Plant Ecology Introduction

The term "ecology" was defined by Ernst Haeckel in 1906 in his book, Principles of General Morphology of Organisms, as follows: "Ecology is the science of relations of the organism to the surrounding environment which includes, in its broadest sense, all 'conditions for existence'. These conditions may be organic or inorganic; both are of the greatest importance for the form of organisms, because they force the organism to adapt."

Haeckel included in the science of ecology the areas physiology, morphology and chorology (the science of the distribution of organisms) to understand the "conditions for existence" and "adaptation". In this book, we try to comply with Haeckel's understanding of plant ecology and to include the breadth of ecology as it was demanded by Haeckel. Adaptation to the environment starts at the molecular and cellular level where environmental conditions are detected and the responses to changes in the environment are accomplished. Starting from these physiological mechanisms, the morphological characteristics of organisms/plants become important at the level of the whole plant. Cellular metabolic reactions and structural (morphological and anatomical) organisation are the biological "tools" with which organisms make use of certain environmental conditions, avoid them or "adapt" to them. The combination of physiological and morphological "adaptation" is particularly important for plants, as they are fixed in their habitat and the conditions for life are determined by the variety and numbers of the organisms of the ecosystem and not by the individual plant alone. These environmental conditions and the interaction of a plant with the environment determine "fitness", i.e. the possibility for growth and reproduction in a spatial and temporal dimension, thus resulting in an association with Haeckel's "chorology". Haeckel's understanding of ecology was broader than our present botanical usage. The present book views ecology in as broad a context as Haeckel did, ranging from molecular stress physiology via ecology of whole organisms and the ecosystem to the temporal and spatial differentiation of vegetation.

Figure 1 shows the relations between (cellular) stress or ecophysiology, whole plant physiology and synecology (i.e. the ecology of vegetation cover) and ecosystem science where other organisms, not only plants, are increasingly considered. The interrelations between stress physiology, whole plant physiology and synecology are very close and obvious. In contrast, the path from stress physiology to ecosystems runs via whole plant ecology and synecology because morphology, i.e. the structure of plants, and the responses of populations are not primarily metabolic. Applied ecology includes all disciplines related to human activities. These include not only agriculture and forestry, but also global change. Agriculture and forestry contain also physiological aspects of high-yielding and pestfree varieties of crops and the biological interactions between crop plants and other organisms,

Ecology Stress physiology

Whole plant ecology


Ecology of ecosystems

Applied ecology, global change

| Fig. 1. Areas of ecology and their position within botany, ecosystem studies and in applied ecology e.g. for pollination. Research on global change also includes the assessment of possible management systems for earth with respect to their effects on climate and maintaining biodiversity. Research on global change leads to model predictions on future effects of human activities.

In this book, an attempt is made, for the first time, to bring together and clearly organise the large subdisciplines of plant ecology. We start from the molecular stress and ecophysiology of plants in the broadest analysis yet attempted. Chapter 1 lays down the molecular basis for ecological "adaptations" to all essential environmental factors. This ranges from climatic factors via salt stress in the soil to environmental pollutants. The stress theory considers the basic possibilities of stress responses resulting from strains and leading to resistance; finally, these provide the basis for understanding adaptive radiation of genotypes, and the processes leading to the evolution of new species. Plants not only react to stress in the sense of a response, the so-called feed-back reponse. There are also preparatory adaptations to changing environmental conditions, the so-called feed-forward reactions, setting off before an organism is stressed (e.g. pre-winter frost hardening). In both cases, signal chains are activated, leading to changes in the physiological/cell biological performance of plants, enabling them to continue to exist under new conditions. The response to one stress factor often protects the organism also from damage by other stresses ("cross-protection"). This results in responses to a variety of stresses resulting from a changing environment where not one single factor (e.g. heat or drought), but multiple stress types are acting in combination. The basic principles of avoidance (as a sort of feedforward response) and tolerance (as a sort of feed-back response) to stress are not only restricted to the level of ecophysiology, but occur also in responses of whole plants, in the distribution of species and plant communities, particularly at extreme sites.

At the level of the organism, we consider the plant as a whole and the relations between its organs from the root to the leaf, flower and seed. At the level of whole plant ecology new, not (primary) metabolic characteristics are added: although these are genetically determined, they may be modified within limits. These include plant structures including size and the life cycle (phenology, life span, strategies for reproduction and distribution). Because of these strategies, certain species are able to avoid extreme conditions and use or change their habitat. Annual plants form the largest proportion of plant species in dry areas. However, their active life is limited to favourable conditions after rain, even if this only occurs sporadically, perhaps only every few years or even decades. In contrast, perennial species have mechanisms that regulate the water relations and enable survival in unfavourable climatic periods. These include, for example, special leaf and root anatomy that allows the species to survive with intact shoot systems or to change their site conditions. Hydraulic lift, for example, enables roots to transport water from deep soil layers to the upper horizons and thus moisten the upper soil layers. In temperate climates, the accumulation of carbon in the soil changes site fertility. The scope for "adaptations" by whole plants is very broad, as are the responses of cells to stress. They range from leaf structure and leaf movement, via the formation of variably dimensioned vessels in the stem, to differentiation of the roots. In Chapter 2, the use of resources by whole plants is discussed. This includes the plant-water relations, the heat and nutrient balances and the carbon relations.

