Sap Flow Density Research Articles

Drought Primarily Reduces Canopy Transpiration of Exposed Beech Trees and Decreases the Share of Water Uptake from Deeper Soil Layers

Research Highlights: During drought, reduced soil water availability and increased vapor pressure deficit diminished transpiration in a mature beech stand (Fagus sylvatica L.). Dominant trees were more affected than suppressed trees. The share of soil water uptake from deeper layers decreased. The ability of individual trees in the forest stand to save water during drought was apparently dependent on their social status. This would be relevant for forest management. Objectives: We investigated which basal area classes of trees contribute more or less to total transpiration under wet and dry conditions, and from which soil layers they took up water. We hypothesized that dominant trees have a better adaptability to drought and diminish transpiration more than suppressed trees. Methods: The water budget of the forest stand was continuously monitored throughout the entire observation period. Xylem sap flux measurements using thermal dissipation probes were performed during the vegetation period at different depths in the trunks of ten representative trees. A radial distribution model of the sap flow density pattern was used to compute whole-tree and stand transpiration. Water budget was simulated using a physiology-based model. Results: During drought, the fraction of suppressed trees to whole-canopy transpiration of the forest stand increased and the share of soil water uptake from deeper layers decreased. Conclusions: The behavior of dominant trees under drought conditions could be interpreted as a water-conserving strategy. Thinning by removing suppressed trees should be employed to stabilize forests.

Whole-plant water hydraulic integrity to predict drought-induced Eucalyptus urophylla mortality under drought stress

Drought-driven tree mortality has been occurring globally and been investigated by many researchers. Yet, while subtropical forests of southern China are predicted to experience an increased frequency and intensity of climate change‐induced drought events in the near future, there still a big knowledge gap in our understanding of the physiological vulnerability of Eucalyptus urophylla S.T. Blake under drought stress. In this study, E. urophylla was selected both for field and greenhouse experiments under controlled soil water conditions. We investigated whole-plant hydraulic conductivity (Kplant), sap flow density (Js), leaf photosynthetic characteristics and non-structural carbohydrates (NSC) to explore how E. urophylla species responded to drought from the perspective of hydraulic and carbon dynamics. Results showed that the percentage loss of stem xylem hydraulic conductance (PLC) caused by drought was up to 84.4%. Both Kplant and Js significantly decreased with the decline of the soil water availability. Also, the interaction of hydraulic conductivity to stomatal conductance was more sensitive under drought. In addition, drought-induced decline of total NSC content in leaf and stem tissue was not distinct. A conversion of starch to soluble sugar was found under drought stress. Our findings suggested that hydraulic failure could be a major threat to the survival of E. urophylla species under drought stress due to a high xylem embolism vulnerability, which improves prediction of the response of E. urophylla species to future climate changes.

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.

Effect of canopy management in the water status of cacao ( Theobroma cacao ) and the microclimate within the crop area

Background: Cacao is an umbrophile species and therefore the handling of shade by producers can cause a microclimatic modification that influences the physiology of the plant.
 Questions: Can canopy management influence the microclimate of the crop area and the water content of cacao?
 Species of study: Theobroma cacao L. (Malvaceae).
 Study site: Comalcalco, Tabasco, Mexico; dry and rainy season 2018.
 Methods: Three sites were selected with an open canopy (OC) and three with a closed canopy (CC), where we determined air temperature and humidity, soil temperature, vapor pressure deficit, photosynthetically active radiation, soil water potential and leaf water potential in 15 cacao trees and the sap flow density in 12 trees, by canopy condition and by season.
 Results: Higher values of solar radiation, air and soil temperature, vapor pressure deficit and lower relative humidity were recorded under OC compared to CC, in both seasons. Differences in soil water potential between 10 and 60 cm depth in CC were recorded during the dry season. There was a lower sap flow density and daily water use in OC. The leaf water potential was similar between canopy conditions, in both seasons.
 Conclusions: Changes in canopy coverage significantly modify the microclimate of the crop area, a less stressful environment being generated under closed canopy conditions, influencing the sap flow density of cacao trees.

