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- W2100697381 abstract "Understanding the drivers and mechanisms of changes in plant richness is a basis for making scientifically sound ecological predictions and land use decisions. Of the numerous factors affecting plant richness, soil has a particularly large influence on the composition and structure of terrestrial flora. Infiltrability is one of the most important factors determining soil moisture, and therefore is of particular interest in semi-arid ecosystems, where water is one of the most limiting resources. Other soil properties, such as clay + silt content, electrical conductivity (EC) and pH may also influence plants. Heterogeneity of these properties creates niches with specific conditions, which in turn affects spatial distribution of plants. An understanding of the relationships between plant richness and soil properties is, however, incomplete. The present study has two main foci. Firstly, relationships between plant richness and soil infiltrability, clay + silt, EC and pH (H2O) were investigated, and secondly, due to the strong influence of infiltrability on plant richness, further investigations were undertaken to improve the understanding of the role of particle size fractions, EC of the soil solution and exchangeable sodium percentage (ESP) on infiltrability. This study only concentrated on the surface 2 cm thick soil layer (known as pedoderm). The study was conducted at a large-scale and was based at 31 study observatories located along a transect stretching from the western seaboard of South Africa to Namibia, and encompassing four biomes, namely: Succulent Karoo, Nama Karoo, Savanna and Woodland. Plant species data for each plot were obtained from the BIOTA South database and categorized into 5 life form categories using Raunkiaer’s (1934) classification system: phanerophytes (trees), chamaephytes (shrubs), hemicryptophytes (grasses), therophytes (annuals) and geophytes. A total of 313 soil samples were analysed for infiltrability, particle size distribution, EC and pH. In order to investigate the effect of soil texture on infiltrability, small intervals of water-dispersible soil fractions were determined. A laser technique was used for particle size determination, which allowed for the determination of smaller particle size fractions than is possible with conventional laboratory techniques. To investigate the effect of dispersion, flocculation, EC and ESP infiltrability was measured using four different infiltration solutions: namely, distilled water; gypsum solution; 1:5 soil suspension in gypsum solution, and 1:5 soil suspension in water. The infiltrability of samples with different particle 1 Abbreviation for: “Biodiversity Monitoring Transect Analysis in Africa”. size distributions and ESP values were compared. A relational envelope approach was used for the data interpretation. The derived envelopes showed ranges along soil property gradients, where plant richness was potentially maximal or predictably restricted. A segmented quantile regression was used to delineate boundary lines representing 0.95 and 0.1 quantiles. These boundary lines circumscribed envelopes in which 85 % of observations occurred. The results of this study revealed that soil infiltrability, water-dispersible clay + silt, EC and pH appeared to influence richness of life forms. Patterns for potentially maximal richness along soil properties gradients differed between life forms. Phanerophytes and hemicryptophytes had potentially maximal richness at high infiltrability, low clay + silt and low EC. By contrast, richness of chamaephytes and geophytes was potentially maximal at low infiltrability values, high clay + silt and high EC. Richness of therophytes showed a humpshaped response to infiltrability and clay + silt with potentially maximal richness at intermediate values. Richness of all life forms was restricted at pH > 9. The observed relationships may be attributed to the effect of an individual soil factor, as well as to the complex effect of a number of soil factors, as they tend to be correlated with one another. This correlation makes it difficult to distinguish which soil factor plays the controlling role. In addition, numerous other factors such as the interaction between species, plant architecture or climate (which were not investigated in the present study) may affect plant richness. Therefore, causality cannot be demonstrated from the relational envelopes, but they do provide an enhanced understanding of ecological processes. Dispersion of soil particles resulting in crust formation on the soil surface was found to be a dominant mechanism reducing infiltrability. Water-dispersible clay + silt showed better correlation with infiltrability than total clay + silt. In terms of soil fractions, soil clay, fine silt, coarse silt, very fine sand and fine sand fractions ( 2 % content. The fraction in the 120-200 μm range showed no clear relationship with infiltrability, in that it could play either a plasmic or skeletal role, depending on its ratio to the 200 μm fraction. Fine, medium and coarse sand fractions (> 200 μm) were found to play a skeletal role i.e., forming pores that promoted infiltrability. At levels above 50 % of these fractions, infiltrability was potentially maximal. This potentially maximal infiltrability was also explained by the concomitant decrease in plasmic fraction content with an increase of the skeletal fraction. Soil texture was found to play a primary role in crust formation with EC and ESP being of secondary importance. In the silty loam group, with clay + silt content above 70 %, infiltrability was restricted to the point where EC and ESP did not play a significant role. In the sand and loamy sand groups with a clay + silt content below 18 %, however, EC and ESP played a significant role. In the sand group, soils with high ESP had lower infiltrability than soils with low ESP. An application of gypsum resulted in an increase in infiltrability. This increase probably related to an increase in EC of the soil solution and a concentration of exchangeable Ca which negated the dispersing effect of high ESP. The effect of gypsum was apparent only in a treatment where crust formation took place (i.e., in treatment with soil suspension), which suggests that the ameliorating effect of gypsum is likely to take place only in soils which have dispersed or are in the process of dispersing in the field. The present study enhanced the understanding of the relationships between richness of life forms of plants and soil properties, as well as the effect of soil particle size, EC and ESP on soil infiltrability. Improving this understanding is of critical importance for planning the sustainable management of semi-arid ecosystems." @default.
- W2100697381 created "2016-06-24" @default.
- W2100697381 creator A5007140008 @default.
- W2100697381 date "2007-11-22" @default.
- W2100697381 modified "2023-09-27" @default.
- W2100697381 title "Soil chemical and physical properties and their influence on the plant species richness of arid South-West Africa" @default.
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