Báo cáo lâm nghiệp: "Cavitation in trees and the stems hydraulic sufficiency of woody"

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of Cavitation in trees and the hydraulic sufficiency woody stems M. Tyree and Northeastern Forest Ex- Department of Botany, University of Vermont, Burlington, VT 05405, periment Station, P.O. Box 968, Burlington, VT 05402, U.S.A. the viscous drag in the xylem Introduction over conduits (trachE or vessels). The water ids 1 stress, manifested as a negative pressure, will cause cavitation events - rapid breaks The cohesion theory of sap ascent (Dixon, in the water column in a conduit followed 1914) forms the basis of our current by rapid (1 us) stress relaxation. Within a understanding of the mechanism of water transport in the xylem of plants. Evapora- few ms, this wiill result in a water vapor- filled conduit. The water vapor-filled void tion from cell wall surfaces in the leaf normally expands to fill only the confines causes the air-water interface to retreat of the conduit in which the cavitation has into the fine porous spaces between cellu- lose fibers in the wall. Capillarity (a conse- quence of surface tension) tends to draw the interface back up to the surface of the pores and places the mass of water behind it under negative pressure. This negative pressure is physically equivalent to a tension (a pulling force) transmitted to soil water by a continuous water column; any break in the column necessarily dis- rupts water flow. In xylem, a break in the water column is induced by a cavitation event, and the xylem of woody plants is highly vulnerable to such events (Tyree and Sperry, 1989). The potential exists for a dynamic cycle of water stress leading to loss of hydraulic conductance and further dynamic stress (see Fig. 1Transpiration produces dyna- mic water stress because of the pressure to maintain sap flow gradient required
occurred because of the surface tension several hundred stem segments, record- effects at pit membranes between ing the length, diameter and leaf area conduits (Bailey, 1916). Over a period of attached to each segment, with a num- minutes, an air embolism forms in the bering system showing the interconnec- cavitated conduit, as gas molecules come tion of the segments; 5) calculate the out of solution from surrounding water- dynamics of y development and embolism filled cells. Built-in pathway redundancy in branches under different transpiration ensures that water conduction can contin- regimes. ue, despite limited numbers of cavitations. Calculations previously published (Tyree But each embolism will cause a small loss and Sperry, 1988) were based on a stea- of hydraulic conductance in the stem. A dy-state model of water flow through a loss of conductance means that sap will branched catena, so no account was have to overcome a larger viscous drag to taken of the water storage capacity of maintain the same transpiration rate, thus leaves and stems or of the temporal dyna- resulting in more water stress. The cycle mics of evaporation. This paper presents of events in Fig. 1 is whatI call an ’embo- results from a more realistic, non-steady- lism cycle’. A number of important ques- state model on a 10 m tall cedar tree tions can be asked about this cycle. Is it (Thuja occidentalis L.). A brief review of inherently stable or unstable? (In a stable the conclusions of the steady-state model embolism cycle, a plant can sustain a lim- and the data used will aid in the under- ited amount of embolism and still maintain standing of the more advanced model pre- normal levels of transpiration without fur- sented here. ther embolism.) How much redundancy is built into the xylem of trees? In other words, how much embolism is too much, Overview of the steady-state model thus making the embolism cycle unstable and leading to ’runaway embolism’? ’Hydraulic architecture’, as used by Zim- Recently, Tyree and Sperry (1988) mermann (1978), describes the relation- attempted to answer these questions by ship of hydraulic conductance of the xylem measuring and calculating what might be in various parts of a tree and the amount called the ’hydraulic sufficiency’ of trees. of leaves it must supply. This is quantified In valuing hydraulic sufficiency, 5 steps by the leaf specific conductance (LSC) used and were repeated for 4 dif- were defined as the hydraulic conductance of ferent species: 1measure the field - h flow rate per unit a stem segment (k extremes in evaporative flux from leaves, = pressure gradient) divided by the leaf area E, and field extremes in leaf water poten- supplied. This definition allows a quick tial, yr, 2) quantify the hydraulic architec- estimate of pressure gradients in stems. If ture of branches by measuring the hydrau- the E is about the same throughout a tree, lic conductance of stems versus the then the xylem pressure gradient (dP/dx) diameter of a tree and relate these mea- in any branch can be estimated from: sures to the amount of foliage supported dPldx= EILSC. by each stem; 3) measure the vulnerability of stems to embolism by measuring a The LSC of minor branches, 1 dia- mm curve of loss in hydraulic conductance meter, is about 30 times less than that of versus y during a dehydration; 4) make a stems 150 mm diameter in cedar major hydraulic map of representative branches (Tyree et aL, 1983). Consequently, trees or small trees, by cutting a branch into most of the water potential drop in the
