DOI: 10.25911/6094cba509e75
title: Top-down rehydration of the mangrove Avicennia marina in response to wetting events caused by deliquescence of salt, accumulation of dew, and interception of rainfall under an arid climate DP150104437

description: Absorption of atmospheric water by branches of Avicennia marina was assessed by field-based studies of sap flow velocity and shoot water relations under dry season conditions along Giralia Bay in arid Western Australia. Two hypotheses were tested: 1) that deliquescence of salt secreted onto leaf surfaces can drive top-down rehydration, and 2) that absorption of atmospheric water from wetting events makes a functional contribution to shoot water balances, subsidising transpiration and reducing stem vulnerability to hydraulic failure. Study design: Three similar trees of the mangrove Avicennia marina (Forssk.) Vierh subsp. marina were studied under field conditions. Four branches of similar size and orientation were selected on each tree and randomly assigned to four treatments: Natural (N), Wet (W), Dry (D) and Ambient (A). Trees are identified by number and branches are identified by the letter denoting the treatment received. For example, an ID of 2D indicates Tree 2, branch allocated to Dry treatment. The Wet and Dry treatments were imposed only during a brief nocturnal manipulation experiment and were otherwise subject to the natural variation in weather. Ambient and Natural treatments were only subject to natural variation in weather but branches in the Natural treatment remained untouched throughout the whole monitoring period. The branches were used in two experiments in which sap flow characteristics were analysed in relation to either natural or manipulated conditions. Measurements of sap flow velocity: Detection of top-down rehydration events was based on reversal of sap flow (ie negative sap flow), indicating downward movement of water from branches to the main stem. A sap flow meter (SFM1, ICT International Pty Ltd, Armidale, Australia) was installed (sensor depth 12.2 mm) in the leafless region of each branch near its junction with the main stem, approximately 70-80 cm from the branch canopy. Branch level sap flow was monitored at 20 min intervals using the heat ratio method (Burgess et al., 2001) together with micro-meteorological measurements to enable analyses of directional water fluxes in relation to air humidity, air temperature and the occurrence of wetting events due to deliquescence, dew or precipitation. Environmental data were recorded at 10 minute intervals throughout the study by a portable weather station (Kestrel 3500 Delta T Meter, Neilsen-Kellerman Co, Boothwyn, PA). Air vapour pressure deficit (VPD) was calculated according to Murray (1967): VPD = Pv � ((RH/100)*Pv), where air temperature (Ta) and relative humidity (RH) were recorded and vapour pressure (Pv) was calculated as Pv = 0.611 exp [17.27 Ta/(Ta + 237.3)]. Branches were harvested when the study concluded, and zero flow baselines were determined for each meter/branch combination as recommended by the manufacturer. All leaves were harvested from each branch and imaged for determination of leaf area using J IMAGEJ software (Schneider et al., 2012). Total leaf dry mass was measured with a field balance (XP 205 Metter Toledo balance, Mettler � Toledo Ltd., Greifensee, Switzerland) after oven drying at 80�C for 72 h. Monitoring sap flow under natural weather conditions: Data sets 1 through 6 Environmental conditions and sap flow velocities wererecorded simultaneously under natural conditions for seven days. During this period, branches were subject to a natural sequence of dry and wet conditions. The whole data set is summarized in Data Set 2 in which reverse sap flow was detected under conditions conducive to leaf wetting by deliquescence of salt, dew formation and interception of intermittent rainfall. Observations of two natural events in which deliquescence of salt on leaf surfaces drove reverse sap flow are highlighted in two data sets. Data Set 3 is a subset of Data Set 2 and contains sap flow data from the three branches in which reverse sap flow was detected under deliquescent conditions in the absence of dew. Data Set 4 is also a subset of Data Set 2 and contains additional data collected when a sudden change in the weather provided an opportunity to study leaf rehydration during a natural foliar wetting event driven by salt deliquescence on leaf surfaces. Once water began to accumulate on leaf surfaces in late afternoon, leaves were harvested from each of the three replicate trees at each time point for measurement of water content and water potential (ΨLeaf) from branches adjacent to those monitoring sap flow. Leaf fresh mass was measured before determining leaf water potential ΨLeaf with a Scholander pressure chamber (1050D, PMS Instruments, Albany, USA), followed by measurements of leaf area and dry mass. Measurements continued until light became too low to work (18:20 pm). Then, one twig with two fully expanded leaves was collected from each of the three trees to measure water uptake by detached leaves. Each twig was incubated in an individual zip-lock plastic bag with moist paper towels for 60 min under darkness at ambient temperature. Then fresh leaf mass and ΨLeaf were determined for one leaf in each pair, followed by determinations of leaf area and dry mass. These data were used to calculate rates of rehydration and water uptake by leaves on detached twigs by assuming that their starting points were the same as those measured at the time of twig collection (18:20). All sap flow data collected under natural or ambient conditions were then pooled to analyse patterns in sap flow velocities in relation to the vapour pressure deficit of the air (Data Set 5) and relative humidity of the air (Data Set 6). Monitoring sap flow under manipulated conditions: data sets 1, 7 and 8. Conditions were manipulated during one night to assess effects of nocturnal Dry, Wet and Ambient treatments on branch sap flow velocity (Data Set 7), and water relations of leaves and twigs together with branch sap flow velocity under common ambient conditions during the following photoperiod (Data Set 8). In late afternoon, six twigs of similar size and orientation were selected on each of the three treatment branches on each tree and allocated to one of six sampling times for measurement of diurnal variation in ΨLeaf. The two youngest, fully expanded leaves on each twig were allocated to either transpiring or non-transpiring treatments. The latter treatment was tightly covered with plastic cling wrap and an outer layer of aluminium foil. Branch treatments were then imposed just before sunset. Wet branches were doused with freshwater to fully wet all leaf and branch surfaces. Six soaking wet sponges each containing approximately 100 g of water were distributed near the designated twigs. Then each branch was sealed inside a heavy gauge black garbage bag. Dry branches each received six envelopes made of 30% shade cloth containing 150 g of dried silica gel. The envelopes were secured near the twigs, and the branches were then covered with a heavy gauge black garbage bag which was then purged with several volumes of dry, compressed air from a scuba tank before the garbage bag was sealed. Ambient branches were left exposed to natural conditions. Each branch was fitted with an i-button (Hygrochron DS1923, Whitewater, Wisconsin) placed in a sheltered canopy position to monitor humidity and temperature during nocturnal treatments. Branches were liberated from Wet and Dry treatments at sunrise (7:30) by removing the garbage bags, sponges and envelopes containing silica gel. Water potentials of paired transpiring and non-transpiring leaves from the designated twigs were measured at six sampling times arrayed at two hour intervals from 30 minutes after sunrise, 8:00 to 18:00 to assess effects of nocturnal treatments on branch water status under common conditions in the subsequent photoperiod. At each sampling time, the designated leaves were collected and water potential measurements were made within minutes at a field lab established approximately 20 m from the study trees. Measurements of leaf fresh mass were followed sequentially by measurements of ΨLeaf, area and dry mass.