Carbon Limitation and Drought Sensitivity at Contrasting Elevation and Competition of Abies pinsapo Forests. Does Experimental Thinning Enhance Water Supply and Carbohydrates?

Stand-level competition and local climate influence tree responses to increased drought at the regional scale. To evaluate stand density and elevation effects on tree carbon and water balances, we monitored seasonal changes in sap-flow density (SFD), gas exchange, xylem water potential, secondary growth, and non-structural carbohydrates (NSCs) in Abies pinsapo. Trees were subjected to experimental thinning within a low-elevation stand (1200 m), and carbon and water balances were compared to control plots at low and high elevation (1700 m). The hydraulic conductivity and the resistance to cavitation were also characterized, showing relatively high values and no significant differences among treatments. Trees growing at higher elevations presented the highest SFD, photosynthetic rates, and secondary growth, mainly because their growing season was extended until summer. Trees growing at low elevation reduced SFD during late spring and summer while SFD and secondary growth were significantly higher in the thinned stands. Declining NSC concentrations in needles, branches, and sapwood suggest drought-induced control of the carbon supply status. Our results might indicate potential altitudinal shifts, as better performance occurs at higher elevations, while thinning may be suitable as adaptive management to mitigate drought effects in endangered Mediterranean trees.

Drought Superimposes the Positive Effect of Silver Fir on Water Relations of European Beech in Mature Forest Stands

Research Highlights: Investigations of evapotranspiration in a mature mixed beech-fir forest stand do not indicate higher resilience towards intensified drying-wetting cycles as compared with pure beech stands. Background and Objectives: Forest management seeks to implement adaptive measures, for example, the introduction of more drought resistant species into prevailing monospecific stands to minimize forest mortality and monetary losses. In Central Europe this includes the introduction of native silver fir (Abies alba) into monospecific beech (Fagus sylvatica) stands. In order to determine, if the introduction of fir would improve the resilience against drier conditions, this study investigates water relations of a mature pure beech and a mature mixed beech-fir stand under natural as well as reduced water availability. Materials and Methods: Sap flow rates and densities were measured in two consecutive years using the heat ratio method and scaled using stand inventory data and modeling. Results: Transpiration rates estimated from sap flow were significantly higher for beech trees as compared with silver fir which was attributed to the more anisohydric water-use strategy of the beech trees. We estimate that stand evapotranspiration was slightly higher for mixed stands due to higher interception losses from the mixed stand during times of above average water supply. When precipitation was restricted, beech was not able to support its transpiration demands, and therefore there was reduced sap flow rates in the mixed, as well as in the pure stand, whereas transpiration of fir was largely unaffected, likely due to its more isohydric behavior toward water use and access to moister soil layers. Thus, we found the rates of evapotranspiration in the mixed beech-fir stand to be smaller during times with no precipitation as compared with the pure beech stand, which was accountable to the severely reduced transpiration of beech in the mixed stand. Conclusions: We conclude that smaller evapotranspiration rates in the mixed beech-fir stand might not be the result of increased water use efficiency but rather caused by restricted hydraulic conductivity of the root system of beech, making mixed beech-fir stands at this site less resilient towards drought.

Variations in sap flow of Zenia insignis under different rock bareness rate in North Guangdong, China

