Plasticity and Tolerance

In explaining and predicting species distribution along environmental gradients, the physiological response curve is generally considered a static characteristic of a given species; the representative individuals of a given species are considered to be in a steady state with the environment. While plant performance can be considered in steady state for slowly changing environmental factors such as soil phosphorus concentration or soil salinity, several environmental factors can rapidly fluctuate. In particular, air temperature and solar radiation vary dramatically during the day, among days, and during the season. This variation is important as plants have large plasticity the potential to adjust, to most environmental factors. Plasticity significantly influences their potential distribution in the field. For example, air temperature that exceeds the potential for survival of plants exposed and acclimated to low temperature can be well within the optimum range for plants of the same species grown and acclimated to higher temperatures (Figure 3 a). As the result of such phenotypic plasticity, plant temperature tolerance is larger than can be predicted from trait versus instantaneous response curves measured in nonacclimated plants. However, acclimation to temperature requires time, with full acclimation to a temperature shift taking between a few days to weeks. Thus, to survive short-term temperature fluctuations, for example, the fluctuations during the day, the instantaneous temperature response function should be broad enough to accommodate such fluctuations.

Plant species are characterized by widely different plasticities for key environmental factors such as temperature and light. Species also vary in the kinetics of acclimation to environment. Such differences in the extent and kinetics of acclimation imply significant differences in the range of habitats a given species can colonize (Figure 3b). Thus, plasticity can importantly modify species optimum range.

Evolutionarily, plasticity is not maximized in situations where environment varies in an unpredictable way (e.g., occurrence of rain in arid habitats), when other co-occurring factors may decrease the fitness of the alternative pheno-types generated by a highly plastic (e.g., drought in low light plants with a poorly developed root system) or when a plastic response to the environment may not improve long-term performance (e.g., elongating strategy in dense vegetation stands when neighbors cannot be easily overtopped). There is evidence from plants from a wide range of ecosystems that tolerance of extreme environments is associated with low plasticity and low growth rates, possibly, because of high carbon and energetic costs associated with reorganization of plant physiology and structure during acclimation. For instance, the most shade-tolerant plant species are known to have low plasticity to light availability.

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