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.

Site description and sampling— 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 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^-1 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^-1 DRM) and root tissue density (RTD, g DRM cm^-3) for each sampled tree.

Anatomical preparation and trait measurements—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., 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.  

Global data set compilation—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 < 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, sensu Soltis et al. 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.