Some shrubs create natural shelters that protect holm oak and cork oak seedlings from livestock browsing, intense sunlight and moisture loss. Their presence can determine whether a young shoot disappears or eventually grows into a mature tree.
Dehesa regeneration often begins in a seemingly insignificant place: beneath the branches of a shrub. There, an acorn can germinate and find more favourable conditions than in open grassland, helping it survive the first years of its life.
These protective plants are known as nurse shrubs because they facilitate the establishment and growth of other species. Like natural nurseries, they provide young holm oaks and cork oaks with a sheltered space during the stage when they are most vulnerable.
The document “Livestock and scrub management for dehesa regeneration”, produced as part of the LIFE ScrubsNet project, highlights that seedlings growing alongside nurse species may have a greater chance of survival than those developing in completely open areas.
The first years: a particularly vulnerable stage
Holm oaks, cork oaks and other species of the genus Quercus grow relatively slowly during their early years. Depending on soil conditions and the availability of water and nutrients, an unprotected specimen may remain only 10 to 20 centimetres tall for three or four years.
During this stage, seedlings are exposed to browsing by livestock and other herbivores, trampling, high temperatures, direct sunlight and soil moisture loss.
A single bite can remove new shoots and delay development. When browsing is repeated year after year, the young tree remains small or eventually disappears without becoming part of the next generation of trees.
Protecting the early stages of growth is therefore essential to ensure the natural renewal of the tree population.
How does a nurse shrub provide protection?
Shrub vegetation modifies the conditions beneath its branches, creating a microhabitat that differs from the open ground around it.
The protocol identifies several mechanisms through which a nurse plant can support the establishment of another species. These include shade, greater moisture retention, nutrient supply and mobilisation, soil aeration, mycorrhizae, support for pollination and protection from herbivores.
Shade reduces direct exposure to solar radiation and can limit extreme temperatures around the seedling. Leaf litter and organic matter accumulated beneath the shrub help improve the soil, while its roots and physical structure contribute to moisture retention.
At the same time, its branches can form a physical barrier that makes it more difficult for livestock to reach the young tree. The shrub does more than simply conceal the seedling: it creates conditions that support its growth.
A natural defence against browsing
Within the dehesa, one of the most important functions of nurse shrubs is protection against herbivores.
Plants that perform this role most effectively tend to be unpalatable to livestock, have dense or thorny branches and be able to spread horizontally around the seedling.
According to the LIFE ScrubsNet document, an effective nurse shrub may reach between one and two metres in height and spread to more than one metre in diameter. These characteristics provide sufficient protection until the holm oak or cork oak reaches a height at which it is less vulnerable to browsing.
The level of protection required will also depend on the type of livestock present. Goats and cattle can reach higher shoots more easily, so the choice and management of nurse plants must be adapted to each farm.
Not all nurse plants need thorns
Thorny species can provide particularly effective protection, but they are not the only plants capable of sheltering young trees.
Some aromatic, resistant or unpalatable species can also perform this role during the earliest stages. Rosemary, for example, is not thorny, but its aromatic nature and low palatability can provide a degree of protection for young trees.
In other cases, several species growing together create a dense, tangled mass of vegetation that restricts access to the seedling. The document itself shows a young holm oak developing between a buckthorn and wild asparagus plant, which together act as a natural shelter.
This diversity makes it possible to adapt the approach to the conditions of each farm, using species that occur naturally or strengthening existing populations when appropriate and permitted.
Choosing species adapted to each area
There is no single nurse plant suitable for every dehesa. The choice must take account of climate, soil, topography, the natural vegetation of the area and the herbivores using the farm.
The protocol recommends checking whether the shrub occurs in nearby areas with similar environmental conditions, consulting information on the potential vegetation of the territory and considering current legislation and any protected status affecting the land.
When introducing or reinforcing a species, it is also important to ensure that it can reproduce easily without having a high invasive potential. The aim is not to replace balanced vegetation with a uniform mass of scrub, but to restore the natural processes that support regeneration.
In many cases, the most suitable nurse shrubs are already present on the farm. Identifying them and avoiding their removal may be the first step towards encouraging tree renewal.
Combining protection with livestock farming
The presence of nurse shrubs does not remove the need for appropriate grazing management. If stocking rates are too high or livestock remain continuously in regeneration areas, they may eventually reach the young trees or damage them through trampling.
Natural protection should therefore be combined with measures such as temporarily excluding livestock from selected patches, providing rest periods, using targeted or rotational grazing and monitoring browsing pressure.
The document explains that natural regeneration based on the controlled recovery of shrub vegetation can protect young Quercus trees during their early growth. This strategy makes it possible to integrate tree renewal into farm management rather than relying exclusively on planting and artificial tree guards.
From a small shoot to a mature tree
A mature holm oak or cork oak may live for generations, but it must first survive a particularly delicate stage. During these early years, the presence of a shrub can make the difference between successful and failed regeneration.
Nurse shrubs demonstrate that relationships between species are fundamental to the functioning of the dehesa. A plant that may appear unproductive could be protecting the tree that will ensure the future continuity of the landscape.
The LIFE ScrubsNet message is clear: regenerating the dehesa does not always have to begin with planting; sometimes, the first step is to conserve the natural nurseries that already exist.
The LIFE ScrubsNet project is co-funded by the European Union through the LIFE programme, LIFE20 NAT/ES/000978.
Recognising, protecting and properly managing these small refuges can help create dehesas with trees of different ages, greater biodiversity and an improved capacity to adapt to climate change.