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Cross-correlation analysis was used to calculate time lags between the measurements.
This result was in good agreement with the cohesion theory of transpiration. For the whole stem diameter measurements, the treetop lagged behind all other heights and the shortest lags were midway along the stem. Time lags calculated separately for the shrinkage morning and swelling afternoon periods indicated shorter time lags during the swelling periods. The non-destructive methods used show promise in the simultaneous study of flow dynamics of xylem and phloem in trees. However, measuring over-bark diameters does not provide information directly about the water flow in the xylem because some of the change in diameter is due to the shrinkage and swelling in phloem and bark cells.
Sap flows from the roots to the leaves in the xylem and sugars are transported from the leaves to the growing parts or reservoirs in the phloem. The sap flow direction in the xylem is upwards and the flow in the phloem is mostly downwards. Besides the vertical movement, water and solutes also moves radially between phloem and xylem along a decreasing water potential gradient Nobel Measuring sap flow in field conditions is not easy because, for example, the equipment tends to break the cell structure, which may lead to embolization and reduction of conductance near the measuring system.
Phloem is fairly thin and there is considerable interaction with the surrounding cells Nobel ; Peuke et al. Stem diameter changes have been previously used in studies of tree growth and water transport.
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However, there are only a few studies where both xylem and whole stem diameter changes have been measured. Also the frequency of data collection has often been only a few times per hour, which means that shorter lags could not be detected. The results are interpreted in terms of water and solute transport.
The station is located in a homogenous year-old in Scots pine Pinus sylvestris L. The total all-sided leaf area index varied from 4 to 6, the maximum occurring in late August. One end of the frame was set to rest on one plate and the head of the sensor positioned on the other. For the whole stem diameter measurements, the bark was smoothed before attaching the aluminium plates. Each frame and transducer was covered with a conical polyethylene shade to avoid heating by direct radiation.
The frame temperatures were measured using copper—constantan thermo-couples and used to correct for the effect of temperature on the expansion of the frame Sandvik Steel, Sandvik, Sandvixen, Sweden. Download Powerpoint slide A schematic picture showing the experimental design and arrangement of the diameter measurement sensors. The sensor labelling and corresponding heights are indicated on the schematic tree not to scale.
S refers to the whole stem diameter measurements and X to the xylem diameter measurements. A side projection of the tree stem right illustrates the construction of the measurement frame. The attachment plates are attached to the bars on the left and right sides of the stem. The LVDT goes through the bar on the left and is in view behind dash line the bar in front of the stem.
For xylem measurements, the bark, phloem and cambium were removed and covered with the small aluminium plates. For the whole stem measurements the aluminium plates are attached to the smoothed bark. The size of the frame was adjusted to the size of the stem at each measurement height. The stem surface was smoothed and a thin layer of silicone grease applied to the surface before fitting the gauges.
The installation and selection of measurement points was carried out as recommended by the manufacturer Dynamax Results The changes in diameter followed a similar pattern to that reported earlier e. The amplitude of the diurnal cycle was about a tenth of a millimetre at its maximum. Growth was seen only in the whole stem measurements. The linear trend was subtracted from the whole stem diameter change measurements to reveal the diurnal change in diameter.
Download Powerpoint slide Measured above-canopy photosynthetic photon flux density PPFD a , sap flow b and xylem c and whole d stem diameter variation for the period 20—30 June In c and d the measurement heights are labelled from the base to the top see Fig. The amplitude of the whole stem measurements did not vary significantly with respect to height Fig. In order to compare the changes in diameter at different heights the changes in diameter were normalized by dividing the values by the equivalent stem diameter at the corresponding height, the sapwood diameter in the case of the change in xylem diameter measurements.
In contrast to the absolute values, the amplitude of the normalized xylem diameter values did not vary much with height but that of the whole stem measurements increased slightly towards the treetop Fig.
Download Powerpoint slide The normalized diameter variations of xylem a and whole stem b diameter during 20—30 June The standardization was made using equivalent diameter of the shrinking material and the value is given in percentages.
The level was adjusted to zero at the beginning of the measurement period. The measurement heights are labelled from the base to the top see Fig. Time lags were calculated by finding the highest cross-correlation between diameter measurements at any two heights when one was moved in time.
Cross-correlation values were calculated for each day separately and averaged over the measurement period for each sensor. The statistical significance of the differences between the values was tested using a t-test.
