Litter decomposability: a neglected component of plant fitness.

The Gaia concept that Lovelock (1979, 1989) proposed and illustrated using his Daisyworld model has met with much scepticism (e.g. Kirchner 1989). His model, however, contains at least one hypothesis that is extremely relevant ecologically and that has not been tested until now. Lovelock assumed that natural selection can favour certain features of organisms because of the effects that these features have on the physical environment (Lovelock 1989, p. 37). Changes in such characteristics of organisms would result in changes in the environment, which in turn would affect the fitness of these individuals. For the past 10 years we have been studying how nutrient availability affects the competitive balance between dominant plant populations in heathlands and what effects these populations have upon soil fertility. Heathland ecosystems in Western Europe are frequently dominated by just one or two higher-plant species and for this reason are sufficiently simple ecosystems for testing Lovelock's assumption. It has now been generally appreciated that plant species may have important effects upon nutrient cycles and upon soil fertility (e.g. Hobbie 1992). Below, I shall provide evidence that natural selection can take place as a result of the influence that dominant plant populations exert on the fertility of the soil. In five series of plots in dry and wet heathlands where above-ground biomass, litter and humus had been removed between 1 and 50 years previously, the amounts of soil organic matter in the litter and humus layer increased significantly with time (Berendse 1990). Annual N mineralization in the upper 10 cm of the soil increased (in one series even from c. 1 to c. 13 g m-2 year-') with increasing amounts of litter and humus. The production of dead organic material by the plants apparently leads to an increase in N supply during succession, as widely shown not only in heathlands but also on glacial moraines in Alaska (Crocker & Major 1955), sand dunes (Olson 1958), and old-fields (Tilman 1988). Reviewing studies on the influence of plants, soil animals and microorganisms on their physical substrate, Van Breemen (1993) concluded that in many cases these organisms appear to affect soil fertility, soil moisture content and other soil features in such a way that with time the substrate becomes more favourable for the growth of plants and soil organisms. In our study the increase in nitrogen mineralization during 50 years' succession was linearly correlated with primary production leading to a five-fold increase in shoot production. Such dramatic changes in nitrogen supply and above-ground productivity must have important consequences for competition between plant species. In a 3-year competition experiment in the field that compared monocultures of the dwarf shrub Erica tetralix L. and the perennial grass Molinia caerulea (L.) Moench with mixtures of the two species at equal total plant densities, Erica out-competed Molinia at the low fertilization levels, whereas at the highest fertilization level (20 g N m-2 year-' and corresponding amounts of P and K) Molinia won (Aerts et al. 1990). On the basis of these results I hypothesized that during secondary succession following sod removal the N supply to the plants increases with time and eventually reaches a level that enables Molinia to replace Erica. I demonstrated mathematically (Berendse 1993) that at high nutrient supply, where the capture of PAR by plants was limiting, fast growing species 1 with a greater potential growth rate (G(max.)) would be able to replace a slower-growing species 2, if