We show that sapflow is a useful tool for studies of water fluxes in forest ecosystems, because (i) it gives access to the spatial variability within a forest stand, (ii) it can be used even on steep slopes, and (iii) when combined with eddy correlation measurements over forests, it allows separation of individual tree transpiration from the total water loss of the stand. Moreover, sapflow techniques are quite easy to implement.

Four sapflow techniques currently coexist, all based on heat diffusion in the xylem. We found a good agreement between three of these techniques. Most results presented here were obtained using the radial flow meter (Granier 1985).

Tree sapflow is computed as sap flux density times sapwood area. To scale up from trees to a stand, measurements have to be made on a representative sample of trees. Thus, a number of trees in each circumference class is selected according to the fraction of sapwood they represent in the total sapwood area of the stand. The variability of sap flux density among trees is usually low (CV. 10–15%) in close stands of temperate coniferous or deciduous forests, but is much higher (35–50%) in a tropical rain forest. It also increases after thinning or during a dry spell.

A set of 5–10 sapflow sensors usually provides an accurate estimate of stand transpiration. Transpiration measured on two dense spruce stands in the Vosges mountains (France) and one Scot's pine plantation in the Rhine valley (Germany) showed that maximum rate was related to stand LAI and to local climate. Preliminary results comparing the sapflow of a stand of Pinus banksiana to the transpiration of large branches, as part of the BOREAS programme in Saskachewan, Canada showed a similar trend.

For modelling purposes, tree canopy conductance (gc) was calculated from Penman-Monteith equation. In most experiments, calculated canopy conductance was dependent on global radiation (positive effect) and on vapour pressure deficit (negative effect) in the absence of other limiting factors. A comparison of the vapour pressure deficit response curves of gc for several tree species and sites showed only small differences among spruce, oak and pine forests when including understorey. Tropical rainforests exhibited a similar behaviour.

Transpiration of trees and forest stands: short and long‐term monitoring using sapflow methods

Axial water flow in the trunks of mature oak trees (Quercus petraea (Matt.) Liebl. and Q. robur L.) was studied by four independent techniques: water absorption from a cut trunk, sap flowmeters, heat pulse velocity (HPV) and thermoimaging. Estimation of the total water flow with sap flowmeters, HPV and water absorption yielded comparable results. We concluded from dye colorations, thermograms and axial profiles of sap flow and heat pulse velocity that, in intact trunks, most of the flow occurred in the current-year ring, where early-wood vessels in the outermost ring were still functional. Nevertheless, there was significant flow in the older rings of the xylem. Total water flow through the trunk was only slightly reduced when air embolisms were artificially induced in early-wood vessels, probably because there was little change in hydraulic conductance in the root-leaf sap pathway. Embolization of the current-year vessels reactivated transport in the older rings.

Axial and radial water flow in the trunks of oak trees: a quantitative and qualitative analysis.

The use of plant indicators may be the ideal method for irrigation scheduling but it is hampered by the dynamic nature of plant water status and by the lack of suitable indicators, relative to established scheduling methods based on atmospheric and soil observations. A study was conducted in an almond orchard located in the San Joaquin Valley of California during the 2001 season using trunk diameter variations as the only indicator for determining the amount of irrigation. The ratio of the maximum daily shrinkage (MDS) of tree trunks relative to a reference MDS, calculated from a relationship between MDS of fully irrigated trees and atmospheric vapor pressure deficit, was used as a signal for modifying the amount of applied irrigation water. Applied water was increased by 10% each time the MDS signal exceeded the prescribed threshold. When the MDS signal went below the threshold, applied water was reduced by 10% in an interactive manner. Two schedules were tested with signal thresholds of 1.75 and 2.75, which generated mild and moderate water stress, respectively, as indicated by their stem water potential (SWP) values. The two irrigation treatments had SWP that varied over the season from around −0.7 to −1.1 MPa and −0.8 to −1.7 MPa, respectively. The two schedules resulted in seasonal water applications of 860 mm for the 1.75 and 525 mm for the 2.75 signal threshold treatments. The grower/cooperator, who based his schedule primarily on SWP measurements but also considered the water balance, applied 900 mm. Estimated crop evapotranspiration was 1,030 mm. The mean coefficients of variation for the two irrigation treatments during the monitoring period were 0.115 and 0.031 for the MDS and SWP measurements, respectively. The stress produced by the irrigation treatments hastened fruit maturation, as evidenced by accelerated hull splitting. This resulted in lower fruit hydration just prior to harvest; 17.3% and 8.0% for the two irrigation schedules, respectively, compared with 27.3% for the grower/cooperator. Based on harvesting selected trees with the same nut load, fresh and dry nut weights in the 2.75 threshold treatment were 9.0% and 10.7% less than those of the 1.75 threshold, which were not significantly different from the results for the grower cooperator. Our results demonstrate that it is feasible to develop an irrigation schedule for almond trees based solely on MDS signals, which may be tailored to any desired stress pattern and be operated in full automation with appropriate software development.

Irrigation scheduling of almond trees with trunk diameter sensors

Transpiration from coppiced poplar and willow measured using sap-flow methods

A new nuclear magnetic resonance imaging (NMRi) method is described to calculate the characteristics of water transport in plant stems. Here, dynamic NMRi is used as a non-invasive technique to record the distribution of displacements of protons for each pixel in the NMR image. Using the NMR-signal of the stationary water in a reference tube for calibration, the following characteristics can be calculated per pixel without advance knowledge of the flow-profile in that pixel: the amount of stationary water, the amount of flowing water, the cross-sectional area of flow, the average linear flow velocity of the flowing water, and the volume flow. The accuracy of the method is demonstrated with a stem segment of a chrysanthemum flower by comparing the volume flow, measured with NMR, with the actual volumetric uptake, measured with a balance. NMR measurements corresponded to the balance uptake measurements with a rms error of 0.11 mg s(-1) in a range of 0 to 1.8 mg s(-1). Local changes in flow characteristics of individual voxels of a sample (e.g. intact plant) can be studied as a function of time and of any conceivable changes the sample experiences on a time-scale, longer than the measurement time of a complete set of pixel-propagators (17 min).

Quantification of water transport in plants with NMR imaging

it provides the means of measuring water uptake by plants under field conditions. The heat pulse method, based on the solution of the convective heat flow equation in a homogeneous medium, can be used to determine heat velocity in plant stems. However, heterogeneity, whether natural or caused by implantation of the sensors in the conducting tissue, results in a measured heat velocity which is always lower than the actual velocity. Therefore, translating the heat velocity into a transpirational flux requires either an experimental calibration procedure or a computational correction of the flow equation. In some cases the calibration procedure is unavoidable. Calibration for several species of trees in lysimeters and on sections of branches in the laboratory produced the same calibration factor, 0.55. Transpirational flux in trees is computed from measured heat velocity, wood physical properties, and the area of water-conducting annulus of stem. In herbaceous plants, however, uncertainties about the water-conducting stem area and its physical properties make it necessary to calibrate for each species. The tests showed that the technique can be calibrated for cotton and soybean. The method produces erratic results with tomato, and its performance with corn has not yet been established.

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M. Fuchs

Papers

Problems in calibrating the heat pulse method for measuring sap flow in the stem of trees and herbaceous plants