El papel facilitador del matorral para la restauración de zonas mediterráneas contaminadas por elementos traza

Experimental sowing of Quercus ilex acorns under shrubs in the Guadiamar Green Corridor (María T. Domínguez in the photo).

Experimental sowing of Quercus ilex acorns under shrubs in the Guadiamar Green Corridor (María T. Domínguez in the photo).

The revegetation of polluted sites and abandoned agricultural soils is critical to reduce soil losses and to control the spread of soil pollution in the Mediterranean region, which is currently exposed to the greatest soil erosion risk in Europe. However, events of massive plant mortality usually occur during the first years after planting, mainly due to the adverse conditions of high irradiance and drought stress. The need for alternative afforestation techniques prompted a number of studies during the last decade to explore the potential application of positive plant-plant interactions for the restoration of degraded sites. Many studies in Mediterranean ecosystems have reported that the presence of pioneer shrub species (often called nurse plants) facilitates the establishment of other late-successional species under their canopies, mainly due to the amelioration of extreme temperature conditions and the improvement of plant water status but also by the concurrence of better soil conditions under the shrubs. To date, very few studies have assessed the role of the chemical stress (i.e. high concentrations of toxic elements in the soils) in the intensity of the facilitation by shrubs. A team from IRNAS, led by María T. Domínguez, has analyzed the role of the nurse shrubs on the restoration of polluted Mediterranean sites. The results have been published in the September number of Journal of Environmental Management.

The study was conducted at the Guadiamar Green Corridor (Sevilla), an area that was polluted by a large mine spill in 1998 and later afforested. We used nurse shrubs (Phillyrea angustifolia and Retama sphaerocarpa) as planting microsites for acorns of Holm oak (Quercus ilex subsp. ballota) along a gradient of soil pollution, and monitored seedling growth, survival, and chemical composition during three consecutive years. Seedling survival greatly increased (from 20% to more than 80%) when acorns were sown under shrub, in comparison to the open, unprotected matrix. Facilitation of seedling growth by shrubs increased along the gradient of soil pollution, in agreement with the stress gradient hypothesis that predicts higher intensity of the facilitation effects with increasing abiotic stress. Although the accumulation of trace elements in seedling leaves was higher underneath the shrubs, the shading conditions provided by the shrub canopy allowed seedlings to cope with the toxicity provoked by the concurrence of low pH and high trace element concentrations in the most polluted sites. Our results show that the use of shrubs as nurse plants is a promising tool for the cost-effective afforestation of polluted lands under Mediterranean conditions.

This study is a contribution of the IRNAS team to the European project RECARE – “Preventing and remediating degradation of soils in Europe through Land Care”.

Domínguez, M.T., Pérez-Ramos, I.M., Murillo, J.M., Marañón. 2015. Facilitating the afforestation of Mediterranean polluted soils by nurse shrubs. Journal of Environmental Management 161: 276 -286.

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