Subject: transcript for vegetative response
From: [email-masked] (Gene Takle)
To: [email-masked]
Date: Mon, 31 Mar 1997 09:16:47 -0600

Matthew:

Here is the transcript

gene
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Vegetation Responses to a Changing Environment
Eugene S. Takle
@ 1997

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 {{
http://www.iitap.iastate.edu/gcp/datasets/images/ecosystems.gif}}

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"
{{http://www.whitman.edu/Departments/Biology/classes/B111/B111OutlinesPhotoF
ood.html}}.  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.

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+                                                                +
+  Eugene S. Takle                                               +
+  Professor of Atmospheric Science                              +
+  Professor of Agricultural Meteorology                         +
+  Coordinator, Environmental and Earth Science Thrust Area      +
+  International Institute of Theoretical and Applied Physics    +
+  3010 Agronomy Hall                                            +
+  Iowa State University                                         +
+  Ames, IA  50011  USA                                          +
+  Tel: (515) 294-9871                                           +
+  Fax: [515] 294-3163                                           +
+  e-mail:  [email-masked]                                  +
+                                                                +
+  Global Change:  Our Environment in Transition                 +
+  http://www.physics.iastate.edu/gcp/gcp.html                   +
+                                                                +
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