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One of the overarching goals of plant ecology is to understand the mechanisms by which plants are adapted to their environment. To grow and persist under regimes of nutrient limitation, plants have developed two main strategies: optimizing nutrient acquisition and reducing nutrient losses. These adaptations are part of a well-known trade-off in and among plants between resource acquisition and conservation (Grime, 1979; Berendse & Aerts, 1987; Aerts, 1990; Reich et al., 1997; Wright et al., 2004). Adaptations that minimize nutrient loss, such as protection against leaching, effective nutrient resorption from senescing plant organs, and high nutrient use efficiency through long lifespans of leaves (Escudero et al., 1992; Reich et al., 1992) or other organs, are thus usually considered to be opposed to adaptations promoting nutrient uptake and growth (Chapin, 1980; Aerts & Chapin, 2000). These aspects of plant nutrient conservation are of predominant importance in nutrient-poor environments (Aerts, 1999).
The nutrient resorption process potentially occurs all year round, especially in evergreens, but is most pronounced during periods of organ senescence leading to plant dormancy such as winter in cold climates, and concerns all senescing plant parts (e.g. Aerts & de Caluwe, 1989; Gordon & Jackson, 2000). It is a dynamic, highly regulated process involving exchanges of nutrients and metabolites from organ to organ (Killingbeck, 1986; Aerts, 1996). The senescence and resorption processes of roots and stems are far less understood than those of leaves. The absence of abscission zones and the less pronounced seasonality of root and stem mortality imply critical differences in the mechanisms involved. Factors driving this process range from growth of new plant parts (new buds and reproductive organs; e.g. Simpson et al., 1983; Milla et al., 2005) or remobilization from underperforming organs (such as excessively shaded leaves; Saur et al., 2000) to self-induced organ senescence (such as autumn senescence of deciduous leaves). The resorption process can involve active (e.g. photosynthetic) processes and stored components, neither of which necessarily involve organ death. However, when interested in how efficiently plants reduce nutrient loss, one should focus on resorption mechanisms associated with organ senescence, for which autumn is the crucial period at higher latitudes.
Leaf nutrient resorption, from the molecular to the whole-plant level, has been remarkably well studied in the past four decades. By contrast, very few root studies and almost no stem studies have been conducted over the same period. The fate of nutrients contained in fine roots, characterized by high turnover rates and high nutrient contents (Gill & Jackson, 2000; Gordon & Jackson, 2000), and in fine stems, especially photosynthetic ones, is of great importance for the whole-plant nutrient budget. However, the question of whether and to what extent nutrients are resorbed from fine stems and fine roots has not been clearly answered. While resorption has been observed for fine roots of annuals (Simpson et al., 1983), perennial grasses (Woodmansee et al., 1981) and woody perennials (Meier et al., 1985), most studies noted no or few changes in nutrient content between live and dead roots (e.g. McClaugherty et al., 1982; Nambiar, 1987; Aerts, 1990). More recently, a meta-analysis by Gordon & Jackson (2000) suggested that roots may be not only a sink but also a source of nutrients for resorption processes. While some evidence exists for resorption of nutrients from stems of single annual species (Simpson et al., 1983; Aerts & de Caluwe, 1989), there is a clear lack of studies that might reveal patterns across multiple species with different growth forms. Renewed effort in this direction is crucial for understanding the carbon (C) and nutrient economy at the whole-plant level, for both terrestrial and aquatic species.
Leaf leaching is the passive removal of substances from leaves by the action of aqueous solutions, such as rain, dew, fog or surface water; this process occurs mostly during the latest phase of leaf maturation and during leaf senescence (Tukey, 1966, 1970; Morton, 1977). Resorption and leaching take place simultaneously during leaf senescence and concern most of the nutrients involved in plant metabolism (e.g. Tukey et al., 1958; Nambiar & Fife, 1991), although nitrogen (N) and phosphorus (P), as the main limiting nutrients to plant growth in most environments, have been the subject of most studies. Because of the generally high nutrient content of leaves as compared with other plant organs, foliar resorption efficiency and leaching resistance are of major importance in plant nutrient use strategy. On average, c. 50% of the maximum N and P content of mature leaves is retained in the plant through the resorption process (Reich et al., 1995; Aerts, 1996; Killingbeck, 1996), and this percentage is substantially increased after correction for the changing mass of senescing leaves (van Heerwaarden et al., 2003). Large amounts of organic metabolites can potentially be leached from plants (Morgan & Tukey, 1964). However, most studies have assumed that resorption is the only significant mechanism responsible for nutrient content changes between mature and senesced leaves.
Resorption and leaching resistance mechanisms, through their role in reducing nutrient losses, are potentially part of the nutrient and C acquisition–conservation trade-off among species. Some evidence exists that resorption efficiency and leaching resistance decrease with increasing leaf nutrient content (e.g. Pastor et al., 1987; Kobe et al., 2005). Nevertheless, it is still unclear whether and to what extent these mechanisms co-vary with other well-known nutrient and C economy traits such as specific leaf area or leaf dry matter content; that is, whether they fit into the ‘leaf economics spectrum’ (cf. Wright et al., 2004).
The aims of this study were: to determine the relative importance of the effects of resorption and leaching on leaf nutrient losses during leaf senescence; to compare the cross-species range of resorption efficiencies occurring in leaves, fine roots and fine stems; to test the hypothesis that resorption efficiency and leaching resistance can be predicted from a combination of other easily measurable plant traits relevant to nutrient and C acquisition-conservation. We addressed these issues in a subarctic flora representing the key species from aquatic, riparian and terrestrial environments and covering the main vascular higher taxa and growth forms in this region.
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Fig. S1 Relationships between leaf carbon (C) and nitrogen (N) potential leaching and the leaf nutrient and carbon economics.
Table S1 Species list, organ resorption efficiency and litter nutrient content
Table S2 Consistency of within-species nutrient resorption efficiency across plant parts
Table S3 Predictions of leaf carbon (C) and nitrogen (N) potential leaching from leaf structural traits
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