Author for correspondence: Roman Zweifel Tel: +41 79 703 04 91 Fax: +41 44 739 22 15 Email: email@example.com
• Ultrasonic acoustic emission (UAE) in trees is often related to collapsing water columns in the flow path as a result of tensions that are too strong (cavitation). However, in a decibel (dB) range below that associated with cavitation, a close relationship was found between UAE intensities and stem radius changes.
• UAE was continuously recorded on the stems of mature field-grown trees of Scots pine (Pinus sylvestris) and pubescent oak (Quercus pubescens) at a dry inner-Alpine site in Switzerland over two seasons. The averaged 20-Hz records were related to microclimatic conditions in air and soil, sap-flow rates and stem-radius fluctuations de-trended for growth (ΔW).
• Within a low-dB range (27 ± 1 dB), UAE regularly increased and decreased in a diurnal rhythm in parallel with ΔW on cloudy days and at night. These low-dB emissions were interrupted by UAE abruptly switching between the low-dB range and a high-dB range (36 ± 1 dB) on clear, sunny days, corresponding to the widely supported interpretation of UAE as sound from cavitations.
• It is hypothesized that the low-dB signals in drought-stressed trees are caused by respiration and/or cambial growth as these physiological activities are tissue water-content dependent and have been shown to produce courses of CO2 efflux similar to our courses of ΔW and low-dB UAE.
Plants emit ultrasonic sounds (ultrasonic acoustic emission (UAE)) that can be detected with adequate sensors (Milburn & Johnson, 1966; Tyree & Sperry, 1989; Kikuta, 2003; Laschimke et al., 2006). A widely accepted theory assumes that the acoustic emissions, which range between 20 and 300 kHz (Tyree & Sperry, 1989; Laschimke et al., 2006), are induced by sounds from the abrupt release of tension in the conduit lumen as liquid water at negative pressure is replaced by water vapour very near to vacuum pressure (Hopwood, 1931; Sandford & Grace, 1985; Rosner et al., 2006). The assumption is based on the cohesion-tension theory (Zimmermann et al., 1995; Milburn, 1996), which states that water transport occurs along water potential gradients in the dead tube-like cells of the xylem, with transpiration, water adhesion to cell walls, and surface tension providing the forces necessary to lift the water against gravity (Koch et al., 2004). If the water potential, equal to the negative of the tension of the water column, exceeds a sapwood-specific threshold, the water column ruptures (cavitation), air enters the flow path and the water column in the affected xylem cell becomes quickly vaporized (embolism). This event causes measurable UAE peaks (Tyree & Sperry, 1989).
The widely accepted interpretation of UAE peaks as sounds from cavitations, however, leaves many questions unanswered. Zimmermann et al. (2004) demonstrated that negative water potentials induced by transpiration are not the only mechanisms by which water moves against gravity in trees. Laschimke et al. (2006) hypothesized a bubble system in the conducting xylem elements, which they claim is responsible for water transport and also for acoustic emissions. They maintain that it is the abrupt regrouping of the wall-adherent bubbles that causes the UAE decibel (dB) peaks. Thus, some investigations even question the accuracy of the cohesion-tension theory and its ability to explain water flow and cavitation in plants (Zimmermann et al., 2004; Laschimke et al., 2006). In contrast to these findings, several recent flow and storage models have been able to almost perfectly simulate the water relations of trees according to the cohesion-tension theory (e.g. Steppe et al., 2006; Zweifel et al., 2007).
An even larger question mark about the physical origin of the UAE arises when UAE below the dB range associated with cavitation is included and the focus is no longer on UAE dB peaks only. Most investigations applying UAE detection techniques have filtered out this low-dB range of sound under the assumption that these signals would not be distinguishable from background noise. Interpretations of UAE in trees have consequently almost exclusively focused on the dB peaks. The only exception we found is in Laschimke et al. (2006), who included UAE signals > 26 dB. The technical set-up in our investigation allowed the detection of continuous UAE between the high dB intensities (gt; 35 dB) associated with cavitation and the lowest detectable intensities associated with the background noise of the measurement system (< 25 dB). These signals in the so-called low-dB range (27 ± 1 dB) appeared to have a consistent pattern, which opens up a completely new perspective on the topic. We explored the question of where the signals in the low-dB range originate and how they fit in with the current understanding of UAE peaks.
