- Holm oak forests form highly biodiverse sclerophyllous forests, with coastal, inland, Cantabrian and mountain variants depending on climate and soil.
- Its dense and tangled structure harbors a rich fauna of birds, mammals, reptiles, invertebrates, and highly specialized mycorrhizal fungi.
- Traditional human activity (charcoal making, pastureland, grazing) has shaped these forests, potentially degrading them or maintaining mosaics of high ecological value.
- Fires, urbanization and dumping threaten holm oak forests, which has led to their protection through Natura 2000 network spaces and other legal mechanisms.
Holm oak forests are one of those landscapes that, although we have been seeing them all our lives, we rarely stop to think about all that they hide. Behind every oak tree lies a story of climate, soil, fauna, human uses, and millions of years of evolution which explain why this type of forest withstands both Mediterranean heat and Cantabrian humidity.
When we talk about holm oak ecosystems, we are not referring to a single type of forest, but to a range of variations that change according to the climate, altitude, the rock on which they grow, or the hand of human beings. From the Mediterranean coastal holm oak forests to the Cantabrian holm oak forests that climb limestone ridges at the edge of the seaThey all share a common protagonist: the holm oak (Quercus ilex), but each one has its own character.
What exactly is an oak forest?
An oak forest is a type of forest dominated by the holm oak, accompanied by a varied array of shrubs, vines, herbs, fungi, and specialized fauna. It is considered a complex forest ecosystem with large biomass, dense structure, and very high biodiversity.especially in the Iberian Peninsula.
From a botanical point of view, we are talking about a forest of broad-leaved, evergreen, hard-leaved trees and shrubs (sclerophyllous broadleaf trees)These forests are adapted to summer drought, intense sunlight, and, in many cases, poor, stony soils. The average height of these forests rarely exceeds 10 meters in many classic Mediterranean stands, although there are holm oak groves where much greater heights are reached.
The dominant species is the holm oak in its two main forms: Quercus ilex subsp. ilex, with bitter acorns and longer leavestypical of wetter Mediterranean and coastal areas, and Quercus ilex subsp. ballota or rotundifolia, with sweet acorns and more rounded and leathery leavestypical of continental and southern Mediterranean environments. In large inland areas, holm oaks with intermediate characteristics can be observed due to hybridization between the two.
The structure of the forest is usually closed and intricate. The undergrowth is usually made up of tall, thorny shrubs and lianas that compete fiercely for lightThis often makes the interior of the holm oak forest dark, difficult to walk through, and with very little grass on the ground.
Main types of holm oak forests
Within the broad umbrella of holm oak forests, we find several distinct typologies depending on the climate, geology, and history of use. Each type of holm oak forest has characteristic accompanying species, different degrees of density, and different processes of degradation and regeneration..
Mediterranean coastal holm oak forest
The Mediterranean coastal holm oak forest is the classic evergreen forest that is associated with the Mediterranean climate near the coast. It is dominated by Quercus ilex subsp. ilex and often shares space with the cork oak (Quercus suber) in suitable soils, forming mixed masses very rich in species.
This type of holm oak forest is characterized by dense undergrowth with typically Mediterranean shrubs such as the mastic tree (Pistacia lentiscus), the Mediterranean buckthorn (Rhamnus alaternus), the butcher's broom (Ruscus aculeatus), the laurustinus (Viburnum tinus) or the strawberry tree (Arbutus unedo)These shrubs usually have hard, glossy, evergreen leaves and in many cases have fleshy fruits that are highly appreciated by seed-dispersing birds.
Vines are also a prominent feature in these holm oak groves. Sarsaparilla (Smilax aspera), ivy (Hedera helix), clematis flammulus (Clematis flammula), wild asparagus (Asparagus acutifolius) or various honeysuckles (Lonicera spp.) They climb the trunks, take advantage of the gaps in the light, and further close the forest, creating veritable tangles of vegetation.
The interior of these holm oak groves is very shady, with a scarcity of grasses due to lack of light. In many coastal Mediterranean mountain ranges, these forests represent the mature climatic stage, that is, the potential vegetation that would occupy the territory if there were no intense disturbances..
Inland Mediterranean holm oak forests and pastureland
Towards the interior of the peninsula, in more continental and dry climates, the sweet acorn holm oaks (subsp. ballota or rotundifolia) prevail. These holm oak groves can be dense forests or, after centuries of human management, become open woodlands.where widely spaced holm oaks coexist with pastures and, sometimes, dryland crops.
