Environmental Engineering Reference
In-Depth Information
moves over the leaf. This is why houseplants have a hard
time surviving the dormant period in heated homes unless
water is added on a frequent basis.
From this it follows that the temperature of the leaf also
becomes a factor that affects transpiration. As the leaf tem-
perature increases, so does the temperature in the leaf air
spaces. Transpiration occurs until the relative humidity of
the leaf air increases to nearly 100%. This explains why, in
arid areas, desert plants have fewer stomata and open them
only during the night, and why, in humid areas, maximum
transpiration occurs when the relative humidity of the air is
lowest and little transpiration occurs when relative humidity
of the air is higher, even though water may be available.
Photosynthesis is at a maximum on sunny days with
relatively cool air temperatures, because the leaf tempera-
ture remains low. The converse also is true. Still, the driving
force for water movement from the leaf to the air is the
water-vapor deficit, or VPD, between the leaf and the air.
As can be seen from the preceding discussion, the need for
plants to open stomata to expose cells to atmospheric CO
2
also results in the loss of water vapor. This balance of water
inputs and outputs can be described in terms of water-use
efficiency,
WUE
, which is the ratio of net photosynthesis, in
grams of CO
2
up taken per gram of transpired water. Desert
plants have higher
WUE
than plants in humid settings.
The age of a tree also affects transpiration. Older trees
tend to have a higher transpiration rate. Tsao (2003) reported
that the transpiration rate for poplars increased with age;
for trees 2, 5, and 30 years, the transpiration rates were a
maximum of 10, 53, and 200 gal/day/tree, (37.8, 200, and
756 L/day/tree), respectively. Two-years-old cottonwoods
had transpiration rates of 3.8 gal/day/tree (14.3 L/day/tree),
and a 19-years-old cottonwood had a rate of 95 gal/day/tree
(359 L/day/tree); for these studies, the source of the water
transpiredwas not delineated. This trend tended to be observed
for other woody plants as well. This observation may be
explained by the increased root density of older plants,
which generally are in contact with additional water sources.
The depth to groundwater also can affect transpiration.
This is more important for plants that have shallow roots
than deep roots when the water table is decreasing, and the
reverse also is true if the water table rises. Gazal et al. (2006)
investigated the transpiration of cottonwood trees growing
along the San Pedro River in Arizona by using sap-flow
methods. Trees growing in areas characterized by different
depths to the water table were compared. Where streamflow
was perennial, groundwater was shallower compared to
deeper groundwater where the streamflow was intermittent.
During drought conditions, the cottonwoods at the intermit-
tent site underwent water stress with little increase in tran-
spiration even though the VPD increased throughout the day.
When recharge from precipitation occurred, however, the
water table rose and these trees increased transpiration.
The hydraulic characteristics of the soil and aquifer
sediments also affect transpiration, as is described in
Chap. 8. The higher the soil porosity the more readily the
soil is accessible by root growth, water infiltration, and air
penetration. Hultline et al. (2006) hypothesized that plants
growing in coarse soils where the water table declined rap-
idly would be more likely to experience xylem cavitation
than plants growing in a more clay-rich soil where the water
table might be less likely to fluctuate. The plants that grow in
coarser soils will be affected more by drought conditions
than those growing in more compacted soils.
3.5.5 Soil-Plant-Atmosphere Continuum
Plants represent an interface between water in the subsurface
and water in the atmosphere. Surface-water bodies, such as
streams and ponds, also link subsurface water with the
atmosphere, but only at specific locations where groundwa-
ter is above land surface. Water used for transpiration and
photosynthesis represents a more widespread transfer of
water from the soil and subsurface reservoir to the atmo-
spheric reservoir. Even during conditions of no flow, plant
biomass represents a standing volume of stored water.
Soil contains at least three major components—solids,
gases, and solutions. The solid is the material that composes
the sediment, such as inorganic sands or organic material
such as peat or lignin. The gases are from the mixing of the
atmosphere in the soil pores spaces with gases generated
abiologically and biologically in the soil itself, such as
methane, CO
2
,orH
2
S. The solution is typically the water
that contains dissolved nutrients and other elements.
The soil is an important part of the plant-water relation
because soil is a source of nutrients and micronutrients and
provides storage for water. We have seen how water in soil
in contact with root hairs enters the plant and travels through
the xylem to be evaporated at the leaf-air interface and
driven by higher to lower water potentials, and how this
removal of soil water into the atmosphere greatly influences
the hydrologic cycle. In fact, this interaction of soil water
with the atmosphere through the water-conducting vessels
of vascular plants has been described generally as the
Soil-Plant-Atmosphere Continuum (SPAC). As was
discussed in the section on plant transpiration, an observable
lag occurs between the removal of water from the leaves and
the uptake of water by the roots, which leads to midday
wilting in plants even when the soil contains water. This
occurs because the plant resists water flow, much like a
copper wire resists the flow of electrons or soil resists the
flow of water.
Up to this point, we have discussed the plant and atmo-
spheric components of the water balance of plants with little
mention of the role that soils play, other than as a system to
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