Aboveground overyielding in a mixed temperate forest is not explained by belowground processes

Lack of effect of admixture proportion and tree density on water acquisition depth for European beech (Fagus sylvatica L.) and sycamore maple (Acer pseudoplatanus L.)

“…Soil samples were collected 10 months after the labelling experiment (i.e. May 2015) by Fruleux et al (2018), down to a depth of 60 cm at 92 locations in the plantation. From our field observations, fine roots were rarely found below a depth of 60 cm.…”

“…For each location, fine root biomass was estimated in three soil sections (0-10, 10-30, 30-60 cm in depth, corresponding to the soil water source depths used in the MixSIAR model, see Section 2.5). To distinguish the species, we used the near infrared spectroscopy method that exploits the difference in biochemical composition of the fine roots from the two species (see Fruleux et al 2018 for a precise description of the method). These data from the plantation were only used to estimate from a linear model the vertical distribution of the fine root biomass of each tree selected in this study as follows.…”

“…Stand density may thus influence the acquisition of soil resources, including water. In addition, the depth of water acquisition (D wa ) is a plastic trait in plants, which responds to both abiotic and biotic conditions such as water availability and the identity of the competitors (Grossiord et al 2014;Fruleux et al 2018;Bello et al 2019;Brinkmann et al 2019). In some cases, competition reduction among trees may emerge from the vertical or temporal differentiation of soil water sources among species or functional groups (Silvertown et al 2015).…”

“…This study was conducted in a 16-year-old mixed plantation of European beech (Fagus sylvatica L.) and sycamore maple (Acer pseudoplatanus L.), which included a double gradient of tree density (number of trees per hectare) and species proportion (percentage of each species). In the plantation, Fruleux et al (2018) studied the profiles of soil water acquisition of trees of both species for three different mixture levels (nearly pure conditions of beech and of maple, and an even mixture) at the highest plantation density during a summer drought. They showed that, on average, both species extracted water from deeper soil layers when they were in an even mixture relative to nearly pure conditions.…”

“…In our study, we performed a water labelling experiment during a wet summer in order to study the interactive effect of mixture and tree density levels on D wa . We determined the water acquisition profiles of trees located along both tree density and species proportion gradients in the plantation and compared them to the fine root vertical distribution of the trees, estimated from soil data collected by Fruleux et al (2018). Our hypotheses were that (1) the water extraction depth of each species would be affected by tree density and species mixture levels, and (2) vertical water acquisition profiles would be consistent with fine root vertical distribution patterns.…”