Peter J. Grubb

THE MAINTENANCE OF SPECIES-RICHNESS IN PLANT COMMUNITIES: THE IMPORTANCE OF THE REGENERATION NICHE

1 According to ‘Gauses 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. 2 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. 3 Several mechanisms not involving regeneration also contribute to the maintenance of species‐richness:

Impacts of root competition in forests and woodlands: a theoretical framework and review of experiments.

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 ...

Colonization, tolerance, competition and seed-size variation within functional groups

Abstract Ecologists interested in seed size have generally contrasted functional groups of plants but, recently, some have focussed on explaining the large range of seed size found within a functional group. A potentially important theoretical advance was the idea that seed number–seedling survival tradeoffs could explain the coexistence of species, in particular colonization–competition tradeoffs where smaller-seeded species are superior colonizers and larger-seeded species are superior competitors. However, recent models have placed limits on the potential of this approach, chiefly by showing that the asymmetry of competition must be strong. Also, although there is evidence from studies within functional groups that seed size does trade off against number of seeds and dispersal of those seeds, and that seed size is correlated with competitive ability among seedlings and tolerance of hazards during establishment, the available evidence suggests that SNSS tradeoffs do not make possible long-term coexistence without other forms of niche differentiation.

Physiological basis and ecological significance of the seed size and relative growth rate relationship in Mediterranean annuals

1. During the first three weeks of growth with plentiful water and mineral nutrients 21 Mediterranean species of annuals showed a linear negative correlation of relative growth rate (RGR) with log initial seedling weight. The lower RGR values resulted mainly from lower specific leaf area (SLA) and occurred despite higher values for unit leaf rate (ULR); they were associated with larger amounts of DNA per cell and probably with larger cell size. 2. The RGR values for a given seed size fell in the sequence Asteraceae > Poaceae > Fabaceae. The Asteraceae had the highest leaf weight ratios, the Poaceae the highest SLA values, and the Poaceae and Fabaceae the highest ULR values

QUANTIFYING THE IMPACT OF COMPETITION AND SPATIAL HETEROGENEITY ON THE STRUCTURE AND DYNAMICS OF A FOUR-SPECIES GUILD OF WINTER ANNUALS

We develop statistical methods appropriate for the analysis of spatially structured population data. The methods are used to study the structure and dynamics of a four-species annual plant guild recorded in 1,000 permanent squares over a 10-yr period. We parameterize models that predict population density from one year to the next. In agreement with theoretical expectation all the models have locally stable equilibria, and overcompensation is rare. We demonstrate that interspecific interactions are extremely weak, relative to intraspecific ones, and that the spatial arrangement of species and individuals within them is critical to the observed dynamics. The impact of spatial density-dependent population growth on observed densities was calculated. In 52% of the cases population size would have been increased by at least a factor of 1.5 had there been no interactions between individuals, and in 9% of these it would have increased by a factor of four or more. This effect is shown to be largely a result of intraspecific interactions. We discuss possible explanations for the weakness of interspecific interactions.

positive distrust in simplicity--lessons from plant defences and from competition among plants and among animals

1. The address is concerned with the degree of complexity needed in a satisfying ecological theory. 2. Using the incidence of spines, it is argued that previous hypotheses intended to explain the development of anti-herbivore defences are inadequate, and a new «scarcity-accessibility» hypothesis is proposed instead. The variables that need to be considered are productivity, accessibility and proportion of the landscape covered, architecture, seasonal behaviour relative to neighbours, nutrient concentration relative to neighbours and kinds of herbivore present. The new hypothesis accounts for completely opposite kinds of plants being armed, for example, particularly slow-growing and particularly fast-growing plants, or the deciduous species in one system and the evergreen in another

A reassessment of the strategies of plants which cope with shortages of resources

Abstract An attempt is made to review the whole range of plants able to cope with shortages of mineral nutrients, water or light for at least a substantial part of their lives. In the past insufficient attention has been paid to quantitative measurements of the tolerance of various species for lack of resources. Growth rate at a low level of supply and mere survival are the variables to be considered. Problems in defining tolerance arise where a collection of species shows a negative correlation between survival at a low level of supply and growth rate under these conditions; in at least some circumstances a balance between survival and growth rate is important. Plants coping with lack of major resources display one of three strategies: ‘low-flexibility’, ‘switching’ or ‘gearing-down’. The essential features of the low-flexibility strategy are long-lived leaves, low maximum relative growth rates, and inflexibility of form and of gas exchange rates when resource-shortage is relieved, both in seedlings and in adults. Plants which show the switching strategy display the low-flexibility strategy as young seedlings, but are flexibile in form as older plants, and commonly have high relative growth rates then. The gearing-down strategy is based on an ability to reduce strongly the respiration rate when resources are in short supply, both as seedlings and as adult. In some cases this involves shedding of parts which would otherwise consume respiratory substrate. Plants showing this strategy have some characteristics which are the opposite of those shown by plants with the low-flexibility strategy: short-lived leaves, and high flexibility in form and in rates of gas exchange. All three strategies are represented among plants tolerant of the most extreme shortages of nutrients, water and light. It is concluded that while ecologists should attempt to reduce the complexities of nature to an oligo-dimensional framework of generalizing ideas, it is not reasonable to expect that any very simple scheme – such as might be represented by three or four reference points in one plane – will have generality on a world scale.

