Species adapted to nutrient-poor habitats are generally assumed to be slow growing (Chapin et al., 1993) and to show less phenotypic plasticity (Grime, 1979; Chapin, 1980) and a slower response to environmental changes than species of nutrient-rich habitats. However, this depends largely on the plant trait considered. For example, grasses with an inherent high growth rate were more plastic than species with a low growth rate in terms of growth and N concentration but not in terms of biomass allocation (Van der Vijver et al., 1993; Garnier, 1998). Owing to the great variation among plant species occurring in nutrient-poor habitats, Grubb (1998) proposed to refine the ‘stress-tolerance’ strategy of Grime (1977, 1979) by dividing it into three different strategies:
Two of those refer to plants that show contrasting traits, viz. a long leaf life span (‘low-flexibility’ strategy) and short leaf life span (‘gearing-down’ strategy), while the third involves a major change in growth rate between the seedling and the adult (‘switching’ strategy) (Grubb, 1998). In our experiment two of the three species collected from a nutrient-poor bog (Calamagrostis and Carex) responded in a plastic way to the imposed treatments (Figs 2, 3). Compared with the forest species these two produced more biomass than the relatively slow growing Deschampsia and Rubus (Fig. 1, Table 1). Calamagrostis and Carex would thus correspond to Grubb’s ‘gearing-down’ strategy, which is exemplified by grasses from nutrient-poor heath systems, which respond quickly when the nutrient limitation is relieved (Aerts et al., 1990; Parsons et al., 1995). The persistence of these species under nutrient-poor conditions is assumed to depend on their ability to gear-down their metabolism (Grubb, 1998). The only species from the infertile habitat showing the low-flexibility strategy was the stress tolerant herb Tofieldia pusilla (Figs 1, 2, 3), which is a slow-growing nonwoody evergreen. In comparison with other life-forms evergreens appear to be least responsive to changes in nutrient availability and temperature, while graminoids are most responsive (Shaver & Chapin, 1986; Parsons et al., 1994; Aerts, 1995, Hartley et al., 1999).
The relationship between MRT and aNP
At the interspecific level, species with a higher average N pool produced more biomass but lost also more N than species with lower average N pool (Fig. 1; Vázquez de Aldana & Berendse, 1997). The same pattern was found within species (Figs 2, 3). A strong link between productivity and N losses (Schläpfer & Ryser, 1996; Vázquez de Aldana et al., 1996; Eckstein & Karlsson, 1997) may be one explanation behind the inverse relationship between productivity and nutrient conservation at the interspecific level. Field data on above-ground N use of species of four different life-forms (Eckstein & Karlsson, 1997) support the proposed trade-off between aNP and MRT sketched in Fig. 4(a). However, within species no such relationship was found, which is in line with other studies looking at the congeneric or intraspecific level (Fig. 4b; Aerts & de Caluwe, 1994; Weih et al., 1998). Summarizing the existing literature, there is, to our knowledge, no convincing evidence for a trade-off between nutrient productivity and mean residence time, when congeneric species, provenances of one species or species of the same life-form were compared (Aerts & De Caluwe, 1994; Vázquez de Aldana & Berendse, 1997; Weih et al., 1998). By contrast, studies dealing with species of different life-forms (including evergreens) found some indications for such a trade-off (Aerts, 1990; Eckstein & Karlsson, 1997, this study).
There is a whole suite of traits related to infertile habitats, including scleromorphic leaves, slow growth, low photosynthetic rate and low leaf turnover rate. It has been suggested that this ‘stress resistance syndrome’ (SRS) may evolve by a relatively simple genetic change in a switch or underlying trait, such as a hormone, which may turn on the SRS (Chapin et al., 1993). Since there are clear physiological links between leaf structure, growth rate and stress resistance (Reich et al., 1992; Chapin et al., 1993) we may ask why no trade-off between aNP (a measure of productivity) and MRT (a measure of nutrient conservation) has been found within-species. However, the apparent lack of a functional link between MRT and aNP within-species may have several reasons.
Secondly, and probably most importantly, MRT and aNP are not intimately/directly linked with each other, but their relationship is mediated through other interdependent traits. MRT is closely related to the leaf life span (Aerts, 1990; Escudero et al., 1992; Garnier & Aronson, 1998; Eckstein et al., 1999). The large variation in leaf life span found at the macro-evolutionary (interspecific) level, for example between life-forms, invariably involves large changes in leaf function. For example, the longevity of a leaf has implications for leaf structure and morphology (Turner, 1994; Ryser, 1996), photosynthesis (Chabot & Hicks, 1982; Field & Mooney, 1986) and growth (Reich et al., 1998). Furthermore, a long life span often motivates larger investments in defence against herbivores and pathogens (Fagerström et al., 1987; Herms & Mattson, 1992) which may have negative effects on productivity. By contrast, within the relatively narrow intraspecific range of leaf life spans (Eckstein et al., 1999) nutrient conservation may be improved by increasing leaf life span without necessarily involving associated changes in leaf structure and productivity. For deciduous species, for example, a change in leaf life span of some days or weeks does probably not motivate increased investments in leaf morphology or defence. Rather, increases in leaf life span probably require changes in characteristics such as frost tolerance since the leaves then merge earlier in the spring and/or are maintained longer into the autumn. Such an increase in leaf longevity probably does not result in any costs as reduced productivity in the short term. However, if a change in leaf longevity is not fully matched by a corresponding change in frost tolerance, it may involve a larger risk of productivity losses due to frost damage at a longer perspective.
We therefore suggest that one may not expect a trade-off between aNP and MRT at the intraspecific level since the relatively small variation in traits possibly improving nutrient conservation may have no negative effects for growth and production. The negative relationship between MRT and aNP appears to reveal an evolutionary trade-off, that is in adaptation to habitats with different nutrient availability species have evolved nutrient use strategies, which consist in having either a long MRT and a low aNP or vice versa. Leaf life span appears to play an important role in this trade-off. The variation of leaf life span within species is low (Eckstein et al., 1999). Therefore, the proposed trade-off between MRT and aNP may be found among life-forms and species, if the variation in leaf life span is large (Aerts, 1990; Eckstein & Karlsson, 1997). By contrast, within species no such trade-off can be expected owing to the small variation in biomass loss rate, biomass life span, MRT or aNP.