Experiment

To evaluate the impact of increasing tree diversity on ecosystem functioning, a highly innovative large-scale forest biodiversity experiment has been established. On three different experimental sites, plots with synthetic communities of single species and plots with increasing tree species richness were installed. When fully grown, differences in various aspects of ecosystem functioning in plots that have developed under the same abiotic circumstances can be related with different tree species richness levels. The sites are located in Zedelgem (Flanders), Gedinne (Wallonia) and Hechtel-Eksel (Flanders).

Basic design

The basic experimental design is the same for each study site and follows a classical synthetic community approach. Monocultures and mixtures of two up to four tree species were planted on an environmentally homogeneous site. All species combinations occur, and there is one (blocked) replication. Each study site has 40 plots, i.e., 5 plots for each of the four diversity levels (1, 2, 3, and 4 species) and two replications. Yet, the overall number of plots in the FORBIO experiment is 127 instead of 120. At the Hechtel-Eksel site, one extra plot is left for spontaneous succession. At the Zedelgem and Gedinne sites, different provenances of oak (Zedelgem) and beech (Gedinne) were used as an extra treatment; two additional monocultures with the extra provenances were added in Zedelgem and four in Gedinne.

Trees were planted on a 1.5 m x 1.5 m grid. In mixed plots, individual species were randomly assigned to homogenous groups of 3 x 3 trees. Five site-adapted but functionally dissimilar species were planted per site. Each species occurs in a similar frequency, i.e., in 20 of the 40 plots, and the two-species combinations occur in similar frequencies as well (10/40). An extensive soil survey prior to the planting enabled attributing treatments and replications to the experimental plots in such a way that there is no covariation between any of the soil variables and the presence/absence of a tree species or the diversity treatments, a major strength of the FORBIO experiment. The overall species pool consists of broadleaved as well as coniferous tree species that are particularly relevant for forestry in Belgium. The species differ in, e.g., root characteristics, temporal growth dynamics, and shade tolerance.

Characteristics of the different sites

Zedelgem (Flanders)
Local name	Vloethemveld
Area	9.5 ha
Owner	Flemish region
Former land use	Agriculture (arable land)
Altitude	11-16 m
Soil type	Relatively dry sandy soil (Zbh) to moderately wet loamy sand soil (SdP)
Planting date	3-5/12/2009 and 24-27/03/2010
Speciespool	Fagus sylvatica, Betula pendula, Pinus sylvestris, Tilia cordata, Quercus robur (1 vs 3 provenances).
Local manager	Agentschap voor Natuur en Bos (ANB)
Site manager	Prof. Kris Verheyen (Department of Forest and Water management, Ghent University)
Gedinne (Wallonia)
Local name	Gribelle and Gouverneurs
Area	2 x 4.5 ha
Owner	Town of Gedinne
Former land use	forest: broadleaved until 1920, spruce plantations until 2005
Altitude	367-376 m (Gribelle) and 421-426 m (Gouverneurs)
Soil type	Moderately dry stony loam soils (Gbb; both sites)
Planting date	April-May 2010
Species pool	Acer pseudoplatanus, Pseudotsuga menziesii, Larix x eurolepis, Quercus petraea, Fagus sylvatica (1 vs 3 provenances).
Local manager	Division de la Nature et les forêts (DNF)
Site manager	Prof. Quentin Ponette (Department of Environmental Sciences and Land Use, University of Louvain-La-Neuve)
Hechtel-Eksel (Flanders)
Local name	Pijnven
Area	8 ha
Owner	Flemish region
Former land use	heathland converted to pine plantation in 1908
Altitude	55-56 m
Soil type	dry sandy soil with gravel substrate (tZbg/Podzol)
Planting date	Fall 2012
Species pool	Betula pendula, Larix kaempferi, Pinus sylvestris, Pseudotsuga menziesii, Quercus petraea
Local manager	Agentschap voor Natuur en Bos (ANB)
Site manager	Prof. Bart Muys (Division Forest, Nature and Landscape, KU Leuven)

Panoramic photo of the Zedelgem site, autumn 2012 (photo: Jan den Ouden)

Details

More details about experimental sites and design can be found in the final report of FORBIO for SSD (outputs). 

Research

Due to the slow development of forest ecosystems, no research results of the experimental sites are available yet. On the longer run, this experiment will potentially yield very valuable outputs. Several research projects have already started on the sites (research).