Plant Physiological Effects of a Changing Environment

Definitions

C3 Plants

The label for this group is the most basic photosynthetic mechanism. The majority of species globally especially in cooler and wetter environments are of this class. Typical examples include most trees and agricultural crops such as wheat, rice, barley, potatoes, soybeans, and cassava.

C4 Plants

These plants have a special mechanism within their leaves by which they are able to increase CO₂ concentration several times higher than ambient levels. These plants tend to be found in warmer and water-limited environments. Typical examples include many tropical grasses and agricultural crops such as maize (corn), sugarcane, and sorghum.

CAM Plants

This is a variant of C4. These plants take up CO₂ at night for normal photosynthetic processes the next day. Plants in this class grow in deserts such as cacti, but also includes pineapple.

GPP

Gross primary production (GPP) refers to the amount of carbon taken from the atmosphere by plants in the photosynthesis process.

Autotrophic respiration (AR) is the amount of carbon that is respired back to the atmosphere as CO₂ by plants during the photosynthesis process.

NPP

Net primary production (NPP) is the carbon that remains stored in the plant after it has taken up CO₂ from the atmosphere in GPP and respired some back by AR: NPP = GPP - AR

Heterotrophic respiration (HR) refers to the loss of carbon due to death of plants or shedding of plant parts to the soil where soil animals, fungi, or bacteria cause decomposition back to atmospheric CO₂.

NEP

Net ecosystem production (NEP) is the amount of carbon that remains in an ecosystem due to NPP but after HR is accounted for: NEP = NPP - HR

Evapotranspiration (ET) describes the combined effect of evaporation of water from soil and standing water and transpiration of water vapor from plants seeking to lower the temperature of their leaves.

WUE

Water-use efficiency (WUE) is the ratio of difference in CO₂ concentration within the stomata to ambient values to the difference between water-vapor concentration at the same locations. Plants having high WUE are efficient (as far as using water is concerned) at converting atmospheric CO₂ to plant carbon for a given humidity condition. WUE = (C_CO₂A - C_CO₂S)/(C_H₂OS - C_H₂OA) where
C_CO₂S is the concentration of CO₂ in the stomata
C_CO₂A is the ambient concentration of CO₂
C_H₂OS is the concentration of H₂O in the stomata
C_H₂OA is the ambient concentration of H₂O

Biome

A biome is a life zone or biogeoclimatic region that shares a common climate, soil and collection of plant communities. Typical biomes include desert, scrubland, tundra, bog, forest, rainforest, woodland, and grassland (Figure 1).

Carrying Capacity

The carrying capacity of the planet is the maximum population of humans that the planet can accommodate and still supply essential food and other ecosystem services to sustain the population.

Climatic Driving Forces

All living organisms in terrestrial ecosystems ultimately depend directly or indirectly on photosynthesis for their energy requirements. We will explore some climatic driving forces for ecosystem processes and then examine some ecological processes on various scales, including the global where we consider the global production of plant carbon.

Solar radiation, temperature, precipitation, air humidity, and atmospheric CO₂ are the key ambient forces that drive ecosystem processes. Of these, changes in temperature, water availability, and CO₂ levels are subject to change in the next 100 years.

Impact of Temperature Change on Plant Growth and Ecosystems

Plant growth and health may benefit from increased temperatures of global warming in that some regions will experience reduced incidence of damage from freezing and chilling. Plants in other regions may suffer from stress due to elevated temperatures. There is some evidence that extreme events (droughts, floods, high winds, etc.) may accompany global warming, in which case plants may experience isolated highly damaging events.

NPP will generally be increased by moderate increases in temperature estimated to occur in the next 60 years, especially in boreal and mid-latitude regions. Estimates are that NPP will increase 1% per degree C in regions where the mean annual temperature is 30 degrees C and 10% in regions where the mean annual temperature is 0 degrees C. Crop yield will be discussed in a future lecture, but the result is that the regions of reduced yields are reasonably balanced by regions of yield gain.

Impact of Precipitation and Water Availability on Plant Growth and Ecosystems

Plant leaves have small openings called stomata that can be adjusted to regulate the exchange of water vapor and CO₂ with the atmosphere. Plants not under water stress keep their stomata open for optimum CO₂ exchange. Under stress, however, plants close their stomata to restrict water loss. They also may allow their leaves to droop to reduce light absorption or they may shed leaves to reduce water loss. C4 plants have higher WUE than C3 plants. Higher atmospheric CO₂ levels will cause stomata to close slightly, increase WUE, and increase carbon gain for plants with limited water supply. Higher temperatures may lead to higher differences in water-vapor concentration inside and outside the stomata, however, and thereby lead to reduced WUE.

