Strategies for soil restoration and sustainable management of cork oak woodlands (STRAW)

In Portugal, cork oak woodlands (COW) presently cover more than 700000 ha. This system play a important role on the rural economy, as are source of cork, high quality fruits (acorns) for swine food, pastures, aromatic and medicinal plants, habitat of a great deal of game, firewood and charcoal. Until the seventies of the last century, COW were mostly used for cereal crops, which led to soils with a low organic matter (OM) and nutrient status. Nowadays, in most of the COW occur natural pastures, which have been extensively used for livestock rearing. The sustainability of this system has been questioned, and soil erosion, OM decline, failure of tree natural regeneration, die-back, and lost of biodiversity are some of the most outstanding threats.

In COW systems, scattered trees create a mosaic of open pastureland and oak/understory plant communities, enhancing soil OM accumulation and nutrient pools beneath their canopy, which may lead to favourable soil structure and conditions to prevent soil degradation. Thus, oak trees play a valuable ecological role on soil quality, which in turn contributes to the long-term ecosystem sustainability. Nevertheless, soil resources and productivity and sustainability of the system are strongly dependent on the management.

Deep studies, at tree level, have been developed in Portugal regarding the ecological features of trees in COW. However, there is scarce information regarding management systems aiming soil restoration system sustainability, taking into account time scales spanning years. For instance, improved sown permanent pastures (mixtures of grasses and legumes) have been installed at low extent to increase stocking rate, but no study has focused so far whether they can increase productivity and, simultaneously, improve soil quality and organic C stock, affect C and N dynamics, and ensure tree regeneration (a requisite for sustainable management). Although vast COW areas have been encroached by shrubs, there is no information whether such encroachment restore soil functions and tree regeneration. Moreover, organic amendment practices aiming to promote soil quality stable organic C accumulation have not been examined.

Within this context, it is expected to get deep knowledge to base guidelines for sustainable management of cork oak woodlands, in Southern Portugal, which have been degraded by cereal crops and overgrazing. It is hypothesized that improved sown permanent pastures and shrub encroachment as well as application of organic amendments may increase system productivity, recover soil quality and promote resistance to land degradation and system sustainability. The project aims to evaluate whether such management systems (i) may reverse degradation trends, evaluating different regionally-adapted management practices focused to land degradation mitigation; (ii) improve soil quality, with especial attention to C and N stocks; enhancing the soil function as a retardant of land degradation, (iii) promote forage yield and affect competition for soil resources; (iv) enhance tree nutrition status and growth and tree regeneration performance; (v) affect temporal and spatial interaction between N mineralization and C dynamics.

Also it is aimed examine whether N inputs through N2 symbiotic fixation affect soil C storage, and whether scattered trees affect such fixation and N recycling relatively to open areas. The project intends to have a very positive social and economic impact in dry sub-humid and semi-arid Mediterranean regions.

The envisaged results will provide information to lay down general guidelines for proper management of COW, in order to optimize the use of natural resources, to protect and improve soil resources and to contribute for environmental quality through C storage in the soil. Expected results will also contribute to strength the economical, social and environmental values of those systems, reducing threaten of land degradation. The information obtained would be useful for the professionals and administration in charge of future elaboration of the managements plans of oak woodland estates, as well as for landowners and respective associations, in order to go forward a sustainable management of this unique system.