Showing papers in "Perspectives in Plant Ecology Evolution and Systematics in 1999"

The factors controlling species density in herbaceous plant communities: an assessment

Abstract: This paper evaluates both the ideas and empirical evidence pertaining to the control of species density in herbaceous plant communities. While most theoretical discussions of species density have emphasized the importance of habitat productivity and disturbance regimes, many other factors (e.g. species pools, plant litter accumulation, plant morphology) have been proposed to be important. A review of literature presenting observations on the density of species in small plots (in the vicinity of a few square meters or less), as well as experimental studies, suggests several generalizations: (1) Available data are consistent with an underlying unimodal relationship between species density and total community biomass. While variance in species density is often poorly explained by predictor variables, there is strong evidence that high levels of community biomass are antagonistic to high species density. (2) Community biomass is just one of several factors affecting variations in species density. Multivariate analyses typically explain more than twice as much variance in species density as can be explained by community biomass alone. (3) Disturbance has important and sometimes complex effects on species density. In general, the evidence is consistent with the intermediate disturbance hypothesis but exceptions exist and effects can be complex. (4) Gradients in the species pool can have important influences on patterns of species density. Evidence is mounting that a considerable amount of the observed variability in species density within a landscape or region may result from environmental effects on the species pool. (5) Several additional factors deserve greater consideration, including time lags, species composition, plant morphology, plant density and soil microbial effects. Based on the available evidence, a conceptual model of the primary factors controlling species density is presented here. This model suggests that species density is controlled by the effects of disturbance, total community biomass, colonization, the species pool and spatial heterogeneity. The structure of the model leads to two main expectations: (1) while community biomass is important, multivariate approaches will be required to understand patterns of variation in species density, and (2) species density will be more highly correlated with light penetration to the soil surface, than with above-ground biomass, and even less well correlated with plant growth rates (productivity) or habitat fertility. At present, data are insufficient to evaluate the relative importance of the processes controlling species density. Much more work is needed if we are to adequately predict the effects of environmental changes on plant communities and species diversity.

Foraging strategies of insects for gathering nectar and pollen, and implications for plant ecology and evolution

Abstract: The majority of species of flowering plants rely on pollination by insects, so that their reproductive success and in part their population structure are determined by insect behaviour. The foraging behaviour of insect pollinators is flexible and complex, because efficient collection of nectar or pollen is no simple matter. Each flower provides a variable but generally small reward that is often hidden, flowers are patchily distributed in time and space, and are erratically depleted of rewards by other foragers. Insects that specialise in visiting flowers have evolved an array of foraging strategies that act to improve their efficiency, which in turn determine the reproductive success of the plants that they visit. This review attempts a synthesis of the recent literature on selectivity in pollinator foraging behaviour, in terms of the species, patch and individual flowers that they choose to visit. The variable nature of floral resources necessitate foraging behaviour based upon flexible learning, so that foragers can respond to the pattern of rewards that they encounter. Fidelity to particular species allows foragers to learn appropriate handling skills and so reduce handling times, but may also be favoured by use of a search image to detect flowers. The rewards received are also used to determine the spatial patterns of searches; distance and direction of flights are adjusted so that foragers tend to remain within rewarding patches and depart swiftly from unrewarding ones. The distribution of foragers among patchy resources generally conforms to the expectations of two simple optimal foraging models, the ideal free distribution and the marginal value theorem. Insects are able to learn to discriminate among flowers of their preferred species on the basis of subtle differences in floral morphology. They may discriminate upon the basis of flower size, age, sex or symmetry and so choose the more rewarding flowers. Some insects are also able to distinguish and reject depleted flowers on the basis of ephemeral odours left by previous visitors. These odours have recently been implicated as a mechanism involved in interspecific interactions between foragers. From the point of view of a plant reliant upon insect pollination, the behaviour of its pollinators (and hence its reproductive success) is likely to vary according to the rewards offered, the size and complexity of floral displays used to advertise their location, the distribution of conspecific and of rewards offered by other plant species, and the abundance and behaviour of other flower visitors.

