1.1. Resources and plant strategies

The availability of soil nutrients sets limits to plant growth rate, and nitrogen and phosphorus are the two most frequently limiting nutrients (DeAngelis 1992, Larcher 1995, Marschner 1995). Trade-offs have been suggested to exist between the abilities to compete for resources in environments of high and low resource availability. Plants seem to be unable to both deplete soil nutrients and grow fast when nutrients are abundant, or tolerate shade and photosynthesise fast under conditions of high amounts of radiation (Tilman 1982, Grace & Tilman 1990). The depletion of nutrients in soil is a slow process but in the long run it is suggested to lead to the exclusion of nutrient demanding plant species or strategies from a nutrient deficient site (Tilman 1982, Grover 1997). When nutrients are abundant, fast growing plants are able to outgrow the slow growing ones. A change in the nutrient availability of a site may thus lead to a change in the species composition of a site, and also to a change within a species from one nutrient uptake strategy to another.

Mycorrhizal colonisation is variable in many plant species and mycorrhizal and non-mycorrhizal strategies respond indifferent ways to the availability of soil nutrients (Pairunan et al. 1980, Thomson et al. 1986, Bougher et al. 1990, Smith & Read 1997, Titus & del Moral 1998). The winning strategy in competition for depleted nutrients often includes symbiotic association with mycorrhizal fungi (Smith & Read 1997). Due to their high surface-to-volume ratio fungi have a greater nutrient uptake capacity than plant roots but they impose an additional energetic cost to a plant, since they may consume up to 10–30 % of a plant"s photosynthates (Fogel & Hunt 1983, Finlay & Söderström 1992, Smith & Read 1997). This will significantly reduce plant growth rate when resource limitation is shifted from nutrients to light and photosynthetic carbon. The cost of mycorrhizae may then exceed the benefits to a plant. Many plants of nutrient rich habitats are therefore non-mycorrhizal or have a lower mycorrhizal colonisation than the plants of nutrient poor habitats (Alexander & Fairley 1983, Smith & Read 1997).

Much theoretical and empirical research has been carried out to study the relationship between plant growth and the availability of soil nutrients. Temporal (e.g. Chapin et al. 1978, Mullen & Schmidt 1993, Grover 1997, Smith & Read 1997, Mullen et al. 1998) and spatial (Smith & Read 1997) variation in nutrient availability have received little attention in experimental designs, which rather attempt to keep nutrients at a constant level (Ingestad & Ågren 1988). The implicit assumption seems to be that growth with a stable availability of nutrients equals the average growth rate with variable nutrients. Plant species and strategies that differ in their response to a stable availability of nutrients are also likely to differ in their response to the variation of nutrients. This can be deduced from Jensen"s inequality (Ruel & Ayers 1999), which predicts the difference between the average of a function and the function of an average. This mathematical property can be used for predicting the response of nutrient use strategies to variation in nutrients.

Plants require several resources, which are taken up with different organs and in variable ratios. Photosynthetic energy is harvested via foliage while nutrients are taken up by roots or mycorrhizal fungi. Plants therefore have to optimise their above and below ground growth allocation according to the ratio of resource availability. The optimal resource use theory (Rapport 1971, Bloom et al. 1985, Tilman 1988) suggests that plants take up resources according to their proportional limitation by each of the resources. For example, if growth is limited by the availability of nutrients rather than by photosynthesis, plants may increase their proportional growth allocation to roots and decrease allocation to shoots (Bloom et al. 1985). At the optimum, all resources will be equally limiting to growth. The ability to optimise requires phenotypic plasticity of allocation (Bradshaw 1965, Grime et al. 1986, Sultan 1987, Scheiner 1993). Plasticity increases the range of resource availabilities a plant can adapt to. According to Chapin (1980), plants with higher maximum growth rates are phenotypically more flexible in their root vs. shoot allocation than slowly growing plants. Phenotypic plasticity increases the tolerance of plants to environmental variation and may therefore extend their geographic distribution.

Ecological interactions among plants often involve competition for nutrients. The nutritional quality of dead organic matter is one of the major factors determining the structure and composition of the soil decomposer community (Visser & Parkinson 1992, Janssen 1996, Mary et al. 1996, Currie 1999) and to a large extent it also determines the rate of nutrient mineralisation (Ågren & Bosatta 1996a,b). Nutrients become available to plants by decomposition and therefore have the potential to determine the rate of primary production of an ecosystem and are perhaps of greater theoretical and practical importance than generally believed (Ågren & Bosatta 1996b). Proper theoretical approaches to soil ecology have been sought and a long lasting debate in plant ecology, the one between J. P. Grime and D. Tilman on plant strategies (Grace 1991, Grover 1997), spread belowground when Grime"s (1977, 1979) theory was suggested to apply to soil systems (Wardle & Giller 1997). Grime (1979) considers plants to be exposed to stress (resource limitation) and disturbance (biomass removal) to a variable extent, which leads to corresponding strategies of stress-tolerators (S) and ruderals (R). The competitive-strategy (C) characterises plants that grow in resource rich (no stress) and undisturbed habitats. Besides these C, S, and R strategies there are combinations and intermediates including the indifferent CSR-strategy. Tilman (1982, 1988), on the other hand, models plant growth as a function of one to several resources and considers plant strategies to be adaptations to different combinations of the availability of resources. Tilman (1982) derives the minimum resource availability R*, that corresponds the needs of a plant, from the interaction of resource uptake by plants and the renewal of resources. From an array of competing plant species, the most competitive is proposed to be the one with the lowest R*. Since nutrient renewal is dependent on soil microbes, the inclusion of nutrient mineralisation and other soil processes seems to be a natural extension of Tilman"s theory.

Grime"s (1979) theory has been criticised for numerous reasons, including the definition of stress, which Grime suggests to be a property of a habitat. The problem is that a low nutrient availability may be experienced as stress by some plant species while it may be optimal for others. Tilman"s (1982) competition is defined as the ability to deplete the availability of resources to a level that is not sufficient for other species to persist. Tilman"s resource competition theory thus predicts the long term competitive outcome and shares the common criticism for all equilibrium theories that nature is not at equilibrium. The properties that make a plant a good competitor under Grime"s definition make it a bad competitor according to Tilman"s criteria, since different traits are beneficial in competition for high relative to low resources. Grime and Tilman also differ in their view of the forces that lead to a successional change. Grime (1979) sees the successional increase in soil nutrient availability as declining stress and increasing competition. Tilman (1985, 1988) argues that the increase in nutrients and decrease in light during a succession leads to a shift from below-ground competition to above ground competition according to the ratio of available resources.