Plant roots typically vary along a dominant ecological axis, the root
economics spectrum, depicting a tradeoff between resource acquisition and
conservation. For absorptive roots, which are mainly responsible for
resource acquisition, we hypothesized that root economic strategies differ
with increasing root diameter. To test this hypothesis, we used seven plant
species (a fern, a conifer, and five angiosperms from south China) for which
we separated absorptive roots into two categories: thin roots (thickness of
root cortex plus epidermis < 247
Plant traits reflecting a tradeoff between resource acquisition and
conservation represent an essential ecological axis for plant strategies
that is important for our understanding of how plants drive ecosystem
processes and responses to environmental change (Cornwell et al., 2008;
Freschet et al., 2010; Reich, 2014; Westoby et al., 2002). On the one end of
this axis, there are species with acquisitive strategies, i.e., fast
acquisition of resources (e.g., CO
Resource acquisition in plant roots is performed by absorptive roots, i.e., the first two or three orders of a root branch with primarily-developed tissues which are part of the commonly used category of “fine roots” (< 2 mm in diameter) (Guo et al., 2008; Long et al., 2013; Pregitzer et al., 2002). For absorptive roots, tissue density, i.e., root dry mass per unit root volume, is a key trait of the root economics spectrum as tissue density is closely linked to the acquisition-conservation tradeoff (Bardgett et al., 2014; Birouste et al., 2014; Craine et al., 2005; Espeleta et al., 2009; Mommer and Weemstra, 2012; Roumet et al., 2006). In general, absorptive roots with higher tissue density are slower in nutrient acquisition and longer in lifespan whereas absorptive roots with lower tissue density may enable faster acquisition but maintain a shorter lifespan (Ryser, 1996; Wahl and Ryser, 2000; Withington et al., 2006). Recently, tissue density for absorptive roots was found to negatively correlate with root diameter. This could be because root cortex is less dense than root stele and because in thicker roots a larger proportion of the root cross-sectional area is accounted for by the cortex (Chen et al., 2013; Kong et al., 2014; Kong and Ma, 2014). On the other hand, compared with thinner absorptive roots, thicker absorptive roots may acquire resources faster because of their greater dependence on mycorrhizal fungi (Eissenstat et al., 2015; Kong et al., 2014; Kong and Ma, 2014; St. John, 1980), and may also have a longer lifespan due to the larger diameter (Adams et al., 2013; Eissenstat and Yanai, 1997; Wells and Eissenstat, 2001). As such, the trait syndrome for thicker absorptive roots would differ from the predictions of faster acquisition and shorter lifespan. This highlights the importance of discriminating thicker and thinner absorptive roots when exploring root strategies. However, few studies have tested for effects of root diameter in driving trait economics spectra in absorptive roots.
In addition to structural traits such as density, the chemical composition of absorptive roots may constitute another important aspect of testing root strategies in relation to root diameter (Hidaka and Kitayama, 2011; Meier and Bowman, 2008; Poorter and Bergkotte, 1992; Poorter et al., 2009). For example, carbon (C) and nitrogen (N), the two most abundant elements in plant tissues, are usually bound to organic compounds which may contain labile fractions (e.g., soluble sugars and proteins in living cells) and recalcitrant fractions (e.g., cellulose and lignin in structural tissues) (Atkinson et al., 2012; Berg and McClaugherty, 2008; Feng et al., 2009; Poorter et al., 2009; Shipley et al., 2006). From the perspective of C and N fractions, absorptive roots with less labile C and more labile N may indicate an acquisitive strategy. This is because high root activity may be accompanied by an increased production of metabolism-related proteins with a high labile N content; such roots may be palatable for herbivores and have a relative short lifespan. On the other hand, conservative roots contain less labile C and N fractions as more of these compounds are used for construction of structural tissues resulting in lower root activity and a longer lifespan. However, compared with thinner absorptive roots, thicker absorptive roots may have higher labile C and N fractions as these labile fractions can be stored in their thick root cortex (Chapin III, 1980; Long et al., 2013; Lux et al., 2004; Withington et al., 2006). As such, the chemical traits of thicker absorptive roots integrate “opposing” effects of root metabolism and storage, suggesting that they have neither a true acquisitive nor a true conservative strategy. Therefore, in evaluating the impact of thickness on root economic strategies it is necessary to examine C and N fractions in relation to root diameter.
