Shalom D. Addo-Danso
Camille E. Defrenne
Daniela Yaffar
Laynara F. Lugli- 1Forests and Climate Change Division, CSIR-Forestry Research Institute of Ghana, Kumasi, Ghana
- 2College of Forest Resources and Environmental Science, Michigan Technology University, Houghton, MI, United States
- 3Functional Forest Ecology, Universität Hamburg, Barsbüttel-Willinghusen, Germany
- 4Land Surface-Atmosphere Interactions, Technical University of Munich, Fresing, Bavaria, Germany
Editorial on the Research Topic
Insights into forest ecosystem belowground processes and functioning in a changing environment
Introduction
Forests cover an estimated 4.06 billion hectares globally (FAO, 2020), and account for about 70 percent of the world's terrestrial biodiversity (Pillay et al., 2022). Much attention has focused on forest ecosystems aboveground, with little emphasis on the belowground components and processes. Hence, our appreciation of forests functioning belowground lags far behind that of the aboveground. Yet, understanding the roles of plant roots, their associated symbionts, and soil microbes in net primary productivity, nutrient and water cycling are key in mediating the supply of ecosystem services including hosting biodiversity, carbon storage and water infiltration (Cusack et al., 2021; Chen et al., 2022; Xie et al., 2023).
Recent conceptual and technological advances in belowground forest ecology are shedding new light on plant root and belowground processes including root traits, root dynamics, rhizodeposition (exudation), mycorrhizal association, and soil respiration partitioning (Freschet et al., 2021a,b). However, there are still gaps in our knowledge on how to efficiently quantify the roles that plant roots and their associated symbionts play in driving forest ecosystems functioning and their responses to climate change, disturbances, and forest management practices. The goal of this Research Topic in Frontiers of Forests and Global Change was to bring together studies on forest ecosystems from a broad range of biomes including temperate, boreal and the tropics to provide new insights into the driving factors of variations in belowground components and processes at the plant species, community and ecosystem levels.
Root traits and soil carbon turnover in tropical forests
Tropical forests play an essential role in global water, carbon, and nutrient cycling (Cusack et al., 2021), but our understanding of the interspecific and intraspecific variations of root traits, and the main drivers of traits expression in the tropical forest biome is still at the nascent stage (Addo-Danso et al., 2020; Giweta, 2020). In this issue, Lee and Andersen examined the current state of root trait representation in the Paleotropics by compiling 421 publications that captured a wide range of root trait categories. They show that root physiological, architectural, and anatomical traits remain heavily underrepresented in the literature, representing only 25% of all the publications. In addition, they highlight gaps in the current trait coverage, with respect to ecosystems and geographical hotspots, a lack of trait data at the species level, and inconsistencies in how publications distinguish between fine roots and coarse roots. Based on their findings, the authors propose four avenues for future research: (i) measuring root anatomical traits (associated with plant hydraulics), (ii) examining “hard” traits such as mycorrhizal colonization intensity and root longevity, (iii) distinguishing between coarse roots, absorptive fine roots, and transport fine roots, and (iv) using a mix of in situ and ex situ experimental designs. Consistent with these recommendations, Weemstra et al. measured five morphological and architectural traits of absorptive fine roots of 218 Neotropical tree species in Brazil. They reveal that tree species that possess a combination of highly mycorrhized, thick roots with low tissue densities may dominate in tropical forests—often associated with arbuscular mycorrhizal fungi, and mostly limited by soil phosphorus. The authors' further highlight that shared ancestry is a weak driver of root trait variation in tropical forests, as opposed to tree species of temperate forests.
Tropical forest succession and the associated shifts in plant community composition and ecosystem services and processes are increasingly gaining attention due to the potential impacts on biodiversity conservation, climate change, and landscape restoration (Chazdon, 2017; Rüger et al., 2023). In this Research Topic, Wallwork et al. explored the relationship among tree functional composition, litter mass loss, and soil respiration rates in a chronosequence of tropical forest regrowth in Panama. They demonstrate that tree functional type (light-demanding vs. shade-tolerant species) influenced leaf litter decomposition and soil respiration rates. Specifically, they observed faster decomposition in light-demanding species than shade-tolerant species. Furthermore, the age of forests and the corresponding litter type interacted in a complex way to influence both leaf litter decomposition and soil respiration.
