Tree of heaven, a pioneer tree species in Italian coppice forest

In the week before Ascension Day, I was south of the Alps as a north-western European forest ecologist. The occasion was appointments in the forest with forest ecologists and forest managers who are all looking for an efficient and effective handling of tree of heaven (Ailanthus altissima) in the forest.

Pioneer thanks to climate change

Tree of heaven is still barely showing its invasive nature in north-western Europe. That is likely to change with warming climate. Young trees of heaven do not survive our winters now. Not because these are too cold (tree of heaven tolerates -35 to -40) but because the saplings are not sufficiently mature (wooded): green seedlings or shoots of heaven trees only tolerate about -5 degrees in winter. To mature, tree of heaven seedlings and shoots need long warm growing seasons. This is also the reason why observations of tree of heaven seedlings have been increasing in recent years.

In an earlier post, I concluded that tree of heaven is a typical pioneer tree species: expands rapidly after which it makes room for other tree species: https://www.dhirubhai.net/feed/update/urn:li:activity:6936759752420945920/

Approach to tree of heaven in coppice forest

In closed-canopy forest, tree of heaven is eventually displaced. But what about it in coppice? The forests in South Tyrol / Alto Adige consist of mixed tree coppice. Until a few decades ago, all trees were cut down every 20-30 years to use the wood for varied purposes, but mainly as firewood. Constantly cutting down coppice creates pioneer situations that increase the proportion of pioneer tree species such as birch (Betula pendula), aspen (Populus tremula), tree of heaven (Ailanthus altissima) and black locust (Robinia Pseudoacacia).

Large-scale experiments have recently been set up in South Tyrol / Alto Adige to reduce the proportion of tree of heaven in the coppice. Not that this is inadequate as firewood but out of the desire for native forest. The approach is to systematically remove the tree of heaven and maintain the other tree species in the coppice.

On the experiments I visited, this happened in 2016 after which no tree of heaven occurred in the canopy at those sites. In May 2023, I could see that due to its rapid growth, the tree of heaven once again had a sizeable, but smaller, share in the canopy. Regularly removing tree of heaven while the other tree species stay in place reduces the proportion of tree of heaven in the canopy because the conditions for establishment of this pioneer tree species are less frequent.

Tree of heaven displaced by doing nothing

But with this, de facto coppice is also abandoned. The forest develops into mature standing forest. This strengthens the resilience against local dominance by the pioneer tree of heaven. This development is in line with the current change in forest management. The coppice is hardly exploited anymore and the main function of the forest is to protect habitation in the valleys from landslides and avalanches.

This forest development affects tree species composition. My main focus here was on what happens in places where the tree of heaven (partly) dominates the canopy. Pronounced pioneer tree species hardly appear in the rejuvenation. This applies to both the non-native

- tree of heaven (Ailanthus altissima) and

- black locust (Robinia pseudoaccacia)

as well as for the native pioneer tree species

- birch (Betula pendula),

- aspen (Populus tremula),

- Scots pine (Pinus sylvestris) and

- European larch (Larix decidua).

The largest share of forest rejuvenation under tree of heaven has

- manna ash (Fraxinus ornus).

This is mainly because this tree usually also co-dominates the crown canopy. A close second in forest rejuvenation is the

- hop-hornbeam (Ostrya carpinifolia).

Furthermore, under the tree of heaven we found scattered rejuvenation of

- small-leaved lime (Tilia cordata),

- sessile oak (Quercus petraea),

- downy oak (Quercus pubescens),

- Norway spruce (picea abies),

- beech (fagus sylvatica),

- whitebeam (Sorbus aria),

- chequers (Sorbus torminalis),

- sorb tree (Sorbus domestica),

- sweet cherry (Prunus avium),

- sweet chestnut (Castanea sativa),

- walnut (Juglans regia),

- silver fir (Abies alba),

- nettle tree (Celtis australis) and

- hazelnut (Corylus avellana).

In mature forest, tree of heaven can provide quality timber

This species-rich forest consistently works away local dominance of the pioneer tree of heaven. The natural succession of tree species, forest succession, ensures this. Knowing that tree of heaven cannot be dominant here for a long time, the question arises whether the positive properties of tree of heaven can be exploited. Therefore, we visited quality wood experiments with tree of heaven from 2018, aimed at producing valuable wood for the furniture industry, for example.

Depending on the application, the wood quality of tree of heaven is comparable to ash (Fraxinus excelsior) and walnut (Juglans regia). The researchers selected trees of heaven at the pole stage. These were on average 11 years old. In addition, comparably thick and tall nettle trees, hop-hornbeams and manna ashes were selected with an average age of 16, 19 and 22 years old, respectively.