The dynamics of cork oak systems in Portugal: the role of ecological and land use factors

Vegetation degradation and desertification occur in many semiarid ecosystems worldwide, particularly in the Western Mediterranean Basin. A peculiar semiarid Mediterranean land use system dominates the landscape of southern Portugal where cork oak (Quercus suber) is the main tree species. This system is the result of both ecological and land use dynamics that shaped the landscape for centuries and created a fine-grained mosaic landscape, which includes four main types of vegetation patches: cork oak woodlands (higher tree density), cork oak montados (lower tree density), shrublands and grasslands. Each patch can be conceived as a vegetation state that is reached and maintained in different ways. Transitions between states are triggered often by a combination of human management and climatic circumstances. I hypothesize that cork oak woodlands and shrublands in southern Portugal represent alternative stable states, which change slowly or not at all in the absence of human intervention. Failures in natural regeneration of cork oak trees triggered by grazing pressure and human overexploitation can be the cause of a shift to the alternative stable state of shrublands, especially in very dry conditions. Stable and expanding shrublands may therefore be a sign of land desertification in southern Portugal, representing an irreversible transition. This PhD proposal aims to evaluate if cork oak woodlands and shrublands represent alternative stable states in southern Portugal and what are the ecological and land use mechanisms underlying their resilience and persistence. Overall methodology The study area is located at Serra do Caldeirão, Algarve, southern Portugal. The persistence and dynamics of cork oak woodlands and shrublands, transition probabilities between vegetation states, and correlation between transitions and environmental variables will be analyzed through the observation of aerial photos between 1958 and 2002 for the study area with the help of a GIS (Geographical Information System). Markov matrix analysis and multivariate analysis will be used. Limitations for seedling establishment will be analysed through an experimental design carried out in different vegetation states. Finally, human management, motivations for land use decisions, behaviour towards the cork oak system, and sociological characterization of landowners will be investigated through landowner interviews

Oak Persistence in Mediterranean Landscapes: The Combined Role of Management, Topography, and Wildfires

Mediterranean ecosystems have been shaped by a history of human and ecological disturbances. Understanding the dynamics of these social-ecological systems requires an understanding of how human and ecological factors interact. In this study, we assess the combined role of management practices and biophysical variables, i.e., wildfire and topography, to explain patterns of tree persistence in a cork oak (Quercus suber L.) landscape of southern Portugal. We used face-to-face interviews with landowners to identify the management practices and the incentives that motivated them. We used aerial photographs and a Geographic Information System (GIS) to classify vegetation patch-type transitions over a period of 45 years (1958-2002) and logistic regression to explain such changes based on management and biophysical factors. The best model explaining vegetation transitions leading to cork oak persistence in the landscape included both biophysical and management variables. Tree persistence was more likely to occur on steeper slopes, in the absence of wildfires, and in the absence of understory management. We identified ecological, ideological, and economical barriers that preclude oak persistence and that are important to consider in implementing efficient environmental policies for adequate conservation and reforestation programs of Mediterranean cork oak landscape

Are drought and wildfires turning Mediterranean cork oak forests into persistent shrublands?

In the Iberian Peninsula Mediterranean oak forests have been transformed into a mosaic landscape of four main patch-types: forests, savannas, shrublands and grasslands. We used aerial photographs over a period of 45 years (1958-2002) to quantify the persistence and rates of transitions between vegetation patch-types in southern Portugal, where cork oak is the dominant tree species. We used logistic regression to relate vegetation changes with topographical features and wildfire history. Over the 45 years, shrublands have been the most persistent patch-type (59%), and have been expanding; forests are also persistent (55%) but have been decreasing since 1985; savannas and grasslands were less persistent (33% and 15%, respectively). Shrublands persistence was significantly correlated with wildfire occurrence, particularly on southern exposures after 1995. In contrast, forest persistence decreased with wildfire occurrence, and forests were more likely to change into shrublands where wildfire had occurred after 1995

Pathways for resilience in Mediterranean cork oak land use systems

Context Loss of woodlands and degradation of vegetation and soil have been described for all Mediterranean-type ecosystems worldwide. In the Western Iberian Peninsula, overexploitation of evergreen cork oak land use systems has led to soil erosion, failures in oak recruitment, and loss of forests. Degraded and dry sites are quickly colonised by pioneer heathland rockrose (Cistus spp.) shrubs forming highly persistent patches. Aims Although traditionally shrublands have been considered as a transient successional state, we present evidence that they can represent persistent alternative states to former cork oak forests. Review trends and conclusions We first describe how Mediterranean vegetation evolved in the Iberian Peninsula and the role of fire and long-term human management as main disturbances. We then discuss alternative pathways through state-and-transition models indicating the ecological and land use variables that halt cork oak regeneration and recruitment and drive vegetation transitions towards persistent shrublands. Unless concerted management actions and restoration programmes are undertaken, the cork oak land use systems will not be sustainabl