‘We believe there is no structure in plants more wonderful, as far as functions are concerned, than the tip of the radicle.’(Darwin & Darwin, 1880)

Plant growth is most often limited by the availability of soil resources, particularly water and macronutrients such as nitrogen (N) and phosphorus (P). Because of this, understanding how plant roots obtain soil resources, as well as how root and shoot functions are coordinated, has long been a focus of research at the molecular, physiological, and ecological levels (Hodge et al., 2009). Moreover, ever since the discovery that roots could sense and respond to their environment (Darwin & Darwin, 1880), plant biologists have sought to understand how root structure and function varies with environmental variation. Historically, climate was considered to be an important determinant of variation in root morphology and architecture, a principle first supported by the comparative studies of John Weaver and his colleagues (Weaver, 1919). Weaver's detailed descriptions of 140 plant species across the prairies, chaparral and forests of the western United States also showed that root architecture is not independent from growth form and taxonomy. These early studies were followed by a more intense focus on fine roots, which are most actively engaged in soil‐resource foraging and uptake, as well as symbiosis formation with soil microbes (Fitter & Hay, 1981). Nevertheless, despite continued surveys of both fine‐root morphology and whole‐root system architecture (Jackson et al., 1997; Schenk & Jackson, 2002), and notwithstanding Darwin's early interest, root traits have only recently begun to be simultaneously evaluated in both ecological and evolutionary contexts (Comas et al., 2012). In this issue of New Phytologist, Valverde‐Barrantes et al. (pp. 1562–1573) provide a much‐needed global synthesis of the ecological and evolutionary factors that have influenced variation in fine‐root traits. By placing a spotlight on the evolutionary ecology of roots, Valverde‐Barrantes et al. provide necessary context for future hypothesis tests of the origin and significance of variation in root structure and function.

et al., 2017 Valverde‐Barrantes et al. et al., 2011 Valverde‐Barrantes et al. Valverde‐Barrantes et al. ‘These factors have long been recognized for their influence on root traits, but this study represents the first attempt to quantitatively partition these sources of variation at a global scale.’ Using a recently published database (Iversen.,),address several questions aimed at strengthening the ability to predict variation in root traits across the globe. They ask whether root traits co‐vary with leaf traits in a manner that is consistent with leaf economics spectrum theory, where leaf function spans a gradient from slow‐growing species that produce long‐lived, structurally expensive leaves with a low N content to fast‐growing species that produce short‐lived, structurally inexpensive leaves with a high N content (Donovan.,). If root variation also falls along this axis, then slow‐growing species should have structurally expensive, low N content roots, whereas fast‐growing species should have structurally inexpensive, high N content roots.also aim to improve the ability to predict root traits from other variables by testing the relative importance of climate, growth form, mycorrhizal status and phylogeny as potential causal factors. These factors have long been recognized for their influence on root traits, but's study represents the first attempt to quantitatively partition these sources of variation at a global scale.

Valverde‐Barrantes et al.'s analysis will be influential, simply because the patterns they uncover raise many follow‐up questions. One of the most consistent findings was that phylogeny explains a substantial amount of variation in root traits, and, therefore, becomes an important consideration in predicting not only global variation in root traits, but also for assessing the degree to which above‐ and below‐ground economics spectrum traits co‐vary. Core predictions about integration between above‐ and below‐ground economics spectrum traits were supported; variation in traits that reflect the structural costs of fine roots (specific root length, SRL) and leaves (specific leaf area, SLA) were positively correlated, as were root and leaf N contents. However, the level of trait integration varied with the traits under consideration and with phylogenetic group. For example, the correlation between SRL and SLA was stronger in clades dominated by herbaceous species (e.g. monocots, Caryophyllales) and weaker in clades where woody plants were more frequent (e.g. Gymnosperms, Magnoliids). By contrast, the correlation between root and leaf N was stronger in clades dominated by woody plants than in predominantly herbaceous clades.

The substantial role that phylogeny has in accounting for global variation in root traits is also one of the most striking findings in Valverde‐Barrantes et al.'s analyses. Phylogenetic effects were stronger than any other factor, including climatic variation and mycorrhizal status, which have been consistently evoked as important drivers of root trait variation (Weaver, 1919; Fitter & Hay, 1981; Brundrett, 2002; Schenk & Jackson, 2002). Of particular interest is that, depending on the trait being examined, different phylogenetic scales accounted for different amounts of variation. For example, divergences at the order level explained much of the phylogenetic signal for root diameter, whereas family‐level divergences were the strongest determinant of phylogenetic signal for root N content. Although a lack of available data prevented assessments of phylogenetic signal at finer scales, recent work in Helianthus suggests that within a genus root traits can be quite variable, and thus have no phylogenetic signal (Bowsher et al., 2016). If phylogenetic effects on root variation are consistently weaker at finer scales, then tests of correlated evolution between leaf and root economics spectrum traits may be more informative when done within a genus.

