Sap Flow Sensors Research Articles

592 Use of Infrared Thermometry and Heat-pulse Technique for Detection of Water Stress in Apple Trees

The recent development of small portable infrared thermometers has made canopy temperature an easily measured characteristc in the field. Our objective was to correlate a reduction of soil water with foliage temperature and to compare it with other indicators of plant stress (Pn, E, gs, leaf expansion, sap flow). During Summer 1998, we evaluated the responses of potted apple rootstocks (cultivars Budagowski 9, M9, and Mark) to soil water deficit. Irrigation was withheld for 7 days, and the canopy temperature (Tc) was measured daily with an infrared camera. Tc was always higher than air temperature (Ta). Tc between control and stress plants began to differentiate from day 3. In Mark, this difference was maintained until the end of the experiment. However, gas exchange in Mark seemed to be less affected by the stress than in the other two cultivars. At day 7, midday stomatal conductance (gs) was 38.0, 32.3, and 72.0 mmol·m–2·s–1 in Budagowski 9, M9, and Mark, respectively (control values varied between 161.6 and 164.3 mmol·m–2·s–1 for all the cultivars). Heat-pulse sapflow sensors installed on Mark indicated that the speed of the xylem sap was affected by the stress from day 4 (19-26 cm/h for the controls vs. 15–21 cm/h for the stressed plants). Specific details on the physiological data will be presented.

Temporal patterns of water flux in trees and lianas in a Panamanian moist forest

Using constant heat sap flow sensors, xylem water fluxes in ten tree species and two liana species were monitored for 5–10 days during the beginning of the wet season in May, 1993. For a subset of the trees, a branch was also monitored at the top of the crown for 5 days. Xylem flux (JS) was related diurnally in all plants to vapor pressure deficit (D) measured within the upper-third of the canopy, and to incoming shortwave radiation RS above the canopy. Cross-correlation analysis was used to estimate time lags between diurnal patterns of JS and D or RS, and between JS in stems and branches. The maximum correlation coefficient from cross-correlation of JS with RS (range=0.57–0.92) was often higher than the maximum of JS with D (range=0.43–0.89), indicating that diurnal JS was more dependent on RS than D. Time lags (lag corresponding to maximum correlation) of JS at stem-base with D was shorter (0–45 min) than with radiation (5–115 min), highly variable within a species, and uncorrelated to the height or exposure of tree crowns or liana in the canopy. On a stand level, not accounting for the diel lag between stem sap flux and canopy flux resulted in errors in estimated canopy transpiration of up to 30%.

Limitations of a compensation heat pulse velocity system at low sap flow: implications for measurements at night and in shaded trees.

Unlike an ideal system, the return time to thermal balance (t(b)) between upstream and downstream thermistors, as measured by the (compensation) heat pulse velocity method, effectively depends on the heat input and the water content of the wood at zero and low sap flow. Even when these factors were held constant and ambient temperature was stabilized, a twofold variation in t(b) at zero flow was observed within and among Greenspan Technology sensors implanted in wooden posts, making it impossible to distinguish zero flow from low sap velocities (< 0.01-0.02 mm s(-1)). This limitation has serious consequences because the contribution of low flow rates to water movement is important during both daytime and nighttime in tropical understory and overstory trees. Measurements in an artificial flow system showed that this technical limitation is exacerbated by erratic variation in sensor response at both zero and low flow rates. The limited sensitivity of the tested sap flow sensors may be caused by their poor thermal contact with wood. Interim procedures are suggested for estimating minimum detectable sap flow and delimiting the hydroactive zone until the sensitivity and interchangeability of sap flow probes are improved.

A Preliminary Evaluation of the Suitability of Sap Flow Sensors for Use in Scheduling Vineyard Irrigation

An experiment was carried out to evaluate the potential for the use of sap flow measurement for scheduling irrigation in vineyards. Sap flow was measured over a 35-day period on 5 different varieties of grape vines (<i>Vitis vinifera</i>), with canopy leaf areas ranging from 3.4 to 16.4 m². Irrigation was withheld from some vines to investigate whether sap flow sensors could be used to detect effects of water deficits on transpiration rates. Data obtained from sap flow sensors under well-watered conditions showed differences in transpiration rates caused by differing canopy sizes and changing evaporative demand due to diurnal and daily changes in meteorological conditions. When transpiration rates were expressed on a leaf area basis, mean transpiration from irrigated and non-irrigated vines were initially similar. With time, transpiration from non-irrigated vines became less than that from the irrigated vines. Also differences in diurnal patterns of sap flow became evident, with significantly more sap flow found at night for non-irrigated vines than for irrigated vines. Thus sap flow sensors were able to detect differences in both the timing and amount of water used by irrigated and nonirrigated vines and hence have potential application in irrigation scheduling.