In provinces such as Guadalajara or in large areas of the central and western peninsula, holm oak forests are mixed with gall oaks (Quercus faginea) and other oaks, generating mixed stands. The undergrowth, depending on the type of soil (siliceous or calcareous) and the humidity, may be dominated by aromatic shrubs such as rosemary (Salvia rosmarinus), thyme (different species of Thymus), lavender and spike lavender (Lavandula stoechas, L. latifolia) and other fragrant labiates that carpet the ground with flowers and aromas in spring.
In many of these systems, the human hand has been decisive. Livestock grazing and firewood extraction have maintained more open holm oak forests, with a low-lying scrubland or pasture structure.Dehesas can be understood as mixed forestry and livestock farms, where trees are thinned to favor grazing, scrubland is removed and, sometimes, cereal crops are interspersed.
This management, when sustainable, can bring benefits: reduction of vegetation fuel and therefore lower risk of fires, conservation of key extensive livestock farming and maintenance of habitats for numerous vertebrates that use the holm oaks as a refuge and breeding ground.
Mediterranean holm oak forests in humid northern environments
It is particularly striking to find holm oak forests in rainy northern regions, such as Bizkaia, Cantabria or areas of the Atlantic Basque Country. There they appear as isolated Mediterranean enclaves embedded in a context of Atlantic deciduous forests, which gives them enormous biogeographical interest.
In the Urkiola Natural Park or in the Urdaibai Biosphere Reserve, holm oak forests grow on reef limestone ridges or highly draining carbonate rocksThe soils are skeletal, with very little capacity to retain water: even with heavy rainfall, the water is lost through rapid infiltration or runoff. This creates xeric conditions that allow the holm oak and all its Mediterranean companions to survive.
These forests, known as Cantabrian or coastal holm oak forests depending on the case, form masses of short stature (often less than 8-10 m) but very dense and tangledwhere the trees branch out from very low down due to centuries of logging for firewood and charcoal. It has the appearance of a low, intricate, evergreen rainforest.
In Cantabria, the coastal holm oak forest linked to Quercus ilex subsp. ilex is located exclusively on limestone outcrops, from sea level up to about 600 meters in altitude. Its mix of evergreen Mediterranean species and shrubs typical of the deciduous Atlantic forest makes it a mosaic of great diversity. and as a refuge for flora and fauna outside their usual areas.
Holm oak forests of the northeast and inland regions (example of Bages and Montserrat)
In inland Catalan regions such as Bages or in mountain ranges such as Montserrat, the holm oak forest appears both on shady rocky slopes and on sunny hillsides. In the cooler, shadier areas, holm oak forests develop with boxwood and a lush undergrowth., while in the more exposed rocky areas the composition is simplified, dominated by species such as the mock privet (Phillyrea latifolia).
In many of these places the holm oak forest has suffered repeated fires and episodes of charcoal production, but it still shows an enormous capacity for regrowth. After large fires, holm oaks sprout again from the stumps and can recover much of their structure in a few decades.although the full maturity of the forest requires much more time.
When the holm oak forest degrades, the first stage is usually a maquis dominated by large shrubs still typical of the holm oak forest, and later a scrubland where species such as rosemary become predominant. The destruction of abandoned crops within the potential range of the holm oak forest also leads to this succession from open fields towards aromatic scrubland, which over time may give way back to the forest.
Accompanying vegetation and dynamics of the holm oak forest
The vegetation that accompanies the holm oak depends largely on the type of soil, the orientation of the slope, the altitude and the degree of human intervention. Siliceous soils are often home to rockrose and broom thickets, while limestone soils are home to more buckthorn, buckthorn, and evergreen rose bushes..
In the Community of Madrid, for example, the holm oak forests on the slopes of the Sierra de Guadarrama and the Tagus depression mix with prickly juniper (Juniperus oxycedrus) and, in degraded soils, give way to replacement shrubs such as the sticky rockrose (Cistus ladanifer) or the broom (Retama sphaerocarpa)The rockrose indicates severe forest degradation, colonizing highly eroded or burned lands, while the broom appears in deeper and less disturbed soils, enriching the pasture and fixing nitrogen.
In Cantabrian and coastal holm oak forests, the shrub and liana-like layer is especially rich. Strawberry trees, laurels, viburnum, butcher's broom, buckthorn, dogwood, hawthorn, evergreen roses, blackthorn, privet and heather They form a complex network. Vines such as ivy, sarsaparilla, or madder (Rubia peregrina) climb trunks and branches, further hindering passage.
