Thermal Dissipation Method Research Articles

Assessing Environmental Control of Sap Flux of Three Tree Species Plantations in Degraded Hilly Lands in South China

Prerequisite for selection of appropriate tree species in afforestation programs is to understand their water use strategy. Acacia mangium Willd., Schima wallichii Choisy, and Cunninghamia lanceolata (Lamb.) Hook are the three main vegetation restoration pioneer species in southern China, but no comparative research on the water use strategy of these three tree species have been reported. Our objective was to gain a detailed understanding of how photosynthetically active radiation (PAR), vapor pressure deficit (VPD), and soil water content (SWC) at different soil depths control the sap flux density (Js) in the dry and wet seasons. We measured the Js of these three tree species by using the thermal dissipation method in low subtropical China. We found that both S. wallichii and C. lanceolata differed clearly in their stomatal behavior from one season to another, while A. mangium did not. The canopy conductance per sapwood area of S. wallichii and C. lanceolata was very sensitive to VPD in the dry season, but not in the wet season. The Js of A. mangium was negatively correlated to SWC in all soil layers and during both seasons, while the other two species were not sensitive to SWC in the deeper layers and only positively correlated to SWC in dry season. Our results demonstrate that the three species have distinct water use strategies and may therefore respond differently to changing climate.

Analytical approach extending the Granier method to radial sap flow patterns

The Granier thermal dissipation (TD) method is probably the most applied method to compute the transpiration flux of trees, due to its simplicity and effective compromise between theory and data availability. Starting from the heat transfer equations at the basis of Granier’s method, the objective of this paper is to derive an analytical solution for the transpiration flux to extend the sap flow equations to the radial domain. We adopted a flexible approach to cope with the differences in radial sapflow density (SFD) profile shapes that are known to occur in relation to wood anatomy (diffuse porous vs. ring- or non-porous xylem). With this purpose, we investigated the robustness of the equations developed on some experimental and reliable radial SFD measurements available in literature to test the influence of considering or not considering the active zone close to the cambium, where most of the species-specific differences are likely to be observed. Moreover, the parameters derived by the extended formulation, are interpreted as descriptive of species-specific radial sap flow patterns. The reliability of the suggested procedure was checked against several experimental SFD profiles from literature: i) monotonically increasing SFD from the centre of the stem towards the cambium, ii) increasing SFD from the centre of the stem and then constant SFD towards the cambium, and iii) increasing SFD from the centre of the stem to a maximum SFD and then decreasing towards the cambium. Results show that according to the suggested procedure, an increasing number of parameters depending on the SFD profile complexity are required to synthetically describe the transpiration flux of different tree species. For the simplest case of monotonically increasing SFD, which could be assumed as standard under conditions of a diffuse porous tree structure, only two parameters with a clear physical meaning are required.

Field scale recalibration of the sap flow thermal dissipation method in a Mediterranean vineyard

The measurement of transpiration at plant level is a challenging topic in crop water requirements research, e.g. for irrigation scheduling and for the partitioning of plant transpiration and soil evaporation in modelling crop evapotranspiration process. The thermal dissipation method (TD) is widely applied for transpiration estimates of tree species, however some studies reported underestimations of transpiration measurements performed with the TD method when using the equation proposed by Granier. In particular, large underestimation of transpiration flux was reported in vineyards. The underestimation is ascribed to the equation, and its parameters, used to calculate the sapwood flux density from the heat transfer equations in the original Granier’s model and site-specific calibration is required. In this study, using measurements performed in a 2-year field experiment, the TD method was recalibrated at field scale in a vineyard cultivated in a Mediterranean region (Sicily, southern Italy), comparing transpiration flux determined by the TD method with transpiration calculated by the partitioning flux-variance similarity (FVS) method. FVS is a relatively new and innovative method for partitioning water fluxes in transpiration and evaporation through EC data. In this study, transpiration calculated through the equation proposed by Granier was, on average, 26% lower than transpiration determined by FVS. The results show that a new exponential function to calculate the sapwood flux density performed better than the power function in the original equation proposed by Granier. The results also showed that species and site-specific relationships are needed to accurately determine transpiration through the TD method. The ratio T/ET when T is calculated by the recalibrated TD function was equal to 0.77.

