In contrast to the BIOTREE-SPECIES approach, the diversity gradient in the BIOTREE-FD experiment will not be established by manipulating the number of species (all experimental assemblages contain four species), but by mixing species with different functional attributes.. Based on the "FD"-approach by Petchey & Gaston (2002), the functional diversity (=FD) of all 1,820 possible four-species mixtures from a pool of 16 tree species were calculated. The trait matrix contained 9 criteria for which data for all species were available. The criteria selected represent attributes which are indicative for complementary resource use and nutrient cycling, which represent the two main functions of interest of this experiment: Leaf type, light requirements, height growth, growth rate, crown architecture, root growth, root architecture, leaf N concentration, C/N ratio of litter.
The full range of FD of all 1,820 mixtures was divided into four even-sized groups, representing "very low", "low", "high" and "very high" functional diversity. Out of each group, 6 mixtures were randomly selected. One mixture will be planted twice to have two replicates of one identical species combination.
Petchey, O.L. and K.J. Gaston, Functional diversity (FD), species richness and community composition. Ecology Letters, 2002. 5: p. 402-411.
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