DOI: 10.5061/dryad.69p8cz916
title: Root anatomy helps to reconcile observed root trait syndromes in tropical tree species

description: Studying the organization of functional traits in plant leaves and stems
 has revealed notable patterns linking function and form; however, evidence
 of similarly robust organization in root tissues remains controversial. We
 posit that anatomical traits in roots can provide insight on the overall
 organization of the root system. We hypothesize that A) size variation in
 the tissue outside the stele relates in a non-linear fashion with
 functional traits associated with direct resource uptake, including a
 negative relationship with root architectural traits; and B) similar
 relationships detected in tropical areas also hold true in other biomes.
 We address our hypotheses using empirical data from 24 tropical tree
 species in French Guiana, including anatomical measurements in first order
 roots, and functional trait description for the entire fine root system.
 In addition, we compiled a global meta-analysis of root trait with 500+
 forest species across tropical, subtropical and temperate forests. Our
 results supported the expected non-linear relationships between cortical
 size and morphological traits, and a negative linear trend with
 architectural traits. We confirmed a global negative relationship between
 SRL, diameter and tissue density, suggesting similar anatomical
 constraints in root systems across woody plants. However the importance of
 factors vary across biomes, possibly related to the unequal phylogenetic
 representation of families across latitudes. Our findings imply that the
 rhizocentric hypothesis can be a valuable approach to understand fine root
 trait syndromes and the evolution of absorptive roots in vascular plants.
description: <b><i>Site description and sampling—
 </i></b>Study sites are located in the French Guiana Space
 Center (CSG), in lowland coastal forest near Kourou, French Guiana, France
 (5° 13′ 51″ N, 52° 46′ 08″ W). We sampled in permanent plots established
 under the revised Gentry protocol (Phillips et al., 2003). Two 2 ha plots
 were established in each of the three main forest habitats: terra-firme
 (clay-rich soils), white-sand and seasonally flooded bottomlands. These
 habitats differ in topography, hydrography and soil nutrient availability
 (Allié et al., 2015; Guitet<i> </i>et al., 2015)
 For root morphological
 analyses, we used five randomly selected 5-10 cm long root systems
 comprised of the three most distal root orders (Xia et al. 2010). Roots
 were weighed to the nearest 0.01mg, scanned with a flat-top digital
 scanner (600 DPI resolution, 256-level gray-scale, TIFF format; Epson
 Scanner Perfection V700 Photo, USA), dried at 65 °C for 48 hr, and then
 reweighed. Image analysis of the scanned roots was performed using
 WinRhizo software (2007 Pro version, Instrument Regent, Quebec, Canada) to
 estimate specific root tip abundance (SRTA, tips
 mg<sup>-1</sup> DRM, (Meinen et al. 2009)), Branchiness Index
 (BI, tips/total length in cm, Eissenstat et al. 2015), average diameter,
 specific root length (SRL, m g<sup>-1</sup> DRM) and root
 tissue density (RTD, g DRM cm<sup>-3</sup>) for each sampled
 tree. <b><i>Anatomical preparation and
 trait measurements—</i></b>Transversal cut sections were
 randomly selected on the most distal root segments, using fully formed
 acquisition roots. Root maturity was visually determined by the presence
 of root cortex and fully formed vascular tissue. A paraffin embedding
 protocol was necessary to assist cutting and was adapted from Morel (2013,
 see Table S4). We cut sections from 8 to 25 µm using a rotating microtome
 (Leica Rotation Microtome, RM 2255; Leica, Vienna, Austria). We dissolved
 the remaining paraffin with heating the blade at 56°C (Premiere slide
 warmer, XH-2002; Daigger, Vernon Hills, IL, USA) and covering the blade
 with Mounting media. We captured the images of the
 sections with an Olympus Microscope BX60 (Olympus Optical Co., Ltd.,
 Tokyo, Japan) with an Olympus camera 750D. The microscopic observation was
 aided by a mercury lamp light source (Olympus, U-RFL-T) – allowing the
 distinction of lignified tissue. Measures of the tissues areas – root
 cortex area, root stele area, and root section area – were done using a
 previous image treatment with Adobe Photoshop CS5 (Photoshop CS3; Adobe
 Systems Inc., San Jose, California, U.S.A.). Each tissue was isolated on a
 single image layer. The area of each layer was automatically calculated
 using a Java script on ImageJ2 software (Rueden et al.<i>,</i>
 2017). Liner measurements of the stele and the tissue outside the stele
 (ToS) was estimated using the square root of the total area of each tissue
 divided by p.   <b><i>Global data set
 compilation—</i></b>We collected a global dataset of
 absorptive root traits (root diameter, RTD and SRL) for woody species. The
 criteria for the selection of papers included: 1) The study focused on
 absorptive root tissues, either first order or lateral root clusters with
 diameter &lt; 2mm; 2) the study included the measurement of the three
 root traits for at least five woody species; 3) the study was performed on
 woody plants in forested areas, excluding arid and alpine ecosystems; and
 4) the study measured individuals growing in natural conditions or outdoor
 collections (see Supplementary Data 1). In total, we included 20 studies
 encompassing 697 entries, including 582 species of trees and understory
 shrubs. A total of 333 entries corresponded to temperate trees, followed
 by 228 subtropical and 136 tropical species. In terms of phylogenetic
 representation, Rosids (including Saxifragales and Vitales,
 <i>sensu</i> Soltis et al<i>.</i> 2003) were the
 most dominant group representing 51% of the species (ranging from 58% in
 the tropics to 53% and 43% in temperate and subtropical forests
 respectively). Asterids (including Caryophyllales and Santalales,
 Manchester et al. 2015) represented 23%, Magnoliids 16% and Gymnosperms 9%
 of the total number of species.