Vertebrate herbivores and plants in the Arctic and subarctic: effects on individuals, populations, communities and ecosystems

Abstract: Plants in the Arctic and subarctic face the problems posed by herbivory in addition to short growth seasons, low temperatures and low nutrient availability. Herbivores control plant performance by removing biomass, by altering resource availability, by altering the physical environment, and by changing the balance of competition. The main difference between effects of herbivores in the Arctic and at lower latitudes may be the relatively greater importance of changes in resource availability and the physical environment resulting from herbivore activity, and their consequences for plant competitive abilities. Species responses to defoliation depend primarily on growth form. Artificial defoliation of graminoids has negative effects on most species, but in the field total effects of herbivores are often neutral or even positive, resulting in increased nitrogen concentrations in shoots in many species. Shrubs are less able to respond positively to herbivory than graminoids, and although there is some evidence that deciduous shrubs recover faster than evergreen ones, the difference is not great. However, effects of herbivores on shrubs are little studied, despite their importance in the herbivore diet. Responses of individual species to increased nutrient availability vary greatly, even within a growth form. Some graminoids and shrubs show strong positive responses to fertilization while others show little or no response. These species-specific effects suggest that herbivores can alter interspecific relationships through differential responses to fertilization. Herbivores may alter plant population dynamics by altering flower or seed production, by consuming seedlings, or by altering the availability of microsites. However, no study has adequately examined this for any arctic species. Changes in community composition following removal of herbivores are the result not only of selective removal of some plant species, but also of changes in microsite availability, nutrient availability, litter accumulation, and soil characteristics. Thus, the view that abiotic factors are the overwhelming determinants of community structure in low-productivity environments is compatible with the view that herbivores exercise their influence to a large extent by altering abiotic factors. Arctic herbivores often increase total above-ground nitrogen availability (and therefore food quality) in the plant community, but increased productivity as a result of herbivores is rare. The increase in nutrient availability is probably due in part to changes in soil temperature and soil moisture following a reduction in litter accumulation. Although our knowledge of effects of herbivory on individual plants and on communities is extensive, we lack information on effects at the population level. We also do not have an adequate understanding of impacts of herbivores at different spatial and temporal scales, something which is needed to be able to make predictions about longer-term impact of herbivores in these systems.

The number and kinds of embryo-bearing plants which have become aquatic: a survey

Abstract: The extant embryo-bearing plants (mosses, ferns and seed plants) have an aquatic ancestor or ancestors. Today they dominate the terrestrial vegetation of the world. Embryo-bearing plants which now live in water have, for a second or even third time, re-invaded water. Aquatic species are found in 440 genera from 103 families of embryo-bearing plants. This survey attempts to find out how often the Event of evolving from land back to water has taken place among the living plants of today. Analysing the morphological and phylogenetic affinities of all aquatics at taxonomic levels from divisio to varieties of species indicate the Event has taken place, at least 222 times but it could have happened 271 or even more times (bryophytes 10–19 times, ferns seven times, seed plants 205–245 times). Aquatic plants have evolved from often very different genetic and ecological backgrounds. Also, they have evolved at different times; some old ones are aquatic at the level of order or family, while others, more recent, are isolated species in otherwise terrestrial genera or races within species. It is, nevertheless, wonderful that such a large variety of plants have evolved solutions to the single problem of remastering life in water.

