Underestimated pioneers

Tree species that largely dominate in early stages of regeneration on open areas are called pioneer tree species. They share the following characteristics:

• They quickly take root naturally on clear areas - even when there are few old trees of their species nearby.
• They flower and bear fruit at a young age.
• They produce seeds almost every year and often in prolific numbers.
• Their seeds are dispersed easily by the wind, water or also animals.
• They are rapidly distributed and sometimes over very long distances.
• They have a low seed weight with only a small stock of nutrients to aid germination and establishment.
• They do not tolerate much shade.
• They are not demanding in terms of site requirements and can thrive well on open forest soil (mineral soil).
• They are hardy species in extreme conditions such as frost, strong direct sunlight and wind.
• They grow rapidly as juvenile trees and reach their highest annual increase in height and diameter at an early age (after 8 to 15 years).
• They usually have a short lifespan because their wood is not very resilient.

As well as relatively short-lived species such as birch, alder, willow, aspen and rowan - which we are going to talk about in this article - some longer-lived tree species such as larch, pine and the non-native robinia are also pioneer tree species. The characteristics of the pioneer tree species, especially their resilience in extreme conditions, give us good reasons to think that our forests might in fact benefit from an increase in the proportion of pioneer tree species in times of climate change. Or better still: these tree species might even play their part in solving silvicultural problems.

Nature is wasteful: every year, many times what we can sow artificially in terms of seed or young plants is dispersed in the form of seed over the forest. Research results from the studies on the development of damage areas after wind storms such as Vivian, Wiebke, Lothar or Kyrill show that forest areas - with very few exceptions - can redevelop all by themselves within a short period of time. And pioneer tree species play an important role in this. The diversity of tree species in this natural reforestation is sometimes astoundingly high. What is critical however, is that there is little pressure from browsing - or that the area is artificially protected. But how can soft deciduous woods be used in silviculture in concrete terms? In fact, there are various possibilities:

One method that has been known for a long time but is seldom used, is to use soft deciduous woods to provide shelter for shade-loving tree species. This is suitable for clear areas. In this case, the soft deciduous wood that regenerates naturally is left, but the number of trunks is reduced through thinning measures to around 1.000 trees/ha. Under these pioneer trees, shade-loving tree species such as beech and fir can be introduced by planting during the given time period.

The pioneer crop protects the regeneration from frost and direct sunlight. Spruce natural regeneration usually develops in addition to this. Once the shade-loving deciduous trees are well established, the pioneer crop can be gradually removed, depending on the light requirements. However, some individual pioneer trees with good trunk formation can also continue to be encouraged and promoted to produce quality timber.

The most cost-effective way to create a new forest after events that have caused damage is to use the natural regeneration over the entire area. This develops whether the area has been completely or partially cleared, or not cleared at all. Pioneer tree species play an important role here. The requirements are that there must be sufficient seed trees available at the appropriate distance, that the accompanying vegetation is not too strong and that there is not too much browsing pressure.

High quality or valuable timber can also be produced with tree species such as birch, alder, rowan or even aspen. So a pioneer tree can certainly also become a target plus tree. The requirements for this are tree vitality and a good trunk formation, site suitability and a good prognosis in terms of the cultivation risk. It is crucial that the target or future crop trees are selected early and consistently and continuously promoted.

Pioneer trees in particular must be promoted consistently and very early, so that their crowns can develop properly at the right time. Trees with well-developed crowns also have a high growth. In this way, valuable timber can be produced on good sites relatively quickly – i.e. within 60 to 70 years. This allows clear areas to be quickly restocked. It has the added advantage that the soil is then also covered, thus preventing erosion and the leaching of nutrients. The risk of nitrate leaching into the ground water is also reduced. The regeneration of the forest, including its soil, is thus achieved naturally.

Fig. 3: Planting options for supplementary planting on succession areas: (From the left) circular cluster, square cluster, and “Nelderrad” method of planting in formation. The main tree species (target trees) are dark green; the secondary tree species (shade-tolerant deciduous trees and/or pioneer tree species) are light green. The diameters shown depend on the tree species: closer spacing for deciduous trees; wider for coniferous trees.

Fig.: 4: Possible silvicultural measures with pioneer trees (example scheme for around 1 ha of forest): left picture: as “fillers and quality drivers” in the qualification phase of final crop tree species introduced in clusters (“cherries”); or right picture: supplementary planting in gaps with sparse natural regeneration.

Another alternative is to use pioneer tree species wherever gaps have appeared in cultures during the establishment phase. Instead of replanting, soft deciduous woods that arrive naturally in these areas can be used to close the gaps in the stand and thus also to promote the quality of the main tree species. If the gaps are not too big, other tree and shrub species such as elderberry, hazel and bird cherry can also be included to form temporary lower and intermediate storeys. This is also very valuable from the conservation point of view. In the subsequent sizing phase, these pioneers and woody shrubs can continue to be used as a lower storey and intermediate storey, contributing to the quality of the target trees. In the course of the consistent thinning to give light to the crown of the target trees, enough light usually passes through the canopy to allow the lower storey to grow as well, so that it can be used to promote trunk quality. If the conditions are right, this integration can be more ecological, cheaper and easier than retrospectively planting beech and lime, for example. There is also no danger of the pioneers growing into the crowns of target tree species later and competing with them, because of the limited light available.

• Pioneer tree species are being reassessed because of their properties in the face of climate change.
• Their robustness, rapid growth, and ability to adapt allow new opportunities, especially after disturbances in the forest.
• If pioneer tree species are to be used successfully, silvicultural measures must be objective-oriented, continuous and consistent.
• Pioneers are often a gift of nature, and they can be part of the solution when it comes to meeting silvicultural challenges. They are not however a cure-all solution to problems, as there are both site and climatic limitations on their use.