To investigate UAE, we selected mature, naturally grown Scots pine (Pinus sylvestris) and pubescent oak (Quercus pubescens) at a site where a wide variety of microclimatic and physiological information was recorded and modelled in parallel. The very dry site at Salgesch in Valais, Switzerland, was equipped with an ecophysiological field station (Zweifel et al., 2005, 2006) that continuously recorded microclimatic profiles in the air and soil, as well as physiological parameters (sap flow, stem radius changes (ΔR), and potential evapotranspiration) of several tree species at 10-min intervals. Of special explanatory value was the so-called tree water deficit (ΔW) (Hinckley & Lassoie, 1981), which is deduced from ΔR by de-trending for growth (Zweifel et al., 2005). The de-trending ‘cleans’ the original stem radius fluctuations from the fraction of radial growth. What remains is the water-related fraction of ΔR, which is proportional to the stem water content (Steppe & Lemeur, 2007) as long as there is no substantial depletion of the sapwood by cavitation (Zimmermann, 1983; Zweifel et al., 2000). In addition, photosynthesis, transpiration, leaf water potential (Haldimann & Feller, 2004) and hydraulic properties (Sterck et al., 2008) were manually measured on selected days. Further, the water potential conditions and stomatal regulation of Scots pine (Pinus sylvestris) and pubescent oak (Quercus pubescens) were accurately simulated with a hydraulic feedback model (Zweifel et al., 2007). The model enabled us to obtain valuable information on the flow and storage dynamics of tree water relations of these two tree species and the underlying species-specific physiological properties that cause them to respond differently to microclimate.
We aimed in this study: to prove the existence of consistent low-dB UAE signals; to relate them to high-dB UAE signals associated with cavitation; to explore the origin of the low-dB UAE; and to propose a potential mechanism to explain the signals of both qualities in the light of current ecophysiological knowledge.
Materials and Methods
Study site and trees
The study site near Salgesch (46°19′27″N, 7°34′40″E, 975 m asl) is located in Central Valais, Switzerland, an inner-Alpine valley characterized by a dry climate mainly caused by inner-valley shielding. The valley is oriented SE–NW, with the main storm tracks coming from the west and the wet air masses from the south. Mean annual precipitation over the past 20 yr has been c. 600 mm yr−1. Annual precipitation at the site was 752 mm in 2001, 899 mm in 2002, 495 mm in 2003, and 500 mm in 2004.
Pubescent oak (Quercus pubescens Willd.) and Scots pine (Pinus sylvestris L.) are the most abundant tree species, and juniper (Juniperus communis L.) is the most abundant woody shrub in the vegetation of the site, which is on a south-exposed slope. Tree age was measured to be between 70 and 110 yr for oak and between 100 and 150 yr for pine at Salgesch (Zweifel et al., 2005). The trees investigated were growing in patches (32 m2 in area) surrounded by grass and bare rock. The eight trees included in the measurements had estimated ages of between 50 and 120 yr. The heights of the six selected oaks and the two selected pines ranged from 3.5 to 4.0 m and from 3.5 to 5.0 m, respectively, the stem diameters from 7.2 to 9.5 cm and from 11.9 to 23.2 cm, respectively, and the crown projection area on the ground from 8 to 18 m2 for both species. The soil on this steep south-facing slope (~25°) is shallow with a maximum depth of 0.1–0.3 m at the site of measurement. Below the organic material is a mostly solid rock layer.
Climate data were collected at the site with a solar-powered logging and steering system (CR10X; Campbell Sci, Shepshed, UK). The microclimate was measured in the north- and south-exposed canopies and also outside the patch of trees. The sensor types and installations are described in Zweifel et al. (2006).