xylem occurs in the small branches and hydraulic path. All shoots are approxi- twigs; of the drop in y! from the soil to the mately equally capable of competing for leaves, about 55% occurs in branches the water resources of the tree. Trees that less than 10 mm diameter, about 35% show strong apical dominance are the from the larger branches and bole, and exception. In these trees, the LSC 15% in the roots (Tyree, 1988). The same remains high or increases towards the hydraulic architecture is observed in other dominant apex (Ewers and Zimmermann, species (Tyree and Sperry, 1989) and this 1984a, b). led Zimmermann (1983) to propose the The vulnerability (see Fig. 2A) curve ’segmentation hypothesis’ to explain the used in model calculations was obtained value of decreasing LSC along with under laboratory conditions by dehy- decreasing stem diameter. Embolism drating cedar branches to known water potentially can occur throughout the tree potentials and then measuring the re- and, by decreasing the xylem conduc- sulting loss of hydraulic conductance by tance, can substantially influence water methods described elsewhere (Sperry et status. According to the segmentation al., 1987). The hydraulic conductivity data hypothesis, embolism will preferentially are shown as a log-log plot (see Fig. 2B) occur in minor branches where LSCs are of conductance versus stem diameter. lowest and consequent xylem tensions are Information on the hydraulic sufficiency greatest. Under severe water stress condi- of cedar was derived by writing a com- tions, peripheral parts of the tree would puter model that calculated the yls that be sacrificed and the trunk and main must develop in different parts of a 2.6 m branches (where most of the carbon sapling under steady-state conditions, that investment has occurred) would remain is, when water flow through each stem functional and permit regrowth. Another segment (kg!s-!), equaled the evaporation important consequence of the hydraulic rate (kg!s-!) from all leaves supplied by architecture is the hydraulic resistance to the stem segment. To do this, the com- water flow from the ground level to all puter model needed an input data set that minor branches, which is approximately amounted to a ’hydraulic map’ of the the same for all twigs, whether the twig is sapling cut into several hundred seg- located near the base of a crown and at the end of a short hydraulic path or at the ments. Each segment was numbered and then coded to show which segment its top of a crown and at the end of a long
base joined. A sample numbering scheme hydraulic map so that it no longer con- for a small branch cut into 11 segments, is tributed to the transpiration stream. A seg- shown in Fig. 3. The hydraulic map also ment was deemed dead when its hydrau- catalogued the leaf area attached to each lic conductance had fallen to 5% of its segment, its length and its diameter. The initial value. In Fig. 2A, a 95% loss of data in Fig. 2B were used to compute a conductance also corresponds to a stem hydraulic resistance of each segment y of about-5.5 MPa. based on its length and diameter. For any Independent field observations on cedar given evaporative flux (E) the computer saplings indicated that E never exceeded model calculated the steady-state water 1.8 x 10- kg-s- - and that rarely fell 5 1M 2 V flow rate through each stem segment; below about -2.0 MPa. The model cor- from the segment’s hydraulic resistance, rectly predicted the observed range of the model calculated the drop in 1/1 across shoot yls for valid ranges of E. The vulner- the segment. Starting with input values of ability curve (Fig. 2A) also predicted that, soil y and root resistance to water flow, under field conditions, the loss of conduc- the model could then calculate the 1/1 of tance ought to average about 10%, if each segment. The model then used shoot y/s never fall much below -2.0. This these values of 1 to calculate the change /1 percent loss of conductance has been in stem resistance (inverse conductance) confirmed on field samples (Tyree and from the vulnerability curves in Fig. 2A. Sperry, 1988). These new resistance values were used At this point important question can an to calculate new y values for the same E. be asked about the hydraulic sufficiency of The calculations were repeated until either cedar stems. Does the vulnerability of stable values of hydraulic resistance cedar stems place a constraint on the (reflecting stable levels of embolism) were maximum rate at which water can flow achieved or until the stem segment was through the stems? If the embolism cycle deemed dead and removed from the (Fig. 1) is unstable, then an increase in water flow rate will result in ’runaway’ embolism and stem death. This question can be answered with the model by either increasing the leaf area attached to the stem segments in the model or increasing E above the maximum rates observed in the field and observing the stability of the embolism cycle. Model calculations that answer this question are illustrated below (see Fig. 4). The solid line in Fig. 4 shows how the average y of all minor branches bearing leaves changed with E, if there were no loss of conductance by embolism. The dotted line shows the average y of all minor branches when embolism was taken into account. The maximum E observed under field conditions is marked by an * the x-axis. The model pre- near dicted that when the loss of conductance in segments exceeded 20-30% then runa-
