Vegetation impacts on ecosystem functioning are mediated by mycorrhizas, plant-fungal associations formed by most plant species. Ecosystems dominated by distinct mycorrhizal types differ strongly in ...their biogeochemistry. Quantitative analyses of mycorrhizal impacts on ecosystem functioning are hindered by the scarcity of information on mycorrhizal distributions. Here we present global, high-resolution maps of vegetation biomass distribution by dominant mycorrhizal associations. Arbuscular, ectomycorrhizal, and ericoid mycorrhizal vegetation store, respectively, 241 ± 15, 100 ± 17, and 7 ± 1.8 GT carbon in aboveground biomass, whereas non-mycorrhizal vegetation stores 29 ± 5.5 GT carbon. Soil carbon stocks in both topsoil and subsoil are positively related to the community-level biomass fraction of ectomycorrhizal plants, though the strength of this relationship varies across biomes. We show that human-induced transformations of Earth's ecosystems have reduced ectomycorrhizal vegetation, with potential ramifications to terrestrial carbon stocks. Our work provides a benchmark for spatially explicit and globally quantitative assessments of mycorrhizal impacts on ecosystem functioning and biogeochemical cycling.
• Although linkages of leaf and whole‐plant traits to leaf lifespan have been rigorously investigated, there is a limited understanding of similar linkages of whole‐plant and fine root traits to root ...lifespan. In comparisons across species, do suites of traits found in leaves also exist for roots, and can these traits be used to predict root lifespan? • We observed the fine root lifespan of 12 temperate tree species using minirhizotrons in a common garden and compared their median lifespans with fine‐root and whole‐plant traits. We then determined which set of combined traits would be most useful in predicting patterns of root lifespan. • Median root lifespan ranged widely among species (95–336 d). Root diameter, calcium content, and tree wood density were positively related to root lifespan, whereas specific root length, nitrogen (N) : carbon (C) ratio, and plant growth rate were negatively related to root lifespan. Root diameter and plant growth rate, together (R2 = 0.62) or in combination with root N : C ratio (R2 = 0.76), were useful predictors of root lifespan across the 12 species. • Our results highlight linkages between fine root lifespan in temperate trees and plant functional traits that may reduce uncertainty in predictions of root lifespan or turnover across species at broader spatial scales.
Since their emergence onto land, terrestrial plants have developed diverse strategies to acquire soil resources. However, we lack a framework that adequately captures how these strategies vary among ...species. Observations from around the world now allow us to quantify the variation observed in commonly-measured fine-root traits but it is unclear how root traits are interrelated and whether they fall along an “economic” spectrum of acquisitive to conservative strategies. We assessed root trait variation and mycorrhizal colonization rates by leveraging the largest global database of fine-root traits (the Fine-Root Ecology Database; FRED). We also developed a heuristic model to explore the role of mycorrhizal fungi in defining belowground exploration efficiency across a gradient of thin- to thick-diameter roots. In support of the expectations of the “root economic spectrum,” we found that root diameter was negatively related to specific root length (Pearson’s r=-0.76). However, we found an unexpected negative relationship between root diameter and root tissue density (Pearson’s r = -0.40), and we further observed that root nitrogen content was largely unrelated to other economic traits. Mycorrhizal colonization was most closely associated with root diameter (Pearson’s r = 0.62) and was unrelated to root tissue density and root nitrogen. The heuristic model demonstrated that while thinner roots have inherently greater capacity to encounter soil resources based on higher surface area per unit mass, the potential for increased associations with mycorrhizal fungi in thicker roots, combined with greater hyphal growth, can result in equally acquisitive strategies for both thin- and thick roots. Taken together, our assessments of root trait variation, trade-offs with mycorrhizal fungi, and broader connections to root longevity allowed us to propose a series of fundamental constraints on belowground resource acquisition strategies. Physical tradeoffs based on root construction (i.e., economic traits) and functional limitations related to the capacity of a root to encounter and acquire soil resources combine to limit the two-dimensional belowground trait space. Within this trait space there remains a diversity of additional variation in root traits that facilitates a wide range of belowground resource acquisition strategies.
Compared with ectomycorrhizal (ECM) forests, arbuscular mycorrhizal (AM) forests are hypothesized to have higher carbon (C) cycling rates and a more open nitrogen (N) cycle
To test this hypothesis, ...we synthesized 645 observations, including 22 variables related to below-ground C and N dynamics from 100 sites, where AM and ECM forests co-occurred at the same site.
Leaf litter quality was lower in ECM than in AM trees, leading to greater forest floor C stocks in ECM forests. By contrast, AM forests had significantly higher mineral soil C concentrations, and this result was strongly mediated by plant traits and climate. No significant differences were found between AM and ECM forests in C fluxes and labile C concentrations. Furthermore, inorganic N concentrations, net N mineralization and nitrification rates were all higher in AM than in ECM forests, indicating ‘mineral’ N economy in AM but ‘organic’ N economy in ECM trees.
AM and ECM forests show systematic differences in mineral vs organic N cycling, and thus mycorrhizal type may be useful in predicting how different tree species respond to multiple environmental change factors. By contrast, mycorrhizal type alone cannot reliably predict below-ground C dynamics without considering plant traits and climate.
