Tree roots are of fundamental functional importance on the individual tree as well as on the ecosystem level. In simplified terms, woody coarse roots (≥ 2 mm in diameter) serve anchorage, transport, and storage functions, while non-woody fine roots (< 2 mm in diameter)1 primarily serve nutrient and water uptake (Helmisaari et al. 2000; Pregitzer 2002). Moreover, roots play a key role in ecosystem functioning with regards to biogeochemical cycling in terrestrial ecosystems (Pregitzer et al. 2002; Yuan and Chen 2010; McCormack et al. 2015): besides their acquisition and transport function for water and nutrients upwards, they are pathways for carbon and nutrients in the downward direction, they facilitate deep water infiltration, affect the weathering of minerals, and they have an impact on the activity of soil fauna (Schenk and Jackson 2002).
Despite their importance, our knowledge on fine root system size, structure, morphology, and anatomy of trees under different environmental conditions is still scarce, thereby limiting our understanding of the role of belowground systems in ecological processes (Reich 2002; Comas and Eissenstat 2009). Since root studies are in the majority confined to the upper soil horizons (Gill and Burke 2002; Schenk and Jackson 2002, 2005), this is particularly true for the subsoil, which is the lower part of the soil above the non-weathered parent material, between topsoil and substratum. The lack of studies on trees’ root systems can be attributed to the methodological difficulties and the enormous work load imposed by the study of fine roots in mature forests (Vogt et al. 1996):
“The fine roots of perennial plants are a royal pain to study.” (Pregitzer 2002, p. 267)
Deep roots
Sampling of deep roots is even more time-consuming, technically demanding, and costly (Maeght et al. 2013) – this is one reason for the scarcity of studies investigating the abundance, distribution, and function of subsoil roots. The usually small share of roots in subsoil layers compared to the bulk of root mass in the topsoil may be another reason for the negligence of deep
1 Although an established definition of fine roots in terms of diameter-size range does not exist, conventionally roots with a diameter smaller than 2 mm are termed fine roots (Fogel 1983; Vogt et al. 1983).
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roots in most studies; however, there is an increasing interest in investigating deep roots since some studies indicate that their activity and functional importance is much more substantive than their abundance may suggest (Stone and Kalisz 1991; Canadell et al. 1996; Lehmann 2003). The large volume of subsoil often constitutes an important reservoir for water and nutrients, which plants can tap with deep roots (Stone and Comerford 1994): soil moisture may be equal or higher in the subsoil than in the uppermost horizons, and the absorption of water by deep-reaching roots can secure trees’ water supply during dry periods (Nepstad et al. 1994). Furthermore, considerable amounts of plant-available Ca, Mg, N, and S may be present below 20 cm soil depth (Jobbággy and Jackson 2001). And subsoils also play an important role in C cycling: > 50 % of the total profile SOC is stored in soils below 20 cm depth (Gill et al. 1999).
Although it is uncontroversial that deep roots may fulfill important roles in plant nutrient and water supply as well as in ecosystem functioning, their function and their development, whether genetically or environmentally driven, is not well understood (Schenk and Jackson 2002; Maeght et al. 2013). The deployment of deep roots appears to be dependent on tree species and their specific strategies to ensure sufficient water and nutrient supply also in stressful environments and in dry periods, modulated by the prevailing environmental conditions, most importantly physical and chemical soil properties (Lehmann 2003): for instance, deep roots are much more likely to occur in coarse- compared to medium-textured soils, most probably because the limited storage-capacities of these soils for plant-available water require to tap greater soil volumes to meet plants’ water demand (Jackson et al. 2000; Schenk and Jackson 2002, 2005; Mainiero and Kazda 2006).
Root system development
Information on the plasticity of mature trees’ fine root systems in terms of biomass, distribution, and morphology is in general still limited, not only with regards to the subsoil (Leuschner et al.
2004). More than 50 years ago Bradshaw (1965) specified that “plasticity is shown by a genotype when its expression is able to be altered by environmental influences”. It is well-recognized that root system development is governed by a combination of endogenous (genetics and hormonal influences (Santner et al. 2009)) and exogenous factors (external physical and biochemical
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factors, soil temperature, moisture and inorganic nutrients, soil organisms (Leuschner and Hertel 2003; Maeght et al. 2013)) (Hodge 2006; Pierret et al. 2007; Hartmann and Von Wilpert 2014).
Most decisively, the plasticity in root system architecture is considered to be a strategy directed at optimizing resource uptake from the soil under different environmental conditions (Yanai et al.
