Seedlings and young trees are particularly vulnerable to browsing, trampling, rooting and other pressures from livestock. Where natural vegetation does not provide sufficient shelter, plant guards can help young plants survive their first years. In the dehesa, protection may be natural, through nurse shrubs, or physical, using structures suited to local conditions.
Getting a holm oak or cork oak to germinate is only the beginning. For that seedling to become a tree, it must survive several years during which its small size makes it especially vulnerable.
The LIFE ScrubsNet document “Livestock and scrub management for dehesa regeneration” shows that excessive browsing can prevent seedlings from growing and threaten young trees that lack protection.
Protecting selected young plants can therefore become an important part of tree regeneration.
Nurse shrubs: protection provided by nature
The first solution does not necessarily need to be artificial.
Shrubs can create a more benign microhabitat beneath their canopy than is available in open ground. The protocol explains that they can act as nurse plants or natural guards, facilitating the establishment of other shrubs and tree seedlings. Natural regeneration and reforestation can be more successful when seedlings develop in association with nurse vegetation.
Their effect is not limited to making plants harder for livestock to reach. Nurse shrubs may provide shade, retain soil moisture, influence nutrients and mycorrhizae, and protect young plants from herbivores.
Thorny, unpalatable and large enough
Not every shrub provides the same level of protection.
Where protection from herbivores is the main objective, the protocol identifies several useful characteristics: low or no palatability, thorny branches or stems, a height of around one to two metres and sufficient lateral growth to form a substantial protective patch.
The required size also depends on the livestock species concerned. The document identifies goats and cattle as particularly challenging because of the height they can reach when browsing.
Even non-thorny plants can sometimes help. Rosemary is cited as an example of a hardy, relatively unpalatable plant that may provide some protection during the early stages of tree regeneration.
When natural protection is not enough
Not every seedling appears beside a suitable nurse shrub. Some may also grow in strategic locations where managers particularly want them to survive.
In these situations, physical plant guards can be placed around young plants or trees to protect them from adverse conditions and livestock pressure.
The protocol identifies browsing damage from herbivores and pressure from pigs through trampling, rooting and rubbing. In some natural regeneration areas, guards may be fitted only to selected individuals that managers want to encourage.
This allows protection efforts to be concentrated on the plants of greatest interest.
Tubes and other structures
One of the systems described in the document is the protective tube placed around newly established seedlings.
As well as limiting herbivore damage, these tubes may provide a more stable microclimate, reduce exposure to wind and lessen competition from vegetation immediately around the seedling.
The protocol also illustrates other structures, including designs that imitate the deterrent effect of a thorny shrub and perimeter guards used where pigs exert particularly high pressure.
The appropriate design therefore depends on both the plant being protected and the livestock likely to damage it.
Biodegradable alternatives
Physical protection does not necessarily have to rely only on conventional materials.
The document mentions biodegradable guards made from paper, cardboard or coconut fibre, designed to protect plants temporarily before gradually decomposing once they have become established.
Pruning material and piles of branches placed around seedlings can also act as a natural, temporary barrier.
Different methods therefore pursue the same objective: buying enough time for a young plant to reach a safer stage of development.
The first years matter
Protection can be especially important for holm oaks and cork oaks.
Young Quercus can grow slowly. Under some conditions, the protocol notes that unprotected individuals may remain only 10–20 centimetres tall for three or four years.
Throughout this period, a shoot only a few centimetres above the ground remains readily accessible to livestock.
Protection does not need to be permanent. Its purpose is to help the tree reach a stage at which it can continue developing with a lower risk of damage.
Protection also requires monitoring
Installing a guard is not the end of the job.
LIFE ScrubsNet states that the type of protector should reflect site conditions and management objectives. Regular monitoring is also needed to ensure that it continues to work properly and to make adjustments where necessary.
Protection should therefore form part of a broader strategy combining grazing management, scrub conservation and observation of regeneration.
Natural or physical: protect in order to regenerate
The LIFE ScrubsNet message is straightforward: if mature trees are to exist in the future, today’s young trees first need the opportunity to survive.
In some places, a nurse shrub can provide sufficient shelter. Elsewhere, tubes, mesh, dedicated structures, biodegradable materials or pruning residues may be required.
There is no single guard suitable for every situation. The important steps are to identify the pressure, choose an appropriate form of protection and maintain it for as long as the young plant needs it.
A few years of protection can make the difference between a seedling that disappears and a tree that becomes part of the future dehesa.
The LIFE ScrubsNet project is co-funded by the European Union through the LIFE programme, LIFE20 NAT/ES/000978.