Mound development as an interaction of individual plants with soil,water erosion and sedimentation processes on slopes

In the Mediterranean region, semi‐natural shrubland communities (named ‘matorral’) often present a discontinuous cover, where isolated perennial plants alternate with bare inter‐plant areas. In such ecosystems, the patchy distribution of the vegetation is usually associated with microtopographic sequences of mounds that develop under isolated plants and break the overall slope continuity. In this study, the influence of three representative species of the Mediterranean matorral (Rosmarinus officinalis, Stipa tenacissima and Anthyllis cytisoides) on slope microtopography is determined and the processes that take part in the development of microtopographic structures beneath the plant canopy are identified. The influence of slope gradient, plant species and plant parameters on the shape and height of microtopographic structures is also studied. The shape of the microtopographic structures is described by using a two‐dimensional microprofilemeter and mound height is determined by measuring in the field a ‘mound height index’ defined as the distance from the top to the bottom of the mound. The results obtained show that plant species play a major role in the shape and height of the microtopographic structures. Whereas terrace‐type structures generally develop under Anthyllis shrubs, microtopographic forms associated with Rosmarinus and Stipa plants vary with slope gradient. The almost symmetric mound‐type structures that develop under these two species on gentle slopes change into terrace‐type structures as slope gradient increases. Moreover, statistically significant differences exist between the three species with regard to mound height. Mean values of mound height are 19·4, 14·6 and 4·3 cm under the canopy of Stipa, Rosmarinus and Anthyllis respectively. Plant parameters, essentially roughness, and slope gradient have a significant influence on mound height index. Four main processes were identified as affecting mound development in the studied field site: sedimentation, differential interrill erosion, differential splash erosion and bioturbation. Plant species interact in different ways with these processes according to their morphologies. Since Stipa and Rosmarinus plants are more efficient than Anthyllis shrubs in controlling water erosion, in retaining sediments and in modifying soil properties under their respective canopies, they give rise to higher microtopographic structures that facilitate water and nutrient storage by plants on slopes. Copyright © 2000 John Wiley & Sons, Ltd     

Comparison of soil bacterial communities associated with actinorhizal, non-actinorhizal plants and the interspaces in the sclerophyllous matorral from Central Chile in two different seasons

The spatial heterogeneity of resources in desert and semi-arid shrubland appears to be important in determining higher soil bacteria abundance around plants than in soil without plant cover. Thus, these bacterial communities could be important contributors to nutrient cycling in arid ecosystems. Bacterial diversity from Chilean sclerophyllous matorral was determined by Terminal Restriction Fragment Length Polymorphism (T-RFLP). Soil samples associated with the actinorhizal plant Colletia hystrix, non-actinorhizal plants and interspace soil without plant cover, were collected in May and October. The non-actinorhizal and interspace soil differed significantly in their potassium content in May and pH in October. The T-RFLP analysis revealed differences in the bacterial community structure from the different habitats. The soil bacterial communities associated with plants were the most similar, whereas the interspace soil community differed in both sampling times. The factors that best explained the groupings were potassium and pH. The greatest diversity was observed in the interspace soil. The Microbial Community Analysis showed a significant proportion of T-RFs identified as Firmicutes and Proteobacteria. Likewise, spatial and temporal differences were observed in the main groups' abundance. The dominance of Firmicutes suggests that the sclerophyllous matorral could be a different ecosystem to other arid and semi-arid soils with respect to the bacterial community structure.     

Germination in tamaulipan thornscrub of north-eastern Mexico

In semi-arid environments plants should be selected to germinate at times most suitable for seedling establishment. Germinability, rate of germination and temperature preference for germination (12 or 28°C constant) were analysed with respect to growth form (grass, forb, woody), longevity and seed size. Short-lived species showed preferential germination at either high or low temperature, whereas long-lived species tended to be indifferent to season. Germinability was >20% for 28 spp., 17 species were <10% and 21 species did not germinate under any circumstances. Growth form or life-span did not influence germination. Woody species germinated rapidly. Germinability and germination rate were positively associated.     

WATER MOVEMENT AND VEGETATION PATTERNS ON SHRUBLAND AND AN ABANDONED FIELD IN TWO DESERTIFICATION-THREATENED AREAS IN SPAIN

Four large-scale rainfall simulation experiments were conducted in Spain to improve the understanding of the relationship between soil water dynamics, soil erosion and land degradation. On different shrublands and an abandoned field, hydrological characteristics were determined in relation to spatial patterns in soil, vegetation and morphology. During the experiments on shrubland, runoff at fine scales occurred shortly after the start of the experiments. Rapid and non-uniform infiltration near vegetation clusters, related to preferential flowpaths of water, was observed. This prevented the development of runoff over distances larger than 1 metre. The surface redistribution of water was not observed on the abandoned land. Here, little vegetation structure was present and infiltration rates were high below crusts as well as stones. We suggest that the development of spatial structures in vegetation and soil forms a positive feedback with non-uniform infiltration and increased soil water retention. The assessment of land degradation could benefit greatly from acknowledging the importance of non-uniformity in hydrological processes. Furthermore, the presented measurements indicate that in discontinuous environments runoff measurements at fine scales cannot be extrapolated directly. In these environments a scaled approach needs to be adopted emphasizing the importance of different hydrological processes at different scales.     

Germination associated with season and sunlight for Tamaulipan thornscrub plants in north-eastern Mexico

Seed germination was investigated for 47 herbaceous and woody species, representative of the Tamaulipan thornscrub flora of north-eastern Mexico. More than half of the species had similar germination under spring and autumn conditions, 17 species showed a higher germination percentage under autumn rain conditions. No species had more seeds germinating under spring rain conditions. Germination season response was independent of plant habit, seed size and dispersal syndrome. Sixteen of the species germinated less under the moderate shade of thornscrub canopy than under direct sunlight, and more than half of the species had similar germination under shade and direct sunlight. Germination in autumn, and away from the root competition of thornscrub, may reflect the importance of soil drought in mortality of young seedlings.