The acronym TWIG stands for Tree Wheels In Geerbos. This 'climate forest' was planted by Ghent University (UGent) as a symbol for its sustainability ambitions and to sequester a little bit of their CO₂ emission. The experiment was established near Geerbos in Melle in 2017. Circular plots were planted with one up to three tree species and from high to low tree density.

The TWIG plantation near the old forest parcel 'Geerbos'

Design
The experiment consists of circular plots that follow the Nelder Wheel design. Trees are planted along concentric circles with increasing radii. Hence, the planting density is higher in the center of the plots than at the plot border. The central area of the wheels is planted as well, but those trees are not part of the Nelder design. The plots are planted with one up to three tree species from a species pool of four species: two associated with ectomycorrhiza (Fagus sylvatica, Quercus robur), two with arbuscular mycorrhiza (Acer pseudoplatanus, Prunus avium). All four monocultures, the six 2-species mixtures, and the four 3-species mixtures are planted. The monocultures and 3-species mixtures are replicated once. Mixing is on a tree by tree level within plots. The plots are grouped into blocks that represent 'close to forest edge' vs. 'non forest edge' and 'deep clay' vs. 'more shallow clay'.

The tree wheels of the TWIG plantation

Site characteristics

location	Melle
former land use	arable field, maize
altitude	16 m
soil type	Retisol (WRB) Ldc - moist, sandy silt with degraded texture B horizon (Belgian Soil Classification System)
area	2.4 ha
no of plots	22
plot size	radius = 15.7 m
no of trees planted	4598
planting date	Feb 2017
diversity variables	species richness functional diversity
diversity gradient	1, 2, 3 sp. AM, EM fungi
size species pool	4
species pool	AM species: Acer pseudoplatanus, Prunus avium EM species: Fagus sylvatica, Quercus robur
contact person	Kris Verheyen

Research
The main research focus is investigating how tree density affects the diversity effects on tree growth and carbon sequestration.

Extra information
Send an e-mail to the contact person, visit the experiment's website, or explore the publications:

• Messier C, Bauhus J, Sousa-Silva R, Auge H, Baeten L, Barsoum N, Bruelheide H, Caldwell B, Cavender-Bares J, Dhiedt E, Eisenhauer N, Ganade G, Gravel D, Guillemot J, Hall JS, Hector A, Hérault B, Jactel H, Koricheva J, Kreft H, Mereu S, Muys B, Nock CA, Paquette A, Parker JD, Perring MP, Ponette Q, Potvin C, Reich PB, Scherer-Lorenzen M, Schnabel F, Verheyen K, Weih M, Wollni M, Zemp DC (2021) For the sake of resilience and multifunctionality, let's diversify planted forests! Conservation Letters e12829 - https://doi.org/10.1111/conl.12829

The experiment was planted on 17 February 2017 with the help of lots of volunteers, BOS+, and Ghent University.