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The Physiological Ecology of Plant Succession F. A. Bazzaz Annual Review of Ecology and Systematics, Vol The Physiological Ecology of Plant Succession F. A. Bazzaz Annual Review of Ecology and Systematics, Vol. 10. (1979), pp. 351-371. Stable URL: http://links.jstor.org/sici?sici=0066-4162%281979%2910%3C351%3ATPEOPS%3E2.0.CO%3B2-U Annual Review of Ecology and Systematics is currently published by Annual Reviews. Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/about/terms.html. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/journals/annrevs.html. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. The JSTOR Archive is a trusted digital repository providing for long-term preservation and access to leading academic journals and scholarly literature from around the world. The Archive is supported by libraries, scholarly societies, publishers, and foundations. It is an initiative of JSTOR, a not-for-profit organization with a mission to help the scholarly community take advantage of advances in technology. For more information regarding JSTOR, please contact [email protected]. http://www.jstor.org Tue Feb 12 09:58:02 2008 Ann Rev. Ecol. Syst. 1979. lO:35l-71 Copyright O 1979 by Annual Reviews Znc. All rights reserved THE PHYSIOLOGICAL ECOLOGY OF PLANT SUCCESSION I? A. Bazzaz Department of Botany, University of Illinois, Urbana, Illinois 61801 INTRODUCTION Succession is a process of continuous colonization of and extinction on a site by species populations. The process has long been central in ecological thinking; much theory and many data about succession have accumulated over the years. Since nearly all species in all communities participate in successional interactions, and because physiological ecology encompasses everything that a plant does during its life cycle, a complete review of physiological ecology of all species in all successions is not possible. Thus in this review I discuss the physiological adaptations of species of one successional gradi- ent-from open field to broad-leaved deciduous forest. I concentrate on the physiological adaptations of early successional plants to environmental variability and collate the literature on tree physiology to make compari- sons with early successional plants. My discussion may not be applicable to seres where there is little difference in physiognomy between early and late successional plants or where the designation of species as early or late successional is unjustified (e.g. for certain desert and tundra habitats). I discuss the nature of successional environments, seed germination, seedling and mature plant development, plant growth, photosynthesis, water use, and the physiological ecology of competition and interference. THE NATURE OF SUCCESSIONAL ENVIRONMENTS The environment of a plant may vary daily, seasonally, vertically, and horizontally. The level of variability is determined by many factors includ- ing climate, geographical location, geomorphological features, the nature of site disturbances, and the number and kind of species present. The influence of the environment on the plant depends not only upon the level of environ- mental variability and the predictability of that variation, but also on the 352 BAZZAZ change in plant size and physiology through time. It is generally thought that environme'ntal variability in open, early successional habitats is higher than in closed, late successional ones. The variability of the physical environment is related mainly to the amount of energy that reaches the soil surface and the way in which it is dispersed from the surface. In an open field, energy exchange occurs at or near the soil surface, light energy reaches the surface unaltered and maximum temperature fluctuations occur there. In a later successional forest the surface of energy exchange is the upper layers of the canopy. Temperature fluctuations below the canopy are buffered by the vegetation itself, and progressively less energy penetrates toward the forest floor; light at the floor is markedly depleted of photosynthetically active wavelengths and is high in far-red wavelengths. Thus seedlings of the late successional species, except in large light gaps, experience a less variable and less extreme environment in the forest with respect to temperature, humid- ity, and wind. However, sunflecks under a canopy result in extremely variable light intensity and perhaps rapid fluctuation of leaf temperature. The extent, frequency, and magnitude of these events, and the physiological response of plants to them, have not been investigated. Maximum fluctuation in temperature and soil moisture occurs at or near the soil surface in open early successional habitats (70, 73). Thus germinat- ing seeds and young seedlings may experience a wider range of fluctuations in these two variables than do mature plants. In open early successional habitats C02 concentrations are higher than ambient just above the soil surface, increase with soil depth, and reach maximum values just above the water table (80). In forests C02 concentrations may rise above ambient levels within the forest canopy, especially early in the day and late at night (29, 100). Although there is general agreement about the relative levels of environ- mental variability in early and late successional habitats, quantification and interpretation of this variability are still rather difficult and may be frus- trated by phenomena such as sunflecks. A more serious problem, however, is the fact that the plant itself and not variation in the physical factors per se determines the effect of variability. It is likely that similar levels of variation of an environmental parameter cause quite different responses in different species: For some a certain level may be of no consequence to their function; for others it may be detrimental or stimulatory. ECOPHYSIOLOGICAL CHARACTERISTICS OF SUCCESSIONAL PLANTS Seed Germination Seeds of many early successional plants live for years in the soil (45, 87). Seeds of early successional trees long dormant in the soil may germinate in PHYSIOLOGICAL ECOLOGY OF SUCCESSION 353 large numbers when the canopy opens (54). In contrast, seeds of late succes- sional trees lose viability quickly (e.g. 2, 83). The relationship between seed germination and various parameters of the physical environment has been reviewed, with emphasis on its adaptive significance (e.g. 46, 90). Early and late successional environments differ primarily with respect to light intensity and spectral quality. Seeds of early successional plants are sensitive to light (37, 38, 79, 94) and their germina- tion is strongly inhibited by vegetation-filtered (high far-red/red) light (30, 44, 81, 85). In contrast, seeds of later successional plants, especially those found in climax forests, do not require light for germinati0n-e.g. Fagus grandifolia (77) and Acer saccharum (59). Furthermore, seeds of species from open habitats require more light for germination than do those of woodland species (34). Fluctuating temperatures also enhance the germina- tion of many species and may be the most important factor in seed germina- tion of annuals (22, 86, 91). Seeds of early successional plants germinate at or near the soil surface. Here the seeds experience unfiltered light, high daytime temperatures early in the growing season, much variation in daily temperature, and low COz concentrations. Thus seed germination of early successional plants is related to disturbance that brings some seeds from deep in the soil closer to the soil surface. Furthermore, both unfiltered high light (rich in red wavelengths) and fluctuating temperatures are associated with disturbance in forests, and the germination of some successional trees is also keyed to this disturbance (e.g. Prunus pensylvanica (54) and Betula alleghaniensis (25). Seed germination of early successional plants may be linked to distur- bance in other ways. For example, KN03and other nitrate salts enhance seed germination of several species, including some early successional herbs (36,46, 66). In devegetated areas a flush of nitrates may occur early in the spring (88) and act as a cue for germination as well as a resource for the young seedlings. Another aspect of germination in early successional plants is the develop- ment of induced (secondary) d0rmancy-e.g. in Ambrosia trzjida (22), A. artemisiifolia (9, and Amaranthus sp. (26). This strategy should protect the seed bank in the event that the site is disturbed again when environmen- tal conditions may be unsuitable for seedling growth. Ambrosia artemisiifolia, perhaps the most common annual of oldfield succession, possesses a complex germination strategy combining several of the features common to colonists. Germination of the species is closely linked to disturbance, which ensures the availability of resources and re- duces the probability of competition with later-successional species (Figure 1). The seeds are dormant when shed. After winter stratification a shift in germinator/inhibitor ratio takes place (97) and the seeds become ready to germinate. If the seeds are brought up to or near the surface
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