Water use of Coffea arabica in open versus shaded systems under smallholder’s farm conditions in Eastern Uganda

Coffee cultivation is a major source of income in East Africa. Increasing temperatures and water scarcity related to climate change are becoming major challenges for coffee production. Therefore, there is an urgent need for sound scientific understanding of the functioning of current coffee cultivation systems and the potential of agroforestry as an adaptation strategy to climate change. In a smallholder coffee farm on Mt Elgon, Uganda, we assessed the effect of three coffee cultivation systems (i.e., Coffee-Open, Coffee-Banana and Coffee-Cordia) on (i) the coffee cultivation environment (e.g. microclimate and soil moisture), (ii) water consumption of coffee, (iii) water consumption of banana (Musa sp.) and Cordia africana and (iv) water competition or complementary use between coffee and shade tree species. To this end, we monitored sap flux density (Js) (g cm−2 hour-1) of coffee, banana and C. africana from March 2015 to April 2016, using Granier thermal dissipation method, along with microclimate, soil moisture and rainfall. Shaded systems reduced irradiance by 70% in Coffee-Cordia system and 58% in Coffee-Banana system compared to Coffee-Open system. Maximum temperatures and daily temperature amplitude were on average reduced by 4 °C in both shaded systems compared to Coffee-Open system. Soil water content (SWC) in shaded systems was reduced by 59% in Coffee-Cordia and 6% in Coffee-Banana compared to Coffee-Open. Daily water consumption of coffee plants was 1.2 ± 0.64 l d-1 and did not differ between systems. Water use of banana was 3.1 ± 1.8 l d-1 and 42 ± 40 l d-1 by C. africana. Coffee-Banana system had the largest daily transpiration rate, 0.9 ± 0.4 mm d-1 per ground area and 0.6 ± 0.4 mm d-1 per unit leaf area, followed by Coffee-Cordia with 0.37 ± 0.1 mm d-1 (per ground area), 0.36 ± 0.1 mm d-1 (per leaf area) and Coffee-Open 0.24 ± 0.1 mm d-1 (per ground area), 0.27 ± 0.1 mm d-1 (per leaf area). Our results showed that differences in microclimate and SWC between cultivation systems did not influence coffee water use during the monitored year. However, water competition between coffee and shade trees could likely occur in drier years, due to the reduced SWC presently observed in shaded systems. Further research is needed to explore the performance of management practices (mulching, pruning and thinning) in interaction with seasonal weather forecast and appropriate selection of shade species (provision of extra products, reduced water use, fast growth and root zone below 80 cm depth) to match the systems’ water requirements with expected soil water availability.

Quantification of uncertainties in conifer sap flow measured with the thermal dissipation method.

Trees play a key role in the global hydrological cycle and measurements performed with the thermal dissipation method (TDM) have been crucial in providing whole-tree water-use estimates. Yet, different data processing to calculate whole-tree water use encapsulates uncertainties that have not been systematically assessed. We quantified uncertainties in conifer sap flux density (Fd ) and stand water use caused by commonly applied methods for deriving zero-flow conditions, dampening and sensor calibration. Their contribution has been assessed using a stem segment calibration experiment and 4yr of TDM measurements in Picea abies and Larix decidua growing in contrasting environments. Uncertainties were then projected on TDM data from different conifers across the northern hemisphere. Commonly applied methods mostly underestimated absolute Fd . Lacking a site- and species-specific calibrations reduced our stand water-use measurements by 37% and induced uncertainty in northern hemisphere Fd . Additionally, although the interdaily variability was maintained, disregarding dampening and/or applying zero-flow conditions that ignored night-time water use reduced the correlation between environment and Fd . The presented ensemble of calibration curves and proposed dampening correction, together with the systematic quantification of data-processing uncertainties, provide crucial steps in improving whole-tree water-use estimates across spatial and temporal scales.

Transpiration and stomatal conductance in a young secondary tropical montane forest: contrasts between native trees and invasive understorey shrubs.