Understanding the impact of rock bareness on the transpiration in karst plants is essential to karst rocky desertification control and sustainable management of plantation in karst area. This study focused on the variations in sap flow of Zenia insignis caused by different rock bareness rate, and the impact of climate factors, soil water content (SWC) and leaf area index (LAI) on transpiration in karst plants, by continuously measuring sap flux densities (Fd) of 12 sample trees using thermal dissipation probes and monitoring micrometeorology and SWC on a typical karst hill in north Guangdong of China during the year of 2016. Results show that: (1) the maximum hourly sap flux density occurred at 11:00–14:00 and the peak daily sap flux density occurred in September. (2) Sap flow density of Zenia insignis increased with rock bareness rate at all hourly, daily and monthly scales, with the sequence of extremely severe > severe > moderate > mild rock bareness. (3) The transpiration of Zenia insignis is controlled by different factors at different temporal scales. At hourly scale, transpiration was highly (n=144, R2>0.72) correlated to Solar radiation (Rs), Air temperature (Ta), relative humidity (RH), and water vapor pressure deficit (VPD). At daily scale, transpiration was greatly (77=366, R2>0.31) affected by Solar radiation (Rs), Air temperature (Ta), and water vapor pressure deficit (VPD). While at monthly scale, transpiration was mainly (n=12, R2=0.8s) controlled by LAI. Our study proved that Zenia insignis has a good physiecological adaption to fragile karst environment, and Zenia insignis plantation has long-term sustainability even in extremely rocky landscapes. The results may provide scientific basis for plantation management and ecological restoration in karst area.

Ecophysiological impacts of Esca, a devastating grapevine trunk disease, on Vitis vinifera L.

Esca is a Grapevine Trunk Disease (GTD) caused by a broad range of taxonomically unrelated fungal pathogens. These attack grapevine wood tissues inducing necroses even in the conductive vascular tissues, thus affecting the vine physiology and potentially leading to plant death. However, the influence of Esca on leaf and whole-plant water transport disruption remains poorly understood. In this paper, a detailed analysis of xylem-related physiological parameters in grapevines that expressed Esca-foliar symptoms was carried out. The experiments were conducted in a vineyard in the Bordeaux region (France) on cv. Cabernet-Sauvignon (Vitis vinifera L.) grapevines, which were monitored for Esca-foliar symptoms over a two-year period. Heat dissipation sap-flow sensors were installed during the summer on grapevines having expressed or not Esca-foliar symptoms. Leaf water potential, stomatal conductance and leaf transpiration were also measured. Physiological monitoring showed that sap flow density and whole-plant transpiration of Esca-infected grapevines decreased significantly a week before the first foliar symptoms appeared. When atmospheric water demand (Vapour Pressure Deficit, VPD) was the highest, both parameters tended to be about twice as low in symptomatic grapevines as in asymptomatic ones. Sap flow density data at the maximum transpiration-time, was systematically 29–30% lower in Esca-infected grapevines compared to control plants before or after the appearance of Esca-foliar symptoms. This trend was observed whatever the temperatures and VPD values measured. In Esca-diseased plants, larger amounts of necrotic wood, mainly white rot, were found in the trunk and cordon of symptomatic grapevines compared to healthy ones, suggesting necroses have an influence in reducing the whole-plant hydraulic capacity. This study reveals that the use of physiological monitoring methods, together with the visual monitoring of foliar symptoms, could prove useful in providing accurate measurements of Esca disease severity.

Artificial Top Soil Drought Hardly Affects Water Use of Picea abies and Larix decidua Saplings at the Treeline in the Austrian Alps

This study quantified the effect of shallow soil water availability on sap flow density (Qs) of 4.9 ± 1.5 m tall Picea abies and Larix decidua saplings at treeline in the Central Tyrolean Alps, Austria. We installed a transparent roof construction around three P. abies and three L. decidua saplings to prevent precipitation from reaching the soil surface without notably influencing the above ground microclimate. Three additional saplings from each species served as controls in the absence of any manipulation. Roofing significantly reduced soil water availability at a 5–10 cm soil depth, while soil temperature was not affected. Sap flow density (using Granier-type thermal dissipation probes) and environmental parameters were monitored throughout three growing seasons. In both species investigated, three years of rain exclusion did not considerably reduce Qs. The lack of a significant Qs-soil water content correlation in P. abies and L. decidua saplings indicates sufficient water supply, suggesting that whole plant water loss of saplings at treeline primarily depends on evaporative demand. Future work should test whether the observed drought resistance of saplings at the treeline also holds for adult trees.

