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.