Hans Lambers F. Stuart Chapin - Plant Physiological Ecology

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Plant ecophysiology is an experimental science that seeks to describe the physiological mechanisms underlying ecological observations. In other words, ecophysiologists, or physiological ecologists, address ecological questions about the controls over the growth, reproduction, survival, abundance, and geographical distribution of plants, as these processes are affected by interactions of plants with their physical, chemical, and biotic environment. These ecophysiological patterns and mechanisms can help us understand the functional significance of specific plant traits and their evolutionary heritage.

The questions addressed by ecophysiologists are derived from a higher level of integration, i.e., from ecology in its broadest sense, including questions originating from agriculture, horticulture, forestry, and environmental sciences. However, the ecophysiological explanations often require mechanistic understanding at a lower level of integration (physiology, biochemistry, biophysics, molecular biology). It is, therefore, quintessential for an ecophysiologist to have an appreciation of both ecological questions and biophysical, biochemical, and molecular methods and processes. In addition, many societal issues, often pertaining to agriculture, environmental change, or nature conservation, benefit from an ecophysiological perspective. A modern ecophysiologist thus requires a good understanding of both the molecular aspects of plant processes and the functioning of the intact plant in its environmental context.

Plant ecophysiology aims to provide causal, mechanistic explanations for ecological questions relating to survival, distribution, abundance, and interactions of plants with other organisms. Why does a particular species live where it does? How does it manage to grow there successfully, and why is it absent from other environments? These questions were initially asked by geographers who described the global distributions of plants (Schimper ).

Although ecophysiologists initially emphasized physiological responses to the abiotic environment [e.g., to calcareous vs. acidic substrates (Clarkson )], physiological interactions with other plants, animals, and microorganisms also benefit from an understanding of ecophysiology. As such, ecophysiology is an essential element of every ecologists training.

A second impetus for the development of ecophysiology came from agriculture and physiology. Even today, agricultural production in industrialized nations is limited to 25% of its potential by drought, infertile soils, and other environmental stresses (Boyer ). A major objective of agricultural research has always been to develop crops that are less sensitive to environmental stress so they can withstand periods of unfavorable weather or be grown in less favorable habitats. For this reason agronomists and physiologists have studied the mechanisms by which plants respond to or resist environmental stresses. Because some plants grow naturally in extremely infertile, dry, or salty environments, ecophysiologists were curious to know the mechanisms by which this is accomplished.

Plant ecophysiology is the study of physiological responses to the environment. The field developed rapidly as a relatively unexplored interface between ecology and physiology. Ecology provided the questions, and physiology provided the tools to determine the mechanism. Techniques that measured the microenvironment of plants, their water relations, and their patterns of carbon exchange became typical tools of the trade in plant ecophysiology. With time, these studies have explored the mechanisms of physiological adaptation at ever finer levels of detail, from the level of the whole plant to its biochemical and molecular bases. For example, initially plant growth was described in terms of changes in plant mass. Development of portable equipment for measuring leaf gas exchange enabled ecologists to measure rates of carbon gain and loss by individual leaves (Reich et al. ). Studies of plant water relations and mineral nutrition provide additional insight into controls over rates of carbon exchange and tissue turnover. More recently, we have learned many details about the biochemical basis of photosynthesis and respiration in different environments and, finally, about the molecular basis for differences in key photosynthetic and respiratory proteins. This mainstream of ecophysiology has been highly successful in explaining why plants are able to grow where they do.

Although there are 270000 species of land plants (Hammond ). Many species are absent from a given plant community for historical reasons . They may have evolved in a different region and never dispersed to the site under consideration. For example, the tropical alpine of South America has few species in common with the tropical alpine of Africa, despite similar environmental conditions, whereas eastern Russia and Alaska have very similar species composition because of extensive migration of species across a land bridge connecting these regions when Pleistocene glaciations lowered sea level 20000100000 years ago.

Of those species that arrive at a site, many lack the appropriate physiological traits to survive the physical environment. For example, whalers inadvertently brought seeds of many weedy species to Svalbard, north of Norway, and to Barrow, in northern Alaska. However, in contrast to most temperate regions, there are currently no exotic weed species in these northern sites (Billings ). Clearly, the physical environment has filtered out many species that may have arrived but lacked the physiological traits to grow, survive, and reproduce in the Arctic.

Biotic interactions exert an additional filter that eliminates many species that may have arrived and are capable of surviving the physical environment. Most plant species that are transported to different continents as ornamental or food crops never spread beyond the areas where they were planted because they cannot compete with native species (a biotic filter). Sometimes, however, a plant species that is introduced to a new place without the diseases or herbivores that restricted its distribution in its native habitat becomes an aggressive invader, for example, Opuntia ficus-indica (prickly pear) in Australia, Solidago canadensis (golden rod) in Europe, Cytisus scoparius (Scotch broom) in North America, and Acacia cyclops (red-eyed wattle) and A. saligna (orange wattle) in South Africa. Because of biotic interactions, the actual distribution of a species (realized niche, as determined by ecological amplitude ) is more restricted than the range of conditions where it can grow and reproduce (its fundamental niche, as determined by physiological amplitude ) (Fig. ).