Acoustic emission detection
UAEs were detected between 100 and 300 kHz using an acoustic signal conditioner with a peak detector (ASCO-P; Vallen System GmbH, Icking, Germany) in combination with a piezoelectric sensor head (VS150-M; Vallen System GmbH). The analogue signal output represented the UAE peak amplitudes of the respective measurement period. The execution interval of the data logger (CR10X; Campbell Sci) was 20 Hz. The measurements were averaged and stored every 10 min.
About 2 cm2 of bark was removed from the stem 50 cm above-ground and the sensor head was fixed in this hole (1–10 mm deep) to the bare xylem with adhesive tape. About 1 mm of silicon gel was applied between the sensor head and the stem to improve the sound propagation between the wood and the sensor, and to prevent the tissue drying out. The background noise of the measurement system was at 24 ± 1 dB.
The sensitivity of the UAE system was tested by breaking a pencil lead on the surface of air-dried pubescent oak and Scots pine branches and checking whether the system registered noise signal (Fig. 1).
Sap flow measurements
Sap flow was continuously assessed in small north- and south-exposed branches on six oaks and two pines with heat balance gauges (Dynagage; Dynamax, Houston, TX, USA). The distribution of the sap flow gauges within the forest patch is described in detail in Zweifel et al. (2006).
Leaf water potential
The water potentials of needles and leaves (Ψleaf) were measured with a pressure bomb (SKPM 1400; SKYE Instruments, Powys, UK) according to Scholander et al. (1965). Two to five samples were measured per record and then averaged.
Stem radius changes and tree water deficit
Stem radius changes (ΔR) were measured with point dendrometers (ZB01; constructed by the authors) on six oaks and two pines (Zweifel et al., 2006). The dendrometers were mounted 0.5 m above-ground on the north-facing side of each stem (facing up the hill). The electronic part of the dendrometer was mounted on a carbon fibre frame, which was fixed to the stem by three stainless steel threaded rods implanted into the heartwood. The sensing rod was kept pressed gently against the tree stem with a spring. The contact point of the dendrometer head was positioned 1 to 6 mm under the bark surface, but still within the outermost dead layer of the bark. The sensitivity of the dendrometers to temperature and humidity was found to be negligible as a result of the use of a weather-insensitive carbon frame and a temperature-insensitive electronic device (Weggeberpotentiometer LP-10F; Pewatron, Zurich, Switzerland). The resolution of the dendrometers in combination with the data logger used (CR10X; Campbell Sci.) was 0.4 µm.
Hydraulic plant model to simulate water potentials
A tree–water relations model that includes species-specific hydraulic properties and stomatal regulation was used to simulate plant water potentials. The model is based on the hydraulic principles of a flow and storage system (Steppe et al., 2006), and uses light intensity and feedback signals from the crown water potential, the tree water deficit and the current stomatal aperture to calculate stomatal regulation dynamics. For the model to be run, the potential transpiration of the crown, light intensity, and soil water potential are needed. The systemic feedback structure of the model design does not determine the course of stomatal resistances by a sophisticated mathematical formula, but rather formulates what factors have a (weighted) feedback effect on stomatal regulation. Thus, the model output for stomatal aperture and tree water potentials is the self-regulated response of the virtual model tree to the input factors in the form of the climatic conditions. Pre-set parameters for hydraulic resistances, storage capacities and further species-specific physiological properties determine the potential range of (model tree) responses. Model parameters, and the detailed functionality and accuracy of the model, are discussed in Zweifel et al. (2007).
Typical UAE patterns
UAE diurnally fluctuated within a low-dB range (27 ± 1 dB) at night and during sunless periods and abruptly increased to a higher-dB range (36 ± 1 dB) on sunny days. The signal remained in the low range all day only on rainy or very cloudy days (e.g. 15 August 2003; Fig. 2). The patterns were similar for the two tree species investigated.
The low-dB range was found to be distinctly above the background noise of the measurement system (23–25 dB). The transition between the low and high-dB ranges was usually very short, so there were few measurement points between the two extremes (Fig. 3). The minimum and maximum peaks of UAE decreased with increasing tree water deficits (Fig. 3).