Air cooling by tree transpiration: A case study of Olea europaea, Citrus sinensis and Pinus pinea in Mediterranean town

Transpiration and surface temperature of Olea, Citrus and Pinus trees in an urban garden in southern Italy were studied from May until August, the hottest period of the year in the investigated area. Transpiration was measured by sap flow heat dissipation method at 10-min scale, while the surface-air temperature difference was estimated by energy balance approach. The radial trend of sap flow density at tree scale was modelled as a function of the sapwood area, while the upscaling to garden level was performed using the quantile method integrated by trunk diameter classification. The results at hourly and daily time scales showed that Olea had the highest transpiration followed by Citrus and Pinus trees. The cooling effect resulted dependent on the species showing Citrus as the most efficient species among the investigated ones. The study might contribute to a better understanding of cooling effect due to tree transpiration under rainfed conditions.

Anatomical and physiological adjustments of pubescent oak (Quercus pubescens Willd.) from two adjacent sub-Mediterranean ecosites

Patterns of drought severity in natural ecosystems depend on ecosite characteristics, particularly on local soil water availability and microclimate (evaporative demand). At the sites with less favourable conditions, the plasticity of fundamental drought adaptive traits can be exceeded under severe drought, even in well adapted species.In our study, anatomical and physiological adjustments to drought were studied in pubescent oak (Quercus pubescens Willd.) growing on adjacent sub-Mediterranean plots (ecosites) differing in bedrock and soil type (limestone with rendzic leptosol, low water retention - L; transition from limestone to flysch with eutric cambisol, high water retention - F).In both years of the study, 2015 and 2016, in the part of the season with sufficient water supply, sap flow density (SFD) was higher in oaks growing on L than those growing on F. With decreasing soil water content (SWC) in July, SFD was reduced on both plots, most in trees growing on limestone. In the period of severe drought, SFD in trees growing on L was decoupled from the changes of VPD, due to increased employment of water conservation strategies, such as decreases in stomatal conductance. This was further translated into stomatal limitation of photosynthesis and contributed to reduced radial stem growth.The seasonal dynamics of traits indicate that Q. pubescens trees use a predisposed anatomical strategy (average size of vessels in different years), adjusted to average ecosite conditions. At the same time, trees respond to local conditions and interanual variability of drought by shortening/prolonging the production and growth of vessels and by adjusting leaf functioning (stomatal conductance, sap flow) in accordance with hydraulic properties.

Responses of water use in Moso bamboo (Phyllostachys heterocycla) culms of different developmental stages to manipulative drought

BackgroundUnderstanding the mechanisms underlying plant water use in response to drought is critical to predicting the trend of ecosystem development under a changing environment, while the responses of water use in clonal plants with interconnected ramets to drought are far less clear than trees.MethodIn this study, we conducted manipulated drought by excluding 50% annual throughfall at a stand level in a Moso bamboo (Phyllostachys heterocycla) forest, to determine how water use of the Moso bamboo responds to drought stress, and whether bamboo culms of different developmental stages show synchronized responses given the existence of physiological integration. Three developmental stages of bamboo culms (i.e., young, mature and old) were investigated and measurements were made on sap flow density (Js), leaf water potential at predawn (ψpre) and midday (ψmid), leaf water potential at turgor loss (ψTLP), leaf stomatal density and size, and the maximum stomatal conductance (gw(max)) was calculated.ResultsUnder the manipulative drought conditions, the values of Js significantly decreased in the mature culms but not in the young and old culms. gw(max) remained unchanged in mature culms, and increased in the young and old culms. Drought significantly reduced ψmid of the young culms, resulting in more negative ψTLP in the young culms compared with the old ones.ConclusionsThe results imply that the young culms adopted the drought tolerance strategy, while the old culms tended to adopt the drought avoidance strategy. Both the young and old culms are capable of maintaining relatively stable Js under drought by structural and physiological adjustments. Our findings demonstrate the variable responses of water use in bamboo culms of different developmental stages to drought, suggesting an unsynchronized responses of water use to water stress among culms in clonal plants.

Calibration of Granier-Type (TDP) Sap Flow Probes by a High Precision Electronic Potometer.

