The platform for European root science, COST action E38: An introduction and overview

Abstract

Globally, forests cover 4 billion ha or 30% of the Earth's land surface and account for more that 75% of carbon stored in terrestrial ecosystem. However, 20 – 40% of the forest biomass is roots. Roots play a key role in acquisition of water and nutrients from the soil, the transfer of carbon to soil, as well as providing physical stabilisation. In temperate forests of Europe, average biomass of trees is estimated to be ca. 220 t ha⁻¹, of which 52 t ha⁻¹ are coarse roots and 2.4 t ha⁻¹ are fine roots. Thus, forests and their soils belong to the planets largest reservoirs of carbon. As an outcome of a recently established European platform for scientists working on woody roots, COST action E38, a series of papers has been initiated in order to review the current knowledge on processes in and of roots of woody plants and to identify possible knowledge gaps. These reviews concentrate on aspects of roots as indicators of environmental change, biomass of fine roots, and modelling of course root systems. The reviews of roots as indicators of environmental change cover a number of aspects including, specific root length, the calcium to aluminium ratio, root electrolyte leakage, and ectomycorrhiza community composition.

Key words:

• Coarse roots
• fine root dynamics
• indicators
• modelling
• tree roots

Introduction

Across the biosphere, rapid and accelerating changes in land use, climate and atmospheric composition driven primarily by anthropogenic forces are known to exert major influences on the productivity, biodiversity and sustainable provision of ecosystem goods and services. Among the terrestrial ecosystems, forests are considered as the most important terrestrial reservoirs of biological diversity, containing as much as two-thirds of all plant and animals species. Globally, forests cover 4 billion ha or 30% of the Earth's land surface (FAO, 2006), and account for more that 75% of carbon stored in terrestrial ecosystems (Schlesinger, 1997). There is a very high confidence that recent warming as a result of the climate change is strongly affecting terrestrial biological systems such as forests. These effects include earlier timing of spring events or shifts in the range of plant species towards the poles or to higher elevations (IPCC, 2007). Forests also play a major role in climate change, as (a) they currently contribute about one-sixth of global carbon (C) emissions when cleared, overused or degraded, (b) they react sensitively to a changing climate, (c) when managed sustainably, they produce woodfuels as a benign alternative to fossil fuels, and finally, (d) they have the potential to absorb about one-tenth of global C emissions projected for the first half of this century into their biomass, soils, and products, and store them – in principle in perpetuity (FAO, 2006). However, mainly due to the conversion of forests to agricultural land, within the last decade the total forest area has continued to decrease by ca. 13 million ha yr⁻¹ (FAO, 2006).

New methods, skills and terminology will be necessary in the future to assess C stocks and their changes in forests. The ‘clean development mechanism’ (CDM) allows industrialised countries to offset part of their C emissions and to contribute towards sustainable development of a developing country through reforestation projects (FAO, 2006). Estimating and accounting for the C in the above- and below-ground compartments, dealing with forest degradation, sink enhancement through global warming, nitrogen (N) emissions, carbon dioxide (CO₂) fertilisation and the natural effect of aging remain issues, which are difficult to resolve. Although the contribution of tree roots is fundamental in these issues, many methods used in root research, for example the determination of belowground biomass lack the harmonisation of methods used for aboveground parameters, as a consequence belowground parameters are often ignored. Thus efforts have to be undertaken to improve the methodology to investigate roots, and to include root data in future studies and scenarios.

COST action E38: An international platform

In 2003, an international platform for the research of roots of woody plants was initiated as a common base for scientists working on woody root processes, allowing exchange of information, ideas and personnel (European Cooperation in the field of Scientific and Technical Research – COST action E38, www.cost38.net). The main objectives of the COST action E38 are to enhance the knowledge base and to improve the methodology of measuring root processes in relation to environmental change. The platform allowed establishment of a concerted network of scientists operating at the cutting edge of woody root processes from cellular to ecosystem scales. Within the E38 framework, available data on the response of root processes to global environmental change is collected, evaluated and categorised. Three research areas currently focussed on are: (a) to identify properties of roots which can be used as indicators for environmental change, (b) to measure the dynamics of fine roots to assess C fluxes to the soils, and (c) to model the coarse roots to estimate the biomass of the root systems. This article provides an introduction and overview of the review articles.

