Monitoring Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 216

influence that plants have on water uptake as related to leaf
area, should be made. In fact, the component of the total leaf
area, or Leaf Area Index, LAI , has been of interest to plant
physiologists for a number of years. An estimate of the leaf
area is important with respect to time, because it changes as
the plant grows and changes seasonally in response to cli-
mate. Plant physiologists have been interested in LAI as a
measure of forest productivity and overall forest ecology.
For example, LAI is directly related to net primary produc-
tion. From the viewpoint of a hydrogeologist concerned with
implementing a potential phytoremediation project or moni-
toring an existing one, LAI can be used as a measure of the
potential magnitude of plant-water loss and its implication
for change in the site water budget.
The LAI is defined as the ratio of leaf surface area (L 2 )of
a plant or grove normalized to ground surface area (L 2 )
covered by the canopy. Measurements of LAI can be made
using a Plant Canopy Analyzer (Li-Cor LAI-2000, LI-COR
Biosciences TM , Lincoln, NE), which measures the extent of
light interception relative to a standard, which can be a
separate system set up in an open area, if room is available.
LAI can be viewed as the total one-sided green leaf area per
unit ground-surface area. By convention, the LAI of a given
site can vary from a high of 10 down to 1. LAI is one of many
vegetation indices that are dimensionless but indicate rela-
tive abundance. The magnitude of LAI varies with such
factors as the size and spacing of trees; LAI also tends to
increase as trees age. For example, the LAI for young trees
for a given area is near 1 and can approach 10 in dense stands
of mature trees.
Because larger trees have more leaves than smaller trees,
it may be appropriate to plant fewer trees per given area at a
site that will grow larger on account of less competition for
nutrients and water, as long as no decrease in LAI is reached
by using fewer trees. A closer spacing of trees does not
necessarily mean that a denser grove with higher LAI will
be produced in a shorter amount of time, because closely
packed plants often can become 'leggy' as they compete for
position relative to each other to catch the sun's rays. At
some well-established phytoremediation sites that are older
than 5 years, the LAI typically is not constant across the
planted area but varies depending upon the location of the
LAI measurement in relation to the health of the trees,
individual mortality, diseases, or increased leaf drop due to
drought.
Measurements of LAI over time may be useful in
quantifying the increases in ET during the development of
a poplar grove at contaminated sites or in quantifying the
effect of contaminant concentrations on tree health. For
instance, if water becomes limited to plants, they drop
older, yellowed leaves to decrease total transpiration water
losses. Some plants will curl their leaves to reduce the
surface area available to evaporation. In some sense, we

Fig. 9.8 Sap flow increases as stomatal conductance increases, and
there is little difference between flow in young plants and 1-year-old
plants (Modified from Vose et al. 2000). One centimeter is equivalent to
0.39 in.

per second; mmol/m 2 /s) or as a resistance (meter squared per
sec per mol; m 2 /s/ mol).
Factors that affect stomatal conductance were examined
at the phytoremediation site in Fort Worth, Texas (Air Force
Plant 4 described in Chap. 8 and earlier in this chapter).
Higher stomatal conductance was measured during the
spring and declined throughout the season for both whips
and 1-year-old trees (Vose et al. 2000). For the most part,
the stomatal conductance of the whips was higher than the
1-year-old trees during the summer, but the average leaf
conductance was relatively similar across the year at about
0.4 in/s (1 cm/s). As might be expected, the stomatal con-
ductance for leaves near the top of the whips was higher than
for leaves near the ground (Fig. 9.8 ). This relation did not
hold for the 1-year-old plants, however (Vose et al. 2000).
The authors speculate that the relation between stomatal
conductance and height was not due to light competition,
because in the 1-year-old plantation the LAI was relatively
low (less than 2), but was due to leaf drop from lower
branches in response to drought conditions that occurred
during August (Vose et al. 2000). In addition, mean daily
sap flow was directly related to mean daily leaf conductance
(Fig. 9.8 ) (Vose et al. 2000).

9.1.5 Leaf Area Index

The total area of leaves has a significant effect on the amount
of transpiration that occurs. Although transpiration is driven
by meteorological factors such as solar radiation and VPD ,
the removal of water from plants is regulated at the leaf
surface, such as by stomatal conductance. In order to
estimate the effect of a phytoremediation planting on
contaminated groundwater, measurements of the total