The Oak And The Mediterranean Basin

The mention of the word “Mediterranean” usually evokes imagery of white sandy beaches with turquoise waters, a gentle summer breeze blowing beneath the green pine trees and the sound of cicadas buzzing. People are often surprised to hear that there are oaks in the Mediterranean.

What if this idea of the ubiquitous pine forests in the Mediterranean is a misconception developed during the last century and the reality is a bit different?

Once, oaks were the keystone species of large and diverse forests covering vast areas, where one predominant oak species gradually succeeded another. To illustrate this point, species such as the Sessile Oak (Quercus petraea) grow natively from Ireland to the west, Sweden in the North, Greece in the South and the Caucasus in the East coexisting with a host of other plant species; a most impressive native range for a tree. Oaks are generalists and thus found all over Europe, North and Central America, Asia and North Africa, compared to other species, such as horse chestnut, which have evolved as specialists and are usually confined to specific climate zones.

The Mediterranean biome, one of the smallest yet most biodiverse in the world, and comprises the following:

  • The forests of the Mediterranean contain about 100 tree species, broadleaf and sclerophyll (hard-leaf), the many oak species of the Mediterranean, evergreen or deciduous, belonging to both categories. River (riparian) areas are inhabited by tree species that demand more water, but can also withstand waterlogged soil, such as Plane trees (Platanus), Poplars and Laurels, the last being a relic of very distant warmer and more humid times. Pines, mainly Aleppo, maritime pine and stone pine are present in lower altitudes and are considered pioneer forest species due to their wind seed dispersal.
  • The Mediterranean woodlands (in essence low density forest with plenty of sunlight) are characterised by oaks or pines as the dominant species.
  • The Savanna/Grasslands of the mediterranean is best represented in the dehesa/montado oak systems of Spain and Portugal, featuring holm and cork oaks.
  • The Shrublands of the Mediterranean Basin, also known as maquis/garrigue, consist of dense thickets of sclerophyll shrubs and, usually small, trees, quite often evergreen oaks. Unfortunately, on a large number of cases, such shrublands are the result of degradation of former forests or woodlands, caused by logging or overgrazing, or disturbance by major fires.

Fire is a driving force for the regeneration of the Mediterranean forest. The high temperatures prevalent during the dry Mediterranean summer make the region prone to lightning-caused fires and many plant species are adapted to it, even depending on it for reproduction. A forest fire may remove old trees which have lost their vigour and make space for new growth, allowing light to reach the forest floor again.

Humans entering this equation, however, changed everything, causing fires with increased frequency over the last 50,000 years. Human population growth and intensive exploitation of the land with upland agriculture, animal domestication, and wood consumption sharply increased in the last 10,000 years. Large-scale human impacts led to increased deforestation and the expansion of maquis. Interestingly, the decline of each major civilization in the Mediterranean was followed by spontaneous forest recovery.

In many cases, Mediterranean succession in the recovery of land begins with a pioneer stage of Aleppo pine colonization, whereby in conditions of moderate humidity and with no further alterations, pine forests are later replaced by evergreen oak forests.

For this succession to take place, however, the presence of oaks nearby is necessary. Pines are wind-dispersed species with a high capacity to colonize open spaces, whereas species such as oaks are usually animal-dispersed, mostly Eurasian Jays and rodents, and therefore, their colonization is expected to be more gradual in time.

When we consider that animals can carry acorns only so far, usually a few hundred meters, the problem is evident. If a jay caches acorns 400 meters away from the mother oak, hypothesizing that an acorn germinates and grows to be a mature tree, it will take around 15-20 years for that oak to reach a sufficient acorn production levels. Repeating that process, in ideal conditions, to expand oak presence over a radius of 2 kilometers, many generations of Eurasian Jays would need to be hard at work for over a century.

As a genus, oaks support a great number of wildlife species, providing home and nutrition to them. Some of these species, weevils and wasps, have evolved along with oaks and depend on them for survival. Oaks can generally resist fungi adequately, even form a symbiotic relationship, whereas other trees would have been mortally infected. It is estimated that the common oak (Quercus robur), may support up to 500 species of insects, birds and mammals.