n the Life project www.resilias.eu/en, we find out which invasive species the Ecosystem Resilience Approach (ERA) might work for. After which we elaborate and demonstrate the integration of the species in question. For forests, the species for which the ecosystem approach is of interest include Prunus serotina, Robinia pseudoaccacia, Pinus radiata, Eucalyptus spp. and Ailanthus altissima.
What these tree species have in common are pioneer characteristics that they combine with a high light requirement and a translucent crown. As a result of the latter properties, native tree species, especially late successional tree species, can establish themselves below these invasive tree species. In the forest succession that follows. the proportion of these pioneer tree species decreases to disappear almost completely from the forest after a tree generation.
Our most recent challenge is to explore whether forest ecosystems also cope in this way with invasive tree species that tolerate and cast more shade. As an example, we took the Boxelder Maple (Acer negundo). We studied it in a region where it is considered invasive, Nouvelle Aquitaine in south-western France.
Acer negundo is a tree that establishes itself mainly in riparian forests. It exhibits pioneer characteristics such as easy establishment, rapid growth, exuberant seed setting at a young age. It combines these traits with average shade tolerance and average shade casting ability. We visited many forests that regularly flood along the Dordogne and Adour rivers.
Our observations showed that many native tree species can establish under mature Acer negundo canopy. Examples include Ulmus laevis, Ulmus minor, Sambucus nigra, Laurus nobilis, Fraxinus excelsior, Fraxinus angustifolia, Corylus avellana, Juglans regia, Prunus avium, Crataegus monogyna, Quercus robur.
It was notable that Acer negundo hardly rejuvenates under its own crown. It is still an open question whether this is due to lack of light or some other negative feedback, such as fungi living with this tree species.
Below are some photos taken during our reconnaissance:
Photo 1 Mature Acer negundo here dominates the forest along the Adour, near Mugron.
Photo 2 Some Acer negundo have been standing in the riverine forest for a long time.
Photo 3 Below Acer negundo is almost exclusively native forest regeneration.
Photo 4 Acer campestre
Photo 5 Corylus avellana
Photo 6 Fraxinus excelsior grows up through the Acer negundo crown
Photo 7 Juglans regia
Photo 8 Tilia heterophyla
Photo 9 On sand dunes along the river mainly rejuvenation of Quercus robur