Reconcilable differences: a joint calibration of fine-root turnover times with radiocarbon and minirhizotrons

Increase in carbon input by enhanced fine root turnover in a long-term warmed forest soil

Fine root litter represents an important carbon input to soils, but the effect of global warming on fine root turnover (FRT) is hardly explored in forest ecosystems. Understanding tree fine roots' response to warming is crucial for predicting soil carbon dynamics and the functioning of forests as a sink for atmospheric carbon dioxide (CO₂). We studied fine root production (FRP) with ingrowth cores and used radiocarbon signatures of first-order, second- to third-order, and bulk fine roots to estimate fine root turnover times after 8 and 14 years of soil warming (+4 °C) in a temperate forest. Fine root turnover times of the individual root fractions were estimated with a one-pool model. Soil warming strongly increased fine root production by up to 128 % within one year, but after two years, the production was less pronounced (+35 %). The first-year production was likely very high due to the rapid exploitation of the root-free ingrowth cores. The radiocarbon signatures of fine roots were overall variable among treatments and plots. Soil warming tended to decrease fine root turnover times of all the measured root fractions after 8 and 14 years of warming, and there was a tendency for trees to use older carbon reserves for fine root production in warmed plots. Furthermore, soil warming increased fine root turnover from 50 to 106 g C m^-2 yr^-1 (based on two different approaches). Our findings suggest that future climate warming may increase carbon input into soils by enhancing fine root turnover. If this increase may partly offset carbon losses by increased mineralization of soil organic matter in temperate forest soils is still unclear and should guide future research.

A starting guide to root ecology: strengthening ecological concepts and standardising root classification, sampling, processing and trait measurements

In the context of a recent massive increase in research on plant root functions and their impact on the environment, root ecologists currently face many important challenges to keep on generating cutting-edge, meaningful and integrated knowledge. Consideration of the below-ground components in plant and ecosystem studies has been consistently called for in recent decades, but methodology is disparate and sometimes inappropriate. This handbook, based on the collective effort of a large team of experts, will improve trait comparisons across studies and integration of information across databases by providing standardised methods and controlled vocabularies. It is meant to be used not only as starting point by students and scientists who desire working on below-ground ecosystems, but also by experts for consolidating and broadening their views on multiple aspects of root ecology. Beyond the classical compilation of measurement protocols, we have synthesised recommendations from the literature to provide key background knowledge useful for: (1) defining below-ground plant entities and giving keys for their meaningful dissection, classification and naming beyond the classical fine-root vs coarse-root approach; (2) considering the specificity of root research to produce sound laboratory and field data; (3) describing typical, but overlooked steps for studying roots (e.g. root handling, cleaning and storage); and (4) gathering metadata necessary for the interpretation of results and their reuse. Most importantly, all root traits have been introduced with some degree of ecological context that will be a foundation for understanding their ecological meaning, their typical use and uncertainties, and some methodological and conceptual perspectives for future research. Considering all of this, we urge readers not to solely extract protocol recommendations for trait measurements from this work, but to take a moment to read and reflect on the extensive information contained in this broader guide to root ecology, including sections I-VII and the many introductions to each section and root trait description. Finally, it is critical to understand that a major aim of this guide is to help break down barriers between the many subdisciplines of root ecology and ecophysiology, broaden researchers' views on the multiple aspects of root study and create favourable conditions for the inception of comprehensive experiments on the role of roots in plant and ecosystem functioning.

Unravelling the age of fine roots of temperate and boreal forests

Fine roots support the water and nutrient demands of plants and supply carbon to soils. Quantifying turnover times of fine roots is crucial for modeling soil organic matter dynamics and constraining carbon cycle-climate feedbacks. Here we challenge widely used isotope-based estimates suggesting the turnover of fine roots of trees to be as slow as a decade. By recording annual growth rings of roots from woody plant species, we show that mean chronological ages of fine roots vary from <1 to 12 years in temperate, boreal and sub-arctic forests. Radiocarbon dating reveals the same roots to be constructed from 10 ± 1 year (mean ± 1 SE) older carbon. This dramatic difference provides evidence for a time lag between plant carbon assimilation and production of fine roots, most likely due to internal carbon storage. The high root turnover documented here implies greater carbon inputs into soils than previously thought which has wide-ranging implications for quantifying ecosystem carbon allocation.

