Environmental Engineering Reference
In-Depth Information
Between the root hairs and epidermal cells are the less
closely spaced cells of the cortex, or parenchymal cells,
which have air spaces between them. The root cortex is
covered by the epidermis. Pore spaces between cortex cells
permit oxygen to enter from underground sources or to pass
to the cells from above ground. These pore spaces constitute
the largest cellular volume of the root. Roots of plants that
are exposed to standing water or periodic flooding
conditions have adapted to the attendant decrease in oxygen
content by using the cortex tissues to transport atmospheric
oxygen by diffusion to the submerged roots—such plants
include the cypress and mangroves. These adaptations are
especially useful if the surface-water body is characterized
by slow to stagnant flow.
On a larger scale than root-tip elongation, overall root
growth is needed for the plant to reach a volume of soil that
contains sufficient amounts of water to support its needs.
A popular myth is that the distribution of roots below ground
is a mirror image of the distribution of shoots above ground.
In fact, the reality is more interesting, because for some
plants, the below-ground mass of roots is greater than the
shoot mass. For example, the production of root material
exceeded that of above-ground wood production by almost
three times for both pine and hardwood forests in the south-
eastern United States (Harris et al. 1977). In addition, the
distribution of roots with depth for many trees is closer to
one third the height of the tree, because lateral roots can
extend beyond the plant crown. This root distribution probably
results from access to rainwater and higher concentrations
of oxygen in the shallow air spaces in the soil. However,
another important control of depth and lateral distribution is
the type of soil; root systems are able to extend farther
laterally in sandy soils than in clayey soils. This may be
due to the lower permeability of clays and the higher poros-
ity and water content or the lower dissolved oxygen levels.
Most woody plants have the majority of roots close to the
surface of the soil; this can be observed after many species of
trees has fallen over. This root distribution permits plants to
access water from infiltrating precipitation, be exposed to
levels of oxygen near that of the atmosphere in the soil pores,
and access nutrients from decaying leaf litter. It has been
reported that 60-90% of the roots of trees are located in the
upper 1-2 ft. of soil (Le Maitre et al. 1999). However, many
trees have roots that penetrate great depths, as long as signifi-
cant amounts of oxygen are available or can be transported
from the air to the roots, and there is sufficient soil permeability
to allow the roots to elongate. In soils that do not receive
frequent precipitation, or if evaporation from the soil surface
is excessive, the only available water for plant growth may be
near the water table. Therefore, the maximum depth that roots
can penetrate is important in determining a plant's ability to
thrive with a lack of water from frequent precipitation. This
phenomenon is discussed further in Chap. 5.
Much as stems branch off of larger growth above ground,
the same branching also occurs in root systems. These root
branches tend to grow at right angles to the parent root. The
rate of root growth varies between species of woody plants,
from a few hundredths of an inch per day to greater than
almost 1 in./day (a few millimeters per day to greater than
25 mm/day). Essentially the existing root mass remains
attached to the soil matrix and only the tip extends outward
into the pore spaces of the soil matrix. Most root growth, as
elongation, occurs in the spring and continues into the fall
even after shoot growth has stopped. A controlling factor is
the temperature of the soil and the availability of soil
moisture.
Not all parts of root systems interact with water. The
outer epidermal layer of cells, especially on older roots,
can be replaced by a tough, bark-like layer of cells filled
with waxes that are less permeable to water, a process called
suberization. Suberin is the same substance used in the
Casparian strip to block water transport into the vascular
system, as described later in this chapter. Suberized roots
comprise a higher percentage, in some cases more than 99%,
of the total root system in older plants (Kramer and Bullock
1966). Limited water absorption can occur through suber-
ized roots even though the permeability is reduced relative to
unsuberized roots.
Speaking of root age, root longevity varies among spe-
cies, so no exact lifespan for roots can be used to encompass
all woody plants. As would be expected, the longer and
larger the root, the older it typically is. For root hairs,
however, considerable turnover occurs, such that 30-90%
of the root hair mass is replaced annually (Fogel 1983).
The food made by the leaves moves throughout the plant
by the phloem and is stored throughout the above-ground
and below-ground structures. But how does this food
get allocated? Do the stems and shoots receive more carbon
because they are closer to the source than the roots, which
are located farther away? In arid areas where water tables are
deep and precipitation infrequent, more carbon is allocated
to the deeper roots relative to the shallower roots. Snyder
and Williams (2003) reported that the defoliation of such
deep-rooted plants that access the water table also rely on
shallower water and allocate carbon to the root system.
Under such a scenario, the plants become carbon limited,
and it appears that available carbon is shunted to shallower
roots, perhaps because the water present near the deeper
roots cannot be extracted by a decrease in the water potential
gradient after defoliation.
Whereas the maximum height of a tree is constrained by
the physical forces of gravity and friction between water and
xylem, these are not constraints on root penetration below
ground. In fact, McElrone et al. (2004) and Jackson et al.
(1999) measured tree-root penetration up to 20 m (66 ft)
below ground surface and was able to make measurements
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