Multiple Environmental Limitations and the Law of the Minimum

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In natural stressful environments, several environmental factors can be simultaneously outside their physiological optimum range (Figure 4) and it is pertinent to ask which of these factors is the limiting factor. Plants colonizing harsh environments like arctic-alpine desert and alpine treelines cope both with extreme low temperatures and with shortages of water and nutrients, while warm deserts are not only characterized by suboptimal rainfall but also nutrient deficiencies, heat stress, and excess light (Figure 4). The 'law of the minimum' formulated in 1820s-1840s on the basis of plant nutritional studies (Sprengel-Liebig law) states that when multiple factors

Figure 3 Illustration of plant response to instantaneous and long-term average temperature (TG). Instantaneous responses to temperature are characterized by a curve with an optimum (Topt). With increasing long-term temperature, plants phenotypically adjust by increasing Topt (a). Thus, Topt is larger in plants acclimated to average temperature TG,2 than in plants acclimated to temperature TG1 and Topt is even larger in plants acclimated to temperature TG,3. 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. Phenotypic plasticity of different species can significantly modify long-term plant responses to temperature gradients (b). Species 1 has larger plasticity in response to temperature than species 2 and as the result, can more effectively track changes in long-term temperature environment. Thus, species 1 has larger tolerance of temperature extremes than species 2 if temperature change is slow enough to allow the plant to adjust to changing temperature, for instance, as is the case with seasonal variations in TG. Sometimes, plants do not acclimate to long-term average temperature, but to average maximum or average minimum temperature. Absolute maximum and minimum temperatures are generally unpredictable, and plants can cope with such extremes only if the extreme temperatures are within the instantaneous tolerance limits.

Figure 3 Illustration of plant response to instantaneous and long-term average temperature (TG). Instantaneous responses to temperature are characterized by a curve with an optimum (Topt). With increasing long-term temperature, plants phenotypically adjust by increasing Topt (a). Thus, Topt is larger in plants acclimated to average temperature TG,2 than in plants acclimated to temperature TG1 and Topt is even larger in plants acclimated to temperature TG,3. 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. Phenotypic plasticity of different species can significantly modify long-term plant responses to temperature gradients (b). Species 1 has larger plasticity in response to temperature than species 2 and as the result, can more effectively track changes in long-term temperature environment. Thus, species 1 has larger tolerance of temperature extremes than species 2 if temperature change is slow enough to allow the plant to adjust to changing temperature, for instance, as is the case with seasonal variations in TG. Sometimes, plants do not acclimate to long-term average temperature, but to average maximum or average minimum temperature. Absolute maximum and minimum temperatures are generally unpredictable, and plants can cope with such extremes only if the extreme temperatures are within the instantaneous tolerance limits.

are operating on an organism, only the factor that is closest to its tolerance limit (the scarcest resource) is the factor controlling organism growth and survival. The 'law of the minimum' implies that at any time there can only be a single limiting factor, even though many environmental factors can be outside the optimum range. For instance, both nitrogen and phosphorus can be in short supply in the soil, but 'the law of the minimum' predicts that if N availability is closer to its tolerance limit, productivity can be increased by adding N and not by adding P. Nitrogen addition affects productivity until P becomes the limiting factor, and productivity can only be increased further by adding P and N until some other factor becomes limiting.

The 'law of the minimum' has been heavily employed in ecology and agriculture due to its appealing simplicity. However, the 'law of the minimum' does not consider that environmental factors often interact in natural ecosystems. Interactions between environmental drivers can arise when the availability of a limiting environmental resource depends on other environmental factors. For instance, total soil N content is very large in mires and bogs on peat soils, but the availability of plant-available mineral N forms - ammonium and nitrate - is low. While plant productivity can be mainly limited by N, fertilization of such ecosystems with P can still enhance plant productivity, because P increases soil N mineralization rate and N availability for the plants.

Interactions between environmental factors can also occur when the use efficiency of a certain resource depends on the status of another factor. For instance, while plant productivity can be enhanced by N fertilization in arctic tundra, suboptimal temperatures reduce biomass production per unit nitrogen taken up, that is, nitrogen-use efficiency. Therefore, productivity enhancement is larger if fertilization is combined by simultaneous increase of temperature. Understanding such interactive effects is crucial for reliable prediction of plant performance in globally changing climate. Recent studies have revealed that response of plants to a combination of two or more different abiotic stresses is unique and cannot be directly extrapolated from the response of plants to each of the different stresses applied individually.

From cost/benefit perspective, efficient harvesting of environmental resources is expensive for the plant due to high carbon and energy costs for construction of efficient uptake systems. Assumption that a single environmental factor is always limiting implies that plant has an excess

Temperature^

Water

Light

_r>- Drought . 7/ Waterlogging '/of'-/.-... Shading '

Excess z..

Warm desert

Arctic-alpine desert

■-►Nutrient Substrate J availability chemistry salinity ^

'> Temperate forest Salt marsh Coastal forest

Arctic-alpine desert

'> Temperate forest Salt marsh Coastal forest

Figure 4 Outline of major abiotic stresses in terrestrial ecosystems. In most communities, different stress factors commonly co-occur as shown for warm desert, arctic-alpine desert, temperate forest, salt marsh, and coastal forest. The photograph shows tropical dry coastal grassland at Papawai Point, Maui, Hawaii, USA (20046'N, 156032'W) during winter. Plant growth in these communities on shallow volcanic soils at complex terrain is limited by low nutrient and water availabilities, salt spray from the ocean, surface movement, and occasionally by heat and excess light stress. Although winter is typically a wet season in Hawaii, winter rainfall is fragmented in southwestern coasts of Maui due to West-Maui Mountains.

capacity to capture and use of the resources that are currently nonlimiting. This would be a wasteful strategy as resources could be redirected from harvesting of non-limiting resources to capture the most limiting resource, thereby enhancing plant performance. In fact, plant performance is optimized when all environmental factors limit plant performance to a similar degree. Of course, such full adjustment to environment is rarely observed in natural communities as environmental factors fluctuate and acclimation is time consuming. Nevertheless, simultaneous adaptation to multiple factors and co-limitation by several factors are frequent in a series of Earth ecosystems (Figures 4 and 5).

Drought Waterlogging

Temperate forest

Mediterranean macchia /

Prairie

Mire forest

Nutrient limitations Arctic-alpine Mire/b^

desert

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