Characteristics of the high-dB UAE range
The course of UAE signals within the high-dB range was complementary to the course of sap flow rates: as soon as UAE strongly increased, sap flow sharply decreased and vice versa (Fig. 4). An artefact resulting from a technical deficiency can be excluded because UAE and sap flow were measured with independent systems (different sensor types and data loggers).
To analyse the initial conditions triggering the transition of the UAE signal from the low to the high-dB range, time-points were selected when the dB increase was gt; 5 dB within 20 min. The corresponding physiological and microclimatic conditions of these time-points were then compared with the tree water deficit (Fig. 5). Four main results were derived from the conditions at the time of transition: (1) high-dB signals occurred over almost the entire range of drought intensities; (2) the higher the tree water deficit already was, the more extreme the microclimatic and physiological conditions had to be to induce more high-dB signals: in other words, high-dB signals occurred under more moderate conditions when the tree was well watered than when it was already drought-stressed; (3) after rehydration, the transition-initiating conditions returned to the pre-drought values; and (4) vapour pressure deficit (VPD) (Fig. 5a,d) and potential evapotranspiration (ET) (Fig. 5b,e) had good predictive power in combination with the tree water deficit (ΔW) for the occurrence of upward transitions. Soil water potentials (ΨSoil) had some predictive power for pine, but none for oak (Fig. 5c,f). No such specific patterns were found with other microclimatic variables such as light, wind or temperature (data not shown).
Characteristics of the low-dB range
When we ignored the high-dB range of the UAE signals and instead focused on the fluctuations within the low-dB range only, an unexpectedly close parallelism between the course of UAE and the inverse course of ΔW became apparent. Both courses showed a harmonic diurnal rhythm for pubescent oak as well as for Scots pine (Fig. 6). At night and when it was raining, UAE signals increased in parallel with the rehydration of the tree stems, and during the day, when the trees were transpiring, the signals decreased in parallel with the shrinking of the stems (e.g. 27 June 2004; Fig. 6a). Moreover, the daily minimum UAE records were closely related to the corresponding mean daily values of ΔW over two seasons (Fig. 7).
Two qualities of UAE signals
Ultrasonic acoustic emissions in trees have been measured and described many times in the past 30 yr (Tyree & Sperry, 1989; Perks et al., 2004; Holtta et al., 2005). Yet, such analyses have always focused on the peaks within the measurable UAE range where these peaks have been counted and interpreted as the ultrasonic sound from cavitations (Zimmermann, 1983). In contrast to these studies, we analysed the UAE dynamics within the entire intensity range of measurable UAE and found not only the well-known old pattern but also a new one. This new pattern appeared in the low-dB range only and has, as far as we know, never been described before. The course of the low-dB UAE gradually fluctuates in parallel with the inverse stem radius changes de-trended for growth (ΔW), which in turn represent the tree water status (Hinckley & Lassoie, 1981; Zweifel et al., 2001, 2005). The origin of these low-dB UAEs is unclear but they seem to be closely connected to tree water relations (Fig. 6). In considering possible mechanisms, we first discuss the characteristics of the two different UAE signals in relation to each other and the respective tree water relations. Secondly, we focus on the question of whether the changing stem water content might explain the attenuation of a stable sound source, and thirdly we discuss possible origins of the low-dB UAE. Finally, we suggest a possible mechanism to describe the phenomenon physiologically.
Dependence on water relations
Tree water relations seem to trigger not only cavitations but also the low-dB UAE. On a typical day, the low-dB UAE course follows the inverse course of ΔW. The more the stem expands as a result of refilling of bark tissue with water (Steppe et al., 2006), the louder the UAE becomes in the low-dB range (Figs 4, 6). The inverse pattern can be observed during the day. With increasing daylight and the onset of transpiration, the stem shrinks and the low-dB UAE starts to decrease. The decrease is gradual and continuous until cavitation occurs, at which point UAE abruptly jumps to the high-dB range.
