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Definitions
C3 Plants
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 |
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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