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

Abstract

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