On the ground of these forests appear ferns such as Asplenium adiantum-nigrum or A. onopteris, shade plants such as Arum italicum, liverworts (Hepatica nobilis) or wild violetsMany of these species seek out the coolest, north-facing corners, fleeing from excessive sunlight.
When the holm oak forest is further degraded by repeated logging or fires, the vegetation evolves into tall scrubland (strawberry trees, tree heaths, broom thickets) and, if these are removed again, into pre-heathlands or esparto grasslands where Tough grasses and dry pasture species dominateThis process involves a loss of soil and ecological complexity that takes decades to reverse.
Fauna of oak forest ecosystems
The fauna of the holm oak forests is as rich as its flora. The dense forest, the old trunks with hollows, the thorny undergrowth and the lianas offer shelter, breeding grounds and food for a multitude of vertebrates and invertebrates.
Forest birds and birds of prey
Among the birds, the Mediterranean holm oak forests of the Iberian Peninsula are home to very important populations of birds of prey. The Iberian imperial eagle (Aquila adalberti), an emblematic and highly threatened species, uses the holm oak forest as its main ecosystem.nesting in the crowns of the most leafy holm oaks. In these same environments appear the red kite (Milvus milvus), the common buzzard (Buteo buteo), the booted eagle (Hieraaetus pennatus), the cinereous vulture (Aegypius monachus) and other large birds of prey, as well as the black stork (Ciconia nigra) on nearby riverbanks.
In the nightlife scene, the following stand out: owls such as the long-eared owl (Asio otus), the little owl (Athene noctua) or the tawny owl (Strix aluco)which find safe places to breed in the hollows of the tree trunks. The dense foliage of the holm oak forest buffers the wind and creates favorable microclimates during the winter.
In the Cantabrian and Atlantic holm oak forests, there appear birds that are less Mediterranean but equally linked to the forest: woodpecker (Picus viridis), great woodpecker (Dendrocopos major), jay (Garrulus glandarius), great tit (Cyanistes caeruleus), great tit (Parus major), barred wren (Regulus ignicapillus), treecreeper (Certhia brachydactyla) and various warblers, among others.
Small insectivorous birds such as the European robin (Erithacus rubecula), the European wren (Troglodytes troglodytes), the common chaffinch (Fringilla coelebs), the European goldfinch (Carduelis carduelis), the European greenfinch (Chloris chloris), or magpies and azure-winged magpies They take advantage of the huge supply of insects, fruits and seeds.as well as hollows and dense branches to hide their nests.
Mammals: from large ungulates to micromammals
Holm oak forests support very remarkable communities of mammals. Red deer (Cervus elaphus), fallow deer (Dama dama) and wild boar (Sus scrofa) They are common in many Mediterranean oak forests, acting as large herbivores that shape the structure of the undergrowth.
On the peninsula, the Iberian lynx (Lynx pardinus) has historically been the large feline species associated with holm oak groves and pastures, following the abundance of rabbits (Oryctolagus cuniculus) and small rodents. Although their populations have suffered a brutal decline, the recovery of the lynx is once again linked to the conservation of holm oak forests and dehesa mosaics.Other carnivores such as the wildcat (Felis silvestris), the genet (Genetta genetta), the beech marten (Martes foina) or the fox (Vulpes vulpes) complete the food web as secondary predators.
Among the mustelids, the badger (Meles meles) stands out, closely linked to dense wooded areas. It builds complex underground burrows and at night travels along fixed paths in search of earthworms, small vertebrates, fruits and crops.Its stocky silhouette and distinctive black and white facial pattern make it unmistakable.
As for micromammals, holm oak forests harbor a remarkable diversity of insectivores and rodents: Millet's shrew (Sorex coronatus), red vole (Clethrionomys glareolus), field mouse (Apodemus sylvaticus), black-faced dormouse (Eliomys quercinus) or gray dormouse (Glis glis) These are some examples. Their high metabolic rate forces them to feed almost continuously, playing a key role in the recycling of organic matter and serving as prey for many predators.
Reptiles, amphibians and invertebrates
Reptiles find ideal conditions in holm oak forests, especially in the more arid ones: green lizard (Lacerta bilineata), rock lizard (Podarcis muralis), lution (Anguis fragilis) or Aesculapian snake (Zamenis longissimus) They are common in stone walls, clearings and sunny edges.
Amphibians are less prevalent in these very dry habitats, although toads such as the common toad (Bufo bufo) or the midwife toad (Alytes obstetricans) They can appear in cool locations, sinkholes, and areas with some seasonal water.
