Expanding the “Root Economics Space”: New traits to
characterize diverse belowground strategies across biomes
The efforts to describe the natural variation in root diversity led to
the definition of the root economics space (RES), stating that
interspecific variation in root traits can be framed along two key
dimensions (Bergmann et al. 2020). First, a ‘collaboration axis’
organizes species based on how much they rely on mycorrhizal fungi for
resource acquisition: on the one end, species produce thin roots with
high specific root length (SRL, root length per unit root dry mass)
through which plants acquire soil resources (i.e., the ‘do-it-yourself’
strategy), while species on the other end of this axis construct thick,
low-SRL roots with ample colonization space for mycorrhizal fungi to
which they outsource soil resource uptake (i.e., the ‘outsourcing’
strategy). A second, independent ‘conservation axis’, in turn, separates
species with mass-dense (and presumably long-lived) roots that permits
long-term resource conservation, from species with roots that are high
in nitrogen concentration indicating active root metabolism and fast
turnover. The establishment of this root economics space – and its four
key traits (root diameter, SRL, root tissue density, and root nitrogen
concentration) – provides novel and important insights into the
formation of diverse belowground strategies and hence, species
coexistence and community diversity.
The studies in this Special Issue not only strengthen the concept of a
RES, but expand it by focusing on novel, largely unexplored systems,
traits, and (a)biotic drivers (Table 1). For example, while the
pioneering work by Bergmann et al. (2020) encompasses species from
around the globe, it mostly reflects trait variation across temperate
species and builds on only sparse information on root trait data from
the tropics. Thus far, it is not known to what extent similar dimensions
underlie root trait variation across tropical species. This Special
Issue presents important contributions to the discussion of how root
systems may work at the tropics (Table 1). Two Forum articles in this
Special Issue suggest that the RES might comprise different trait axes
in tropical than temperate regions. Firstly, Weemstra et al. (2022
speculate that the ‘collaboration axis’ may be even more important for
plants on highly weathered and phosphorus-limited tropical soils than in
nitrogen-limited temperate systems, where the deposition of highly
mobile (inorganic) nitrogen may reduce the need for investments in
mycorrhizal symbiosis. Secondly, Dallstream et al. (2022) argue that
phosphorus limitations in tropical regions may select for an even larger
variety of root trait combinations than those reflected by the
collaboration axis. For example, tropical trees may vary in the
formation and traits of cluster roots, exudation rates and profiles, and
root lifespan depending on the chemical forms in which phosphorus occurs
in (tropical) soils. These conceptual advances highlight the need for
exploring how and why a large variety of fine-root systems coexist in
hyper-diverse tropical plant communities.
Besides adding information from underexplored biomes, this Special Issue
further highlights the importance of incorporating additional traits
covering all belowground trait categories (McCormack et al. 2017) –
albeit to different extents – in the context of a RES (Table 1). For
example, using meta-analyses, Stiblíková et al. (2022) report that root
foraging precision (i.e., the percentage increment of root biomass in a
nutrient-rich patch relative to a nutrient-poor patch) across 123
herbaceous species was unrelated to the expected traits based on the RES
assumptions (i.e., SRL, root diameter, mycorrhizal colonization rate,
root tissue density and root nitrogen concentration) Being able to
rapidly exploit nutrient hotspots may thus be an important resource
uptake strategy independent of current RES, for at least some herbaceous
species. In contrast, among ten shrub species, root foraging precision
was associated with the collaboration axis, as those species with thick
roots and low SRL had lower foraging precision and this may be explained
by the greater mycorrhizal dependency of thick-rooted species
circumventing their need to produce highly exploitative, branched-out
roots (Yang et al. 2021). In addition to root architectural traits, Mao
and co-workers (2022) examine root mechanical traits among 12 herbaceous
species in relation to the RES. Those species that displayed the
do-it-yourself strategy also had more mechanically robust roots (e.g.,
higher tissue quality investment) than species that relied more on
mycorrhizal colonization. The authors argue that species relying on
their own roots rather than on fungal symbionts should develop stronger
and tougher roots that are better protected against root herbivores and
able to penetrate compact soils to ensure sufficient resource
acquisition. These studies provide further evidence that a large variety
of root traits might feature in the RES in order to understand e.g., how
plants acquire resources (e.g., through root foraging success) or
determine ecosystem processes such as soil stability (e.g., through root
mechanical traits).
Whereas the work of Yang et al. (2021), Stiblíková et al. (2022), and
Mao et al. (2022) in this issue expanded the RES with fine root traits,
others delved further, beyond the absorptive roots traditionally
emphasized in fine-root studies. Klimešová and Herben (2021) study
non-acquisitive belowground plant organs, like rhizomes or bud-bearing
roots, of close to 1500 herbaceous species with different lifespans and
clonality. Here, non-clonal perennial species built roots with
mass-denser tissue and a higher degree of mycorrhization than annual and
clonal species. Weemstra et al. (2022) argue that besides mycorrhizalroot traits, mycorrhizal fungal traits (e.g .,
specific hyphal length [i.e., hyphal length per unit hyphal dry
mass]: the fungal analogue of specific root length) determine fungal
resource uptake and thus, plant nutrition, but explicitly warn against
treating and interpreting them similar to mycorrhizal root traits
because mycorrhizal fungal traits serve fungal but not
necessarily plant’s fitness interests. Emerging evidence further
suggests that not only mycorrhizal but also soil-borne pathogenic
interactions shape the RES (Xia et al. 2021). The work of Dai et al.
(2022) shows that (arbuscular mycorrhizal) tree species adopting the
do-it-yourself strategy had higher root pathogen richness than species
exhibiting the outsourcing strategy, potentially because high-SRL roots
have greater investments in root mechanics (Mao et al. 2022) and thus
lower investments in root chemical defense against infections by
soilborne pathogens (Xia et al. 2021). Together, these studies focusing
on interspecific variation in a broad array of belowground traits and
functions highlight the multidimensionality of the belowground, and
bring forward relevant candidate (biotic) traits that would play a
prominent role in expanding the current RES.