Here, we selected a variety of plant species common to tropical and subtropical forests in south China with contrasting phylogeny and root structure. The aim of our study was two-fold. First, we examined the influence of root diameter on the root economic strategies in absorptive roots. We hypothesized that the root economic strategies differ between thinner and thicker absorptive roots, with trait relationships indicating acquisitive-conservative trade-off for thinner roots but not for thicker roots. The hypothesis was tested using a series of trait relationships involving both structural and chemical traits. Second, root C and N fractions have been suggested to vary in predictive ways across branch orders (Fan and Guo, 2010; Goebel et al., 2011). However, we hypothesized that patterns of root C and N fractions across branch orders differ in species varying in absorptive root diameter.
We selected seven plant species with contrasting phylogeny and root
structure (Table S1 in the Supplement) in tropical and subtropical forests in south China.
Three species were sampled at the Heshan Hilly Land Interdisciplinary
Experimental Station (22
Roots were collected at a soil depth of 0–10 cm in June and July 2011. For
each species, at least three mature trees were selected. We first tracked
the main lateral roots by carefully removing surface soil at the base of
each plant with a specially manufactured fork. Root branch order was defined
according to Pregitzer's study with the most terminal branch as the
first-order (Pregitzer et al., 2002). The intact roots
were collected and soil adhering to the roots was carefully removed. We
distinguished all four root orders for
For each species, 50 root segments for the first order, 30 segments for the
second order, and 20 segments for the third to the fifth order were randomly
picked for measuring root diameter and length. Depending on root size, the
root diameter was measured under a 40
Root segments from the FAA solution were cleaned with deionized water (4
The frozen root samples were put into deionized water to carefully remove
any soil particles or dead organic matter that adhered to but was not part
of the root (Pregitzer et al., 2002). The samples of
each root branch order were then oven-dried (65
A 5mg subsample of residue left after the above acid-digestion procedure was used to measure N concentration and N allocation in the acid-insoluble C fraction. The N in the extractive fraction was too low to measure. Thus, estimates of N in the acid-soluble fraction were calculated as the difference between total N and N in the acid-insoluble fraction.
Relationships between root tissue density and root N concentration and each
of the three C fractions were assessed by linear regressions. Here, we
introduced a new term, “root EC” referring to tissues outside the stele
including the epidermis and cortex. Root EC was used for two reasons. First,
the thickness of root EC can be a proxy of the size of root diameter
(
To explore the effect of root diameter on root ecological strategies, the
above analyses were repeated for thin and thick absorptive roots,
respectively. A mean thickness of 247
We acknowledge that the seven species we used represent a relatively small
species pool. To validate the results of our study, another data set of 96
woody species from one of our previous studies was used where only the
first-order roots were measured (Kong et al., 2014). For these 96
species, we did not use the average root EC thickness as the cut-off between
thin and thick absorptive roots. This was because root EC of these species
followed a skewed normal distribution with abundant species having thinner
root EC (
Relationships between root tissue density and root N concentration for total (black line), thin (solid circles, grey line), and thick (open circles) absorptive roots.
To test interspecific differences of root chemical fractions among root
orders, two-way ANOVAs were used with plant species and root order as fixed
factors. Tukey's HSD test was conducted to evaluate differences in chemical
fractions among root branch orders within species (Long et al.,
2013). All statistical analyses were carried out in SPSS (version 13.0; SPSS
Inc. Chicago, USA) with significant level at
Relationships between root tissue density and extractive C fraction
Root tissue density was negatively correlated with root N concentration for total and thin but not for thick absorptive roots (Fig. 1). Similarly, using a larger species pool, negative relationships between root tissue density and root N concentration were found for total and thin but not for thick absorptive roots (Fig. S3).
For thin absorptive roots, the extractive C fraction peaked at medium root
tissue density (Fig. 2a). Moving average analysis revealed a quadratic
relationship between the extractive C fraction and root tissue density in
thin absorptive roots (Fig. S4a), while no relationships were found between
acid-soluble and acid-insoluble fractions and root tissue density. The
recalcitrant C fraction (acid-soluble C
Relationships between thickness of root EC and root N concentration
Relationships between root tissue density and thickness of root EC for total, thin (solid circles, black line), and thick (open circles) absorptive roots.