Even though phylogeny accounts for substantial variation in root traits, this finding does not identify mechanisms responsible for the diversification of root structure and function. Instead, evolutionary hypotheses still need to be tested to explain the phylogenetic patterns. For example, the observation that extant variation in root traits is associated with a few deep divergences in the phylogeny suggests that there may be genetic constraints on evolutionary change in the descendent clades. These may arise from a lack of genetic variation or genetic trade‐offs in populations, such that a response to natural selection on root traits is weak or absent. Although genotypic variation in root morphology and architecture has only been measured in a handful of species, such variation appears to be abundant (Ristova & Busch, 2014), which suggests that adaptation is unlikely to be limited by this factor. Alternatively, stabilizing selection to preserve a particular optimum may be common, resulting in stasis. To test this hypothesis, the form, magnitude and direction of natural selection on root traits needs to be quantified. Ideally, such studies should be carried out with replicate populations in contrasting soil and climate environments to causally link putative agents of selection to specific root adaptations (Wade & Kalisz, 1990; Donovan et al., 2011).

Another emergent pattern from Valverde‐Barrantes et al. that deserves further study is that root‐trait variation is quite homogenous in some clades, whereas it is heterogeneous in other clades. To illustrate differences in within‐clade variability, order‐level standard deviations in root diameter were plotted against order‐level means and departures from the least‐squares regression line used to identify clades with higher, or lower, than expected variation in root diameter (Fig. 1). This analysis indicates that the Ericales, Malpighiales, Apiales, and Liliales have exceptionally high interspecific variation, suggesting that root evolution may be quite labile in these lineages. Further studies in lineages where root traits are highly variable could help to identify the ecological causes of diversification in root morphology. It should also be noted that the Asterales, Fabales and Magnoliales have relatively low interspecific variation in root diameter, and studying these lineages could yield insights into the mechanisms that constrain diversification in root structure and function.

Figure 1 Open in figure viewer PowerPoint Valverde‐Barrantes et al. New Phytologist (pp. 1562–1573), and include only those orders that had at least five species sampled. Orders with standard deviations higher than the confidence interval have higher than expected interspecific variation relative to the least squares prediction, and orders with standard deviations lower than the confidence interval have lower than expected interspecific variation. Symbol colours represent sample sizes for each order, ranging from blue (low) to red (high) as indicated on the legend. The relationship between the standard deviation of root diameter and mean root diameter for 22 plant orders. A least squares regression along with a 95% confidence interval (shaded region) was fit to the data. Data were obtained from, in this issue of(pp. 1562–1573), and include only those orders that had at least five species sampled. Orders with standard deviations higher than the confidence interval have higher than expected interspecific variation relative to the least squares prediction, and orders with standard deviations lower than the confidence interval have lower than expected interspecific variation. Symbol colours represent sample sizes for each order, ranging from blue (low) to red (high) as indicated on the legend.

Valverde‐Barrantes et al. also test a long‐held hypothesis that mycorrhizal status and fine‐root morphology are correlated. This hypothesis is based on the expectation that because plants with coarser roots have lower intrinsic ability to absorb nutrients than plants with finer roots, they should benefit the most from a nutritional symbiosis with mycorrhizal fungi (Brundrett, 2002; Maherali, 2014). However, Valverde‐Barrantes et al. find very little to no association between mycorrhizal status and any measure of fine‐root morphology. This finding suggests that evolutionary losses of the mycorrhizal symbiosis may have occurred without a shift in root morphology, and this is consistent with a recent meta‐analysis showing that root traits and growth response to mycorrhizal colonization are also not correlated (Maherali, 2014). Valverde‐Barrantes et al. are careful to point out that other aspects of the mycorrhizal symbiosis, such as the intensity of root colonization, have been associated with the diameter and anatomical configuration of fine roots. More work on this topic is necessary, and as noted elsewhere (Hodge et al., 2009; Maherali, 2014; Kong et al., 2017), other traits such as root‐hair length, root‐hair frequency, fine‐root branching and the ratio between cortical and vascular tissue could be associated with mycorrhizal status.

The global scale of Valverde‐Barrantes et al.'s analysis and the inclusion of species that span nearly two‐thirds of all seed plant orders (40 of c. 60 defined orders; Stevens, 2001) provides necessary context for future research in specific communities or taxonomic groups, and also helps us recognize that much more remains to be done. For example, root traits for only a fraction of plant species have been sampled (c. 600 out of >300 000), and even orders that rank highly in terms of species sampled still only include a small subset of the total diversity available (e.g. Poales, Fig. 1, with 75 of an estimated c. 19 000 species sampled; Stevens, 2001). The need for more sampling is highlighted by Valverde‐Barrantes et al. for improving the ability to predict root variation in certain biomes, and one could also add a call for more data collection from all biomes and taxonomic groups. As global root trait data accumulate to match observations for above‐ground traits (Diaz et al., 2016), our understanding of the evolutionary causes and ecological consequences of root variation can only improve.