The experimental site is located in the Caxiuanã National Forest, Pará, Brazil (1°43′3.5′′S, 5°27′36′′W). The forest is a lowland terre firme rain forest. The mean annual rainfall is 2272 mm (± 193 mm), but with a pronounced dry season between July and December, when on average only 555 mm (± 116 mm) of rainfall is recorded (data from 1999 to 2003). The soil is a yellow oxisol (Brazilian classification latosol), with a 0.3–0.4 m thick stony/laterite layer at 3–4 m depth. The soil texture (0.0–0.5 m) is 75–83% sand, 12–19% clay and 6–10% silt (Ruivo & Cunha 2003). The soil consists of mainly kaolin in the clay fraction and quartz in the sand fraction (Ruivo & Cunha 2003). The site elevation is 15 m above river level, and the water table has been observed at a depth of 10 m during the wet season.
To investigate the limitation of soil water on forest gas exchange in drier conditions than those normally experienced, an artificial soil drought was created by using TFE. This work was carried out as part of the LBA (Large-Scale Biosphere Atmosphere Experiment in Amazonia) Ecology program (Avissar & Nobre 2002). Two 100 × 100 m plots, a control and a treatment TFE plot, were established, and the borders trenched to a depth of 1 m to reduce the lateral flow of water. In the TFE plot, a roof of transparent plastic sheeting and wooden guttering was installed at a height of approximately 2 m height in November 2001, to keep the soil free from rainfall.
A 30-m-tall canopy access tower was installed in each plot. Nine trees were accessible from each tower. Of these trees, five in the TFE plot and four in the control plot were equipped with sap flow monitors. The species, canopy heights and diameter at breast height (DBH) were recorded (Table 1). The sap-flow-equipped trees were the tallest trees accessible from the towers, and their leaves ranged from 11 to 28 m in height in the control plot and 16–28 m in the TFE plot. Trees were measured up to the top of the canopy. A meteorological station (Campbell Scientific, Loughborough, UK), installed on a 55-m-tall tower located 700 m from the experimental site, recorded climatic conditions [wet and dry bulb temperatures, rainfall, wind speed and direction, incoming and outgoing photosynthetically active radiation (PAR), short-wave and long-wave radiation] every 15 min.
Table 1. Details of the intensively studied trees equipped with sap flow monitors and accessible from the canopy tower (C trees are the trees in the control plot, T trees are the ones in the TFE plot)
|Tree code||Species||DBH (m)||Ψcrit MPa||Measurement height (m)||Canopy height (m)|
|C1||Mezilaurus mahuba||0.156||−1.9||11|| 6–21|
Tree physiology measurements
Canopy leaf area index (LAI) was measured with an LAI-2000 Plant Canopy Analyser (Li-Cor, Inc., Lincoln, NE, USA) on a 10 × 10 m grid in both plots, in May 2003 and November 2003. The grid covered the full extent (100 × 100 m) of each plot. Diurnal courses of leaf water potential were monitored by using a digital pressure bomb (Skye Instruments, Llandrindnod Wells, UK) on 17 and 19 May 2003 (late wet season) and on 19 and 20 November 2003 (late dry season). On each of these days, four to five leaves were sampled from each of the intensively studied trees at 0600, 0900, 1100, 1300, 1500 and 1630 h, and leaf water potential was determined for each leaf by using the pressure bomb. Sap flow rates were measured for each of the intensively studied trees by using the trunk segment heat balance method (Environmental Measuring Systems, Brno, Czech Republic) (Cermak, Deml & Penka 1973; Cermak, Kucera & Nadezhdina 2004). The heat balance sensors measure sap flow over an entire sector of circumference, therefore they do not require calibration for xylem depth if the sensors (which are 30–50 mm long) penetrate through all of the active xylem tissue. Xylem depth was estimated in wood cores both visually and using dye previously injected below the point of measurement, to confirm that water was not transported beyond 30 mm depth. Xylem depth measurements of 47 trees, which ranged from 0.1 to 1.3 m in diameter, indicated that the xylem rarely extended beneath 20 mm depth, irrespective of tree size (data not shown); therefore, the 30-mm-long sap flow sensors cut through all of the conductive tissue. Water flux was logged every 15 min throughout each day.
