Sierra y pico del Aljibe. Los Alcornocales Natural Park.

Field trip and text by Fernando Ojeda (Universidad de Cádiz, Spain) and Lorena Gómez-Aparicio (IRNAS-CSIC, Spain)

Photo album: https://goo.gl/photos/sMWWp9pprfWsGsTP6

The western Mediterranean Basin is characterized by a markedly seasonal, winter rainfall climate regime (Mediterranean climate). Dominant soils are loamy to marly, have a neutral to basic pH, and moderate to high fertility. Such climatic and edaphic conditions explain the preponderance of garrigue-like shrublands and woodlands in this region. They are dominated by sclerophyllous woody species and represent the paradigm of Mediterranean vegetation in the Mediterranean Basin.

By contrast, the Strait of Gibraltar region is characterized by mountains and hills composed of OligoMiocene siliceous sandstone (“Aljibe sandstone”) that produce highly acidic, nutrient-poor, sandy soils. These mountains appear in the form of edaphic islands, surrounded in the lowlands by more fertile, non-acid, loamy or marly soils. Climate is mild Mediterranean owing to the oceanic influence determined by its geographic location between two seas. Mean annual rainfall ranges from ca. 750 to over 1500 mm. In summer, a prevailing southeastern wind (locally known as Levante) coming directly from the sea brings moisture to these sandstone mountains, which condenses into thick, low clouds and contributes to somewhat alleviate in this region the characteristic severity of summer drought of the Mediterranean Basin. Such contrasting climatic and edaphic features, together with its particular Plio-Pleistocene history (Rodríguez-Sánchez et al. 2008) explain the replacement of typical garrigue-like shrublands in these sandstone mountains by a distinct flora and vegetation. Extensive cork oak woodlands, moist semi-deciduous oak forests and spectacular canutos in deep gorges, together with Mediterranean heathlands or herrizas covering sandstone crests and ridges, are major vegetation formations in the Strait of Gibraltar region.

Most of the northern (European) side of this region is preserved under the umbrella of Los Alcornocales Natural Park. It covers ca.1700 km2 of rugged relief formed by not very high mountains and hills (highest elevation of 1092 m) mainly composed of Oligo-Miocene siliceous sandstone. This natural park was formally declared in 1989 so as to promote the sustainable management of forest resources and to maintain its outstanding biodiversity. Main natural resources in the region are cork extraction, free-range livestock (mainly Retinto cattle), game hunting (red- and roe-deer) and, more recently, eco-tourism.

In its shady slopes facing north, cork oak woodlands are replaced by semi-
deciduous Quercus canariensis forests (quejigares). The understorey of these close
forests is formed by broadleaf tall shrubs such as Phillyrea latifolia, Viburnum tinus and Rhamnus alaternus, among others. Erica arborea is the only heath species able to tolerate the shady conditions of these forest understoreys.

After a bit, the trail overlooks the Puerto Oscuro Gorge (Garganta de Puerto Oscuro, which conceals a very interesting example of warm-temperate forest or canuto. These canutos are very diverse in tree and arborescent shrub species, including Tertiary (premediterranean) relict species such as Rhododendron ponticum subsp. baeticum, the bay-tree Laurus nobilis, the alder buckthorn Frangula alnus subsp. baetica and the holly Ilex aquifolium, highlighting the role of the Strait of Gibraltar region as a premediterranean refuge (Rodríguez-Sánchez et al. 2008). The herriza is dominated by heath species such as Erica australis and Calluna vulgaris, the rockrose Cistus populifolius, gorses such as Genista tridentata or Stauracanthus boivinii, and the prostrated oak Quercus lusitanica. In the herriza is where we can find the flypaper-trap species Drosophyllum lusitanicum a unique carnivorous plant species (Bertol et al. 2015; Salces-Castellano et al. 2016).

Although the herriza is floristically and fisiognomically similar to other European dry heathlands, it shows much higher levels of species richness and narrow endemism. But they are not only of high conservation value because of the number of narrow endemic species but, more remarkably, because of the singularity of its endemism (Rodríguez-Sánchez et al. 2008). Nevertheless, they have been traditionally and inexplicably disregarded or considered as mere “tree-less” or degraded “pre-forest” stages, and have been systematically “restored” by afforestation with pine trees (Andrés and Ojeda 2002). More about the herriza in the 8-min documentary HERRIZA - The Cinderella of the Strait of Gibraltar (https://www.youtube.com/watch?v=u0YpOcl27Nw). Only Quercus pyrenaica, another semi-deciduous oak tree species, can form small scattered treelet patches in the herriza, always in wind-sheltered locations above 800 m of elevation.

References
Andrés C & F Ojeda (2002). Effects of afforestation with Pinus pinaster on biodiversity of Mediterranean heathlands in South Spain. Biodiversity and Conservation 11: 1511-1520.
Bertol N, M Paniw & F Ojeda (2015). Effective prey attraction in the rare Drosophyllum lusitanicum, a flypapertrap carnivorous plant. American Journal of Botany 102: 689-694.
Gómez-Aparicio L, B Ibáñez, MS Serrano, P De Vita, JM Ávila, IM Pérez-Ramos, LV García, ME Sánchez & T Marañón (2012). Spatial patterns of soil pathogens in declining Mediterranean forests: implications for tree species regeneration. New Phytologist 194: 1014-1024
Ibáñez B, L Gómez-Aparicio, P Stoll, JM Ávila, IM Pérez-Ramos & T Marañón (2015). A neighborhood analysis of the consequences of Quercus suber decline for regeneration dynamics in Mediterranean forests. PLOS One, e0117827.
Rodríguez-Sánchez F, R Pérez-Barrales, F Ojeda, P Vargas & J Arroyo (2008). The Strait of Gibraltar as a melting pot for plant biodiversity. Quaternary Science Reviews 27: 2100-2117.
Salces-Castellano A, M Paniw, R Casimiro-Soriguer & F Ojeda (2016). Attract them anyway: benefits of large, showy flowers in a highly autogamous, carnivorous plant species. AoB Plants 8: 1-10.