It has been suggested that vigorous secondary tropical forests can have very high transpiration rates, but sap flow and stomatal conductance dynamics of trees and shrubs in these forests are understudied. In an effort to address this knowledge gap, sap flow (thermal dissipation method, 12 trees) and stomatal conductance (porometry, six trees) were measured for young (5-7 years) Psiadia altissima (DC.) Drake trees, a widely occurring species dominating young regrowth following abandonment of swidden agriculture in upland eastern Madagascar. In addition, stomatal conductance (gs) was determined for three individuals of two locally common invasive shrubs (Lantana camara L. and Rubus moluccanus L.) during three periods with contrasting soil moisture conditions. Values of gs for the three investigated species were significantly higher and more sensitive to climatic conditions during the wet period compared with the dry period. Further, gs of the understorey shrubs was much more sensitive to soil moisture content than that of the trees. Tree transpiration rates (Ec) were relatively stable during the dry season and were only affected somewhat by soil water content at the end of the dry season, suggesting the trees had continued access to soil water despite drying out of the topsoil. The Ec exhibited a plateau-shaped relation with vapour pressure deficit (VPD), which was attributed to stomatal closure at high VPD. Vapour pressure deficit was the major driver of variation in Ec, during both the wet and the dry season. Overall water use of the trees was modest, possibly reflecting low site fertility after three swidden cultivation cycles. The observed contrast in gs response to soil water and climatic conditions for the trees and shrubs underscores the need to take root distributions into account when modelling transpiration from regenerating tropical forests.

Comparisons of xylem sap flux densities in immature hybrid rubber tree clones under varied environmental conditions

The objective of this research was to evaluate the diversity of xylem sap flux density in four immature hybrid rubber clones, including the main mother clone RRIM600, planted in northeast Thailand. These hybrid clones, bred from the original RRIM600 (female) and RRII105 (male) clones, differed in trunk diameters. The age of the sample trees was 4 years. Measurements were taken from April to October 2016, within the full-leaves stage. The experimental design consisted of a randomized block design of hybrids and blocks for performing measurements at four environmental conditions: well-watered and sunny day (VPD 2.2 kPa), well-watered and extremely sunny day (VPD 3.2 kPa), well-watered and cloudy day (VPD 0.9 kPa) and dry soil and extremely sunny day (VPD 4.2 kPa). Sap flow density was recorded through the transient thermal dissipation method (10 min heating and 20 min cooling in a half hour period; TTD10) with a Granier's type probe. The results show that the maximum and daily sap flux densities were reduced by 40-45% on the cloudy day and soil drought conditions, compared to the well-watered and sunny day condition, while daily sap flux density in the extremely sunny day condition slightly increased. Differences in trunk size were insignificant in relation to the varying sap flux densities.

Difference of water relationships of poplar trees in Zhangbei County, Hebei, China based on stable isotope and thermal dissipation method

The water sources and transpiration of poplar trees in Zhangbei County were measured using stable hydrogen isotope and thermal dissipation method. The differences in water relationships between dieback and non-dieback poplar trees were analyzed. The results showed that the dieback trees mainly used shallow water from 0-30 cm soil layer during growing season while the non-dieback trees mainly used water from 30-80 cm soil layer. There was a significant difference in water source between them. The non-dieback trees used more water from middle and deep soil layers than that of the dieback trees during the dry season. The percentage of poplar trees using water from 0-30 cm soil layer increased in wet season, and the increase of dieback trees was higher than that of non-dieback trees. The contributions of water from 30-180 cm soil layer of dieback and non-dieback trees both decreased in wet season. The sap flow rate of non-dieback trees was higher than that of dieback trees. There was a similar variation tend of sap flow rate between dieback and non-dieback trees in different weather conditions, but the start time of sap flow of non-dieback trees was earlier than that of dieback trees. Correlation analysis showed that the sap flow rate of either dieback or non-dieback poplar trees strongly related to soil temperature, wind speed, photosynthetically active radiation, relative humidity and air temperature. The sap flow rate of die-back poplar trees strongly negatively related to soil temperature and relative humidity, and strongly positively related to the other factors. The sap flow rate of non-dieback poplar trees only strongly negatively related to relative humidity but positively related to the other factors. The results revealed transpiration of both poplar trees was easily affected by environmental factors. The water consumption of dieback trees was less than non-dieback trees because the cumulative sap flow amount of dieback trees was lower. Reduced transpiration of dieback trees couldn't help to prevent poplar forest declining due to shallow water source.

