3.5 Conclusions
5.2.2 Sap flow measurements with the TDP method
From the clump edge of B. vulgaris, we selected three pairs of established and attached freshly sprouted culms during the pre-leafing period and installed
82 Water transfer between bamboo culms in the period of sprouting TDP sensors on each of them (Fig.5.1). Interconnectivity between established and freshly sprouted culms was verified by partially removing the topsoil and directly observing the rhizomes. On each culm, three TDP sensors (1 cm length) were inserted into the culm walls at the breast height. Each TDP sensor consisted of a heated and an unheated reference probe, and the heating probe was installed 10 cm above the reference probe. The heating probe was powered by 120 mA (Mei et al.,2016). The temperature differences between the two sensors (recorded as voltage signals) were used for calculating sap flux density (Js, g cm−2h−1). To capture potentially occuring circumferential variability of water use in bamboo culms, we installed three pairs of TDP evenly around the culm. The signals from the three sensors were averaged connecting the signal wires in parallel. Voltage signals were recorded every 30 seconds and averaged every 10 minutes with data loggers and multiplexers (CR1000, AM16/32, Campbell Scientific Inc., USA). The TDP voltage signals were used to calculate Js with the Granier’s original TDP sap flux equation (Granier, 1987), which were subsequently corrected with available species-specific calibration parametersMei et al.(2016). Sap flow rates (SF) per hour (g h−1) were derived by multiplying correctedJsby the cross-section area of the culm walls at breast height. Daily SF was obtained by summing up the hourly SF of a day.
To measure Js and detect direction of sap flow in the rhizome between each pair of culms, we built self-made, modified TDP sensors with three probes instead of two. This modified TDP consisted of one central heating and two unheated reference probes. The heating probe was installed at the mid-point of the rhizome between the culms, and the two reference sensors were installed at 5 cm distance from the heated probe, one on each side. The temperature differences between each reference probe and heated probe were recorded as voltage signals and stored in the same way as described above for the "standard" TDP sensors for measurements of culmJs. In the analysis, the two derivedJswere compared, and the reference probe with the lowerJs was assigned the downstream position (to determine direction of flow). This was based on the assumption that sap flow brought the heat energy to the downstream sensor and that this heat would increase the temperature of the downstream sensor. In this case, the signal value from the downstream sensor was smaller than that from the upstream sensor. To verify this assumption, we simulated the heat field around the heating sensor and the two reference sensors under different sap flow density with the ANSYS model (Academic version, CFX 17.0, ANSYS Inc., Pennsylvania, USA). TheJsof the rhizome
5.2 Materials and methods 83 Table 5.1 The information of studied bamboos and trees (3 culms per species but 4 culms ofB. vulgarisused). The adjusted WUD2Owas WUD2Oadjusted with deuterium transfer on bamboos. The values were represented as the means(SD) which were in parenthesis.
Species Height (m) DBH (cm) D2O injected Tarrival Tresidence B. vulgaris 17.9(0.8) 7.0(0.4) 5.8(1.6) 1.5(1.0) 5.5(1.3)
D. asper 21.1(0.9) 10.7(0.9) 8.0(1.1) 1.0(0.0) 5.0(1.7) G. apus 16.2(2.7) 7.9(1.1) 5.7(0.1) 1.0(0.0) 6.3(0.6) was calculated with the same calibrated formulas as for culms, and hourly SF was derived as the product ofJs and the cross-section area of the rhizomes.
As for culms, daily SF of rhizomes was obtained by summing up the hourly SF of a day. The contribution of rhizomes to the freshly sprouted culms was calculated as a ratio of daily SF of the rhizomes to those of the freshly sprouted culms.
To explore how freshly sprouted culms were influenced when established culms were removed, we cut and removed all established culms in one clump and kept them in a different clump ofG. apus. Culm Js of the left freshly sprouted culms was monitored by TDP in the same way as described above.
The monitoring was conducted on five culms with an initial height of around 2 meters; it stopped after about one month due to the falling of the culms.
Until then, the culms had reached 5 - 8 meters in height without any leaf development.
5.2.3 Deuterium tracing
To explore if there was water transferred from established culms to freshly sprouted culms (Fig. 5.1C), we conducted a deuterium tracing experiment on three clumpy bamboo species (B. vulgaris,G. apusandD. asper) in the leafing and well-leafed periods of freshly sprouted culms. From the edge of the clumps, we selected four pairs of established and attached freshly sprouted culms forB. vulgarisand three pairs forG. apusandD. asper(Table5.1). In the morning (6:00 - 7:00) on 8th March 2013, we injected deuterium oxide (D2O, 99.90% D, euroiso-top, Gif sur Yvette, France) into the established culms at a height of≈50 cm. Before labeling, we glued four plastic tubes on each culm and filled them with 40 mM KCl solution. We then drilled holes into the tubes and replaced the solution with 1.5 - 3 g D2O. The tubes were
84 Water transfer between bamboo culms in the period of sprouting refilled with KCl solution several times. The residual D2O in the tubes was collected with syringes after sunset.
Sampling was conducted on both the labeled established culms and the attached freshly sprouted culms. On each labeled established culm, we sampled from five regions of the crown, sealing five leaves each with one transparent plastic bag per leaf. The leaf condensate water from the leaves of one culm was collected and mixed at around 7:00 am every day of the first ten days and every 4 - 6 days for three weeks after that. On each attached freshly sprouted culm, we drilled holes on the culm and extracted water from the holes at around 6:00 in 3 - 5 day intervals. Additionally, after about 40 days since the deuterium labeling, all labeled established culms were harvested, and water samples were obtained from leaves, branches, culms and rhizomes. All water samples from both the labeled established culms and the attached freshly sprouted culms were kept in 1.5 ml glass vials which were stored in a refrigerator .
The water samples were analyzed in the Center for Stable Isotope Re-search and Analysis (KOSI) at the University of Göttingen, Germany. The analysis was conducted with a high-temperature conversion/elemental analyzer coupled via a ConFlo III interface to a Delta V Plus isotope ratio mass spec-trometer (Thermo-Electron Cooperation, Bremen, Germany). The deuterium enrichment (δD,h) with 2hprecision was derived as below (Coplen,1995):
δD= ( Rsample
RV SMOW −1)×1000 (5.1)
WhereRsampleis D/H in the water samples whileRVSMOWis D/H in the Vienna Standard Mean Ocean Water.