1995), a continuous response to the variability in soil resource availability in space and time (Harper et al. 1991). Soils are in the majority markedly heterogeneous on a spatial as well as on a temporal scale with regards to resource distribution, which has led the scientific community to describe soils as “patchy” environments (Hodge 2006). It is well established that root form is determined by root function (within the limits of a species’ genetic make-up), and that particularly fine roots can be considered to be the modular unit of plants’ belowground systems (Pregitzer et al. 2002; Pierret et al. 2007; Maeght et al. 2013): plants may increase their absorptive area via increasing their root system size or via alteration in morphological traits in order to optimize the acquisition of essential nutrients (Hodge 2004; Ostonen et al. 2007; Comas and Eissenstat 2009). Morphological parameters like specific root length (SRL, cm g-1)and (SRA, cm2 g-1) can be thought of as factors indicating the ratio of root benefit (resource acquisition) to root cost (root construction and maintenance) (Eissenstat and Yanai 1997; Eissenstat et al. 2000;
Pregitzer et al. 2002; Ostonen et al. 2007). Developing this conceptual model further, fine roots’
morphological traits are thought to be shaped by the soil conditions they meet and are therefore indicative for the mineral nutrition of trees at certain sites, since nutrient uptake from the soil solution is a function of both soil and root properties (Yanai et al. 1995): roots might react to low nutrient availability with the production of thinner roots, which have a larger specific surface area per unit of carbon expenditure compared to thicker roots and can take up more nutrients at a given resource investment. How the variation in specific root traits and variation in soil chemical and/or physical characteristics are exactly linked is not well understood (Pregitzer et al. 2002;
Comas and Eissenstat 2004; Pierret et al. 2007; Pregitzer 2008; Chen et al. 2016), for the main part because intra-species comparative studies particularly on fine root system morphology under different environmental regimes are rare (Leuschner et al. 2004). Furthermore, plants may, but were not always shown to (Caldwell et al. 1996) respond to soil heterogeneity with root proliferation into patches, where resources are available (Hodge 2004), or with physiological,
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morphological, and / or anatomical adjustments in the root system in order to optimize resource capture (Fitter 1994; Forde and Lorenzo 2001; Pregitzer et al. 2002).
Overall, our knowledge about the linkages between structural, physiological, morphological, and anatomical root system adjustment and root function is still extremely limited (Pregitzer et al.
2002) and a generalization of strategies in terms of plastic responses in root traits could not be established yet (Ryser 2006).
The role of roots in the C cycle of forests
In terms of terrestrial C cycling, fine roots play a major role in forest ecosystems (Rasse et al.
2005; Comas and Eissenstat 2009): although their share in tree biomass may be less than 2%, they consume up to 75% of forests’ annual net primary production (Keyes and Grier 1981; Fogel and Hunt 1983; Fogel 1985; Vogt et al. 1996; Gill and Jackson 2000). Dead fine roots and rhizodeposits are a major source of soil organic carbon (SOC) in soils, particularly in subsoils (Rasse et al. 2005; Comas and Eissenstat 2009; Tefs and Gleixner 2012); however, how tree roots exactly impact the spatial distribution, turnover and storage of soil organic matter (SOM) as well as its chemical composition is not fully understood, yet (Angst et al. 2016). The SOC concentration in subsoils is most often comparably low, but because the volume of subsoils generally exceeds that of topsoils by several magnitudes, 30-60% of the global SOC is stored in the horizons below the topsoil (Chabbi et al. 2009; Harrison et al. 2011; Koarashi et al. 2012;
Harper and Tibett 2013). Soils contain the largest terrestrial organic carbon (OC) pool (Jobbágy and Jackson 2000; Janzen 2005), and forest soils contain up to 70% of all SOC (Jobbágy and Jackson, 2000), which emphasizes the need to quantify belowground C-fluxes, including subsoil properties and deep root systems, of forests to fully understand global C cycling (Jackson et al.
1997; Pollierer et al. 2007). Despite a comparably high number of studies investigating SOC contents and stocks, most studies are confined to the topsoil and therefore quantitative information on subsoil SOC stocks, cycling, and storage mechanisms are scarce (Rumpel and Kögel-Knabner 2011). In this regard, only little information is available on the effects of contrasting parent material on the SOC cycle (Barré et al. 2017; Heckmann et al. 2009).
7 Fagus sylvatica
European beech (Fagus sylvatica L.) is the most prevalent broadleaf tree species in Germany: it covers about 1.680.072 ha or 15.4 % of the forest area in Germany (Bundeswaldinventur 2012) - only pine and spruce cover a higher percentage of the forested area. Prior to deforestation and management of forests, more than 300 000 km2 of Central Europe were covered by European beech (Leuschner et al. 2006). Today, beech and mixed beech forests are still the most important and characteristic alliance in terms of spatial extension in Central Europe (Fig. 1.1) (Leuschner and Ellenberg 2017). The success of the tree species is due to its tolerance of a very broad range of site conditions and its ability to outcompete other native tree species owing to its shade-tolerance at juvenile stage and by the production of shade at adult stage (Hertel 1999; Leuschner and Ellenberg 2017). It thrives on almost all geological substrates and Central European forest soil types, whether highly acidic or alkaline, if sufficient drainage is given. Due to its sensitivity to hypoxia, European beech is absent on gleysols or other hydromorphic soils and does not grow in waterlogged depressions either. European beech also tolerates a broad range of climatic conditions: naturally, it would cover around 2/3 of the land area of Central Europe, apart from azonal habitats with too cold or dry conditions (Leuschner et al. 2006; Bohn and Gollub 2007).
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FIGURE 1.1: Distribution map of Fagus sylvatica (adapted from EUFORGEN 2018)
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