Vanda Acácio Vanda the role of ecological and land use factors land ecological and of the role The dynamics of cork oak systems in Portugal: Portugal: in oak systems of cork dynamics The The dynamics of cork oak systems in Portugal: the role of ecological and land use factors V. Acácio 2009 The dynamics of cork oak systems in Portugal: the role of ecological and land use factors Vanda Acácio Thesis committee Thesis supervisors Prof. dr. ir. G.M.J. Mohren Professor of Forest Ecology and Forest Management Wageningen University Prof. dr. Francisco Castro Rego Associated Professor at the Landscape Architecture Department Instituto Superior de Agronomia Universidade Técnica de Lisboa, Portugal Thesis co-supervisor dr. Milena Holmgren Assistant Professor at the Resource Ecology Group Wageningen University Other members Prof.dr. P.F.M. Opdam, Wageningen University Prof.dr. Stefan A. Schnitzer, University of Wisconsin, USA Prof.dr. B.J.M. Arts, Wageningen University Prof.dr. B.G.M. Pedroli, Wageningen University This research was conducted under the auspices of the C.T. de Wit Graduate School for Production Ecology and Resource Conservation (PE&RC). The dynamics of cork oak systems in Portugal: the role of ecological and land use factors Vanda Acácio Thesis submitted in partial fulfilment of the requirements for the degree of doctor at Wageningen University by the authority of the Rector Magnificus Prof. dr. M.J. Kropff, in the presence of the Thesis Committee appointed by the Doctorate Board to be defended in public on Tuesday 17 November 2009 at 4 PM in the Aula. Vanda Acácio The dynamics of cork oak systems in Portugal: the role of ecological and land use factors, 210 pages. Thesis, Wageningen University, Wageningen, NL (2009) With references, with summaries in Dutch and English ISBN 978‑90‑8585‑504‑0 Ao meu pai To my father Abstract 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 degra‑ dation 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 per‑ sistence. 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, par‑ ticularly on south ‑facing slopes. Persistence of Cistus shrublands is also explained by multi‑ ple 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 un‑ derstorey 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 se‑ verely 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 re‑ covery 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 encroach‑ ment, agroforestry, Mediterranean, landscape changes, tree recruitment,­arrested succession, forest transitions, Portugal Table of Contents Chapter 1 – General Introduction .............................................................. 9 Chapter 2 – Are drought and wildfires turning Mediterranean cork oak forests into persistent shrublands? ........................................... 43 Chapter 3 – Multiple recruitment limitation causes arrested succession in mediterranean cork oak systems .......................................... 67 Chapter 4 – Cork extraction as a key factor determining post ‑fire cork oak survival in a mountain region of southern Portugal .................. 93 Chapter 5 – Interactive effects of human and ecological factors determine persistence of cork oak forests in southern Portugal ................. 115 Chapter 6 – Synthesis ................................................................................ 143 References .................................................................................................. 161 Summary ................................................................................................... 189 Samenvatting ............................................................................................. 195 Acknowledgements .................................................................................. 201 Short Biography ........................................................................................ 203 List of Publications ................................................................................... 205 PE&RC PhD Eduction Certificate ......................................................... 207 Chapter 1 General Introduction The dynamics of cork oak systems in Portugal: the role of ecological and land use factors Today’s Mediterranean ‑type ecosystems have evolved through the major geological and climatic changes that took place during the Pleistocene period. Their vegetation, which has adapted successfully to natural dis‑ turbances throughout millennia, is considered highly resilient. The dis‑ turbance has been significantly increased by human activity such as tree and shrub clearing, livestock grazing, and ploughing for cultivation, especially in the Mediterranean Basin (Joffre et al., 1999; Pulido et al., 2001); this has contributed greatly to the present condition of the vegeta‑ tion, sometimes jeopardizing its resilience. Vegetation degradation and loss of woodlands has been observed in Mediterranean ‑type ecosystems worldwide, particularly in the Western Mediterranean Basin (Pignatti, 1978; Puigdefábregas, 1995; Puigdefá‑ bregas and Mendizabal, 1998). It is considered to be mainly caused by a combination of human management and climatic disturbances (Puig‑ defábregas and Mendizabal, 1998; Scarascia‑Mugnozza et al., 2000). Fire has also played a major role in shaping the landscape, having contributed to the present dominance of fire‑resistant species, which have been able to survive due to their morphological traits, such as thick fire‑resistant bark, resprouting and seed germination stimulated by fire. The landscape in southern Portugal is dominated by a distinctive Medi‑ terranean land use system. This area, where cork oak Quercus( suber L.) is the main tree species, is the largest cork oak habitat in the world. The cork oak is a sclerophyllous evergreen tree that develops a special bark: the cork. The cork oak land use system of today is the result of long ‑term combined ecological and land use dynamics that have shaped the landscape. In some cases, overexploitation of the land led to soil degradation and erosion and a lack of natural regeneration of cork oak (Natividade, 1950; Gonçalves, 1991; Montero et al., 1994a). Areas of degraded soil where cork oak natural regen‑ eration becomes difficult 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. 10 The dynamics of cork oak systems in Portugal: the role of ecological and land use factors 1. General Introduction Succession has traditionally been thought to be a relatively linear process, but evidence from a variety of ecosystems indicates that persistent alter‑ native vegetation or ecosystem states may occur, particularly after distur‑