SUMMARY According to ‘Gause's hypothesis’ a corollary of the process of evolution by natural selection is that in a community at equilibrium every species must occupy a different niche. Many botanists have found this idea improbable because they have ignored the processes of regeneration in plant communities. Most plant communities are longer‐lived than their constituent individual plants. When an individual dies, it may or may not be replaced by an individual of the same species. It is this replacement stage which is all‐important to the argument presented. Several mechanisms not involving regeneration also contribute to the maintenance of species‐richness: differences in life‐form coupled with the inability of larger plants to exhaust or cut off all resources, also the development of dependence‐relationships, differences in phenology coupled with tolerance of suppression, fluctuations in the environment coupled with relatively small differences in competitive ability between many species, the ability of certain species‐pairs to form stable mixtures because of a balance of intraspecific competition against interspecific competition, the production of substances more toxic to the producer‐species than to the other species, differences in the primary limiting mineral nutrients or pore‐sizes in the soil for neighbouring plants of different soecies, and differences in the competitive abilities of species dependent on their physiological age coupled with the uneven‐age structure of many populations. The mechanisms listed above do not go far to explain the indefinite persistence in mixture of the many species in the most species‐rich communities known. In contrast there seem to be almost limitless possibilities for differences between species in their requirements for regeneration, i.e. the replacement of the individual plants of one generation by those of the next. This idea is illustrated for tree species and it is emphasized that foresters were the first by a wide margin to appreciate its importance. The processes involved in the successful invasion of a gap by a given plant species and some characters of the gap that may be important are summarized in Table 2. The definition of a plant's niche requires recognition of four components: the habitat niche, the life‐form niche, the phenological niche, and the regeneration niche. A brief account is given of the patterns of regeneration in different kinds of plant community to provide a background for studies of differentiation in the regeneration niche. All stages in the regeneration‐cycle are potentially important and examples of differentiation between species are given for each of the following stages: Production of viable seed (including the sub‐stages of flowering, pollination and seed‐set), dispersal, in space and time, germination, establishment, and further development of the immature plant. In the concluding discussion emphasis is placed on the following themes: the kinds of work needed in future to prove or disprove that different...

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Light is widely considered to be the most important factor limiting the performance of plants on the floors of forests and woodlands, but the roles of nutrient availability and water supply remain poorly defined. We seek to predict the types of forest in which root competition affects seedling performance, and the types of plants that respond most strongly to release from root competition. We then test our predictions by reviewing experiments in which tree seedlings and forest herbs are released from belowground competition, usually by cutting trenches to sever the roots of surrounding trees. First, we provide a worldwide review of changes in canopy form and fine‐root mass along gradients of soil fertility and seasonal drought, keeping in mind the stages of forest development. Our review shows that penetration of light is least in forests on moist soils providing large amounts of major nutrients. The changes are far more complex than those considered by allocation models. Dry woodlands typically allow 20 times as much light to penetrate as do wet forests, but there is surprisingly little evidence that they have greater fine‐root densities in the topsoil. Tropical rain forests on highly infertile soils have only slightly more open canopies than those on fertile soils, but much greater fine‐root densities. Northern temperate forests on highly acidic peats and sandy soils are often dominated by early‐successional, open‐canopied conifers (generally pines), mostly as a result of recurrent fires, and transmit about five times as much light as surrounding deciduous forests. A review of trenching experiments shows that light alone limits seedling growth in forests on moist, nutrient‐rich soils, but competition for belowground resources becomes important on infertile soils and in drier regions. Secondly, we consider how root competition alters species' shade tolerances. Shade‐house experiments demonstrate that species differ markedly in the minimum irradiance at which they respond to nutrient addition, but there generally tends to be a sizable response at >5% daylight and little response in <2% daylight. There is some evidence that species that have high potential growth rates and that respond markedly to increased irradiance are also most responsive to nutrient addition in 2–3% daylight. T. Smith and M. Huston have hypothesized that species cannot tolerate both shade and drought; this appears to be the case for species that tolerate shade chiefly by maximizing leaf area. However, many shade‐tolerant woody plants in tropical and mediterranean‐climate forests have thick, tough, long‐lived leaves and a relatively high allocation to roots, and these species are much more drought tolerant. A few studies indicate that root trenching allows species to persist in deeper shade than that in which they are normally found and allows species from mesic sites to invade more xeric sites. Usually, the impact of trenching on growth rate is much greater in gaps than in the understory. Finally, we discuss the ways in which life‐form composi...

To test whether the impact of drought on the growth and biomass allocation of first-season shade-tolerant woody seedlings in low irradiance differs from that in high irradiance, seedlings of Viburnum lantana, V. opulus, V. tinus and Hedera helix were grown in pots at two watering frequencies × three irradiances. Hypotheses in the recent literature variously predict that drought will have a stronger, weaker or equal impact on seedling relative growth rate (RGR) in deep shade relative to that in moderate shade. Experimental irradiance levels were selected in the typical range for temperate deciduous forest seedlings in either understorey or clearings: 3-4% daylight (low red: far-red shade), 3-4% daylight (neutral shade), and 30-40% daylight (neutral shade). Watering was 'frequent' (every 3-4 days) or 'infrequent' (five times during the 8-week experiment), producing soil matric potentials as low as -0.03 MPa, and -2 MPa. To prevent the interaction of irradiance and watering treatments, each seedling was grown in a 'shade tower' that was surrounded by an uncovered sward of grass (Festuca rubra), which depleted pot water at the same rate regardless of the species of seedling, or its irradiance treatment. Shading affected all species: seedlings in 3.5% daylight grew at 56-73% of their dry-mass RGR in 35% daylight. Low red: far-red shade reduced the RGR of Hedera to 68% of its value in neutral shade. Infrequent watering significantly reduced the RGR of only V. lantana and V. opulus, by approximately the same proportion across irradiance treatments. Infrequent watering did not significantly alter any species' biomass allocation across irradiance treatments. Such orthogonal impacts of deep shade and drought on seedling growth and biomass allocation indicate a large potential for niche differentiation at combinations of irradiance and water supply for species of forest seedlings, and suggest a multiplicative-effects approach for modelling seedling performance in microsites with different combinations of irradiance and water supply.