The extractive C fraction
Root N concentration
Across total absorptive roots, thickness of root EC was positively
correlated with total root N concentration (Fig. 3a) and negatively with
root N in the acid-insoluble fraction (Fig. 3b). Thickness of root EC was
also positively correlated with the extractive C fraction (Fig. 3c) and
negatively with the acid-insoluble fraction (Fig. 3e). However, in each of
thin and thick absorptive roots, no relationships were found between
thickness of root EC and either of these chemical fractions (all
Thickness of root EC decreased linearly with root tissue density (Fig. 4), but no relationships were found when separated between thin and thick absorptive roots. Using a large species pool we found a very similar pattern: a significant relationship between thickness of root EC and root tissue density for total absorptive roots, a weaker relationship for thin absorptive roots and no relationship for thick absorptive roots (Fig. S5). In addition, we found exponential relationships between SRL and thickness of root EC for the species in our current study as well as for the larger species pool from a previous study (Fig. S6).
All chemical fractions except the extractive fraction showed significant
differences among species and root orders (
The extractive C fraction tended to increase with increasing root order for
species with thin absorptive roots such as
The negative relationship between root tissue density and root N
concentration supports the acquisition-conservation tradeoff, and hence, the
existence of economic strategies in absorptive roots because absorptive
roots with higher tissue density usually have a longer lifespan (Eissenstat
and Yanai, 1997; Ryser, 1996; Withington et al., 2006) while their lower N
concentration indicates slow resource acquisition (Kong et al., 2010;
Mommer and Weemstra, 2012; Reich et al., 2008). However, our results further
showed that the negative relationship between root tissue density and root N
concentration held for thin but not for thick absorptive roots (Fig. 1).
Although these results were based on a relatively small number of species,
reanalysis of data from a previous study including 96 species
(Kong et al., 2014)
The trait relationships between root tissue density and root C fractions
provide further support for our hypothesis. Theoretically, absorptive roots
with lower tissue density would have higher activity, while higher root
activity also consumes more labile C thus leaving less labile and more
recalcitrant C fractions in these roots. In contrast, in absorptive roots
with higher tissue density, more C is used for structural tissues demanding
recalcitrant C fractions (Fan and Guo, 2010). Therefore, we would
expect an inverted U-shaped relationship for labile C fractions and a
U-shaped relationship for recalcitrant C fractions when these C fractions
would be correlated with root tissue density. As expected, for thin
absorptive roots we found an inverted U-shaped relationship between the
labile C fraction and root tissue density (Fig. S4a) and a U-shaped
relationship between recalcitrant C fractions (acid-soluble C
Furthermore, observed relationships between thickness of root EC and root C and N fractions provide the third piece of support for our hypothesis of contrasting economic strategies with root diameter. Across total absorptive roots, thickness of root EC was positively correlated with root N concentration and the extractive C fraction while negatively correlated with the acid-soluble C fraction and N in the acid-soluble C fraction. This suggests that compared with thin absorptive roots, thick absorptive roots acquire resources at higher rates as indicated by their higher N concentration and lower C and N in recalcitrant fractions. Meanwhile, thick absorptive roots may also have longer lifespan because of their larger root diameter (Adams et al., 2013; Anderson et al., 2003; McCormack et al., 2012; Wells and Eissenstat, 2001). These findings seem to contrast with an acquisition-conservation tradeoff. Further, we showed that relationships between thickness of root EC and root chemical fractions only held across the full spectrum from thin to thick absorptive roots. Nevertheless, it was also noted that root tissue density showed a greater range of variation for thin than for thick absorptive roots. For thin absorptive roots, variation in root tissue density might arise from secondary thickening of root EC cell walls (Eissenstat and Achor, 1999; Long et al., 2013; Ryser, 2006; Wahl and Ryser, 2000). This could be associated with lower root activity and hence lower root N concentration (Figs. 1 and S3). As such, an acquisition-conservation tradeoff in thin absorptive roots would be expected. However, for thick absorptive roots, the cell size, as well as the cortical cell file number (Chimungu et al., 2014a, b), may be more important than cell wall thickening in determining root activity. If so, root activity may be less affected by thickening of root EC cell walls than by changing the size or number of these cells. As such, there would be no obvious economic strategies for thick absorptive roots.
