Images Biogeoclimactic regions for typical biomes. Vegetation model simulation of changes in potential natural vegetation in the US. Simulated changes in equilibrium terrestrial carbon storage in the US.

Much of this information has been taken from: Watson, Robert T., Marufu C. Zinyowera, Richard H. Moss, 1996: Climate Change 1995, Impacts Adaptations and Mitigation of Climate Change: Scientific-Technical Analyses. Cambridge University Press. 879 pp. The lecture on plant physiological effects of a changing environment addresses the impact of various influences, particularly climate change, on an individual plant. In this lecture we overview some of the interrelations among plants in plant communities and ecosystems and how they might be affected by climate change. Some Definitions Different plant groups 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 such plants as pineapple. GPP Gross primary production (GPP) refers to the amount of carbon taken from the atmosphere by plants in the in photosynthesis process. AR 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 HR 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 ET 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 = (CCO2S - CCO2A)/(CH2OS - CH2OA) where CCO2S is the concentration of CO2 in the stomata CCO2A is the ambient concentration of CO2 CH2OS is the concentration of H2O in the stomata CH2OA 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 (see this image). 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. Introduction All living organisms in terrestrial ecosystems ultimately depend directly or indirectly on photosynthesis for their energy requirements. To review the essentials of the photosynthesis process, check the source on "how plants make food" 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. Climatic Driving Forces 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 1 degree C in regions where the mean annual temperature is 30 C and 10% in regions where the mean annual temperature is 0 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 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 Concentration 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 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 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. The accompanying figure 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. The next figure 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.

Transcription by Theresa M. Nichols