Age-related changes in survival and turnover rates of balsam fir (Abies balsamea (L.) Mill.) fine roots

Fine-root (≤2 mm) demographics change as forests age, but the direction and extent of change are unknown. Knowledge of the change and understanding of causes will improve predictions of climate change impacts. We used minirhizotrons at three young and three mature balsam fir (Abies balsamea (L.) Mill.) sites to measure median lifespan (MLS) for each site and for annual cohorts. We computed turnover rate from the inverse of MLS (Tinv) and calculated a second turnover rate (T) from annual mortality, annual production and previous year-end standing crop. Median lifespan at mature sites (436 days) was half that at young sites (872 days). Median lifespan of annual cohorts varied widely at all sites. Age-class distributions of fine roots seen by minirhizotrons changed with increasing years of observation, with older age classes accumulating more slowly at mature sites. Our findings highlight the need to determine whether the proportional contributions of absorbing and transporting fine roots to annual production and their median lifespans change during stand development. Due to its variation among annual cohorts, we believe robust estimates of MLS at our sites require 5-7 years of observation, and reliable estimates of Tinv are reached earlier than T.

Fast-cycling unit of root turnover in perennial herbaceous plants in a cold temperate ecosystem

Roots of perennial plants have both persistent portion and fast-cycling units represented by different levels of branching. In woody species, the distal nonwoody branch orders as a unit are born and die together relatively rapidly (within 1-2 years). However, whether the fast-cycling units also exist in perennial herbs is unknown. We monitored root demography of seven perennial herbs over two years in a cold temperate ecosystem and we classified the largest roots on the root collar or rhizome as basal roots, and associated finer laterals as secondary, tertiary and quaternary roots. Parallel to woody plants in which distal root orders form a fast-cycling module, basal root and its finer laterals also represent a fast-cycling module in herbaceous plants. Within this module, basal roots had a lifespan of 0.5-2 years and represented 62-87% of total root biomass, thus dominating annual root turnover (60%-81% of the total). Moreover, root traits including root length, tissue density, and biomass were useful predictors of root lifespan. We conclude that both herbaceous and woody plants have fast-cycling modular units and future studies identifying the fast-cycling module across plant species should allow better understanding of how root construction and turnover are linked to whole-plant strategies.

Redefining fine roots improves understanding of below-ground contributions to terrestrial biosphere 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.

How tree roots respond to drought

The ongoing climate change is characterized by increased temperatures and altered precipitation patterns. In addition, there has been an increase in both the frequency and intensity of extreme climatic events such as drought. Episodes of drought induce a series of interconnected effects, all of which have the potential to alter the carbon balance of forest ecosystems profoundly at different scales of plant organization and ecosystem functioning. During recent years, considerable progress has been made in the understanding of how aboveground parts of trees respond to drought and how these responses affect carbon assimilation. In contrast, processes of belowground parts are relatively underrepresented in research on climate change. In this review, we describe current knowledge about responses of tree roots to drought. Tree roots are capable of responding to drought through a variety of strategies that enable them to avoid and tolerate stress. Responses include root biomass adjustments, anatomical alterations, and physiological acclimations. The molecular mechanisms underlying these responses are characterized to some extent, and involve stress signaling and the induction of numerous genes, leading to the activation of tolerance pathways. In addition, mycorrhizas seem to play important protective roles. The current knowledge compiled in this review supports the view that tree roots are well equipped to withstand drought situations and maintain morphological and physiological functions as long as possible. Further, the reviewed literature demonstrates the important role of tree roots in the functioning of forest ecosystems and highlights the need for more research in this emerging field.