The functional morphology of juvenile plants tolerant of strong summer drought in shaded forest understories in southern Spain

It has been hypothesized that plants cannot tolerate combined shade and drought, as a result of morphological trade-offs. However, numerous plant species are reportedly widespread in shaded forest understories that face drought, whether seasonal or occasional. We studied juveniles of six plant species that cope with strong summer drought in the understoreys of mixed Quercus forests in southern Spain: the tall-shrubs Phillyrea latifolia and Viburnum tinus, the perennial herb Rubia peregrina, the small shrub Ruscus aculeatus, and climbers Hedera helix and Smilax aspera. All of these species persist in evergreen shade (c. 3% daylight). Two other species were studied as comparators, Ruscus hypoglossum, less tolerant of drought, and Ceratonia siliqua, less tolerant of shade. Morphological and chemical variables relevant to shade and drought tolerance were measured for juveniles in a range of sizes, and also for the leaves of mature plants. The species converge in features that confer tolerance of shade plus drought by reducing demand for resources. Demand for water is reduced through a moderate to high below-ground mass fraction and low to moderate specific leaf area (respectively 0.22–0.52 and 112–172 cm2 g−1 at 1.00 g total dry mass). Demand for both irradiance and water is reduced through a low to moderate foliar nitrogen concentration and long-lived, physically protected leaves (≥2 yr). The species also converge in features that confer tolerance of either low irradiance or drought through specialized capture of resource, without precluding the other tolerance. These features include deep roots relative to shoot size, moderately higher specific leaf area in shade (1.2–2.0 × that in sun) and higher chlorophyll:nitrogen ratio in shade. Foliar chlorophyll per unit mass was higher in shade, but chlorophyll was not necessarily synthesized in greater amounts; rather, it was higher apparently due to shade effects on structural features linked with specific leaf area. In contrast, N per unit mass was higher in sun leaves independently of specific leaf area. Despite these convergences, the species diverge considerably in their root mass allocation and architecture, leaf saturated water content, density of stomata and guard cell size. No single narrowly defined functional type is needed for tolerance of shade plus drought.

Rainforest dynamics: the need for new paradigms

A desire to understand the maintenance of the very high species-richness of tropical lowland rain forest provides the setting for this paper. It is widely accepted that both niche-differentiation and chance effects among ecologically very similar species play an important part in maintaining richness. The purpose of the paper is to provide an improved background of ideas on rainforest dynamics, against which to consider differentiation in the regeneration niche, and against which to plan management and conservation. Paradigms deemed to need review are the following: the all-importance of canopy gaps, the existence of a spectrum of tolerance in which the relative positions of species are not dependent on size of individual, the association of large seed size with shade tolerance, the idea that soil seed banks are composed of species demanding sizeable canopy gaps, and the all-importance of shade as opposed to competition for water or mineral nutrients in limiting regeneration. Recent evidence bearing on these ideas is reviewed briefly, and an alternative set of paradigms is offered.

Responses to nutrient addition among shade-tolerant tree seedlings of lowland tropical rain forest in Singapore

1 Two bioassays of growth limitation were carried out for seedlings of four shadetolerant tree species (Antidesma cuspidatum, Calophyllum tetrapterum, Dipterocarpus kunstleri and Garcinia scortechinii) growing in P-deficient soil taken from lowland dipterocarp forest in Singapore, as a test of the hypothesis that growth would be limited by the availability of phosphorus. 2 Seedlings of only one species, Antidesma cuspidatum, showed increased growth in response to increased nutrient supply and in that case the limiting nutrient was not P. A majority of seedlings of Antidesma, Calophyllum and Garcinia in this experiment possessed VA mycorrhizas. 3 For seedlings of Antidesma, addition of magnesium led to an increase in the concentration of Mg in all fractions and a positive relation between Mg concentrations and dry mass yield. Addition of potassium and calcium resulted in reductions in concentrations of these elements in the leaves of Antidesma. 4 Seedlings of Antidesma, Calophyllum and Dipterocarpus responded to P by altering distribution of dry mass between different plant parts; the pattern of response varied between species. Phosphorus taken up in excess of requirements for vegetative growth was transferred to plant stems rather than leaves. 5 The outcome of pot bioassays may be dependent on factors such as pot size, irradiance and soil moisture conditions; therefore conclusions drawn here need to be tested by field fertilization experiments.