Roots, mycorrhizal fungi, and environmental changes in boreal forests
Boreal peatlands store large amounts of carbon in the peat layers (Beaulne et al., 2021) and are regarded as critical ecosystems for climate change mitigation (Eyvindson et al., 2023). Information on fine root dynamics and controlling factors of forested peatlands is limited, despite the belowground components contribute over 30% of the total net primary productivity (Xie et al., 2023). In this issue, Lampela et al. examined the effect of persistent water-table level drawdown in forested boreal peatlands, and called attention to the context dependency of fine-root biomass and production responses to drought. In particular, in forested fens, drainage decreased fine-root biomass, whereas in forested swamps, drainage increased fine-root biomass. The authors emphasize that both site wetness and nutrient regime should be accounted for when assessing the impact of drought on belowground plant traits in boreal peatlands. Bucher et al. went further to conduct a meta-analysis on root responses to water level drawdown and warming considering three plant functional types (tree, shrub, and graminoid) common in boreal peatlands. From the 65 observations, they showed an increase in fine root growth with both warming and water drawdown. When they considered the individual plant functional types, trees showed the greatest response (approximately +375% for warming and +867% for water level drawdown) and graminoids the least (approximately +60% for warming and −60% for water level drawdown), but these responses were mediated by topographic differences within sites.
Roots, mycorrhizal fungi, root exudation, and environmental changes in temperate forests
Most of the data and information that have shaped our understanding of root dynamics and belowground ecosystems have come from temperate forest ecosystems. However, there are still knowledge gaps when it comes to the responses of the belowground components of dominant tree species of temperate forests to changing climatic conditions, particularly drought and climate warming (Zwetsloot and Bauerle, 2021; Brunn et al., 2022; Kengdo et al., 2022). In this issue, Danzberger et al. rewetted beech and spruce forest stands, after subjecting them to 5 years of experimental summer drought, to explore the recovery of belowground plant roots and their associated fungal communities. One of their major findings was that the soil fungal community was more resilient to drought in mixed stands, as opposed to monocultures. In particular, ectomycorrhizal fungal species richness and relative abundance were higher in mixed stands within 3 months after rewetting.
Root exudates are key belowground components that determine plant interactions within the soil environment (Prescott et al., 2020), but how exudates change in response to increasing temperature remains unclear (Brunn et al., 2022). Heinzle et al. addressed this by delving into this crucial belowground component using an in-situ incubation method set up in a long-term soil warming experiment to measure carbon exudation rates from roots of spruce-dominated forests. The authors did not find any changes in root exudation per unit root dry mass in the warmed plots over a 15-year period when contrasted with control plots. Nevertheless, owing to increased root biomass stocks, the overall fine root carbon exudation experienced a 30% increase in the warmed plots. However, a +10°C increase in soil warming within 4 days led to a rapid increased root exudation. They demonstrate that root carbon exudation is temperature-sensitive in the short term.
Outlook
This Research Topic of papers represents the belowground component dynamics of forests in the boreal, temperate, and tropical biomes. The papers show that there are still knowledge gaps that need to be filled, especially on species-level root trait expression from diverse tropical forest ecosystems and sites. Furthermore, there is the need to shift attention to less studied belowground traits including those that are associated with root anatomy, architecture and physiology such as root exudation rate, specific root respiration and mycorrhizal colonization intensity, which often have direct link with plant and ecosystem functioning. Focusing on the belowground processes of peatland forests is critical for improving our understanding on the management practice of draining peatlands, and potential consequences on the long-term storage of carbon. Furthermore, expanding studies on exudation will help us to better understand how exudates mediate the interactions among above- and belowground ecosystem components, and their interactions with rhizosphere microbes. Finally, considering changes in forest community composition and structure during forest regrowth recovery could help us understand belowground dynamics of these forests, and predict their succession trajectory. Together, this Research Topic shows new research on some of the driving factors of belowground processes in different forest ecosystems and their responses to changing environmental conditions, and it serves as an avenue toward filling gaps for a more holistic view of the belowground component.
Author contributions
SDA-D: Conceptualization, Writing – original draft, Writing – review & editing. CED: Conceptualization, Writing – original draft, Writing – review & editing. DY: Conceptualization, Writing – original draft, Writing – review & editing. LFL: Conceptualization, Writing – original draft, Writing – review & editing.
Funding
The author(s) declare financial support was received for the research, authorship, and/or publication of this article. SDA-D is supported by the Royal Society Leverhulme Africa Postdoctoral Fellowships Grant No. LAF\R1\180025. LFL acknowledges the Bavarian State Chancellery (Project Amazon FLUX).
Acknowledgments
We thank all the authors and reviewers who participated in this Research Topic.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Publisher's note
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References
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Keywords: tropical forest, root traits, decomposition, forested peatlands, temperate forest, boreal forest, exudation, environmental changes
Citation: Addo-Danso SD, Defrenne CE, Yaffar D and Lugli LF (2024) Editorial: Insights into forest ecosystem belowground processes and functioning in a changing environment. Front. For. Glob. Change 7:1363762. doi: 10.3389/ffgc.2024.1363762
Received: 31 December 2023; Accepted: 12 January 2024;
Published: 23 January 2024.
Edited and reviewed by: Frank Hagedorn, Snow and Landscape Research (WSL), Switzerland
Copyright © 2024 Addo-Danso, Defrenne, Yaffar and Lugli. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Shalom D. Addo-Danso, shalomdanso@gmail.com