DOI: 10.5061/dryad.2v6wwpzs3
title: Data from: Climatic water availability mainly drives context-dependency of tree functional diversity effects on soil organic carbon storage in European forests

description: The interplay of forest stand and environmental factors shape soil organic
 C (SOC) storage in forest ecosystems but little is known about their
 relative impacts in different soil layers. Moreover, how environmental
 factors modulate the impact of stand factors, particularly species mixing,
 on SOC storage, is largely unexplored.  In this study conducted
 in 21 forest triplets (two-species mixed stand and respective monocultures
 nearby) distributed in Europe, we tested the hypothesis that stand factors
 (functional identity and diversity) have stronger effects on topsoil
 (FF+0-10 cm) C storage than environmental factors (climatic water
 availability, clay+silt content, oxalate-extractable Al - Alox) but that
 the opposite occurs in the subsoil (10-40 cm). We also tested the
 hypothesis that functional diversity improves SOC storage under high
 climatic water availability, clay+silt contents, Alox. We characterized
 functional identity as the proportion of broadleaved species (beech and/or
 oak), and functional diversity as the product of broadleaved and conifer
 (pine) proportions. The results show that functional identity was the main
 driver of topsoil C storage while climatic water availability had the
 largest control on subsoil C storage. Contrary to expectations, functional
 diversity decreased topsoil C storage under increasing climatic water
 availability but the opposite was observed in the subsoil. Functional
 diversity effects on topsoil C increased with increasing clay+silt
 content, while its effects on subsoil C was negative at increasing Alox
 content. This suggests that functional diversity effect on SOC storage
 along environmental gradients depends on the specific environmental factor
 and the soil depth under consideration.
description: This study was conducted in 21 forest triplets across Europe. A
 triplet consisted of two-species mixed stand and their corresponding
 monocultures at the same site. The three forest stands in each triplet
 were of similar ages (based on tree cores and forest archives) and had
 homogenous soil conditions based on texture analyses on soil samples in
 the 10-20 cm depth. The triplets were of three types: five beech-oak
 (<em>Fagus sylvatica</em> L. - <em>Quercus
 petraea</em> (Matt.) Liebl.), eight pine-beech (<em>Pinus
 sylvestris</em> L. - <em>Fagus sylvatica</em> L.), and
 eight pine-oak (<em>Pinus sylvestris</em> L. -
 <em>Quercus robur </em>L. / <em>Quercus
 petraea</em> (Matt.) Liebl.). These tree species are widely
 distributed in Europe and are very important for forestry.
 We placed ten (10) sampling points in each mixed stand and five
 (5) points each in the corresponding monocultures. At each sampling point,
 we sampled the forest floor (organic layer above the mineral soil) with 30
 cm x 30 cm metal frame. Subsequently, we dug sampling pits in 10 cm
 interval until 40 cm depth. We estimated total volume (soil + voids +
 stones) of soil samples in each 10 cm pit by the volume replacement method
 (Al-Shammary and others 2018) with glass beads. Samples were air-dried,
 crushed, then passed through 2 mm sieve to separate fine soil (&lt;2
 mm), coarse roots (&gt;2 mm), and stones. We picked visible roots in
 fine soil to reduce their influence on C contents. We separately weighed
 all the fine soil and the stone fractions. We determined stone volume by
 water displacement method. Sub-samples of fine soils were ground into
 powder with Vibratory Disc Mill (Retsch RS 200, Germany) for C and N
 analyses on all samples (totaling 2080) using CN Analyzer (FlashEA® 1112,
 USA). Computation of SOC stocks have been described in Osei and others
 (2021). Soil pH, particle size distribution, and oxalate-extractable
 metals (Al<sub>ox</sub>, Fe<sub>ox</sub>) were
 determined on samples from the 10-20 cm depth. We determined soil pH in
 deionized water at a ratio of 1:10 using inoLab pH Level 1 (WTW GmbH,
 Germany). Particle size distribution was determined by sedimentation
 method following protocol NF X31-107. The oxalate-extractable metals
 (Al<sub>ox</sub>, Fe<sub>ox</sub>) were extracted
 by 0.2M ammonium-oxalate at pH 3 according to Blackmore and others (1981),
 and the concentrations of Al and Fe were determined by ICP. We
 characterized the combined effect of precipitation (P, mm) and temperature
 (T, °C) by the de Martonne aridity index (DMI; P/T+10)
description: It is saved in txt, which is accessible by almost all
 softwares.