Invertebrates literally form the functional heart of the holm oak forest. Bees and bumblebees are responsible for the pollination of many flowers, also producing honey.Meanwhile, thousands of species of phytophagous, detritivorous, and predatory insects support a highly complex food chain. Nearly 200 species of phytophagous beetles have been cataloged in the Cantabrian holm oak forests, with particularly interesting findings such as the stag beetle (Lucanus cervus) and large longhorn beetles associated with dead wood.
Fungi and mycorrhizae: the subsoil of the holm oak forest
Beneath the carpet of oak leaves lies an invisible network that is key to the ecosystem. Many oak trees establish ectomycorrhizal relationships with soil fungiThis improves the uptake of water and nutrients and, in turn, provides sugars to the fungus.
In deciduous forests linked to holm oak groves or in mosaics where holm oaks and oaks coexist, well-known fungal species are found: Amanita rubescens, Amanita phalloides, Boletus erythropus, Russula cyanoxantha, Russula virescens, Lactarius spp. and so on, some of them edible and others very toxic.
In well-developed holm oak forests, fungi more exclusive to these environments also appear, such as Boletus queletii, Boletus satanas, different Cortinarius linked to Quercus, Hydnum rufescens, Hygrophorus persoonii, Lactarius ilicis or Russula persicinaThe diversity of mushrooms is especially notable in mature stands with larger trees and relatively stable soils.
In the scrubland and grasslands that replace the holm oak forest, the fungal communities change. Saprobic and heliophilous macromycetes dominate in heathlands and grasslands, in addition to an almost constant presence of endomycorrhizal fungi of the Glomales group, associated with herbaceous roots.
Historical relationship with human beings
The holm oak forests have been exploited by rural communities for centuries. Holm oak firewood, with a very high calorific value, has been used intensively as domestic fuel and to fuel furnaces and forges.Traditional charcoal production yielded light and highly energetic charcoal, from logging rotations of about 10-15 years.
This exploitation generated masses of scrubland where the trees sprouted again and again from the stump, leaving multiple trunks of similar size emerging from each stump. In landscapes like Bages, Urkiola or Urdaibai, old charcoal kiln sites can still be seen as black stains on the ground and oak forests with low trees, highly branched from the base.
In limestone areas of the north, oak wood was also burned in lime kilns to transform the limestone into quicklime, essential as an agricultural amendment. With the advent of fossil fuels and modern energy sources, this use has plummeted.allowing many holm oak forests to regenerate and reach greater heights than in the recent past.
At the same time, livestock management has given rise to the dehesas, a traditional model of high ecological and cultural value. In these systems, the trees are thinned and acorn production, firewood collection, livestock grazing and, sometimes, cereal cultivation are made compatible.These are iconic landscapes linked to products like Iberian ham, but also to first-rate biodiversity.
Threats, degradation and protection figures
Despite their resilience, holm oak forests are not indestructible. Excessive logging, repeated fires, overgrazing, urbanization, and the abandonment of balanced traditional practices have caused significant degradation in many areas.
Forest fires are especially critical in these ecosystems. After a large fire, the soil, already shallow in holm oak forests on limestone, can be lost to erosionthe system then regresses to impoverished stages of scrubland or arid pasture. Even if the holm oak regrows vigorously, recovering its structural and faunal complexity takes many decades.
In karst areas, the use of sinkholes as waste dumps has a serious impact. Leachate contaminates aquifers and disrupts scavenging animal communitiesradically altering the ecological balance. This is compounded by fragmentation caused by infrastructure and the planting of exotic species.
In response to these threats, many key holm oak forests have been protected through various means. The Natura 2000 network includes Special Protection Areas for Birds (SPAs) and Special Areas of Conservation (SACs) based precisely on well-preserved holm oak forests, such as those of the Alberche and Cofio rivers in Madrid or the Cantabrian holm oak forests of Urdaibai in the Basque Country.
In some cases, such as in Urdaibai, regional legislation has declared the Cantabrian holm oak forest a Special Protection Area and subsequently a SAC, recognizing its rarity, its value as a climatic relic, and its essential role as a reservoir of biodiversity in a highly humanized region.
Holm oak ecosystems combine enormous biological richness with a long history of human use, becoming true pillars of the Iberian and Mediterranean landscape. Understanding their types, their dynamics, the fauna and flora they harbor, their threats, and the environmental services they provide —from climate regulation and soil protection to resource production and support for traditional activities— is essential to be able to conserve and manage them wisely in a context of climate change and transformation of the rural environment.