Thermal dissipation probe (TDP) method (Granier, 1985) is widely used to estimate tree transpiration (i.e., the water evaporated from the leaves) because it is simple to build, easy to install, and relatively inexpensive. However, the universality of the original calibration has been questioned and, in many cases, proved to be inaccurate. Thus, when the TDP is used in a new species, specific tests should be carried out. Our aim was to propose a new method for improving the accuracy of TDP on trees in the field. Small hazelnut trees (diameter at breast height 5 cm) were used for the experiment. The response of TDP sensors was compared with a reference water uptake measured with an electronic potometer system provided with a high precision liquid flow meter. We equipped three stems where we measured the sap flow density, the sapwood area (by using fuchsine), the total tree water uptake (reference), and the main meteorological parameters during summer 2018. Results confirmed that the original Granier’s calibration underestimated the effective tree transpiration (relative error about −60%). We proposed a new equation for improving the measurement accuracy within an error of about 4%. The system proposed appeared an easier solution compared to potted trees and particularly suitable for orchards, thus contributing to improve the irrigation management worldwide.

Determining the independent impact of soil water on forest transpiration: A case study of a black locust plantation in the Loess Plateau, China

Transpiration (Tr) is influenced by environmental factors, vegetation properties, and anthropogenic management. The effect of environmental factors on Tr are taking place from two aspects: evaporative demand (i.e., potential evapotranspiration, PET) and water supply (i.e., soil water). Soil water is one of the most important factors that limit plant transpiration in terrestrial ecosystems, especially in semi-arid and arid regions. Investigating the relationship between Tr and soil water is crucial for an improved understanding of plant survival strategies and predicting hydrological cycles and water resources under climate change. Although the relationships of soil water and Tr have been widely studied, the independent effects of soil water on Tr are difficult to separate because soil water and PET occur concurrently under natural conditions. This study carried out field observations of sapflow density, meteorological factors and soil water in a black locust (Robinia pseudoacacia) plantation in the semi-arid Loess Plateau of China. This information was used to develop a model integrating the daily PET and relative extractable soil water (REW, an index that represents the available soil water): Tr = (0.27 × PET − 0.02 × PET2 − 0.32) × (1 − e−5.70×REW). The model fitted the measured data well (R2 = 0.65 and RMSE = 0.06 mm day−1). We found that the daily Tr increased as the REW increased under varying PET levels. Additionally, the independent effects of soil water on Tr were analysed using the factorial experiment analysis method. The REW was manipulated and PET varied naturally during the measurements to separate the independent effects of the REW on Tr. The results showed that Tr increased with the REW during the study period at a rate of 0.53 mm day−1 per 0.1 REW when the REW < 0.4 and 0.09 mm day−1 per 0.1 REW beyond the threshold (REW = 0.4). Tr increased by 27.3 mm (43.7%) compared to the controlled Tr (observed PET and lowest REW) due to the effects of increasing REW (ranging from 0.14 ∼ 0.81) during the study period (growing seasons of 2015 and 2016). Furthermore, canopy-level stomatal conductance was calculated using a simplified inverted Penman-Monteith equation. The reference canopy-level stomatal conductance (Gsref), which represents the Gs capacity and its sensitivity to soil water, showed an increasing response with increasing REW, which is the possible reason for the Tr dynamics with the REW. Our study provided an improved understanding of the soil water-Tr relationship and predicted effects of climate change on tree water use. Although the method required abundant field observation data, we have provided a feasible method of accurately quantifying the independent contribution of soil water on forest Tr.