Plant roots have greatly diversified in form and function since the emergence of the first land plants, but the global organization of functional traits in roots remains poorly understood. Here we ...analyse a global dataset of 10 functionally important root traits in metabolically active first-order roots, collected from 369 species distributed across the natural plant communities of 7 biomes. Our results identify a high degree of organization of root traits across species and biomes, and reveal a pattern that differs from expectations based on previous studies of leaf traits. Root diameter exerts the strongest influence on root trait variation across plant species, growth forms and biomes. Our analysis suggests that plants have evolved thinner roots since they first emerged in land ecosystems, which has enabled them to markedly improve their efficiency of soil exploration per unit of carbon invested and to reduce their dependence on symbiotic mycorrhizal fungi. We also found that diversity in root morphological traits is greatest in the tropics, where plant diversity is highest and many ancestral phylogenetic groups are preserved. Diversity in root morphology declines sharply across the sequence of tropical, temperate and desert biomes, presumably owing to changes in resource supply caused by seasonally inhospitable abiotic conditions. Our results suggest that root traits have evolved along a spectrum bounded by two contrasting strategies of root life: an ancestral 'conservative' strategy in which plants with thick roots depend on symbiosis with mycorrhizal fungi for soil resources and a more-derived 'opportunistic' strategy in which thin roots enable plants to more efficiently leverage photosynthetic carbon for soil exploration. These findings imply that innovations of belowground traits have had an important role in preparing plants to colonize new habitats, and in generating biodiversity within and across biomes.
Patterns of plant biomass allocation and functional adjustments along climatic gradients are poorly understood, particularly belowground. Generally, low temperatures suppress nutrient release and ...uptake, and forests under such conditions have a greater proportion of their biomass in roots. However, it is not clear whether ‘more roots’ means better capacity to acquire soil resources.
Herein we quantified patterns of fine-root anatomy and their biomass distribution across Scots pine (Pinus sylvestris) populations both along a 2000-km latitudinal gradient and within a common garden experiment with a similar range of populations.
We found that with decreasing mean temperature, a greater percentage of Scots pine root biomass was allocated to roots with higher potential absorptive capacity. Similar results were seen in the common experimental site, where cold-adapted populations produced roots with greater absorptive capacity than populations originating from warmer climates.
These results demonstrate that plants growing in or originated from colder climates have more acquisitive roots, a trait that is likely adaptive in the face of the low resource availability typical of cold soils.
Arbuscular mycorrhizal fungi (AMF) mediate plant interspecific competition and community structure. However, the magnitude and direction of AMF effects and underlying mechanisms are not clear. Here, ...we synthesized the results of 304 studies to evaluate how AMF affect plant competition and community structure and which abiotic and biotic conditions in experimental design modify these AMF effects. The magnitude and direction of AMF effects on plant competitive ability (in terms of competitive response) differed markedly among plant functional groups. When AMF inoculum was added, competitive ability was strongly enhanced in N‐fixing forbs and was significantly suppressed in C₃ grasses, whereas no effect was observed in C₄ grasses, non‐N‐fixing forbs and woody species. Furthermore, AMF inoculation increased competitive ability of perennial species when their competitors were annual species. AMF inoculation differentially influenced separate aspects of plant community structure and species composition. AMF inoculation significantly increased plant diversity but had no effects on plant productivity. Response of dominant plant species to AMF inoculation was the determining factor in explaining variations in how and to what degree plant diversity was influenced by AMF inoculation. When dominant species derived strong benefits from AMF, their dominance level was increased by AMF inoculation, which consequently decreased plant diversity. We did not find stronger AMF effects on plant diversity and productivity when greater numbers of AMF species were used in the inoculation. Synthesis. Despite large variations in AMF effects among studies, a unifying mechanism was observed that the mycorrhizal responsiveness (differences in plant growth between AMF and non‐AMF colonization treatments) of target and neighbouring plant species can determine AMF effects on the competitive outcome among plant species, which in turn influenced plant species diversity and community composition. Given that plant traits, soil nutrient conditions and probably mycorrhizal fungal traits are all factors determining the degree of mycorrhizal response of plant species, future studies should explicitly consider each of these factors in experimental design to better understand AMF effects on plant coexistence, plant community dynamics and ecosystem processes.
Fine roots acquire essential soil resources and mediate biogeochemical cycling in terrestrial ecosystems. Estimates of carbon and nutrient allocation to build and maintain these structures remain ...uncertain because of the challenges of consistently measuring and interpreting fine-root systems. Traditionally, fine roots have been defined as all roots ≤ 2 mm in diameter, yet it is now recognized that this approach fails to capture the diversity of form and function observed among fine-root orders. Here, we demonstrate how order-based and functional classification frameworks improve our understanding of dynamic root processes in ecosystems dominated by perennial plants. In these frameworks, fine roots are either separated into individual root orders or functionally defined into a shorter-lived absorptive pool and a longer-lived transport fine-root pool. Using these frameworks, we estimate that fine-root production and turnover represent 22% of terrestrial net primary production globally – a c. 30% reduction from previous estimates assuming a single fine-root pool. Future work developing tools to rapidly differentiate functional fine-root classes, explicit incorporation of mycorrhizal fungi into fine-root studies, and wider adoption of a two-pool approach to model fine roots provide opportunities to better understand below-ground processes in the terrestrial biosphere.
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