Nitrogen fixation and growth of non-crop legume species in diverse environments

Abstract: Of the 643 legume genera, few have been exploited in agriculture and 40% have not even been evaluated for their ability to nodulate and fix nitrogen. Most of these are in tropical/subtropical regions, with habitats ranging from extremely dry to flooded. Recent work in some of these areas shows that plants can nodulate under conditions previously thought to be disadvantageous. The accepted dogma that nitrogen fixing legumes have a high demand for P is challenged and examples of how legumes can extract P from soils with low available P described. Species tolerant to shading and high soil Al are cited, although the mechanisms of adaptation are not yet clear. Some tropical soils have high nitrate levels and, contrary to perceived wisdom, there are legumes which can nodulate under such conditions. Many tropical tree legumes prefer ammonium to nitrate and are able to fix nitrogen and assimilate ammonium at the same time. In all these cases, there are genotypic differences, both within and among species. Large areas of tropical fresh water, such as the Brazilian Pantanal and the Orinoco floodplain have nodulated legumes predominant in their flora. The ecological potential of these has not been evaluated. One of the sites of nodule evolution is likely to have been in such areas. Modes of infection of legumes by rhizobia vary with taxonomic tribe and may represent evolution for survival in different environments. As more legumes, from more ecosystems are studied, a wider range of adaptations is likely to be found. Work is urgently needed to study these, especially in areas being cleared for agriculture or by logging.

Long-term change in growth, mortality and regeneration of trees in Denny Wood, an old-growth wood-pasture in the New Forest (UK)

Abstract: Long-term changes in stand composition and structure were recorded in Denny Wood (New Forest, UK) by means of a permanent transect covering 2 ha. Denny is an ancient, mixed deciduous wood-pasture dominated by beech ( Fagus sylvatica ), pedunculate oak ( Quercus robur ) and holly ( Ilex aquifolium ) whose canopy trees ranged in age from approximately 70 years to over 300 years when the study began in 1956. Individual trees, shrubs and saplings were mapped and measured at irregular intervals until 1996. During the 40 years of observations, storms and drought disrupted the stand. Considerable volumes of dead wood accumulated, and canopy gaps extended to 30% of the transect area. Small trees and saplings were severely damaged by ponies and grey squirrels. Regeneration ceased after 1964, due principally to heavy grazing and browsing by deer and ponies. Despite the disturbances, most mortality was due to competitive exclusion within well-stocked parts of the stand. Historical records from the 17th century onwards demonstrate a long-term change from oak dominance with groups of beech before 1800 to beech dominance in the late 20th century. The stand through which the transect now runs was enclosed in 1870, and this allowed beech to regenerate abundantly, but in the nearby unenclosed part of Denny Wood holly regenerated more abundantly than beech. The patterns of growth, mortality and regeneration are compared with natural temperate deciduous woodland. The long-term relationship between beech and oak is likely to involve periodic oak regeneration after major disturbances, interspersed with steady increases in the proportion of beech. The implications for managing and monitoring the “Ancient and Ornamental Woods” of the New Forest are considered.

The role of plant resources in forest succession: changes in radiation, water and nutrient fluxes, and plant productivity over a 300-yr-long chronosequence in NW-Germany

Abstract: In the past insufficient attention has been paid to quantitative measurements of resource fluxes in ecosystems that undergo successional change. In this study, simultaneous changes in seven plant resources (photosynthetically active radiation (PAR) , water, nitrogen, phosphorus, calcium, magnesium and potassium) are quantified by a chronosequence approach for a 300-yr-long secondary succession on poor soil from Calluna vulgaris heathland to Fagus sylvatica-Quercus petraea late-successional forest (heathland-to-forest succession). Above-ground net primary production increases sevenfold, and total above-ground phytomass about fortyfold during heathland-to-forest succession. Plant organs that capture resources increase much more slowly (leaf area index: threefold; fine root biomass: 1.3-fold). The increase in productivity is based both on higher absorptivity and conversion efficiency of PAR by the canopies of the successional plants. Accumulation of organic material on the forest floor significantly improves soil water availability. Evapotranspiration losses increase early in succession as the growing vegetation increases in both height and leaf area but tend to decrease again in the late-successional community. Drainage losses are at their minimum at the conifer-dominated pioneer forest stage. Accumulation of available nutrients in the soil is a key process in heathland-to-forest succession that significantly improves plant nutrient availability but leads to only minor changes in carbon/nutrient ratios and humus quality. Litter decomposition rates increase and result in a more rapid nutrient turnover in late successional stages. External nutrient inputs (from the atmosphere and soil weathering) significantly contribute to plant nutrient supply early in succession, whereas the internal cycling of nutrients through litter fall and nutrient mineralisation by far exceeds external inputs at the late stages. Vitousek & Reiners' (1975) ecosystem nutrient loss hypothesis is supported by the heathland-to-forest succession data. Odum's (1969) hypotheses on how nutrient cycles change during the course of succession is, in one part, rejected, in part supported. Tilman's (1988) hypothesis on nutrient limitation early, and light limitation late in primary succession is rejected.