Canopy transpiration of a Larix sibirica and Pinus sylvestris forest in Central Siberia

Russian boreal forests represent the largest forested region on Earth and comprise one-fifth of the world's forest cover. The two most common genera in Siberia are Larix and Pinus, which together cover more than 80% of the region’s forested area. One observable ongoing effect of climate warming is that natural populations of Siberian larch are gradually being replaced by Scots pine. The present work focuses on comparing effects of environmental variables on sap flow density in two even-aged stands of Larix sibirica and Pinus sylvestris. While the two study stands were identical in age (49 years) with similar basal areas and leaf area index, they exhibited very different transpiration rates and response mechanisms to environmental signals. Stand water use was higher for larch than it was for pine, even though transpiration for deciduous larch trees occurred over shorter time periods. The cumulative annual transpiration of the larch stand was 284 ± 4 mm measured over two consecutive growing seasons (2015–2016), while for pine this was 20% lower. Seasonal transpiration accounted for 50% and 40% of the reference evapotranspiration and 91% and 67% of growing season precipitation for larch and pine, respectively. Water stored in soil provided an important source of water for transpiration, observed as roughly 100 mm, which was then replenished from snowmelt the following spring. The greatest difference between two species related to how well they controlled transpiration, notably in the context of high vapor pressure deficit; under these conditions, pine maintained greater control over transpiration than larch. For all soil moisture levels measured, larch transpired more water than pine. Importantly, our results point to potential future effects of global warming, most notably an increasing decline of larch forests, changes in the ratio between latent and sensitive heat fluxes, and significant modifications in ecosystem water availability.

New insights into Dutch Elm Disease: cell wall compositional, ecophysiological, vascular and nanomechanical assessments

Comprehensive assessments were made of the chemical profiles of woody cell wall components, and also leaf growth, ecophysiological, vascular and nanomechanical traits for two Dutch elm hybrids 'Groeneveld' and 'Dodoens' which possess contrasting tolerances toward Dutch elm disease. Upon infection with Ophiostoma novo-ulmi ssp. americana × novo-ulmi, medium-molecular weight macromolecules of cellulose were degraded in both hybrids. A loss of crystalline and non-crystalline cellulose regions occurred in parallel. In 'Groeneveld' plants, syringyl-rich lignin provided a far greater degree of protection from cellulose degradation, but only guaiacyl-rich lignin in 'Dodoens' plants was involved in a successful defence against the fungus. Unexpectedly, we found a very high proportion of non-significant differences between the infected and non-infected plants of 'Dodoens', including similarities in leaf growth, leaf gas exchange and leaf midrib vascular traits, as well as in the nanomechanical properties of the cell walls of tracheary elements such as modulus of elasticity, adhesion and energy dissipation. Three years after initial inoculations, except for a few traits such as leaf slenderness, relative chlorophyll content, transpiration rate and sap flow density in branches, we found no evidence of a decrease in leaf trait performances among the infected plants of 'Dodoens', despite the occasional persistence of fungal hyphae in the lumens of leaf midrib tracheary elements.

Stem Radius Variation in Response to Hydro-Thermal Factors in Larch

The response mechanism of the tree stem radius variation to hydro-thermal factors is complex and diverse. The changes of TWD (tree water deficit-induced stem shrinkage) and GRO (growth-induced irreversible stem expansion) are respectively driven by different factors, so that their responses to hydro-thermal factors are different. The stem radius variation and its matching hydro-thermal factors experimental data was measured and determined at 0.5 h time scale in larch (Larix gmelini Rupr.) forest of the Daxing’anling region of the most northeastern part of China. Response characteristics of the stem radius variation to hydro-thermal factors have been found by analyzing the data under different time windows. The stem radius variation mainly responded to the changes in precipitation and relative humidity. The main driving factors for TWD were sap flow density and solar radiation. The response of GRO to hydro-thermal factors was complex, varied a lot under different time scales. During the analysis of the response of tree radial growth, changes of the stem radius can be divided to TWD and GRO to implement separate studies on their responses to hydro-thermal factors. In this way, it becomes easier to discover the response of TWD under drought stress and the responding mechanism of GRO to hydro-thermal factors.

