Kyotaro Noguchi

Biomass and production of fine roots in Japanese forests

To better understand the control of fine-root dynamics in Japanese forests, we reviewed studies conducted in Japan on fine-root biomass and production. Most of the data on fine-root biomass were obtained for conifer plantations in limited regions; the average fine-root biomass of dominant trees ranged from ∼50 g m−2 for Pinus species (n = 3) to ∼600 g m−2 for Cryptomeria japonica (n = 4) and Chamaecyparis obtusa (n = 3). These values are comparable with or less than those reported for other temperate forests mainly in North America or Europe. Information on fine-root production in Japanese forests remains limited. Fine-root production accounted for ∼30% of the net primary productivity in two deciduous forests, but similar data was not reported for coniferous forests in Japan. In Japanese forests, slope position is a key parameter controlling fine-root biomass that is greater on upper slopes than on lower slopes, probably because soil resource availability decreases upslope. Studies in manipulated soil environments (e.g., removing throughfall to simulate drought) also suggested that fine-root biomass and production were greatly affected by altered soil environments. Physiological control of fine-root dynamics was recently discussed via anatomical analyses of Chamaecyparis obtusa. Findings from Japanese studies generally support data on fine-root biomass and production obtained from other temperate regions. Further attempts to elucidate the influence of slope position (soil resource availability) on fine-root production would be useful to gain a more detailed understanding of the fine-root dynamics in Japanese forests.

Effects of simulated drought stress on the fine roots of Japanese cedar (Cryptomeria japonica) in a plantation forest on the Kanto Plain, eastern Japan

Drought stress was simulated in a 28-year-old Japanese cedar plantation (Kanto Plain, Japan) between April and October 2004 by removing throughfall using rain shelters. Changes in fine-root parameters caused by this drought treatment were examined by sequential soil coring. Drought effects on fine roots were analyzed separately for particular soil depths (0–5, 5–15, and 15–25 cm) and root diameters (<1 and 1–2 mm). Generally, fine-root biomass and root tip numbers decreased by the drought treatment. Drought stress was most intense for fine roots in the topsoil and weakest for fine roots in the deepest soil layer. Fine roots less than 1 mm in diameter were affected more severely than 1- to 2-mm roots. The effect of drought treatment was most remarkable for the number of white root tips, which decreased to 17% of the control at the soil depth of 0–5 cm. These results suggest that white root tip is the most suitable indicator of drought stress. Simulated drought reduced production of fine roots less than 1 mm and 1–2 mm in diameter. Fine-root mortality was stimulated for roots less than 1 mm, but not for 1- to 2-mm roots. These results suggest that fine roots with larger diameters can survive drought stress at a level simulated in this study, but processes of fine-root production were inhibited regardless of the diameter classes. The duration of drought stress and phenology of fine roots should also be considered in diagnosing the effects of drought on fine-root parameters.

Estimating the production and mortality of fine roots in a Japanese cedar (Cryptomeria japonica D. Don) plantation using a minirhizotron technique

Fine roots are a key component of forested ecosystems, but available information is still limited. This study examined the production and mortality of fine roots less than 1 mm in diameter in a Japanese cedar (Cryptomeria japonica D. Don) plantation located on the Kanto Plain in central Japan. We used a minirhizotron technique in combination with soil coring, and collected data for 1 year (May 2002–May 2003). Fine root production and mortality were determined from changes in the lengths of individual fine roots on minirhizotron tubes. Both fine root production and mortality rates were greater in the upper soil than in lower soil levels. Both rates were seasonal, with higher values in summer than in winter; this trend was more pronounced in upper soil levels. These results suggest that environmental conditions, such as temperature or soil properties, affect the production and mortality rates of fine roots. Fine root production and mortality occurred simultaneously, and their rates were similar, which may have led to unclear seasonal changes in fine root standing crop estimates. Soil coring indicated that the fine root biomass of this stand was about 120 g m−2, of which 40% was from Japanese cedar. The estimated rates of dry matter production and mortality of total fine roots, including understory plants, were both approximately 300 g m−2 year−1.

Estimation of the fine root biomass in a Japanese cedar (Cryptomeria japonica) plantation using minirhizotrons

We estimated fine root biomass in a Japanese cedar (Cryptomeria japonica) plantation using a minirhizotron technique. Since data obtained from minirhizotrons are limited to the length and diameter of fine roots observed on minirhizotron tubes, data conversion is necessary to determine the fine root biomass per unit soil volume or unit stand area. We first examined the regression between diameter squared and weight per unit length of fine roots in soil core samples, and calculated the fine root biomass on minirhizotron tubes from their length and diameter. Then we determined conversion factors based on the ratio of the fine root biomass in soil core samples to that on minirhizotron tubes. We examined calculation methods, using a single conversion factor for total fine root biomass in the soil for depths of 0–40 cm (Cal1), or using four conversion factors for fine roots in the soil at 10-cm intervals (Cal2). Cal1 overestimated fine root biomass in the lower soil or underestimated that in the upper soil, while fine root biomass calculated using Cal2 better matched that in soil core samples. These results suggest that minirhizotron data should be converted separately for different soil depths to better estimate fine root biomass.

