Is fragmentation affecting regime shifts occurrence in Mediterranean oak woodlands? A case study in southern Portugal

In southern regions of Iberia, in Portugal and Spain, the evergreen cork oak woodlands are human-made scattered-tree ecosystems. These woodlands mimick natural savannas and are truly cultural landscapes, where trees form... [ view full abstract ]

In southern regions of Iberia, in Portugal and Spain, the evergreen cork oak woodlands are human-made scattered-tree ecosystems. These woodlands mimick natural savannas and are truly cultural landscapes, where trees form socio-economic and ecological keystone structures, adapted to limited water availability. Recent disturbances such as abnormal tree mortality and lack of tree recruitment under a changing Mediterranean climate (in particular related to drought) resulted in the gradual disappearance of trees. In some regions, it seems that these woodland ecosystems are undergoing complex ecological regime shifts, leading to a disrupted forest-cycle. At the same time, cork oak woodland ecosystems are increasingly fragmented, with new boundaries with scrublands and other land uses.
A key management challenge is to maintain the trees and their functional traits to the ecosystem’s services conservation. In this study, through dendrochronological dating archives in cork samples from trees at contrasting fragmented woodlands in two distinct regions, undergoing a fragmentation process, we derive resilience of trees to cork harvesting. We then assessed the amplified influence of the forest fragmentation on the occurrence of a regime shift (disruption of forest cycle) of these scattered-tree ecosystems. Similarly, the ecosystem recovery, when occurring was also assessed, enabling to identify oak woodlands ecological thresholds for regime shift occurrence. Thus we addressed three main questions for this ecosystem’s conservation: 1) Can we detect ecological regime shifts in Mediterranean evergreen oak woodlands?; 2) Are there imminent regime shifts indicators? And; 3) Are there spatial fragmentation thresholds bellow which woodlands become vulnerable to regime shifts?
Basic knowledge on the ecological resilience of Mediterranean oak woodlands will increase our ability to anticipate irreversible changes in the ecosystem may contribute to the design of sustainable land management strategies, to be considered by policy-makers, forest owners and forest managers.



Keywords: Quercus suber L.; Patch size–frequency distribution; Dendrochronology; Resilience to cork harvesting