Assessing the thermal dissipation sap flux density method for monitoring cold season water transport in seasonally snow-covered forests.

Productivity of conifers in seasonally snow-covered forests is high before and during snowmelt when environmental conditions are optimal for photosynthesis. Climate change is altering the timing of spring in many locations, and changes in the date of transition from winter dormancy can have large impacts on annual productivity. Sap flow methods provide a promising approach to monitor tree activity during the cold season and the winter-spring and fall-winter transitions. Although sap flow techniques have been widely used, cold season results are generally not reported. Here we examine the feasibility of using the Granier thermal dissipation (TD) sap flux density method to monitor transpiration and dormancy of evergreen conifers during the cold season. We conducted a laboratory experiment which demonstrated that the TD method reliably detects xylem water transport (when it occurs) both at near freezing temperature and at low flow rate, and that the sensors can withstand repeated freeze-thaw events. However, the dependence between sensor output and water transport rate in these experiments differed from the established TD relation. In field experiments, sensors installed in two Abies forests lasted through two winters and a summer with low failure. The baseline (no-flow) sensor output varied considerably with temperature during the cold season, and a new baseline algorithm was developed to accommodate this variation. The Abies forests differed in elevation (2070 and 2620 m), and there was a clear difference in timing of initiation and cessation of transpiration between them. We conclude that the TD method can be reliably used to examine water transport during cold periods with associated low flow conditions.

Contribution of lianas to community‐level canopy transpiration in a warm‐temperate forest

Summary Lianas (woody climbers) have a greater amount of leaves relative to basal area or standing biomass than trees, and very wide vessels that permit efficient water transport. These features suggest that lianas possibly consume proportionally more water through transpiration than trees. Despite their potential importance, researchers have made only limited attempts to evaluate effects of lianas on forest water dynamics. We conducted sap flow measurements for 1 year using a thermal dissipation method for four species each of lianas and trees in a liana‐rich, warm‐temperate forest in Japan and estimated the contribution of lianas to stand canopy transpiration. Based on a calibration measuring water uptake rates from cut‐stem ends, the actual sap flux (Fd) in liana stems was several times greater than those estimated from the original calibration provided for the method. In the field, lianas showed an average of 2–4 times greater Fd than trees throughout the year. Except for this difference, diurnal and seasonal patterns of relative changes of Fd were similar in both groups. The whole‐plant transpiration (Qt) of sample plants was exponentially related to basal diameter for both lianas and trees; Qt of lianas increased more steeply with basal diameter than that of trees. By extrapolating the relationships between Qt and basal diameter to the inventory data of the study plot, we estimated that lianas contributed 12·8% to the annual stand canopy transpiration while comprising 2·3% of stand basal area, which probably reflected the top‐heavy architecture of lianas. Our results indicate that the contribution of lianas to forest water dynamics may be several times greater than their contribution to forest basal area. This implies that a slight increase of liana abundance might have greater effects on water dynamics and, through competitions with trees for limited water, the carbon sequestration capacity of forests than expected from the increase in basal area. This study underlines the necessity of evaluating the relative importance of lianas to forest water dynamics in forests world‐wide. A lay summary is available for this article.

Quantifying differences in thermal dissipation probe calibrations for Eucalyptus globulus species and E. nitens × globulus hybrid

Calibration of sap flux density equations for Eucalytus is required when using sapflow thermal dissipation probes to avoid large underestimations of transpiration and water use. Eucalyptus plantations are expanding in response to global wood demand but are raising concerns about their impacts on water supplies. Sustainable plantation management in areas with water conflicts will require accurate assessments of tree and stand water use. Thermal dissipation probes have been used to estimate tree water use, but recent work suggests that species-specific calibrations may be required to obtain accurate results. In this study, we quantified sap flux density (SFD) in 2-year-old Eucalyptus globulus Labill (Eg) and E. nitens × globulus (Eng) species using the thermal dissipation method developed by Granier. For each species we compared the original Granier equation with species-specific calibrations using whole tree potometers over a 36-h period. Our results showed that on average, Granier’s original equation significantly underestimated SFD in both species, and when scaled to the stand level, tree transpiration (Ec) was significantly lower compared to onsite calibrations. The Granier method also underestimated nocturnal transpiration for both genotypes. Measured calibration coefficients were similar and not statistically different between Eg and Eng. These results highlight the importance of species-specific calibrations using thermal dissipation probes for Eucalyptus species to improve stand water use estimates and inferences about ecological impacts.