Cellular metabolism and structural characteristics are not only the basis for the spatial and temporal patterns of plant species, dealt within synecology (Chap. 4), but also the basis for element cycles in ecosystems, which are characterised by the diversity of species and forms of organisation. These include indirect interactions between individual plants and other plant species. Here applies the wisdom that: "Even the most pious cannot live in peace if it does not please the nasty neighbour". Competition exists in the effective use of resources on limited space. If the resources become scarce, "efficiency" means a better use of limiting resources at the cost of the neighbour. This, of course, does not always imply saving resources or using them most economically. Indeed, it may be more useful to use more resources than required, if this brings advantages in competition with the neighbours. Growth also plays an important role in ecosystems, and the "ecological equilibrium" of an ecosystem probably does not exist in "nature" as it is. Metabolic cycles in ecosystems are not as closed as previously assumed. This means that the actual status as it may be observed as a momentary picture of a system is in the long term very dynamic. Not all processes of a system move alkaline shade

Limits of a positive carbon balance cold alkaline shade

Limits of a positive carbon balance cold

wet rig. 2. Distribution of a species depends on different environmental factors. The actual distribution area is significantly smaller than the potential areas of distribution which are reached without competition at the extreme limits of flowering or at the boundaries of a positive material balance. In the example shown, temperature is the dominant factor, but this may differ in other cases. According to the species, the limits of distribution change wet rig. 2. Distribution of a species depends on different environmental factors. The actual distribution area is significantly smaller than the potential areas of distribution which are reached without competition at the extreme limits of flowering or at the boundaries of a positive material balance. In the example shown, temperature is the dominant factor, but this may differ in other cases. According to the species, the limits of distribution change linearly in one direction. There are strengthening and weakening processes with a consequential complex, non-linear temporal behaviour. The scientific basis of ecosystems is, for the first time, presented in Chapter 3 of this volume. General conclusions are drawn about how individual organisms interact within the diversity of vegetation. This leads to the question about the degree to which vegetation is more than the sum of its individual plants. Many characteristics of vegetation result from material fluxes which dictate the performance of individual plants. For example, the "aero-dynamic roughness" of the surface of vegetation determines coupling to the meteorological conditions in the atmosphere and thus determines the survival of plant individuals or a species in the vegetation. It is only by large numbers of individuals of the same species which, through distribution of seeds or other ways of propagation, determine the habitat in relation to a dynamic diversity of other species.

Synecology is the next higher level of plant ecology, extending to populations based on the strategies of propagation and distribution. Synecology does not consider the fate of a single individual, but the dynamic spatial and temporal behaviour of populations, including population growth, homoeostasis and decline. Only in exceptional cases does a single species form a vegetation. Generally, natural vegetation includes a diversity of species which make complementary use of the available resources. In synecology, the broad spectrum of responses at the cellular and whole plant level is replaced by the enormous diversity of species (350,000 species of vascular plants) which determine in different proportions the composition of the vegetation cover of the earth. In Chapter 4, the historical and spatial dimensions of species distributions and their biological interactions are discussed.

Combining the ecology of ecosystems with the field of synecology enables us to understand also the distribution of species. Both the potential and actual areas (Fig. 2) are determined by several parameters. For example, considering only the carbon balances, a plant species could grow on a much larger area than the region in which it actually reproduces. However, even this region is limited by different types of competition with other species, so that the area eventually occupied is even further restricted. Agricultural crop plants (maize, beans, wheat, potato, soya and many others) are an interesting example of evolution in a geographically limited area (the so-called genetic centres of origin), but these species are now distributed worldwide after domestication and management by humans.

The science of geobotany relates to global aspects in plant ecology, which are included in the term global change, where the direct and indirect influences of man through land use, changes in land use and the subsequent changes in climate are becoming increasingly felt. The pivotal question in Chapter 5 is: "How are conditions for human existence affected by the reactions of the vegetation cover of the earth?" In this book, plant ecology is broadened to include the effects of plant life on global element cycles. At present, the primary focus is on the "management" of the global carbon cycle by humans. It is obvious that in future questions concerning global management of water and nutrient cycles will have to be considered as being just as important. Here, ecology is no longer "pure" science and will play an important and even vital role in providing information for politicians on questions concerning humanity as a whole. Research into global change is developing very fast at present, similarly to the development of molecular plant ecophysiology.

Based on these considerations, the book deals with the main areas of plant ecology in the following chapters:

• Stress physiology

• Whole plant physiology

• Ecology of ecosystems

• Synecology

• Global aspects of plant ecology

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