Modification of Tolerance by Multiple Environmental Limitations and Polytolerance

We showed above that acclimation to environment can alter tolerance. In addition, suboptimal values of other environmental factors can affect the use efficiency of specific resource and thereby the absolute tolerance to the deficiency and excess of this resource. Thus, tolerance to a certain environmental factor estimated at optimal conditions of other environmental factors can be hardly transferred to natural environments exhibiting multiple limitations. In practice, tolerance to a given environmental limitation is not a constant, absolute feature of a given species, but depends on a number of internal and external factors that influence the final tolerance to a given factor. Tolerance to most environmental factors depends on the status of other environmental factors. For instance, reduction in water or nutrient availability commonly increases the minimum light availability for survival, that is, reduces shade tolerance (Figure 5). As co-occurring environmental drivers can influence light requirement

Figure 5 Interactions between shading and water availability in communities with occurrence of frost during the cold season (a) and communities without freezing temperatures (b). Leaf area and plant height increase with increasing water availability until the maximum values and decrease with increasing the degree and duration of soil inundation. The dashed lines indicate reduction of leaf area and shading with increasing nutrient limitations. At similar water availability, larger leaf areas that result in heavier shading in understory are generally observed in tropical than in temperate forests.

Figure 5 Interactions between shading and water availability in communities with occurrence of frost during the cold season (a) and communities without freezing temperatures (b). Leaf area and plant height increase with increasing water availability until the maximum values and decrease with increasing the degree and duration of soil inundation. The dashed lines indicate reduction of leaf area and shading with increasing nutrient limitations. At similar water availability, larger leaf areas that result in heavier shading in understory are generally observed in tropical than in temperate forests.

of different species differently, species distribution along light gradients depends on the degree of other ecological limitations as well. This evidence collectively emphasizes that for a given species, ecological tolerance to a certain environmental limitation is not an absolute but a relative species potential.

Our knowledge of simultaneous tolerance to more than one stress is fragmentary, challenging our capacity to anticipate the potential impact of climate change on plant communities. For instance, responses of Mediterranean plants to drought, heat, and high light have been intensively studied, but the impacts of low temperatures remain poorly known, though they exert a strong influence on community assemblage and plant distribution along altitu-dinal and latitudinal gradients. In general, plants can effectively handle an environmental stress and its typical correlates. For example drought and heat co-occur in warm deserts (Figure 4), and plants growing in deserts can normally cope with both of these stresses simultaneously. In addition, as physiological acclimation to both of these stress factors is associated with preservation of membrane integrity, acclimation to only one of them enhances plant tolerance to the other.

However, many habitats such as those in continental Mediterranean regions impose multiple, uncorrelated stresses such as freezing temperatures in winter and heat and drought over the summer. Polytolerance, that is, the capacity to cope with more than one distinct stress, like shade, drought, and waterlogging, is almost absent in the woody flora of the Northern Hemisphere due to functional tradeoffs and constraints. As the result of limited species capacity to tolerate different stresses simultaneously, distinct differentiation of species along combined environmental gradients occurs (Figure 5).

Global change exacerbates the multiple stresses and represents a complex challenge for plants dwelling in already stressful ecosystems. Furthermore, new combinations of abiotic stresses arise in Earth ecosystems due to overall warming, changes in the frequency and timing of heat waves and cold snaps, and changes in timing and amount of rainfall. Changing timing of frost events is already now affecting many forest tree species well known to tolerate freezing temperatures over winter dormancy, but not once the dormancy has been broken by periods of unusually warm temperatures. Periodic episodes of strong selection such as that caused by the late-season frost may be disproportionately important in evolution, and are likely becoming more common because of human alterations of the environment.

See also: Succession.

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