The cavitation-inducing conditions were found to be associated with shifting thresholds for VPD and potential transpiration (Fig. 5), depending on the current tree water status. The higher the ΔW was, the more extreme the microclimatic conditions had to be to induce more cavitations. Thus, cavitation did not necessarily occur only in extremely dry weather. A reasonable explanation for the shifting threshold for cavitation is given in Tyree & Sperry (1989). They concluded that, during a drought period, the most vulnerable xylem elements embolize first at a certain water potential and are then followed by the less vulnerable xylem elements at lower water potentials over the following days. Modelled leaf water potentials (Zweifel et al., 2007) support this idea and suggest an almost constant cavitation threshold at c. −3 MPa for pubescent oak and c. −1 MPa for Scots pine (Fig. 8). Similar values for oak and pine species have been reported in other studies (Jackson et al., 1995; Cochard et al., 1996; Perks et al., 2004; Holtta et al., 2005). When there is enough water to rehydrate the stem tissues, the embolized xylem elements become refilled and the threshold for further cavitation is re-set to more moderate conditions. This explanation is also supported by early studies of Milburn (1973) and Pena & Grace (1986).
In contrast to the high-dB UAE, the low-dB UAE showed no abrupt changes and seemed to depend completely on ΔW (Fig. 7). During periods with cavitations, the measurements suggested that the low-dB sound was continuous without interruption. The low-dB UAE restarted its own course almost exactly on the continuously measured course of the inverse ΔW after a ‘cavitation break’. Even very slight changes in ΔW, for example between two cavitation periods, were detectable in the low-dB UAE, and were particularly evident in pubescent oak on 27 June 2004 (Fig. 6a). However, it was not possible to monitor the exact course of low-dB UAE during cavitation, as it was covered by the high-dB signals.
No attenuation as a result of the changing tissue water content
To find the origin of the low-dB UAE, it is essential to know whether there could be a constant sound source whose intensity is changed by attenuation, for example by the changing tissue water content. If this cannot be excluded, it can be assumed that the low-dB UAE could also be of abiotic nature. However, Tyree & Sperry (1989) concluded that the velocity of sound propagation through water-saturated wood is about the same as in dry wood, but higher than in pure water. This means that the ultrasonic waves measured on the surface of tree stems are mainly propagated through cellulose and not through water. Further, our test of the sensitivity of the measurement system indicates that sound sources detected as UAE are likely within a distance of < 30 cm (Fig. 1). We therefore exclude the possibility that attenuation continuously changes with stem water content and maintain that it is more likely to be attributable to a change in the low-dB UAE source itself.
Possible origins of the low-dB UAE
The high-dB UAE is very likely to be induced by cavitations, which can be explained by the cohesion-tension theory. There is, however, no comparable theory to explain the pattern of the low-dB UAE and the corresponding tissue water content. To find possible origins of the low-dB-UAE, we looked for physiological dynamics that produce a similar diurnal pattern to that of the low-dB UAE course. Dynamics such as those of sap flow, which could produce a water flow noise in the low-dB range, did not fit at all (Fig. 4). The highest low-dB UAE occur before dawn, which is when the least water movement occurs because the water potential gradients from the day before will have mostly levelled off within the tree by then.
Another possible origin of sound could be the mechanical noise of the stem radius expanding and shrinking. Kikuta & Richter (2003) reported a slight increase in UAE signals depending on the hydration status of living Evonymus latifolia. However, they counted UAE peaks and did not continuously record low-dB UAE. Their finding might support our result that high-dB signals occur sooner in well-watered plants rather than being a strong indication of a mechanical noise. Further, this theory does not explain why the low-dB UAE does not stop when the stem stops expanding towards dawn (Fig. 6). If stem expansion or contraction really produced a UAE-relevant sound, we would expect the diurnal low-dB UAE course to resemble the first deviation of ΔW, which is obviously not the case.
