Monitoring tools to assess vegetation successional regression and predict catastrophic shifts and desertification in Mediterranean rangleland ecosystems

NATO Mediterranean Dialogue Workshop on Desertification in the Mediterranean Region - A Security Issue -- DEC 02-05, 2003 -- Valencia, SPAINWOS: 000235906500020The relationship between grazing intensity and ecosystem performance is complex and can depend on the prevailing ecological conditions. Previous studies have revealed that, in traditional grazing ecosystems, grazing can reduce ecosystem diversity ill poor soils, but increase diversity and productivity in rich ecosystems subject to moderate grazing pressure. We are interested in detecting long-term structural changes or drift in an ecosystem before it is too late to prevent irreversible degradation. We analyzed vegetation spatial patterns and complexities of four Mediterranean communities: Tihmadit Region (Middle Atlas, Morocco), Camiyayla (Namrum) Region (Taurus Mountain, Turkey), Sykia Region (south of the Sithonia Peninsula, Greece), and Cabo de Gata Nijar Natural Park, Spain. Grazing disturbance was most intense near shelter and water points, which lead to gradients in soil surface disruption, compaction, and changes ill the composition and cover of perennial vegetation. Dense matorral was more resistant to species loss than were moderately dense and scattered matorral, and grassland. Information fractal dimension decreased as we moved from a dense matorral to a discontinuous matorral, and increased as we moved to a more scattered matorral and to a grassland, which resulted in two opposing processes (interaction declining with ecosystem development, and immigration increasing with degradation) in a common pattern, i.e., small patches homogeneously distributed in the landscape. Characteristic species of the natural vegetation declined in frequency and organization in response to higher grazing disturbance, while species of disturbed areas exhibited the opposite trend. Overall, the spatial organization of the characteristic plants of each community decreased with increasing vegetation degradation, with the intensity of the trend being related to the species' sensitivity to grazing. Developmental instability analyses of key species were used to determine the sensitivity of dominant key species to grazing pressure. Palatable species, which are better adapted to being eaten, such as Periploca laevigata, Phillyrea latifolia and Genista pseudopilosa, were able to resist moderate grazing pressure, while species of disturbed, grazed sites did not change developmental instability in response to increasing grazing pressure, such as Thymus hyemalis, Teucrium lusitanicum and Cistus monspeliensis. The usefulness of these monitoring tools in preventing land degradation is discussed.NATO Sci Comm, US EPA, Ctr Desertificat Res, Desert Res Inst, NATO Comm Challenges Modern Soc, European Soc Soil Conservat, Dept Territory & House, Spanish Minist Environm, United Nat Convent Combat Desertificat, Secretariat, City Art & Sci Valenci

Spatial vegetation patterns and imminent desertification in Mediterranean arid ecosystems

Humans and climate affect ecosystems and their services(1), which may involve continuous and discontinuous transitions from one stable state to another(2). Discontinuous transitions are abrupt, irreversible and among the most catastrophic changes of ecosystems identified(1). For terrestrial ecosystems, it has been hypothesized that vegetation patchiness could be used as a signature of imminent transitions(3,4). Here, we analyse how vegetation patchiness changes in arid ecosystems with different grazing pressures, using both field data and a modelling approach. In the modelling approach, we extrapolated our analysis to even higher grazing pressures to investigate the vegetation patchiness when desertification is imminent. In three arid Mediterranean ecosystems in Spain, Greece and Morocco, we found that the patch-size distribution of the vegetation follows a power law. Using a stochastic cellular automaton model, we show that local positive interactions among plants can explain such power-law distributions. Furthermore, with increasing grazing pressure, the field data revealed consistent deviations from power laws. Increased grazing pressure leads to similar deviations in the model. When grazing was further increased in the model, we found that these deviations always and only occurred close to transition to desert, independent of the type of transition, and regardless of the vegetation cover. Therefore, we propose that patch-size distributions may be a warning signal for the onset of desertification

Change in plant spatial patterns and diversity along the successional gradient of Mediterranean grazing ecosystems

In this study, we analyze the complexity of plant spatial patterns and diversity along a successional gradient resulting from grazing disturbance in four characteristic ecosystems of the Mediterranean region. Grazing disturbance include not only defoliation by animals, but also associated disturbances as animal trampling, soil compaction, and mineralization by deposition of urine and feces. The results show that woodland and dense matorral are more resistant to species loss than middle dense and scattered matorral, or grassland. Information fractal dimension declined as we moved from a dense to a discontinuous matorral, increasing as we moved to a more scattered matorral and a grassland. In all studied cases, the characteristic species of the natural vegetation declined in frequency and organization with grazing disturbance. Heliophyllous species and others with postrate or rosette twigs increased with grazing pressure, particularly in dense matorral. In the more degraded ecosystem, only species with well-adapted traits, e.g., buried buds or unpalatable qualities showed a clear increase with grazing. Indeed, the homogeneity of species distribution within the plant community declined monotonically with grazing impact. Conversely, the spatial organization of the characteristic plants of each community increased in the better-preserved areas, being also related to the sensitivity of the species to grazing impact. The degree of autocorrelation of plant spatial distribution at the species level and the information fractal dimension at the community level allow us to quantify the degree of degradation of natural communities and to determine the sensitivity of key species to disturbance. © 2004 Elsevier B.V. All rights reserved.Comisión Interministerial de Ciencia y Tecnología: AMB1998-1017 European Commission: ERBIC18-CT98-0392The work was performed under the DRASME (Desertification Risk Assessment in Silvopastoral Mediterranean Ecosystems) collaborative research project and CICYT (Evaluación de la presión ganadera para la conservación de las estepas y matorrales Mediterráneos, project number AMB1998-1017). DRASME is funded by the EC under its INCO-DC programme, contract number ERBIC18-CT98-0392. The support from both programs is gratefully acknowledged. We are grateful to John Emlen, Carl Freeman and Bai-Lian Li for critically reading and helpful suggestions on the manuscript. Software for data analysis was developed by Juan Escós. Appendix