Full paper Climate and phylogenetic history structure morphological and architectural trait variation among fine-root orders M. Luke McCormack1,2* , Matthew A. Kaproth3,4* , Jeannine Cavender-Bares4 , Eva Carlson2, Andrew L. Hipp1,5 , Ying Han6 and Peter G. Kennedy2,4 1Center for Tree Science, The Morton Arboretum, Lisle, IL 60523, USA; 2Department of Plant and Microbial Biology, University of Minnesota, St Paul, MN 55108, USA; 3Department of Biological Sciences, Minnesota State University Mankato, Mankato, MN 56001, USA; 4Department of Ecology, Evolution, and Behavior, University of Minnesota, St Paul, MN 55108, USA; 5The Field Museum, Chicago, IL 60605, USA; 6School of Life Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China Summary Authors for correspondence: Fine roots mediate below-ground resource acquisition, yet understanding of how fine-root M. Luke McCormack functional traits vary along environmental gradients, within branching orders and across phy- Tel: +1 630 725 2072 logenetic scales remains limited. Email: [email protected] Morphological and architectural fine-root traits were measured on individual root orders of Matthew A. Kaproth 20 oak species (genus Quercus) from divergent climates of origin that were harvested after Tel: +1 507 389 2787 three growing seasons in a glasshouse. These were then compared with similar measurements Email: [email protected] obtained from a phylogenetically diverse dataset of woody species from the Fine-Root Ecol- Received: 1 April 2020 ogy Database (FRED). Accepted: 30 June 2020 For the oaks, only precipitation seasonality and growing season moisture availability were correlated to aspects of root diameter and branching. Strong correlations among root diame- New Phytologist (2020) 228: 1824–1834 ters and architecture of different branch orders were common, while correlations between doi: 10.1111/nph.16804 diameter and length were weakly negative. By contrast, the FRED dataset showed strong pos- itive correlations between diameter and length and fewer correlations between root diameter and architectural traits. Key words: climate adaptation, Fine-Root Our findings suggest that seasonal patterns of water availability are more important drivers Ecology Database (FRED), oak, plant functional traits, plant trait variation, of root adaptation in oaks than annual averages in precipitation and temperature. Further- Quercus, root economics, species distribu- more, contrasting patterns of trait relationships between the oak and FRED datasets suggest tion. that branching patterns are differentially constrained at narrow vs broad phylogenetic scales. especially root diameter, across species at large spatial scales. For Introduction example, multiple studies have observed smaller average fine-root Trait coordination and variance, both within and across species, diameters among species growing in colder, higher latitude determine the range of functional strategies exhibited by organ- biomes compared to those found in more tropical systems (Chen isms across diverse environments (Reich, 2014; Dıaz et al., 2016). et al., 2013; Freschet et al., 2017). Ma et al. (2018) also observed While the majority of plant trait assessments have focused on similar differences across biomes, but further noted the impor- leaves and stems, the traits of fine roots are increasingly recog- tance of plant evolutionary history as a codeterminant of climate- nized as important codeterminants of plant functional strategies related patterns. In addition, previous studies have reported (Comas et al., 2013; Eissenstat et al., 2015). Despite this growing strong differences in root morphology when comparing earlier recognition, relatively little is known about fine-root trait varia- diverging lineages of angiosperms (e.g. Magnoliids) to more tion across species and along environmental gradients compared recently derived groups (Baylis, 1975; Comas et al., 2012; Chen to variation in the traits of above-ground plant structures et al., 2013; Valverde-Barrantes et al., 2016). Together, these (McCormack et al., 2017). Given the importance of fine roots results suggest that the reduced prominence of evolutionarily ear- for both soil resource acquisition and total plant resource alloca- lier diverging plant families (e.g. Magnoliaceae and Lauraceae) at tion (Eshel & Beeckman, 2013), it is critical to understand how higher latitudes may be the proximate driver of decreased root fine-root traits differ among species and in relation to varied envi- diameter in woody species under colder climates. ronmental conditions. Given the aforementioned effects of phylogeny on key fine- Recent analyses suggest a pivotal role for climate and plant root traits, efforts to identify linkages between root trait variation