Direct Effects of CO₂

Photosynthetic rates in C3 plants increase by 25-75% for a doubling of CO₂. For C4 plants the data are less conclusive and range from no response to an increase of 10-25%. Results likely are temperature dependent. Increases in CO₂, with accompanying increases in photosynthetic rate and decreased water requirement, translate into increased growth and crop yield in C3 plants, increased growth in C4 plants, and increased tree seedling growth. The response to elevated CO₂ will be most pronounced in regions where water availability is a limiting factor.

The net responses of ecosystems to increases in CO₂, both directly and indirectly through changes in temperature and water availability, are quite complex and only poorly understood. The actual growth enhancements expected in response to gradually increasing CO₂ concentrations are likely to have only a small and gradual impact on terrestrial ecosystems globally.

Soil Processes and Properties

Temperature changes will have only minimal effects on reaction rates for inorganic processes in soils, but changes in soil moisture could have significant effects on rates of diffusion and supply of nutrients to plants.

Carbon Dynamics

The global pool of carbon is available for cycling by natural processes on interannual to centery timescales (i.e., all except fossil carbon) in reservoirs as follows:

Soil	1500 Gt
Aboveground biomass	600-700 Gt
Atmosphere	800 Gt
Ocean	40,000 Gt

Both NPP and organic-matter decomposition likely will increase under increasing temperature. If moisture is readily available, decomposition of organic matter is likely to be enhanced more than NPP under global warming, thereby adding more CO₂ to the atmosphere. However, if moisture becomes more limiting then decomposition will be reduced. Models that take both temperature and moisture into account suggest that increased NPP would lead to increases in soil carbon under increasing atmospheric CO₂.

Land use is a much more important factor than changes in NPP for determining soil carbon. Typically about half of the native carbon is lost from soils when they are put under cultivation over a period of 50-100 years. Minimum tillage practices reduce carbon loss from soils.

Soil Biodiversity

Climate change, specifically changes in temperature and water availability, could change soil microbial and faunal populations, but changes in land-use practices are likely to have much greater impact. However, another element of global change, namely increased deposition of nitrogen from industrial NOX emissions, is being more widely associated with major losses of fungi in the root zone in some (particularly forest) biomes.

Ecological Processes

Organisms interact with their physical environment and with other organisms to form a complex set of dependencies and interrelationships sometimes called the "web of life". This interconnectedness makes the study of impacts of changes in external factors on ecosystems very difficult. The combinations of all environmental factors and interactions with other organisms determine the preferred places for each organism to live, i.e., its "niche". Some niches are more vulnerable to climate change than others.

Interactions within ecosystems include competition, herbivory, and actions of parasites, disease, and mutualists (ecosystem components that provide mutual benefit such as pollinating bees and flowering plants).

Communities and Community Dynamics

The collection of different species that interact in a variety of ways in a defined patch of land is called a community. Communities are always changing and are subject to "succession", which may be a complete changeover to another collection of plants or a more incremental series of species losses and gains. Loss of one species may provide opportunities for changes in populations of existing species or gain of new species. Communities may migrate and disperse as their environmental conditions change. The rate of change compared to the ability of the community to move determines whether the community will survive under such changing conditions.

Ecosystems and Biomes

The community and its abiotic environment constitute an ecosystem. The biotic and abiotic components may have significant interactions. A biome is a life zone or biogeoclimatic region that shares a common climate, soil and collection of plant communities and hence ecosystems. Typical biomes include desert, scrubland, tundra, bog, forest, rainforest, woodland, and grassland.

Ecosystem Breakdown

Models are used to study the interactions within and between ecosystems. Such studies ultimately are useful to determine the vulnerability of ecosystems to break-down due to loss of some species or invasion by others. This may impact the functioning of the ecosystem in terms of its ability to efficiently use water, light, and nutrients in the production of plant carbon.

Figure 2 shows the changes in natural vegetation for the US as simulated by two different vegetation models under a doubling of atmospheric CO₂. Maps on the right consider both the climatic and physiological effects of the enhanced CO₂, whereas the maps in the center consider only the climatic effects.

Figure 3 gives the change in terrestrial carbon storage simulated for the US under a doubling of atmospheric CO₂ for various combinations of climate models, biogeochemistry models, and vegetation models.

Calculate Your Ecological Footprint

The choices we make in defining our individual lifestyles determine the amount of productive land and water required to support this demand. The Ecological Footprint Quiz created by the EarthDay Network uses 15 easy questions to provide an estimate of the productive land and water required to support your lifestyle. Take the quiz and see how you rate compared to others in this country and elsewhere. Take the quiz more than once by changing some choices to see where you could do better in minimizing your footprint.

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2-15: Plant Physiological Effects of a Changing Environment