Roots as indicators

Fine roots are in intimate contact to the soil and provide the plants with water and nutrients. Thus, root parameters are related and closely linked to soil chemical and soil physical properties. However, root parameters are only poorly used in the indication of stress or/and environmental change. The term stress has often been used to describe a number of poorly defined negative conditions. For tree roots stress factors are mainly defined by soil physical and chemical properties, such as waterlogging or soil acidity. However, only rarely do stress factors occur singly, and in most cases plants roots are exposed to a number of stress factors simultaneously. Grime (1991) defined stress as “Constraints which limit the utilisation of resources, growth and reproduction”. Using the concept of limiting resources, constraints stress can be extended to cover any environmental factor, and can thus include chemical, physical and also biotic factors (Godbold, 1998). Through a cascade of physiological and morphological response mechanisms, tree root systems are able to respond flexibly to steadily occurring changes induced by variable environmental conditions. If the limits of this adaptation are exceeded either due to the intensity of the stress factor, or the speed of change, damage will occur. Both response mechanisms and damage symptoms are used as indicators of stress. Fränzle (2006) suggested two types of physiological bioindicators: effect indicators and accumulation indicators. In the COST action E38, the indicators, root electrolyte leackage, root Ca/Al molar ratio, specific root length, and mycorrhizas have been focussed on. If these indicators are catagorised, root electrolyte leackage (Radoglou et al., 2007) can be considered as an effect indicator, and the root Ca/Al molar ratio (Brunner et al., 2004; Vanguelova et al., 2007) as an accumulation indicator, whereas specific root length (Ostonen et al., 2007) and ectomycorrhizas (Cudlin et al., 2007) are morphological reactions. Although, the ICP Forests Programme objectives are to contribute to a better understanding of the relationships between the condition of forest ecosystems and anthropogenic (in particular air pollution), as well as natural stress factors through intensive monitoring on a number of selected permanent observation plots spread over Europe (Level II; ICP Forests Manual, 2004), root indicators are not currently applied in long-term forest monitoring programmes.

Fine root biomass and dynamics

In the last decades, humans have strongly affected ecosystems due to atmospheric and climatic changes, such as rising atmospheric CO₂ concentrations, elevated temperatures, altered precipitation, and N deposition (Norby & Jackson, 2000). Soils of forest ecosystems, in particular, have been affected by high atmospheric inputs of acidifying pollutants (S and N compounds) originating from the combustion of fossil fuels in power generation, industry and transportation (Fowler et al., 1999; Nadelhoffer et al., 1999; Rennenberg & Gessler, 1999). These inputs have led to an acceleration of soil acidification, loss of basic cations and release of Al ions into soil solution as a consequence of proton-buffer processes (Blaser et al., 1999). In addition to the acidifying effects, excessive inputs of atmospheric N result in nitrate leaching and in relative shortage of other nutritional elements for plants (Magill et al., 1997; Högberg et al., 2006). Changes in atmospheric CO₂ concentrations, soil N availability, soil acidification as well as soil physical properties such as soil temperature and drought, all can potentially effect fine root biomass and fine root dynamics. Estimations from temperate forests of Central Europe reveal that C storage in trees accounts for about 110 t C ha⁻¹, of which 26 t C ha⁻¹ is in coarse roots and 1.2 t C ha⁻¹ is in fine roots (Perruchoud et al., 1999). Compared with soil C, which is about 65 t C ha⁻¹ (without roots), the contribution of the root C to the total belowground C pool is about 42%. Recent analysis in experiments with elevated CO₂ concentrations have shown increases of the forest net primary productivity (NPP) by about 23% (Norby et al., 2005), and, in the case of poplars, an increase of the standing root biomass between 47 – 76% (Lukac et al., 2003) and 113% (King et al., 2001). Thus estimating changes in fine root biomass and dynamics, is crucially important in determining global C budgets. The COST action E38 has focused on both the biomass (Finér et al., 2007) and the turnover (Majdi et al., 2005) of fine root systems in relation to forest types and latitudinal gradients.

Root modelling

The structural course root systems of trees are on average between 20 – 30% of the total biomass of a tree, and the structure of the root system is critically important in the stability of trees (Coutts et al., 1999; Danjon et al., 2005). Indeed, research into architectural aspects of root systems has been driven by silviculural interests of tree establishment and subsequent stand stability (Tobin et al., 2007). Root system architecture is a result of a number of processes including branching, elongation, gravitropic response, and secondary thickening. All aspects of the dynamics of 3D root architecture cannot generally be measured using one method, and several measurement methods are usually required. Of primary importance, for the estimation of course root properties, and the biomass of course root systems, are modelling and in particular the three-dimensional (3D) characterisation of structural roots (Pellerin & Pagès, 1996). Three-dimensional visualisation is carried out by modelling of roots systems after, excavation of root systems and direct measurement of geometric parameters (Oppelt et al., 2001) or spatial digitization (Danjon et al., 1999) In addition the structure of root systems can be predicted using 3D root growth simulation models (Pagès et al., 1989). In the COST action E38, emphasis has been placed on reviewing the current modelling climate, assessing the types of input data available from the various branches of woody root research and their impact on future dynamic modelling of structure, and linking of coarse root models with fine root distribution. The results of this work are presented in the article of Tobin et al., (2007).