Recent studies have revealed different nutrient foraging strategies for thin and thick absorptive roots with the former depending on roots themselves and the latter depending more on mycorrhizal fungi (Baylis, 1975; Eissenstat et al., 2015; Liu et al., 2015). These observations are supported by the SRL-thickness relationship we found in our study where thin roots had larger SRL than thick roots (Fig. S6). Here, our results further indicate that thin and thick absorptive roots may follow different economic strategies when foraging for nutrients. These findings may have important implications for the emerging debate on the root economics spectrum. For example, the existence of an economic strategy for plant roots has been commonly accepted (Craine et al., 2005; Espeleta et al., 2009; Freschet et al., 2010; Reich, 2014). However, some recent studies have challenged the ubiquity of root economics spectra by showing no (Chen et al., 2013) or positive (Kong et al., 2014) relationships between root diameter and root N concentration. One possible explanation for these contrasting findings is the inclusion of many species with thick absorptive roots. Including these species may potentially obscure trait relationships indicating acquisition-conservation tradeoffs. On the other hand, the lack of evidence of an acquisition-conservation tradeoff may have resulted from the larger proportion of root cross-section area accounted for by root EC compared to the stele (Table S2; Kong et al., 2014). Notably, for species like monocots, the area of root stele is much larger than the area of root EC. We did not include monocots in our study, but it would be interesting to test whether the contrasting economic strategies for thin and thick absorptive roots, as presented here, can be applied across mono-dicots. Furthermore, our findings of different economic strategies for thin and thick absorptive roots are important for understanding plant impacts on soil processes. Acquisitive species are usually associated with bacterial-dominated soil microbial communities, faster carbon and nutrient cycling, and stronger plant-soil feedbacks, while conservative species are usually associated with fungal-dominated soil microbial communities, slower carbon and nutrient cycling, and weaker plant-soil feedbacks (Bardgett et al., 2014; Kardol et al., 2015; Wardle et al., 2004). This suggests that the impact of absorptive roots on soil processes would depend on root diameter.
Besides the prominent role in influencing root strategy, root thickness may also affect patterns of root chemical traits among root branch orders. The extractive C fraction increased with increasing root order for species with thin absorptive roots, whereas it declined for species with thick absorptive roots. Although both the acid-soluble and acid-insoluble fractions showed no consistent trends across root branch orders, the total recalcitrant fraction (sum of acid-soluble and acid-insoluble fractions) showed a pattern opposite to that of the extractive fraction. On the other hand, root N concentration and N in recalcitrant C fractions showed relative consistent patterns across root orders. As such, our findings provided only partial support of our second hypothesis. These patterns of root chemical fractions, however, are important in understanding soil ecosystem processes. For example, it is increasingly recognized that lower-order roots, compared with higher-order woody roots, are faster in root turnover but slower in root decomposition which makes the former a disproportionally greater source of soil organic matter (Clemmensen et al., 2013; Fan and Guo, 2010; Goebel et al., 2011; Xiong et al., 2013). This has been ascribed to higher recalcitrant C fractions in lower-order compared with higher-order woody roots (Goebel et al., 2011). However, our results may challenge the generality of slower decomposition of lower-order relative to higher-order roots as some lower-order roots had less recalcitrant C fractions and hence faster decomposition than higher-order roots.
In conclusion, the results of our study suggest an acquisition-conservation tradeoff for thin absorptive roots but not for thick absorptive roots. In addition, we found different patterns of root chemical fractions with root diameter and root order. The contrasting economic strategies between thin and thick absorptive roots are important in advancing our understanding of root ecology and the links with aboveground plant counterparts. Yet, our knowledge on the functioning of plant roots and their roles in driving soil ecosystem processes is still limited. We hope that our study presents an instructive perspective on the root economics spectrum that will stimulate further research in this field. Future studies may test to what extent our results hold for other (groups of) plant species (e.g., monocots, ferns, or conifers), including a larger spectrum of functional traits (including those associated with interactions with rhizosphere biota), and unravel the mechanisms underlying the “non-economics strategy” for thick absorptive roots. Further, we anticipate that the mycotrophy (i.e., plant species association with mycorrhizal fungi for resource acquisition) may underlie economic strategies in thick absorptive roots; however, empirical studies are needed to confirm this.
We thank Zhengxia Chen and Yingqian Long for their assistance in measuring root chemicals and anatomical structures, and Chengen Ma and Xin Jing (Peking University) for their valuable contribution to this work. We also appreciate two anonymous reviewers, Alexandra Weigelt and the editor Michael Bahn for their valuable comments on earlier versions of this manuscript. This study was sponsored by the open fund of Key Laboratory of Tropical Forest Ecology in Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences and Natural Science Foundation of China (No. 31200344). Edited by: M. Bahn