phylogeny in determining patterns of fine-root trait variation, and broader climate variables may also benefit from case studies within a narrower phylogenetic range (Ackerly, 2004; Mason & * These authors contributed equally to this work. Donovan, 2015). For example, working within the genus 1824 New Phytologist (2020) 228: 1824–1834 Ó 2020 The Authors www.newphytologist.com New Phytologist Ó 2020 New Phytologist Trust New Phytologist Research 1825 Rhododendron, Medeiros et al. (2017) found climate of origin, began by testing whether morphological and architectural fine- and most often mean annual temperature (MAT), of an individ- root traits of oaks were correlated to MAT, mean annual precipi- ual species’ origin to be significantly correlated to several root tation (MAP), precipitation seasonality and an index of moisture traits including root diameter and specific root length. Similarly, in the growing season (ImGS; Kaproth & Cavender-Bares, 2016) recent observations within the single species, Scots pine (Pinus relating to each species’ climate of origin. We specifically mea- sylvestris) have also identified climate-driven patterns of fine-root sured the diameter and length of individual root orders within trait variation and resource allocation within fine roots (Zad- the branching hierarchy as well as the branching ratios of lower worny et al., 2016, 2017). Intriguingly, the results of these genus- order roots to higher order roots. These traits were selected due and species-level studies suggest an increase in root diameter in to their connection to root construction costs and resource acqui- cooler climates, whereas the phylogenetically diverse, cross-biome sition (McCormack & Iversen, 2019; Bergmann et al., 2020; G. datasets indicate that diameter tends to decrease from warmer to T. Freschet et al., unpublished). We hypothesized that root diam- cooler climates (Freschet et al., 2017; Ma et al., 2018). These eter and length would be lower but that branching ratios would together suggest that root trait organization and variation along be higher in species from cooler, drier climates compared to environmental gradients may differ at relatively narrow vs broad species from warmer, wetter climates. Next, we tested whether phylogenetic scales, although this has not been tested. morphological and architectural traits were coordinated within Complicating assessments of root trait relationships to climate, the fine-root branch hierarchy across the 20 oak species, indicat- most previous comparative fine-root studies have assessed traits ing that traits of higher order roots could be inferred from lower- measured for many roots pooled together (e.g. all roots < 2mm order roots. We then contrasted these observations with a phylo- in diameter), which can mask variation among species. genetically diverse suite of 60 woody plant species representing Researchers have more recently emphasized an order-based all major forest biomes using the Fine-Root Ecology Database approach to facilitate more direct comparisons of functionally (FRED; Iversen et al., 2017) to determine whether patterns of equivalent roots among species (Pregitzer et al., 2002; Guo et al., within-branch trait coordination were consistent at relatively nar- 2008; Chen et al., 2013; Kong et al., 2014). Order-based com- row (i.e. within Quercus) vs relatively broad phylogenetic scales. parisons are usually made at the level of first-order roots, the most For both datasets, we hypothesized that root length and root distal and metabolically active roots in the fine-root system (Chen diameter would be positively correlated within and across fine- et al., 2013; McCormack & Iversen, 2019). Although this facili- root branch orders. By contrast, we hypothesized that root tates a robust basis for comparison across species, it also excludes branching ratio would be negatively correlated to root diameter higher order roots (i.e. second- and third-order) from analysis but still positively correlated to root length. despite their importance as structural components of the absorp- tive root system. As a result, there has been little direct assessment Materials and Methods of how morphological and architectural traits are correlated, not just within species, but also within the branching hierarchy (Pre- The oak root trait data (20 species) were collected from individ- gitzer et al., 2002; Guo et al., 2008; Sun et al., 2016). In particu- ual plants grown in a glasshouse under common garden condi- lar, it is unknown whether variation in root morphology and tions using seed collected from native habitats while additional branching are predictable within the branched fine-root system fine-root trait data (60 species, 29 families) were obtained using based