Edition
The dynamics of cork oak systems in Portugal: the role of ecological and land use factors
Acácio, V. (2009). PhD thesis Wageningen University, the Netherlands
ISBN 978-90-8585-504-0; 208 pp.
With references, with summaries in Dutch and English
The landscape in southern Portugal is dominated by a distinctive Mediterranean land use system, in which cork oak (Quercus suber L.) is the main tree species. This area constitutes the largest cork oak habitat in the world. The cork oak land use system of today has a patchy appearance, which is the result of long-term combined ecological and land use dynamics that have shaped the landscape. In some cases, overexploitation of the land has led to soil degradation and erosion and a lack of natural regeneration of cork oak. Areas of degraded soil are commonly invaded and dominated by rockrose (Cistus spp.) shrubs; the resulting shrubby vegetation is very persistent and can be interpreted as a stage of regressive succession of the original forest. Succession has traditionally been thought to be a relatively linear process, but evidence from a variety of ecosystems indicates that persistent alternative vegetation states may occur, particularly after disturbances. The overall objective of this thesis was to evaluate if cork oak forests and Cistus shrublands represent alternative stable states in southern Portugal, and to ascertain the ecological and land use mechanisms underlying their resilience and persistence. All the data analysed for this thesis were collected at a study site in southern Portugal.
It is shown that Cistus shrublands have been the most persistent patch-type in the study area for 45 years (1958-2002) and have been expanding. Cork oak forests were also persistent but, contrary to shrublands, have been decreasing since 1985. Wildfires triggered transitions from forests to shrublands after 1995 and contributed to maintain shrubland patches, particularly on south-facing slopes. Persistence of Cistus shrublands is also explained by multiple mechanisms that severely constrain cork oak recruitment in shrubland patches (namely, limitation of seed source, dispersal, germination and establishment). On the other hand, cork oak persistence was more likely in areas with a combination of certain factors: where the understorey had not been managed before 1975, no cork oak had been planted since the 1980s, wildfires were absent and slopes were steeper. It was also found that fire resistance in cork oak is essentially determined by management decisions. Cork oak survival after a wildfire was lower in stripped trees, trees with thinner bark and trees with larger diameter (correlated with the number of stripping operations). Survival was also lower with increasing fire damage and on south to east oriented slopes in the study area, which are more xerophitic. The results also show that over half of the landowners interviewed hold traditional beliefs that run contrary to current scientific knowledge.
It is concluded that the future of the cork oak land use system in the study area is severely threatened by loss of resilience and that shrubland is an alternative stable state of lower vegetation biomass on the most degraded soils and dry areas, where conditions for forest recovery are extremely hostile. The findings suggest that cork oak land use systems in the study area are not sustainable if adequate management actions and restoration programmes are not enforced in the short term.
Key-words:
Quercus suber, cork oak, Cistus, alternative stable states, wildfire, shrub encroachment, agroforestry, Mediterranean, landscape changes, tree recruitment, arrested succession, forest transitions, Portugal
Vanda Acácio defended her thesis on November 17, 2009