Experimental design
ORPHEE is a tree diversity experiment part of the international tree diversity network TreeDivNet. The experimental design was first described by Castagneyrol et al. (2013, Journal of Ecology). From 2015 on, we added an irrigation treatment, described by Castagneyrol et al. (2017, Ecology and Evolution). The outlines below are copied and pasted from these papers.
A tree diversity experiment...
"The experimental plantation was established in 2008 on a clear cut of former maritime pine stands. Stumps were removed and the site, on a sandy podzol, was ploughed and fertilized with phosphorus and potassium before planting. In total, 25 600 trees of five native species (European birch, Betula pendula; pedunculate oak, Quercus robur; Pyrenean oak, Quercus pyrenaica, holm oak, Quercus ilex; and maritime pine, Pinus pinaster) were planted within a 12 ha area.
Eight blocks were established, with 32 plots in every block, corresponding to the 31 possible combinations of one to five species, with an additional replicate of the combination of five species.
Photo: What a (holm) oak - birch mixture looked like in 2010.
Each plot contains 10 rows of 10 trees planted 2 m apart, resulting in 100 trees per plot, with a plot area of 400 m². Tree species mixtures were established according to a substitutive design, keeping tree density equal across plots. Within plots, individual trees from different species were planted in a regular alternate pattern, such that a tree from a given species had at least one neighbour from each of the other species within a 2 m radius. Plots are separated by a distance of three metres and were randomly distributed within blocks. Blocks cover an area of 100 × 175 m.
The entire experimental site is fenced to prevent grazing by mammalian herbivores. The understorey vegetation was mowed once per year in 2009 and 2010 between trees. Since, it is only mowed between plots, once a year
... manipulating water constraint
Since 2015, half of the blocks are irrigated. Irrigation consists in sprinkling ca 42 m³ per night and per block from early May to late September, corresponding to ca 3 mm/day per plot. This volume was calculated based on regional climatic data (evapotranspiration) and is assumed to avoid any soil water deficit in the irrigated blocks during the entire growing season.
Photo: Irrigation at work in an oak monoculture in 2015.
Site management
In 2009 and 2010, understory vegetation was mowed between trees to ensure initial tree installation and survival.
Between plot vegetation is mowed once a year in summer.
In autumn 2017, birches and pines at plot edges (i.e., most external trees) were pruned to allow the circulation of machines and people.
Photo: ORPHEE from the above, April 2019. (c) Raphael Segura