Determinants of water circulation in a woody bamboo species: afternoon use and night-time recharge of culm water storage

To understand water-use strategies of woody bamboo species, sap flux density (Fd) in the culms of a woody bamboo (Bambusa vulgaris Schrader ex Wendland) was monitored using the thermal dissipation method. The daytime and night-time Fd were analyzed in the dry and rainy seasons. Additionally, diurnal changes in root pressure, culm circumference, and stomatal conductance (gs) were investigated to characterize the mechanisms used to maintain diurnal water balance of woody bamboos. Both in the dry and rainy seasons, daytime Fd responded to vapor pressure deficit (VPD) in an exponential fashion, with a fast initial increase in Fd when VPD increased from 0 to 1 kPa. The Fd and gs started to increase very fast as light intensity and VPD increased in the morning, but they decreased sharply once the maximum value was achieved. The Fd response of this woody bamboo to VPD was much faster than that of representative trees and palms growing in the same study site, suggesting its fast sap flow and stomatal responses to changes in ambient environmental factors. The Fd in the lower and higher culm positions started to increase at the same time in the morning, but the Fd in the higher culm position was higher than that of the lower culm in the afternoon. Consistently, distinct decreases in its culm circumference in the afternoon were detected. Therefore, unlike trees, water storage of bamboo culms was not used for its transpiration in the morning but in the afternoon. Nocturnal sap flow of this woody bamboo was also detected and related to root pressure. We conclude that this bamboo has fast sap flow/stomatal responses to irradiance and evaporative demands, and it uses substantial water storage for transpiration in the afternoon, while root pressure appears to be a mechanism resulting in culm water storage recharge during the night.

Sap flow of black ash in wetland forests of northern Minnesota, USA: Hydrologic implications of tree mortality due to emerald ash borer

Black ash (Fraxinus nigra) mortality caused by the invasive emerald ash borer (EAB) is of concern to land managers in the upper Great Lakes region, given the large areas of ash-dominated forest and potential alteration of wetland hydrology following loss of this foundation tree species. The importance of changes in evapotranspiration (ET) following black ash mortality is currently unknown and is the focus of this study. Sap flux density rates were evaluated at three black ash stands with differing moisture regimes within the Chippewa National Forest, Minnesota, USA using the Granier thermal dissipation method. Sapwood area and sap flux density were combined to determine sap flow. Tree level sap flux density estimates were comparable to other reported values and averaged 4.59, 2.31, and 1.62m3m−2day−1, respectively, for the very wet, wet, and moderately wet field sites. However, black ash exhibited small sapwood area in general, resulting in lower overall sap flow values. Scaled stand-level transpiration followed a similar trend as the tree-level estimates; mean daily transpiration over 10 weeks was 1.62 (80% of PET), 1.15 (53% of PET), and 0.90 (42% of PET) mm for the very wet, wet, and moderately wet site, respectively. Sap flux density was positively related to vapor pressure deficit when soil moisture was at or near saturation and negatively related when soil moisture content was lower. There was also a significant positive relationship between sap flux density and relative soil moisture saturation at the stand scale. Our results indicate that hydrologic regime has substantial influence on sap flow with highest transpiration when soil moisture is at saturation, underscoring the unique ecological role that black ash plays in these wetland forest types. The effects of EAB-induced black ash mortality on overall ET and related hydrologic processes will likely be greatest in the wettest hydrologic regimes.