Plants as amphibians

Abstract: From the poles to the tropics flooding is a powerful discriminator in plant distribution. Although plants can be divided globally as to whether or not they are tolerant of high water tables, it does not follow that all flood-tolerant species achieve their ability to survive flooding by similar adaptations. Flooding implies a periodic but temporary rise of the water table, hence plants that live in such areas have an amphibious life style. Amphibious plants have to adjust, not only to inundation and the dangers of oxygen deprivation, but also to the eventual lowering of water tables and often sudden re-exposure to a fully aerated environment and the lack of the physical support that is provided by flooding. In this respect they are distinct from aquatic species that live constantly in water. It is often tacitly assumed that for amphibious species flooding is the stressed condition and non-flooding the norm. This pre-judgement is not appropriate, particularly as in many habitats the flooded condition predominates for a longer part of the year than the unflooded. For amphibians, re-adapting from the aquatic to the terrestrial habitat requires specialised adaptations, just as much as a change from unflooded to flooded. Many flood-tolerant species, including surface-rooting grasses and sedges, may not be tolerant of anoxia, and instead prevent the accumulation of an oxygen debt in submerged organs by aeration mechanisms, including oxygen diffusion through aerenchyma, thermally induced mass movement of air, and the elongation of submerged shoots. In other species, and particularly in perennial plants with buried perennating organs, flooding can impose prolonged periods of anaerobiosis (anoxia). Being able to survive such oxygen deprivation requires (1) energy reserves sufficient for cell maintenance, (2) the prevention of cytoplasmic acidosis under anoxia, and (3) the anaerobic mobilisation of starch reserves. Re-entry to the aerobic habitat is facilitated by (4) the dispersal and excretion of products that transfer hydrogen from anoxic or hypoxic tissues, either to the external environment, or to parts of the plant with access to oxygen, before the anaerobic tissues return to air, and (5) anti-oxidative activity to minimise post-anoxic injury.

Ecological communities: conceptual problems and definitions

Abstract: In the preceding chapter I have noted that there are no examples of reduction of laws and theories about communities and ecosystems for the simple reason that there are no general laws and theories about communities and ecosystems. I have also noted that one particular cause of this might be that the concepts of community and ecosystem have not been defined adequately and unambiguously. The terms ‘community’ and ‘ecosystem’ are being used for a wide variety of objects at different levels of organization (see, amongst others, Shrader-Frechette und McCoy 1993). Considering that one of ecology’s central goals is to describe and explain the structure of communities and ecosystems, as given by such properties as species number and composition, this situation is deplorable.

Evolution, polymorphology and multifunctionality of the phloem system

Abstract: Current research discloses that the phloem system is not only responsible for the allocation of photoassimilates, but has several other functions. Despite the knowledge acquired recently, the phloem remains the most puzzling plant tissue due to its inaccessability to experimental approach. Since well-preserved fossile remnants of phloem tissue are rare, evolution of sieve elements and the whole phloem was inferred from the phloem structure in present plant taxa. Special attention is paid to the evolution of the sieve elements being the conducting modules of the phloem. Development of sieve elements probably was a polyphyletic event. It may have occurred independently in various groups of algae and in the land plants. The emergence of highly specialized accessory cells sustaining the sieve element is restricted to the Spermatophyta. An attempt is made to explain the presumptive evolutionary development of the phloem system in terms of physiological fitness. In particular, the diversity of the leaf phloem in dicotyledons is discussed. It is an example of progressive phloem evolution in a plant organ that is permanently challenged by daily variations and more persistent environmental changes.