Two processes recently reported in the literature suggest themselves as possible origins of low-dB UAE: the respiration and the metabolic growth activity of cambium or ray parenchyma cells. These processes, however, were not measured in this study. The two processes are coupled to each other (Maunoury et al., 2007; Teskey et al., 2008) and, according to Saveyn et al. (2007a,b), produce diurnal courses of CO2 efflux from the stem similar to the courses of the bark water content under dry conditions. What seems to be crucial is the actual degree of drought stress. As long as the water content, and thus the turgor pressure, are sufficient for growth, the diurnal course of respiration follows temperature (Maunoury et al., 2007; Saveyn et al., 2007b, 2008; Teskey & McGuire, 2007). However, as soon as drought stress becomes more severe, respiration no longer follows temperature but is more closely related to stem water content (Saveyn et al., 2007a,b). The missing water in the cambium and phloem cells is assumed to inhibit the biochemical activity of growth and respiration, independent of the current temperature. This finding has been recently corroborated at a higher level of organization by a report of distinct increases in total ecosystem respiration during wet periods after a drought (Granier et al., 2007).
The measurements obtained from Scots pine and pubescent oak at our site indicate that the trees were at a drought stress level at which respiration corresponded to tree water deficit and not temperature. Under such conditions, the low-dB UAE and the stem water content match stem respiration, which, in turn, is coupled to metabolic activity. At night and during periods when the turgor pressure in the cambium increases, radial growth occurs (Steppe et al., 2006) and leads to an increase in the maintenance metabolism. As a consequence, respiration increases, which probably causes the increase in UAE within the low-dB range. The low-dB signal we measured could, it seems, be produced either by the expansion of newly built cells (not water-related tissue expansion) or by an increased concentration of CO2 (Saveyn et al., 2008; Teskey et al., 2008).
A potential mechanism to explain low-dB emissions
One recently published theory by Laschimke et al. (2006), which partially contradicts the cohesion-tension theory, suggests that a bubble system causes the ultrasonic emissions. They demonstrated how air-saturated water forms clusters of gas bubbles in the pits and holes of the wall in the water-conducting systems of both a 4-yr-old Ulmus glabra and a perforated glass capillary. These micro-bubbles are initially spherical (c. 1–50 µm in diameter, depending on pore size), and with increasing size coalesce to form flat bubble aggregates. The abrupt regrouping of the wall-adherent spherical bubbles may well be the reason for the UAE peaks (gt; 26 dB) they detected. Further, the authors concluded, from studying the temporally highly resolved waveforms of UAE, that coalescing bubbles lead to a ‘violent vibration’ which might be another source of UAE in the low-dB range. Their theory could probably help to explain our high- and low-dB UAE in a mechanistic way.
In combination with our observations of the timing of increased low-dB UAE, the increased production of CO2 during the night, as a result of the turgor-dependent physiological activity of the cambium (growth and maintenance metabolism), could explain why more gas is infiltrated into the water-conducting system. The additional CO2 leads to more bubbles or increases the size of bubbles, which in turn increases the number of coalescences and thus the intensity of the low-dB UAE. Our speculation that the low-dB UAE is not affected by the high-dB UAE, but is just acoustically drowned, is consistent with the fact that a water-conducting cell undergoing cavitation is well insulated from its neighbouring cells and, thus, the low-dB-producing bubble system is not disturbed.
We found convincing evidence that, in addition to the well-documented cavitation-induced UAE, there is another biotic source of UAE that leads to low-dB signals which closely follow the stem radius changes de-trended for growth. The origin of the low-dB UAE is uncertain. However, there is some evidence that these signals are produced by stem tissue processes that depend on water saturation such as growth, respiration and maintenance metabolism. If this assumption is correct, we would expect a change from the water dependence of the low-dB UAE to a temperature dependence, with consequently a different temporal pattern under less drought-stressed conditions. To test this and to obtain a more complete mechanistic explanation, further investigation of low-dB UAE in trees is needed.
We would like to thank David Newbery, Ernst Bhend and Lukas Zimmermann from the Institute of Plant Sciences, University of Bern, for their support. The work was part of the project ‘Tree response to climate change’. The manuscript was edited by Silvia Dingwall.