This assumes that the data for separate days are independent, which may not be the case. The apparent inconsistency of the xylem time lags the sum of the lags of adjacent measurement heights does not equal the largest time lag results from the averaging and rounding of the values. The data were collected from 20 June to 12 July The measurement heights are illustrated schematically on the tree on the left see also Fig.
The figure is not to scale. The time differences the upper value below each arrow and the corresponding standard errors the lower value were calculated for each sensor pair indicated by the arrows.
The first group of six arrows from the left refer to xylem diameter measurements labelled with X on the tree and the following group of six arrows to the whole stem diameter measurements labelled with S on the tree. The third group of four arrows refer to the comparison of xylem and whole stem diameter measurements at each height. The time differences are averages of daily values over the measurement period and a positive value means that the lower measurement lags behind the upper one.
In the case of the xylem versus stem measurements, a negative time difference means that the stem measurement is behind. A comparison of xylem and stem measurements in each sensor pair showed that at every level the stem measurement was behind the xylem one. Only the differences between the time lags of the three highest xylem measurements were not significant.
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Neither were there any significant differences in the time lags between days with moderate and low relative humidity moderate: Discussion According to the cohesion theory of water movement in plants, transpiration should create a tension gradient that pulls water up inside the xylem.
Xylem is more rigid material than sapwood and contracts less than elastic living tissue, as our results indicate.
However, the large amplitude of the whole stem diameter variations may partly result from water exchange between the xylem and the outer tissue. The cells of the phloem, cambium and bark may serve as a store of water. When transpiration is highest, water is taken from the store most actively and during the swelling period the store is refilled Herzog et al.
The similarity of the amplitudes of the whole stem shrinkage at all heights Fig. The normalized amplitudes Fig. On some occasions, the diameter changes at height S3 Fig. The focus of this study was on the timing of the diurnal diameter change along a tree stem and between the xylem and the outer tissue, and its relation to sap flow dynamics.
Theoretically, the maximum speed of a pressure signal inside some material corresponds to the sound velocity, which, in this case, would give an instantaneous response. The lower limit to the speed of signal propagation is the actual stream velocity, which in this case was of an order of centimetres per hour. The time lags of xylem diameter change were found to increase with increasing distance from the treetop, the lower parts of the stem being behind in most cases.
However, the elastic properties of wood material would also contribute to signal propagation and affect the timing of diameter changes. Taking this into account, the values presented support the cohesion theory, i. The magnitude of the lags on one hand and the similarity between the lags of the shrinkage and swelling periods and those of the whole day on the other hand suggest that storage in the xylem cannot be large.
Comparing the diameter variation of the xylem and the whole stem at the same height revealed that the whole stem diameter change always lagged behind. However, the longer lag we observed at the treetop would suggest that the hydraulic conductivity between the store and the transpiration stream close to the transpiring surfaces was low.
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Furthermore, considering that wood permeability decreases towards the distal parts of the tree in Scots pine Zimmermann , the hydraulic connection between the store and the transpiration stream would be expected to have a high resistance. The observed time lags and diameter variation may also be related to the transport of carbohydrates in the phloem. Since the change in xylem diameter is a minor part of the total stem diameter change and only a thin layer of bark was left to cover the phloem, the stem diameter changes are considered to mainly represent changes in the phloem.
The loading of sugars at the treetop lowers the osmotic potential in the phloem and results in a water flow from the xylem to the phloem. This increases the phloem volume but the net effect on the stem diameter remains small.
The result would be a large time lag between the xylem and the stem measurements at the top of the tree. Lower down in the stem, the growing meristem uses sugars and the osmotic potential increases.
Water would readily flow into the xylem as the tension gradient builds up due to transpiration. This study also revealed some problems in the use of cross-correlation in time lag analysis. Data with periods when the diameter decreased or increased quite linearly, the noise in the measurements affected the correlation coefficient relatively more than the overall trend and the determination of the actual time lags was difficult. This problem was most evident with afternoon and cloudy-day values, although the periods were selected to include at least one of the daily maximum or minimum points of the diameter change curve.
For this reason, the two negative xylem time lags for swelling periods seen in Fig. Nevertheless, this study showed that time lag studies is a useful tool in analysing the flow dynamics within a tree. More detailed and longer-lasting experiments are required.