Acorn sowing, sapling planting and natural regeneration offer different possibilities for ensuring the replacement of holm oaks and cork oaks. The LIFE ScrubsNet protocol highlights the low investment and high effectiveness of natural regeneration, provided that grazing is controlled for the time required.
The absence of young trees is one of the main problems threatening the continuity of many dehesas. As mature holm oaks and cork oaks age and disappear, it is essential to ensure that new trees become established to replace them.
But what is the best way to achieve this?
The document “Livestock and scrub management for dehesa regeneration”, produced within the LIFE ScrubsNet project, compares three approaches to renewing tree cover: sowing acorns, planting saplings and natural regeneration.
Each option has different advantages, limitations and management requirements. The choice will depend on the characteristics of the holding, the condition of the existing trees, the resources available and, above all, the ability to protect young trees during their earliest stages of growth.
Sowing acorns: making use of the farm’s own trees
The first option involves sowing selected acorns, preferably from healthy, long-lived trees with good production.
One of its main advantages is that it can produce trees originating from the holding itself and therefore adapted to local environmental conditions. It also provides an opportunity to select specimens with desirable characteristics, such as longevity or high acorn production.
However, this technique produces results only in the long term. Success rates can be low, so it is usually necessary to sow a large number of acorns to ensure that some germinate and survive.
Germination is only the beginning. The new seedlings must also withstand drought, competition from other plants, trampling and livestock browsing before they can become trees.
Planting saplings: faster results, but greater investment
Planting one- or two-year-old saplings can provide visible results more quickly than sowing acorns directly.
The saplings, usually obtained from nurseries, can be placed in areas where tree density needs to be increased or where no regeneration is taking place. In some cases, this method may also be supported through public grants or reforestation programmes.
However, the protocol identifies several limitations. Nursery-grown saplings may be less hardy and significant losses can occur after planting. There may also be plant-health risks associated with moving plant material.
Tree guards must also be installed, and monitoring and maintenance are required. Planting can therefore involve considerable financial and labour investment.
Natural regeneration: working with what is already growing
Natural regeneration is based on identifying and supporting germinated acorns, seedlings, young shoots and root suckers that are already appearing on the holding.
Instead of introducing all trees from outside, this strategy makes use of existing natural processes and creates the conditions required for young trees to develop.
The document identifies low investment and high effectiveness as the main advantages of this approach. Trees germinate and grow under the specific conditions of the holding, which may improve their adaptation to its climate, soil and water availability.
However, natural regeneration does not mean a lack of intervention. Its success depends on active management that protects the areas where young trees are growing.
Protecting enclosures, patches and boundaries
For young holm oaks and cork oaks to develop, they must be protected from repeated browsing by livestock.
The protocol proposes setting aside or protecting selected enclosures, patches and boundaries for several years, until the young trees are sufficiently developed to withstand seasonal grazing.
The time required will depend on the initial conditions in each area. The presence of small shrubs, scrub seedlings, seeds, young trees and sufficiently diverse vegetation can accelerate the process.
During this period, grazing should either be excluded or take place only occasionally, with low stocking rates and outside the most sensitive seasons. Alternatively, selected patches can be enclosed with fixed or electric fencing while grazing continues across the rest of the field.
An investment in the future of the holding
Protecting an area from grazing can temporarily reduce the amount of pasture available. The document indicates that this cost may continue for at least the first five years.
This is one of the main limitations of natural regeneration. It is also possible that some of the trees that become established will not be high acorn producers.
Even so, temporarily setting aside part of the holding should be understood as an investment in its future. Without young trees to replace those that are disappearing, tree cover will gradually decline, along with many of the productive and environmental services it provides.
Three techniques that can be combined
Acorn sowing, sapling planting and natural regeneration do not have to be treated as incompatible alternatives.
A holding can protect the young trees that are already present while also sowing acorns or planting saplings in areas where regeneration is insufficient. The document recognises these measures as possible alternatives or complements to natural processes.
The key is to assess each part of the holding and apply the most appropriate solution. In some areas, reducing livestock pressure may be enough. In others, it may be necessary to protect patches, introduce selected seeds or reinforce tree cover through planting.
Regeneration also means managing livestock
The success of all three approaches depends to a large extent on livestock management.
Sowing or planting does not guarantee tree renewal if young trees remain exposed to browsing and trampling. Likewise, natural regeneration cannot progress if livestock remain continuously in the same areas and vegetation is not given adequate rest periods.
The LIFE ScrubsNet message is clear: before planting, it is worth observing what is already trying to regenerate naturally and considering which management changes could help it thrive.
Protecting a germinated acorn, a young shoot or a small holm oak today may mean having the tree that secures the future of the dehesa tomorrow.
The LIFE ScrubsNet project is co-funded by the European Union through the LIFE programme, LIFE20 NAT/ES/000978.