Use of Sap-Flow Sensors to Schedule Vineyard Irrigation. I. Effects of Post-Veraison Water Deficits on Water Relations, Vine Growth, and Yield of Shiraz Grapevines

Different levels of irrigation based on transpiration data measured using sap-flow sensors were applied after veraison to Shiraz grapevines grown on a two-wire vertical trellis in the Barossa Valley, Australia. Data for the vines of the upper and lower wire were studied separately. The intensity and duration of stress in each treatment was expressed as a water stress integral (Sψ) from the predawn leaf water potentials. Leaf area, yield and post-veraison water use of vines in the different treatments was closely related to Sψ. Irrigation increased grape yields and differences in vine water status after veraison led to differences in the leaf area to fruit weight ratio. It was concluded that data from the sap-flow sensors could be used as a basis for calculating irrigation amounts to influence vine water status, canopy size, and grape yield.

Use of Sap-Flow Sensors to Schedule Vineyard Irrigation. II. Effects of Post-Veraison Water Deficits on Composition of Shiraz Grapes

Transpiration rates of Shiraz vines grown on a two wire vertical trellis were measured using sap-fow sensors and used as a basis for applying different levels of post-véraison irrigation. The effect of this differential irrigation on plant water status and grape composition were investigated with responses of vines on the upper and lower wires of the trellis studied separately. The concentration of juice total soluble solids and pH was unaffected by irrigation treatment but the concentration of berry anthocyanins and phenolics, expressed as mg/g berry mass and absorbance units/g berry mass respectively, decreased with increase in water application, mainly due to an increase in berry weight. Predawn leaf water potential was measured from veraison to harvest and the water stress integral (S_ψ) was calculated as an indication of the intensity and duration of water stress in the three irrigation treatments. S_ψ was found to be closely correlated with post-veraison water use and with grape composition.

HEAT UNIT-BASED CROP COEFFICIENT FOR GRAPEFRUIT TREES

The onset and rate of sap moving up the branches of grapefruit (Citrus paradisi Macfadyen) trees weremonitored hourly using portable sap flow sensors at Waddell, Arizona. Hourly reference evapotranspiration (ETo)estimates were calculated using data from a nearby weather station. Crop water use was estimated from soil moisturemeasurements using a neutron probe. These data were used to first delineate the upper and lower temperature thresholdvalues for the determination of heat units. A heat unit-based crop coefficient was then derived from a correlation of thecrop coefficient with heat units over a crop year. The heat unit-based crop coefficient was found to be similar to cropcoefficients derived by other researchers.

Variation of sapflow velocity in Eucalyptus globulus with position in sapwood and use of a correction coefficient.

Sapflow sensors were used to investigate variation in sapflow velocity at different positions in the sapwood of three-year-old Eucalyptus globulus ssp. globulus Labill. trees. Sapflow velocity was measured at 5-mm intervals across the sapwood by moving two probe sets simultaneously on two opposite radii. Another probe set placed in a fixed position at right angles to the first two sets acted as a control. A sapflow velocity ratio was defined as the velocity given by each moving sensor divided by that given by the static sensor. Correlation between observations of sapflow velocity at different positions exceeded 95%, and the ratio of velocity between any pair of sensors was constant. We observed radial variation in sapflow velocity across the sapwood with the lowest velocities at the center of the tree. Variation due to sensor position was high implying the need for large numbers of sensors for accurate estimates of sap flux. To overcome this need, we used a correction coefficient, namely a simple weighted average of the sapflow ratios with depth in the sapwood, for each fixed sensor. We recommend the use of three probe sets to estimate the correction coefficient. Subsequently, two probe sets can be placed at two fixed positions for routine measurements of sap flux.

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

AbstractWe 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.

