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Climate
Meteorological measurements were made at the study site using an automated weather
station consisting of a data logger (CR10, Campbell Scientific Ltd., Leicestershire,
England), a temperature and relative humidity probe (MP300, Campbell), a tipping bucket
rainguage (ARG100, Campbell), a quantum sensor (SKP215, Campbell) and a wind monitor
measuring both speed and direction (Campbell) all mounted 2m above ground level.
Instruments were situated in an open site 50m from the edge of the lake, approximately 600
m from the location of the Eddy covariance equipment. The measurement interval for each
sensor was set according to manufacturers recommendations and the data average recording
interval was set to 15 minutes to coincide with Eddy covariance measurements.
Ten year, monthly mean records of maximum, mean and minimum temperatures and solar
radiation were also available from a weather station located 10 km from the study site.
These have been reported by Muthuri (1985) and Muthuri et al. (1989). |
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Eddy covariance
Ecosystem fluxes of CO2, H2O, momentum, sensible and latent heat were measured using
the Eddy covariance technique . The system used consisted of commercially available
instrumentation: a 3 axis symmetrical sonic anemometer (Solent A1012r, Gill Instruments,
Lymington, UK), an infra-red gas analyser (IRGA Li 6262, Li-COR, Lincoln, Nebraska) in
closed path mode and custom written software (EddySol, University of Edinburgh)
which calculated the fluxes in real time, performing complex co-ordinate rotation and flux
averaging as required. A full description of the equipment and EddySol software used in
this work is given by Moncrieff et al. (1994), Grace (1995) and Moncrieff et al. (1997).
Eddy covariance measurements were made at a height of 7 m above ground level with air
drawn at 6l (litres) min-1 down to the IRGA. This was 2-3 m above the tallest papyrus
culms and gave a flux footprint in which 95% of the footprint area was contained within
the region of homogenous papyrus canopy (Schuepp, 1990). The mast was located on a
concrete base (approx. 3 m x 3 m) at a distance of 150 m from the landward edge of the
papyrus swamp. Averaged fluxes were recorded at 15 minute intervals for an initial 4 week
period from 2 August, 1995 followed by a second period of one week starting on 11 March,
1996. These periods corresponded with, respectively, the coolest (monthly mean @ 15.0° C
) and warmest (monthly mean @ 17.9° C ) times of the year at the experimental site
(Muthuri, 1985 and Muthuri et al., 1989).
Span calibration of the CO2 and H2O channels of the IRGA was performed at 3 day
intervals using chemical absorption columns for zero values and a compressed gas source of
CO2 at 600 m mol mol-1 + 10 m mol mol-1 (Linde Gas, England) and a portable dew point
generator (Li-610, Li-COR) for span values. |
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Organ gas exchange
(i) Bracteole photosynthesis
Initial characterisation of papyrus bracteole photosynthesis was performed by Jones
(1987), Jones (1988), Jones and Muthuri (1984, 1985). In addition a CIRAS infra red gas
analyser (PP Systems, Hertfordshire, England) with a Parkinson leaf chamber was used tmake
bracteole CO2 response measurements on
plants at the experimental site. Measurements were made on 20 bracteoles from different
stand members throughout a 3 day period to establish their A/Ci response (Long and
Halgren, 1993).
(ii) Tissue Respiration
Papyrus culms show a range of developmental stages which have been classified
previously according to age and nutritional status . Total above ground respiration was
estimated from measurements of respiration rates of material from each of the culm age
classes which included juvenile, mature, senescent and standing dead. These tissue
respiration rates were determined by modifying a soil respiration chamber (SRC-1, PP
Systems, Hertfordshire, England), used in conjunction with an IRGA (CIRAS, PP Systems), by
the addition of an air-tight plate to the bottom of the chamber. This allowed the
insertion and sealing of cut plant sections into the chamber and determination of
respiration rate by monitoring the rate of increase of CO2 within the chamber (Parkinson,
1981). Respiration on a dry matter basis was calculated after oven-drying the samples to
constant weight. (iii) Detritus Respiration The soil respiration chamber was used to
measure respiration of level areas of detritus at 20 locations around the study site. Dry
and dead papyrus umbels and culms were first cleared from the sampling area and the
chamber was pushed firmly down into the detritus so that the stainless steel perimeter
ring was partially imbedded to give a seal. Measurements were repeated three times at each
location. Detritus temperature was 18°C at the time of measurements which was middle to
late afternoon. It should be noted that because the soil contained living root and rhizome
material the measurements made by the chamber included respiration from both detritus and
root/rhizome fractions. Standing biomass Above ground standing plant biomass was measured
in five 3 m x 3 m quadrats placed at random at the study site. The quadrats were sampled
by measuring the basal diameter of each shoot or 'culm unit' (culm & umbel) within the
quadrat. Dry weights of the 'culm units' were derived from a linear regression previously
fitted to a log culm girth and log dry weight relationship for a wide range of culm-unit
sizes by Jones and Muthuri (1985).
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