The heat balance sap flow measurement method suffers from calibration errors around zero, such that when there is zero flow, a slight positive flow is recorded and a calibration is necessary. This is typically achieved by taking the minimum point over a period of several days and subtracting it from the raw data, so that the minimum becomes the zero point. This method is problematic if sap flow data are used to establish that the trees and soil have reached equilibrium, based on the achievement of zero sap flow during the night. However, if flow continues through the night as the leaves refill, then the flow will constantly decline as the soil–leaf water potential gradient becomes smaller. If flow were to stop altogether, then the apparent flow would be constant. For all of the trees studied the refilling period appeared to last only until between 2200 h and midnight. Thereafter, sap flow values remained constant to within 0.002 kg s−1 cm−1. This constant value was used as the zero point in all cases, and the existence of unchanging sap flow for several hours was used as evidence of the existence of zero flow.
Stomatal conductance was measured using an LI-1600 leaf porometer (Li-Cor, Inc.). Diurnal measurements of the ambient transpiration rates, stomatal conductance and other associated meteorological variables (humidity, photon flux density, leaf and cuvette temperature) were made on the control plot on 27 May 2003 and on 31 October 2003, and in the TFE plot on 23 May 2003 and on 2 November 2003. (The May dates represent the late wet season while the October and November dates represent the late dry season.) Four to five leaves were sampled from each of the intensively studied trees. Measurement times were 0900, 1030, 1200, 1330, 1500 and 1630 h. Prior to 0900 h, very high (> 90%) humidity prevented accurate readings from being obtained from the porometer, because of low transpiration rates. Leaves were not divided into shade and sun leaves, because the sun/shade definition of a mid-canopy leaf changes very frequently as the position of the sun shifts throughout the day. It was assumed that at a given canopy level, all leaves experienced a similar proportion of sun and shade conditions.
Stem psychrometers (Plant Water Systems, Guelph, Ontario, Canada), in conjunction with a manual microvoltmeter (Wescor, Logan, UT, USA), were used to measure the water potential of the xylem at the base of each of the intensively studied trees. We collected these measurements at the same time as the leaf water potential measurements, in order to compare the water potentials of the leaves and stem. Prior to installation, the psychrometer sensors were calibrated against the pressure bomb measurements of leaf water potential. Nine leaves were collected from trees at different levels in the canopy at midday. From each leaf, a piece of the lamina was removed and measured with the psychrometers according to the Wescor protocol, while the water potential of the remaining leaf was measured by using the pressure bomb. After the calibration, we installed the sensors between the height of 0.2 and 0.3 m at the base of the intensively measured trees. The sensors were insulated with a foam with depth of 0.1 m and an aluminium foil radiation shield. This insulation was highly effective at removing temperature gradients between the two thermocouple junctions – the main source of error in psychrometer measurements – and the voltage gradient was never higher than 0.1 µV, which was within the range recommended by the manufacturers.
We measured the ambient hydraulic resistance of the excised segments of terminal branches during November 2002, May 2003 and November 2003 as another means of observing changes in above-ground hydraulic resistance. Four branches were collected from each intensively measured tree over several days. Branches were collected between 1400 and 1500 h to ensure that embolism risk was maximal. The leaves and petioles were removed immediately to prevent further water loss, and measurements were made within 3 h of collection to minimize the effects of cavitation recovery (Zwieniecki & Holbrook 2000). A low-pressure hydraulic resistance measurement system similar to that described by Sperry & Tyree (1988) was used to measure hydraulic resistance. Branch segments were between 0.09 and 0.15 m in length and 10–14 mm in diameter. Leaf area distal to each measured segment was found by measuring the area/mass ratio of a subset of leaves from each branch, using digital photography and Scion Image software (Scion Corporation, Frederick, MD, USA).