Pattern and driving factor of intense defoliation of rubber plantations in SW China

The pattern of intense defoliation of rubber trees (Hevea brasiliensis) in Southwest China is assumed to mainly be attributable to drought stress. However,thishas never beenproven, and the pattern remains unclear. Our objectives were to reveal the driving factor and pattern of intense defoliation, and to explore the characteristics of sap flow density (Js) and relate them to climatic factors before, during and after the intense defoliation period. We continuously measured Js and related climatic factors in a rubber plantation in Southwest China from 2013 to 2016 and found that (1) the cold stress (<10 °C) was more relevant to intense defoliation than drought stress, and after encountering cold stress the rubber trees might enter dormancy for approximately 50 days during the defoliation period; (2) Js showed a self-regulating ability to respond to high air temperature (Ta) and water vaporpressuredeficit (VPD) with atime lag of approximately one hour during all periods except during the defoliation period; (3) the sensitivity of Js to Ta weaken (slope = 0.14, r = 0.17, P > 0.05) during the defoliation period, but it rebounded back to a relative high level (slope = 1.44, r = 0.55, P < 0.01) during the refoliation period which was similar to that during the remaining periods; and (4) the relationship between Js and Ta was generally positive, while both SWC5 and SWC100 exerted inhibitory effects on Js, and the interactive effects among Ta, SWC5 and SWC100 on Js were significant during the refoliation period (P < 0.01). Based on these findings, we evaluated Js as a crucial physiological indicator of the processes underlying important phenological transition periods, and the cold stress rather than drought stress was likely the driving factor of the pattern of intense defoliation occurred in Southwest China rubber plantations.

Responses of sap flow to natural rainfall and continuous drought of tree species growing on bedrock outcrops

This study focused on bedrock outcrops, a very common habitat in karst region of southwest China. To reveal the responses of plant transpiration to natural rainfall and continuous drought, two tree species typical to this habitat, Radermachera sinica and Triadica rotundifolia, were selected as test materials. A rainout shelter was used to simulate continuous drought. The sap flow dynamics were monitored using the method of Granier's thermal dissipation probe (TDP). Our results showed that sap flow density increased to different degrees after rain in different stages of the growing season. Sap flow density of the deciduous species T. rotundifolia was always higher than that of the semi-deciduous species R. sinica. After two months without rainfall input, both species exhibited no obvious decrease in sap flow density, indicating that rainfall was not the dominant source for their water uptake, at least in the short-term. Based on the regression relationships between sap flow density and meteorological factors before and after rainfall, as well as at different stages of continuous drought, we found that the dynamics of meteorological factors contributed little to plant transpiration. The basic transpiration characteristics of both species were not changed in the circumstance of natural rainfall and short-term continuous drought, which would be closely related to the special water storage environments of bedrock outcrops and the reliance on deep water sources by tree species.

Sapwood Area Related to Tree Size, Tree Age, and Leaf Area Index in Cedrus libani

Sapwood includes the water conducting part of the stem which transports water and minerals from roots to leaves. Studies using sap flow gauges have to determine the area of the sapwood in order to scale measured sap flow densities to the tree or stand level. The aim of this study was to investigate the relationship between sapwood area at breast height and other tree parameters which are easy to measure of the montane Mediterranean conifer Cedrus libani, including a total number of 92 study trees of different size and age. The study was conducted at four different stands situated between 1000 and 2000 m altitude in the Elmalı Cedar Research Forest of Antalya, SW-Turkey. Sapwood area of the sample trees was determined by extracting two tree cores from opposite directions of the stem using an increment borer and by visually assessing the wet part of the core. Parameters measured besides sapwood area were diameter at breast height, stem radius without bark, sapwood depth, tree basal area, tree height, tree age, and projected crown area. Furthermore, at each stand, leaf area index (LAI) was determined using hemispherical photographs of the forest canopy. The most significant relationship was found between stem radius without bark and sapwood area (R2: 0.94) followed by tree basal area and sapwood area (R2: 0.90). Although it was the second best predictor, tree basal area should be used to estimate sapwood area when estimating stand transpiration since it can be measured faster and without giving damage to the tree. Mean sapwood area and mean site-specific LAI showed a significant positive correlation. The findings of this study can be used in ecophysiological studies when transpiration rates of C. libani are measured using sap flow gauges.