Sap flow and water use sources of shelter‐belt trees in an arid inland river basin of Northwest China

AbstractThe knowledge of plant water use characteristics was essential for ecosystem management and water resources distribution in arid inland river basin. This study was conducted to quantify the sap flow variations and water use sources of shelter‐belt trees at the oasis–desert ecotone in the middle of China's Heihe River Basin. Sap flow was measured by the thermal dissipation method on eight Gansu Poplar (Populus gansuensis) trees with different diameter at breast height in 2012 and 2013. The mean sap flow density increased linearly with diameter at breast height. The sap flow density exhibited linear relationship with solar radiation, and it increased logarithmically with vapour pressure deficit and air temperature, whereas the water table had negative impact on sap flow. The relationship between sap flow and soil relative extractable water at 0–220 cm depth was implicit during the whole growing season, whereas the soil water had apparently positive influence on sap flow after the shelter‐belt trees irrigation. The stand transpiration rate represented as a logarithmic function of reference crop evapotranspiration. The total transpiration of Gansu Poplar during growing season was 599.3 mm, whereas only 58.1% of which was provided by the precipitation and irrigation. The groundwater and cropland irrigation were critical water sources of shelter‐belt trees. The contribution of groundwater to tree transpiration was estimated to be 35.1 and 19.0% in 2012 and 2013, respectively. The great precipitation in 2013 weakened the dependence of tree transpiration on groundwater. The estimated threshold distance of cropland irrigation influencing the tree transpiration was about 8 m. Copyright © 2015 John Wiley & Sons, Ltd.

How significant is nocturnal sap flow?

Nocturnal sap flow (Qn) has been found to occur across many taxa, seasons and biomes. There is no general understanding as to how much Qn occurs and whether it is a significant contribution to total daily sap flow (Q). A synthesis of the literature and unpublished data was made to determine how significant is Qn, as a proportion of Q (%Qn), across seasons, biomes, phylogenetic groups and different thermometric sap flow methods. A total of 98 species were analysed to find that %Qn, on average, was 12.03% with the highest average dataset of 69.00%. There was significantly less %Qn in winter than in other temperate seasons, and significantly less %Qn in the wet season than in the dry season. The equatorial and tropical biomes had significantly higher %Qn than the warm temperate and nemoral biomes. The heat ratio method (HRM) and the thermal dissipation (TDP) method had significantly higher %Qn than the heat balance method. Additional analysis between HRM and TDP found HRM to have significantly higher %Qn in winter, wet season and various biomes. In all but one out of 246 cases Qn occurred, demonstrating that Qn is significant and needs to be carefully considered in sap flow and related studies.

TRANSPIRATION OF POPLAR BASED SHORT ROTATION COPPICE UNDER DROUGHT STRESS

This study compares canopy transpiration (T) of two treatments with different levels of water availability calculated using sap flow (SF) measurements based on thermal dissipation (TD) method. The study was conducted in a poplar based short rotation coppice (SRC) plantation in Czech-Moravian highlands during the summer of 2012 (DOY 154-274). There are 3 pairs of experimental plots under research. Each pair comprises a control plot, and a neighboring plot, equipped with a roof system, draining 40% of incoming rain water away (the water reduced plot). Sap flow of 4 trees within the control plots and 4 trees within the water reduced plots was measured. Data from both treatments were scaled up to stand level separately. Then, potential transpiration (Tp) of SRC for the period DOY 154-183) was successfully (R 2 =0.89) modeled, as a function of global radiation and vapor pressure deficit (VPD), using the Penman-Monteith formula and a parameterized semi-empirical stomatal conductance model. Finally, canopy transpiration from controls (Tc) and water reduced plots (Ts) were compared with modeled Tp and further analyzed. In the period June-September the mean daily Tc=1.86 mm, Ts=1.38 mm and Tp=2.27 mm; daily maximum occurred on 24 June – Tc=3.75 mm, Ts=2.84 mm and Tp=3.89 mm. The T totals over the study period reached Tc= 226 mm, Ts=168 mm and Tp=276 mm. T of both treatments differed depending on actual soil water availability. Our results show that a 40% reduction in throughfall resulted in a 26% reduction in transpiration over the four months study period. Results also confirmed that even a low number of sensors (4+4) can be used for credible estimates of canopy transpiration.

