Principles of terrestrial ecosystem ecology.pdf

96 4. Terrestrial Water and Energy Balance
somewhat higher in crop systems. Climate influences
evapotranspiration by determining the
driving forces for evapotranspiration and the
water availability in the soil, which determines
stomatal conductance. Vegetation influences
evapotranspiration through plant height and
aerodynamic roughness, which govern boundary
layer conductance, and through stomatal
conductance, which influences surface conductance
and the plant response to soil moisture.
The partitioning of water loss between evapotranspiration
and runoff depends primarily on
water storage in the rooting zone and the rate
of evapotranspiration. Runoff is the leftover
water that drains from the ecosystem at times
when precipitation exceeds evapotranspiration
plus increases in water storage. Human activities
alter the hydrologic cycle primarily through
changes in land cover and use, which affect
evapotranspiration and soil water storage.
Review Questions
1. What climatic and ecosystem properties
govern energy input to an ecosystem?
2. What are the major avenues by which
energy absorbed by an ecosystem is
exchanged with the atmosphere? What
determines the total energy exchange? What
determines the relative importance of the
pathways by which energy is exchanged?
3. What are the consequences of transpiration
for ecosystem energy exchange and for the
linkage between energy and water budgets
of an ecosystem?
4. How might global changes in climate and
land use alter the components of energy
exchange in an ecosystem?
5. What are the major pathways of water
movement in an ecosystem? What determines
the balance among these pathways,
for example, between evaporation, transpiration,
and runoff? How do climate, soils,
and vegetation influence the pools and
fluxes of water in an ecosystem?
6. What are the mechanisms driving water
uptake and loss from plants? How do plant
properties influence water uptake and
loss?
7. How do the controls over water loss from
plant canopies differ from the controls at the
level of individual leaves?
8. Describe how grassland and forests differ
in properties that influence wet canopy
evaporation, transpiration, soil evaporation,
infiltration, and runoff. What will be the
consequences for runoff and for regional
climate of a policy that encourages the
replacement of grasslands with forests so as
to increase terrestrial carbon storage?
Additional Reading
Campbell, G.S., and J.M. Norman. 1998. An Introduction
to Environmental Biophysics. Springer-
Verlag, New York.
Dawson, T.E. 1993. Water sources of plants as determined
from xylem-water isotopic composition:
Perspectives on plant competition, distribution,
and water relations. Pages 465–496 in J.R.
Ehleringer, A.E. Hall, and G.D. Farquhar, editors.
Stable Isotopes and Plant Carbon-Water Relations.
Academic Press, San Diego, CA.
Jarvis, P.G., and K.G. McNaughton. 1986. Stomatal
control of transpiration: Scaling up from leaf to
region. Advances in Ecological Research 15:1–49.
Jones, H.G. 1992. Plants and Microclimate: A Quantitative
Approach to Environmental Plant Physiology.
Cambridge University Press, Cambridge, UK.
Kelliher, F.M., R. Leuning, M.R. Raupach, and E.-D.
Schulze. 1995. Maximum conductances for evaporation
from global vegetation types. Agricultural
and Forest Meteorology 73:1–16.
Monteith, J.L., and M. Unsworth. 1990. Principles of
Environmental Physics. 2nd ed. Arnold, London.
Oke, T.R. 1987. Boundary Layer Climates. 2nd ed.
Methuen, London.
Schulze, E.-D., F.M. Kelliher, C. Körner, J. Lloyd, and
R. Leuning. 1994. Relationship among maximum
stomatal conductance, ecosystem surface conductance,
carbon assimilation rate, and plant nitrogen
nutrition: A global ecology scaling exercise.
Annual Review of Ecology and Systematics
25:629–660.
Sperry, J.S. 1995. Limitations on stem water transport
and their consequences. Pages 105–124 in B.L.
Gartner, editor. Plant Stems: Physiology and Functional
Morphology. Academic Press, San Diego,
CA.
Waring, R.H., and S.W. Running. 1998. Forest Ecosystems:
Analysis at Multiple Scales. Academic Press,
New York.