Three Methods for Monitoring the Gas Exchange of Individual tree Canopies: Ventilated-Chamber, Sap-Flow and Penman-Monteith Measurements on Evergreen Oaks

Functional Ecology 1994 TECHNICAL REPO RT Three methods for monitoring the gas exchange of individual tree canopies: ventilated-chamber, sap-flow and Penman-Monteith measurements on evergreen oaks M. L. GOULDEN*H and C . B. FIELDt *Department of Biological Sciences, Stanford University and tcarnegie Institution of Washington, Department of Plant Biology, Stanford, California 94305, USA Summary I. Physiological methods applicable to scales between individual leaves and whole forests have the potential to improve substantially our understanding of ecosystem gas exchange. 2. We compared three approaches for determining the canopy gas exchange of individuals representing a pair of mediterranean-climate oak species. 3. We estimated transpiration from the Penman-Monteith equation, measured sap flow with heat-balance sensors, and also measured net C0 2 assimilation, transpir- ation and conductance with a whole-canopy gas-exchange system. 4. Simultaneously measured sap flow and chamber transpiration were qualitatively similar, provided that the sensors were designed to compensate for thermal gradients along the tree trunk. Both in situ and bench-top measurements indicated that the quantitative relationship beween transpiration and the signal from the sap-flow sensor varied among stems. The sap flow of individual trees measured on consecutive days with the tree in the chamber 1 day, and out the next, was similar, indicating that enclosure had only a small impact on transpiration. Total daily sap flow, which was similar during atmospherically moist period to the Penman- Monteith transpiration calculated assuming a fixed stomata! conductance, became almost insensitive to further increases in evaporative demand during hot and dry intervals. S. While the application of each approach is limited by experimental considerations, these shortcomings may be overcome by using the techniques in combination. Key-words: Conductance, evaporation, Quercus agrifolia , Quercus durata, transpiration Fu11ctio11al Ecology (1994) 8, 125-135 Introduction Researchers need a better understanding of the role plants play in controlling the flux of water vapour and carbon dioxide between forests and the atmosphere (Jarvis & McNaughton 1986; Committee on Global Change 1990). Achieving this understanding will require an interdisciplinary effort to link leaf-level physiology with canopy measurements based on micrometerological techniques. One possible strategy is to scale gas exchange directly from the leaf to the canopy, bypassing explicit consideration of :j: Present address: Dr M. L. Goulden, Division of Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA. processes that occur at the whole-plant level (Baldoc- chi, Luxmoore & Hatfield 1991; Baldocchi 1993). While this approach holds promise, it precludes the possibility of drawing upon a body of theory, derived from community (Tilman 1988) and evolutionary (Bloom, Chapin & Mooney 1985) studies, that is based on the response of the individual to the environment. In contrast, approaches that explicitly consider whole-plant processes have the potential to explore the consequences of plant-to-plant variation in physiological characteristics and access to resources, as well as capitalize on possible simplifi- cations resulting from physiological integration at the level of the individual. Unfortunately, past progress in this direction has been limited by the unavailability

TRANSPIRATION OF A POTTED ROSE PLANT USING A HEAT BALANCE STEM FLOW GAUGE

Recently developed stem flow gauges that allow for direct, accurate, non-invasive, and continuous measurement of plant sap flow rates have not been used to monitor transpiration of floricultural plants grown in greenhouses.A Dynamax SGA10 heat-balance sap-flow sensor was mounted on a potted rose plant's main stem containing a total leaf area of 0.52 m in order to monitor transpiration. The sensor was connected to a CR21X Micrologger for data calculation and temporary storage. The results showed average midday sap-flow rates range from 20-30 g·hr-1 to 50-70 g·hr-1 at low and high levels of PPF, respectively. Nighttime levels of 4-7 g·hr-1 persisted throughout early winter trials. Monitoring transpiration of the same rose stem using a lysimeter revealed a significant linear correlation (r2 = 0.999) between the lysimeter and the stem flow gauge values.In the future, research will be conducted with the gauge to investigate relationships between microclimatic variables, photosynthesis, and transpiration.

Une méthode de mesure du débit de sève brute dans de petits arbres par bilan de chaleur

it is assumed that the heat flow between the trunk surface and the surrounding air is negligible. The method was tested : a) in the laboratory, by forcing water passage in a branch segment under stationary conditions and by comparing these known water fluxes with the estimated values; b) in the greenhouse, with small potted trees (1 - 8 cm diameter), by comparing the actual transpiration rate measured by weighing and the estimated values. The experimental results indicate that the computed sap flow rate closely follows the measured transpiration rate and the daily transpiration flow is evaluated by excess with a variable error not greater than 30%. Analysis of the results shows that this method can be improved by measuring the lateral heat flow and the heat power accumulated in the volume of trunk limited by the heating jacket An improved sap flow sensor is now being tested.