Special feature: development and function of roots of forest trees in Japan

This issue of the Journal of Forest Research is the fi rst ever to feature research on roots of forest trees in Japan. Interest in the dynamics of tree roots has been increasing globally because of the importance of belowground carbon pools and fl uxes in predicting the effects of global change on forest ecosystems (Jackson 2000). In general, large coarse roots provide anchorage; transport carbohydrates, water, and nutrients; and are responsible for most of the C storage in roots. Fine roots are important for water and nutrient uptake, and they contribute signifi cantly to belowground C fl uxes via root turnover and exudations. However, few data on tree-root dynamics are currently available when compared with the information on aboveground parts, which is due to the diffi culty of studying roots hidden in the soil. An international platform, “Woody Root Processes,” to support research on roots of woody plants has been ongoing since 2003 within a framework of European COoperation in the fi eld of Scientifi c and Technical Research (COST Action E38) (Brunner and Godbold 2007). COST E38 have held workshops on roots of woody plants fi ve times from 2004 to 2006 to enhance the knowledge base, improve methodology, and establish a network of scientists conducting cutting-edge research on woody roots. We, as coordinators of this special issue, have attended these workshops continuously and confi rmed the value of this platform for tree-root researchers in Japan. We decided to hold a special session of the Japanese Forest Society focused on “Development and function of roots of forest trees” during the 117th Annual Meeting in April 2006. This session had 26 presentations of tree-root studies that ranged in scale from the cellular level to that of the ecosystem. Here, a part of the above-mentioned session is presented as peerreviewed articles. The goals of this issue are to describe Japanese tree-root research and to compare available data with those from other countries. We hope that these articles contribute to the further construction of the platform required for designing future root research and facilitating research collaborations in Japan or between Japan and other countries. First in this special issue, Brunner and Godbold (2007) review recent research of tree roots focusing especially on the global importance of the contribution of roots to C cycling in forest ecosystems. They show that tree-root C accounts for about 20%–40% of forest biomass globally; C fl uxes simulated using ecosystem models need to include accurate estimates of root-turnover rate. These insights into the contribution of roots to forest C dynamics provide distinct reasons why tree-root research is being carried out. Following this review article, the feature presents eight articles in two categories of “Root development” and “Root function” of forest trees.

Editorial note for the special issue “Dynamics and physiological processes of tree roots”

Plant roots are well known to have key roles in water and nutrient acquisition from soils. Recent studies using molecular techniques have facilitated our understanding of these physiological processes in tree roots (Mclean et al. 2011; Alber et al. 2012; Laur and Hacke 2013). On the other hand, a number of studies in the past decades have focused on tree roots as essential in carbon dynamics in terrestrial ecosystems, in which fine roots are a key component of below-ground carbon flux, whereas coarse roots function as a carbon reservoir (Brunner and Godbold 2007). In addition, recent studies have started to examine the effects of biodiversity on root dynamics or interactions between roots and other organisms, which are important topics to deepen our comprehensive understanding about ecosystem functions of tree roots (Brassard et al. 2013; Meier et al. 2013). Although studying roots that are hidden in the soil is often difficult, new techniques such as non-destructive imaging have been applied to better understand the dynamics of tree roots (Nakaji et al. 2008; Hirano et al. 2012). In September 2014, the ‘‘Sixth international symposium on physiological processes in roots of woody plants’’ was held at Nagoya University, Japan. In this symposium, current woody root studies were reported by 37 oral and 77 poster presentations. The topics discussed in the symposium were quite diverse including root growth dynamics, water and nutrient uptake, below-ground carbon allocation and biological interactions (e.g. symbiosis, biodiversity effects). This special issue ‘‘Dynamics and physiological processes of tree roots’’ was provided by 21 excellent papers, most of which were from the symposium in Nagoya. In the first part of this special issue, CO2 flux in roots and effects of elevated atmospheric CO2 on root dynamics were highlighted. Although a number of papers were published on root dynamics related to below-ground carbon flux in the past decades as mentioned above, there are still many uncertainties. Bloemen et al. (2016), in their review, showed that a substantial proportion of CO2 derived from root respiration remained in root systems and was transported through the transpiration stream. Although these findings were obtained from a limited number of species so far, this phenomenon could impact our current understanding of carbon dynamics in terrestrial ecosystems or CO2 metabolism in trees. Agathokleous et al. (2016) and Wang et al. (2016) reported results obtained by free-air-CO2 enrichment (FACE) experiments using deciduous broadleaved species (Betula and Quercus species) in Hokkaido, northern Japan. They examined the effects of elevated CO2 on the trees grown on two different types of the soil [fertile brown forest soil (BF) and immature volcanic ash soil (VA)]. Agathokleous et al. (2016) showed that elevated CO2 increased the growth of whole roots in nutrient poor VA soil, but not in fertile BF soil. Wang et al. (2016), on the other hand, reported that elevated CO2 decreased standing crop of fine roots and increased their life spans especially in the VA soil. The responses of tree roots to elevated CO2 varied with factors such as soil resource availability and periods of CO2 fumigation (Eissenstat et al. 2013; Pritchard et al. 2014). In their case, not only soil fertility, but also other factors such as soil porosity or growth stage of the trees may have affected the responses to CO2 fumigation. & Kyotaro Noguchi kyotaro@affrc.go.jp