4 SSU ha^?1) influenced all soil parameters most favourably. From a hydrologic point of view, grazing levels and rotation schemes need to be tailored for sustainable utilization of arid subshrub/grass vegetation by livestock.