Turnover of Root Systems
Turnover of tissues is a fundamental process that operates continuously in all vascular plants. In some, especially those in seasonal environments, entire organs may be shed at the end of the growing season. Winter or drought deciduous trees are a clear e
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3.1 Introduction Turnover of tissues is a fundamental process that operates continuously in all vascular plants. In some, especially those in seasonal environments, entire organs may be shed at the end of the growing season. Winter or drought deciduous trees are a clear example of this process. Herbaceous plants may also episodically lose all or most of their aboveground organs at the onset of a cold or dry season, but most of them also und ergo continuous replacement of leaves throughout a growing season (Larcher 1995; Lambers et al. 1998). Many grasses are specifically adapted to withstand high turnover of their aboveground organs be it by grazing, fire, or drought because of their evolutionary histories of exposure to such selection pressures (Mack and Thompson 1982; Coughenour 1985; Milchunas et al. 1988). In the case of deciduous trees, turnover of the canopy is 100 % per year. Similarly, grasses in seasonal environments often have essentially 100 % canopy turnover per year. The concept of turnover, i.e. the replacement of a particular standing stock, is equally applicable to belowground plant organs as to those aboveground. While assessment of canopy turnover is relatively straightforward, evaluation of the turnover of belowground organs (e.g. roots, rhizomes, corms, tubers, etc.) is very difficult and continues to be one of the most challenging issues in root system ecology. Our knowledge of the anatomy, morphology, and physiology of individual belowground organs is quite good even though much of our knowledge has come from studying roots removed from the field environment (Esau 1965; Davis and Haissig 1994; Altman and Waisel 1997). We know considerably less about belowground organ systems (we will refer to these as root systems), but we have relatively good information about the static aspects of root systems. Our gaps in knowledge about root systems are related to their dynamic aspects of which root system turnover is of central importance. The objectives of this chapter are to evaluate the state of our knowledge about root system turnover under field conditions. Specifically, we will disEcological Studies, Vol. 168 H. de Kroon, E.J.W. Visser (Eds.) Root Ecology © Springer-Verlag Berlin Heidelberg 2003
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W.K. Lauenroth and R. Gill
cuss the structure of root systems and characteristics of individual roots, methods used to estimate root turnover in the field, published results for root turnover and net primary production in forest, grassland and shrubland ecosystems, relationships between root turnover and environmental variables, and end with an assessment of future needs in research on root system turnover.
3.2 Overview of the Structure of Root Systems The major benefits to a plant of investing in a root system are the connections it provides with the substrate and the ability it confers to absorb water and mineral nutrients. While not all members of the plant kingdom have true roots, our current analysis is limited to those that do (vascular plants). Plants have "solved" the problems of anchorage an
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