The bell-shaped physiological response curves developed in the pioneering work of Shelford are widely used in current ecology and physiological ecology textbooks to outline the concept of tolerance. However, bell-shaped symmetric response curves are observed rarely. In fact, organism response to common environmental drivers is more frequently described by asymmetric response functions. For instance, plants in temperate ecosystems are characterized by optimum temperatures for growth and photosynthesis between 15 and 30 ° C, and can tolerate temperatures around 0 ° C during the growing season and minimum temperatures around —20 to —40 ° C during dormancy. In contrast, heat stress limit of these plants is commonly around 35-40 ° C, implying that heat tolerance is significantly lower than the tolerance of low temperatures. Accordingly, the whole response function is negatively skewed (Figure 1b).
Differently from temperature, growth and survival of most plant species is more strongly limited by the deficiency of mineral nutrients than by the nutrient excess, and the response curves are positively skewed (Figure 1b and Figure 2 a). Analogously, most plants can grow in full sunlight, while few species can tolerate dense overstory shade, again implying that plant survival versus light response is positively skewed. In contrast, nonessential mineral nutrients such as sodium or chloride can have no negative influence on plant survival at low concentrations, but their effect on plant performance gradually increases with increasing concentration. In the latter case, only the tolerance of excess is relevant for species distribution.
While a curve with an optimum is appropriate to characterize the response to most environmental factors, tolerance of extreme low- and high-factor values is often determined by a suite of different traits. Thus, species low-factor tolerance limit is generally a poor estimate of high-factor tolerance limit and vice versa. Therefore, studies on tolerance do not generally focus on the tolerance range per se, but separately on the determinants of the tolerance to extreme low- and high-factor values. For instance, to understand plant distribution across environments of different temperatures, information of both heat- and frost-tolerance limits is needed.
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