Tree species with a short lifespan are occupying more and more space in our forests. They primarily include pioneer tree species such as goat willow, aspen, silver birch, bird cherry and rowan. These tree species can quickly take root on clear-cut and storm-damaged areas and initiate the forest succession.
These pioneer tree species have the following things in common:
- They produce prolific numbers of seeds
- They grow rapidly as young trees
- They regenerate easily
- They are relatively undemanding in terms of site requirements
- They have mechanisms allowing them to distribute seeds widely
Taking the goat willow as an example, we aim to show here just how important pioneer tree species are ecologically for variety and biodiversity in our forests.
Our willow species play a significant role particularly along streams and rivers or as shrubs in marshes and on flood plains. The goat willow (Salix caprea) is the only common willow species that is not dependent on being close to water or on riparian forests, and it often adapts on succession areas in forests or on the edges of forests.
The goat willow occurs from the lowlands to approx. 1800 metres above sea level and is widespread in central and western Europe - its natural area of occurrence stretches as far as northern Asia. It prefers loose clay soils with a good water supply and grows well on the edges of forests, in clearings and on clear-cut areas, but also in quarries and gravel pits.
In times when the land rent theory or "net soil yield theory" dominated in forestry, soft deciduous wood trees were dismissed as "not real wood" and considered to be "suppressive" of other species, and as such they tended to be removed during tending measures. Goat willow and other soft deciduous wood tree species were not used for silvicultural purposes to fill gaps or to promote the height growth of wanted trees, either.
The carbon-nitrogen ratio of the leaves of goat willow is as good as that of the leaves of lime trees and it thus improves the quality of the humus and the topsoil much better than oak or beech leaves.
Willows are generally dioecious, which means that male and female flowers are to be found on distinct specimens. Almost all willows are pollinated by insects and thus secrete nectar. As well as bumblebees, wasps and bees, it is mainly hoverflies, beetles and butterflies that visit the flowering willow catkins. As the goat willow blossoms early and has a lot of flowers, it is a valuable bee pasture. For the honey bee, the goat willow is the first mass forage source for bees in the year.
In Germany, around 500 species of insect are dependent on willow species as a food source. There are also predators and parasitoids that feed on the primary consumers. This raises the number of insect species to be found on willows to over 1000. 179 species of butterflies and moths alone are to be found on Salix. 18 % of all large butterfly species in central Europe are linked in some way with Salix species. The goat willow in particular plays a key role for the insect world in succession areas in low and high mountain ranges.
It is a caterpillar food plant for 37 butterfly and moth species, including the puss moth (Cerura vinula) (Fig. 2). Bugs suck on the leaves of the goat willow, and the larvae of beetles and saw flies (Tenthredinidae) also feed on them. The caterpillars of the goat moth (Cossus cossus), which can grow to 10 cm in length, live in the wood of the willows. The larvae of the highly endangered weaver beetle (Lamia textor) develop in the base of the trunk and the roots.
When it comes to nectar-sucking birds, we automatically think of humming birds, but there are also bird species here that like to suck flower nectar. Blue tits (Parus caeruleus) thus specifically seek out flowering goat willows in order to suck nectar from the blossoms and to consume the protein-rich pollen (Fig. 4). In Europe around 30 bird species up to now are known to seek out blossoms at least from time to time, especially tits, warblers (Sylvia) and leaf warblers (Phylloscopus).
Because it is so widespread, the goat willow thus plays a important role for some bird species as a valuable, high-energy source of food. Older, partly decaying pioneer trees are an important feeding biotope for birds such as the lesser spotted woodpecker (Dryobates minor).
The pioneer tree species not only boost the variety of tree species in our forests. They also contribute significantly towards increasing biodiversity in the forest. For ecological reasons, goat willows in particular should therefore not be removed across larger areas in forest tending measures. We should take account of the importance of pioneer tree species for the diversity of species in our forests.