Plant roots are able to exploit below-ground resource heterogeneity in several ways. An increase in root length allocation within nutrient-rich patches is one of the most striking and frequently demonstrated (Hutchings & de Kroon 1994; Robinson 1994). However, species differ in root system morphology (Fitter 1985), and in the scale and precision of their response to patches of different size and quality (Einsmann et al. 1999; Farley & Fitter 1999). Such differences may affect the outcome of interspecific competition, although there is relatively little experimental evidence to support or refute these suggestions (but see Cahill & Casper 1999; Fitter 1982; Robinson et al. 1999). The ability of species to proliferate roots within a nitrogen-rich, organic patch can increase nitrogen capture, and hence competitive ability (Hodge et al. 1999).
Tilman (1982) suggested that soil heterogeneity should lead to an increase in species coexistence, and that this effect should be most marked in nutrient-poor habitats. The concentration of nutrients in surface horizons, caused by organic matter accumulation and nutrient deposition at the soil surface, is a widespread and predictable source of nutrient heterogeneity in undisturbed soils. This is a particularly strong feature of heathland soils, characterized by organic upper horizons above leached, mineral, lower horizons (Gimingham 1972). Grass roots are found at greater depth when co-occurring with ericaceous dwarf shrubs (Gimingham 1972), and this has been suggested as a mechanism that reduces resource competition in heathland communities (Smith & Read 1997).
The ericaceous dwarf shrub Calluna vulgaris (L.) Hull and the coarse grass Nardus stricta (L.) are naturally co-occurring species of Scottish upland heaths. In recent decades, large areas of Calluna heath have been replaced by Nardus-dominated grassland. Preferential grazing of Calluna over Nardus is though to be the main mechanism of this replacement, particularly at high nutrient levels (Hartley 1997).
Calluna and Nardus have contrasting root systems (Genney et al. 2000), in common with other ericaceous dwarf shrubs and grasses (Aerts 1993; Read 1996). Calluna produces an abundance of fine hair-roots (≈ 0·1 mm diameter) that proliferate in the upper organic soil horizons (Aerts 1993; Gimingham 1960). These roots are normally heavily colonized by ericoid mycorrhizal (ErM) fungi. This is of functional significance because ErM fungi can access N and P from decaying plant and fungal matter within the litter and humic horizons, and consequently increase host-plant nutrition (Kerley & Read 1998). In contrast, Nardus is a relatively coarse-rooted grass that produces numerous, thick, unbranched roots (1–2 mm diameter) that penetrate to depths of 50 cm or more, along with shallower, fine lateral roots (<0·5 mm diameter) (Chadwick 1960). Nardus roots may be colonized by arbuscular mycorrhizal (AM) fungi (Ali 1969), which improve uptake of immobile nutrients such as phosphorus (Smith & Read 1997) and have recently been shown to increase decomposition of, and nitrogen capture from, patches of organic matter (Hodge et al. 2001).
In a previous study (Genney et al. 2000) we demonstrated that, when grown alone in a layered organic/sand substrate, Nardus allocated the greatest proportion of its root length to the organic layer. However, Calluna was the superior competitor when the species were grown together (competition with Calluna reduced Nardus mass more than competition with other Nardus), and this was attributed to the ability of Calluna partially to exclude Nardus roots from the surface organic layer. It was not, however, possible to determine whether this was caused by the superior ability of Calluna to respond selectively to the nutrient-rich layer by plastic root allocation, or because it had predetermined root allocation. The study reported here was designed to distinguish between these two possibilities. Our objectives were to measure (i) the vertical root length allocation of Calluna and Nardus when grown alone (monocultures) in a layered or in a mixed peat/sand substrate; and (ii) the comparable root length allocation patterns when the species are grown together (mixtures), and also to determine how root allocation patterns affect Calluna/Nardus competition in the layered and mixed substrate.