Transpiration rates of urban trees, Aesculus chinensis

Transpiration patterns of Aesculus chinensis in relation to explanatory variables in the microclimatic, air quality, and biological phenomena categories were measured in Beijing, China using the thermal dissipation method. The highest transpiration rate measured as the sap flux density of the trees took place from 10:00 am to 13:00 pm in the summer and the lowest was found during nighttime in the winter. To sort out co-linearity, principal component analysis and variation and hierarchical partitioning methods were employed in data analyses. The evaporative demand index (EDI) consisting of air temperature, soil temperature, total radiation, vapor pressure deficit, and atmospheric ozone (O3), explained 68% and 80% of the hourly and daily variations of the tree transpiration, respectively. The independent and joint effects of EDI variables together with a three-variable joint effect exerted the greatest influences on the variance of transpiration rates. The independent effects of leaf area index and atmospheric O3 and their combined effect exhibited minor yet significant influences on tree transpiration rates.

Sap Flow Sensors: Construction, Quality Control and Comparison

This work provides a design for two types of sensors, based on the thermal dissipation and heat ratio methods of sap flow calculation, for moderate to large scale deployments for the purpose of monitoring tree transpiration. These designs include a procedure for making these sensors, a quality control method for the final products, and a complete list of components with vendors and pricing information. Both sensor designs were field tested alongside a commercial sap flow sensor to assess their performance and show the importance for quality controlling the sensor outputs. Results show that for roughly 2% of the cost of commercial sensors, self-made sap flow sensors can provide acceptable estimates of the sap flow measurements compared to the commercial sensors.

Nighttime sap flow of Acacia mangium and its implications for nighttime transpiration and stem water storage

Nighttime sap flow of trees may indicate transpiration and/or recharge of stem water storage at night. This paper deals with the water use of Acacia mangium at night in the hilly lands of subtropical South China. Our primary goal was to reveal and understand the nature of nighttime sap flow and its functional significance. Granier's thermal dissipation method was used to determine the nighttime sap flux of A. mangium. Gas exchange system was used to estimate nighttime leaf transpiration and stomatal conductance of studied trees. Nighttime sap flow was substantial and showed seasonal variation similar to the patterns of daytime sap flow in A. mangium. Mean nighttime sap flow was higher in the less precipitation year of 2004 (1122.4 mm) than in the more precipitation year of 2005 (1342.5 mm) since more daytime transpiration and low soil water availability in the relatively dry 2004 can be the cause of more nighttime sap flow. Although vapor pressure deficit and air temperature were significantly correlated with nighttime sap flow, they could only explain a small fraction of the variance in nighttime sap flow. The total accumulated water loss (E-L) by transpiration of canopy leaves was only similar to 2.6-8.5% of the total nighttime sap flow (E-t) during the nights of July 17-18 and 18-19, 2006. Therefore, it is likely that the nighttime sap flow was mainly used for refilling water in the trunk. The stem diameter at breast height, basal area and sapwood area explained much more variance of nighttime water recharge than environmental factors and other tree form features, such as tree height, stem length below the branch, and canopy size. The contribution of nighttime water recharge to the total transpiration ranged from 14.7 to 30.3% depending on different DBH class and was considerably higher in the dry season compared to the wet season.

Characteristics and modelling of canopy conductance and transpiration of Platycladus orientalis (L.) Franco in Loess Plateau of China

Estimating plant water use is an important step in assessing the effects of increasing vegetation cultivation on the hydrological cycle especially in arid and semi-arid regions. In this study, meteorological measurements combined with sap flow techniques provided a low-cost option to study the canopy physiological transpiration of Platycladus orientalis response to environmental factors on a continuous basis. Canopy transpiration (Ec) was measured by thermal dissipation method of Granier, and canopy conductance (gc) was calculated by inverting the Penman-Monteith equation. The results showed that the transpiration ofP. orientalis was strongly controlled by stomatal conductance, and gc was a comprehensive and compounded environmental variable. An improved Jarvis-type model, based on a series of environmental control functions, explained 85% of the variation observed in gc. Cross validation showed that this model provided good predictions of canopy conductance and transpiration for P. orientalis. Such a methodology offers a reasonable estimation of water use in the determination of water balance for land water resources planning, vegetation management and impact assessments of rehabilitation. Key words: Sap flow, canopy transpiration, canopy conductance, model, Penman-Monteith equation, Platycladus orientalis, semi-arid region.