23. Large-Scale Water Manipulations
tries followed by similar quantities added back with
altered chemistries. Other roofed catchment studies
(Rasmussen et al. 1995; Beier et al. 1995) involve
somewhat smaller manipulation areas (i.e., typically less than 500 m 2 ) and they most often use
100% removal of throughfall as the primary manipulation although the duration of the treatments
is variable. In most cases in which 100% throughfall removal has been applied, attempts are made to
resaturate the soils periodically, especially during
dormant periods.
Pilon et al. (1996) report on the use of a sealed
tarpaulin around the base of individual trees as a
method to eliminate throughfall inputs to the root
zone of trees, but their approach was not consistently effective in the first year of their study, and
the tarpaulin approach had no effect on soil water
content in the second year of their study. They point
out that the presence of a tarpaulin directly on the
soil surface does preclude normal evaporation processes. This is a serious limitation to the ground
tarpaulin approach to throughfall manipulations.
In a recent study of grasslands (Fay et al., in
press), investigators employed a number of permanently installed plastic greenhouse roofs
(9 X 14 m) as simultaneous shields from ambient
rainfall and throughfall collectors. Together with
large rainfall collection tanks, pumps, irrigation
nozzles, and barriers to horizontal rooting and
water movement (7.6 X 7.6 m plots under each
roof), their roofed plots represent a cost effective
approach for manipulating both the quantity and
timing of rainfall events over a 6 X 6 m treatment area.
Verification of Water Treatments
A key feature of any water manipulation experiment should be an adequate and comprehensive
characterization of the soil water status. This is important both from the standpoint of verifying the
treatments applied and for the characterization of
the level of soil moisture stress experience by plants
and microorganisms. Measurements of soil water
status need to have adequate temporal resolution to
cover important seasonal patterns, and for largescale experiments, a clear understanding of the spatial variation in soil water status across the experimental site is essential. While these points may
345
seem obvious, adequate soil water data are not always collected. For example, in the throughfall roof
study conducted at Klosterhede, Denmark (Gundersen et al. 1995), natural throughfall was removed and replacement throughfall was added by
a sprinkler system underneath the roof, and extensive measurements of the artificial throughfall
reaching the forest floor were conducted both temporally and spatially. Unfortunately, neither soil
water content or soil matric potential data were reported leaving us unsure of the soil water status
during the experiment. As a result, direct comparisons of plant, decomposition, and nutrient cycling
responses between the Klosterhede studies (Beier
et al. 1995; Gundersen et al. 1995) and other largescale water manipulation studies are hampered due
to a lack of published data on soil water status.
Future soil water manipulations should include adequate measurement approaches at appropriate spatial and temporal scales to avoid this problem.
Measurement Approaches
Soil water observations can be conducted using a
variety of methods. Destructive gravimetric sampling, neutron probe measurements, and time domain reflectometry can be used for measurements
of soil moisture content. Tensiometers and soil resistance blocks can provide a direct measurement
of soil matric potential with appropriate calibration,
and soil psychrometers can be installed in the field
for a direct measurements of soil water potential.
A detailed discussion of the advantages and disadvantages of each technique can be found in
Chapter 13 and in a previous review by Rundel and
Jarrell (1989). When not directly measured as matric or total soil water potential, measured soil water
content data need to be translated into water potential terms (i.e., soil moisture retention curves) so
that direct intercomparisons among studies can be
facilitated. In many published studies it is difficult
to judge the severity of drought because soil water
content data are often not expressed as matric or
total water potential.
Although destructive gravimetric sampling is often prohibited in an experimental setting where removal of soil over time may compromise the natural state of the soil profile, a certain amount of
gravimetric soil sampling should always be conducted to complement (and if necessary calibrate)
tries followed by similar quantities added back with
altered chemistries. Other roofed catchment studies
(Rasmussen et al. 1995; Beier et al. 1995) involve
somewhat smaller manipulation areas (i.e., typically less than 500 m 2 ) and they most often use
100% removal of throughfall as the primary manipulation although the duration of the treatments
is variable. In most cases in which 100% throughfall removal has been applied, attempts are made to
resaturate the soils periodically, especially during
dormant periods.
Pilon et al. (1996) report on the use of a sealed
tarpaulin around the base of individual trees as a
method to eliminate throughfall inputs to the root
zone of trees, but their approach was not consistently effective in the first year of their study, and
the tarpaulin approach had no effect on soil water
content in the second year of their study. They point
out that the presence of a tarpaulin directly on the
soil surface does preclude normal evaporation processes. This is a serious limitation to the ground
tarpaulin approach to throughfall manipulations.
In a recent study of grasslands (Fay et al., in
press), investigators employed a number of permanently installed plastic greenhouse roofs
(9 X 14 m) as simultaneous shields from ambient
rainfall and throughfall collectors. Together with
large rainfall collection tanks, pumps, irrigation
nozzles, and barriers to horizontal rooting and
water movement (7.6 X 7.6 m plots under each
roof), their roofed plots represent a cost effective
approach for manipulating both the quantity and
timing of rainfall events over a 6 X 6 m treatment area.
Verification of Water Treatments
A key feature of any water manipulation experiment should be an adequate and comprehensive
characterization of the soil water status. This is important both from the standpoint of verifying the
treatments applied and for the characterization of
the level of soil moisture stress experience by plants
and microorganisms. Measurements of soil water
status need to have adequate temporal resolution to
cover important seasonal patterns, and for largescale experiments, a clear understanding of the spatial variation in soil water status across the experimental site is essential. While these points may
345
seem obvious, adequate soil water data are not always collected. For example, in the throughfall roof
study conducted at Klosterhede, Denmark (Gundersen et al. 1995), natural throughfall was removed and replacement throughfall was added by
a sprinkler system underneath the roof, and extensive measurements of the artificial throughfall
reaching the forest floor were conducted both temporally and spatially. Unfortunately, neither soil
water content or soil matric potential data were reported leaving us unsure of the soil water status
during the experiment. As a result, direct comparisons of plant, decomposition, and nutrient cycling
responses between the Klosterhede studies (Beier
et al. 1995; Gundersen et al. 1995) and other largescale water manipulation studies are hampered due
to a lack of published data on soil water status.
Future soil water manipulations should include adequate measurement approaches at appropriate spatial and temporal scales to avoid this problem.
Measurement Approaches
Soil water observations can be conducted using a
variety of methods. Destructive gravimetric sampling, neutron probe measurements, and time domain reflectometry can be used for measurements
of soil moisture content. Tensiometers and soil resistance blocks can provide a direct measurement
of soil matric potential with appropriate calibration,
and soil psychrometers can be installed in the field
for a direct measurements of soil water potential.
A detailed discussion of the advantages and disadvantages of each technique can be found in
Chapter 13 and in a previous review by Rundel and
Jarrell (1989). When not directly measured as matric or total soil water potential, measured soil water
content data need to be translated into water potential terms (i.e., soil moisture retention curves) so
that direct intercomparisons among studies can be
facilitated. In many published studies it is difficult
to judge the severity of drought because soil water
content data are often not expressed as matric or
total water potential.
Although destructive gravimetric sampling is often prohibited in an experimental setting where removal of soil over time may compromise the natural state of the soil profile, a certain amount of
gravimetric soil sampling should always be conducted to complement (and if necessary calibrate)