into 4107 segments. The model took full way embolism would occur leading to seg- account of hovu water storage in stems and ment death if E did not change (that is, leaves affected the tempo of change of yr stomates did not close). The percentage throughout the crown. Tyree (1988) has pre- of all leaf area lost by stem death is shown viously shown that the non-steady-state model by the solid line with triangles. When stem correctly predicted field-observed ranges of yr from field-measured rates of E. The water stor- death starts (shown by points to the right age capacity of leaves was assigned values of the * the water balance of the living ), obtained from pressure-volume curves and in segments improves. In the dotted line, the this paper stem capacitances were assigned a y of dead segments is not included in the value of 0.1 kg-d The following . 1 -MPa- 3 - M changes in the previous model were made for average for points to the right of the * The . this paper: 1) computations were started at mid- water flow rate, v kg-s- through a stem , 1 night of d 1 after an adjustment for the level of segment is given by Ex A, where A is the embolism that might have occurred on previous area of leaves fed by the segment. Prior to days; initial values of stem conductance were stem death, as E increases, then v in- adjusted upward by 4% for stem segments with diameters between 0.2 and 1.0 cm and by 8% creases in proportion causing a propor- for stem segments of
meter in the mid-crown of the cedar tree. increased to 22% and 3% loss of leaf area On d 1 the E values used were those on a had occurred due to runaway embolism. typical sunny day (Tyree, 1988}; on d 2 By the end of d 3, when E peaked at 2.Ox and 3 the values of E were 1.5x and 2.0x higher than on d 1, the loss of conduc- higher, respectively. After the 1 st d (with tance of minor branches reached 40% typical E values), the loss of conductance with a 16% loss of leaf area. The model was about 9 and 5% in minor branches predicted runaway embolism only in (
tained in minor branches compared to that has increased to 18.3% and the mini- area predicted by the steady-state model with a average stem y! has eased off to mum consequent reduction in loss of leaf area. - 2.39 MPa from -2.76 MPa shown in Fig. 5 at 65 h. It is rare to find individual trees that suf- fer significant leaf loss due to drought. This is presumably because stomates close and reduce E before runaway embo- Discussion and Conclusion lism causes leaf loss. When the steady- state model was modified to allow for sto- -2.0 MPa, then matal closure at y The results of the non-steady-state and = embolism was not predicted. The runaway steady-state models were qualitatively value of the model (without stomatal regu- quite similar. When E exceeded a critical lation), is that it shows that, due to embo- threshold value, then runaway embolism lism, cedar operates near the point of caused a patchwork dieback in minor catastrophic xy failure. Similar conclu- em ! twigs as predicted by the plant segmenta- sions (Tyree and Sperry, 1988) have been tion hypothesis. Normal rates of evapora- drawn for maple (Acer saccharum) and 2 tion in cedar closely approached this criti- tropical species: red mangrove (Rhlzo- cal level. The quantitative differences phora mangle) and a moist forest relative between models were in the direction (Cassipourea elliptlca). It is common to expected. In the non-steady-state model, see a patchwork pattern of brown foliage the values of E peaked at 1.8, 2.4, 2.6 and in cedar trees. This might be due to runa- 2.8 x 10- kg-s- on the north, east, Si 2 -m- way embolism in the summer or due to west and south quadrants of the crown, winter dehydration of the sapwood which, respectively. Using these E values in a if not reversed in spring, would lead to steady-state model led to a predicted loss runaway embolism at modest evaporation of leaf area of about 29% over the entire rates in spring. crown. In the non-steady-state model, the Runaway embolism might be a potential loss of leaf area was less (18%). As threat for all woody species. If so, this expected, water storage capacity of stems would suggest a strong selective process and leaves reduced the extremes in y at-
to their theoretical limit of hydraulic suffi- fora number of diverse morphological and ciency. Trees must evolve mechanisms to physiological properties that keep the keep an appropriate balance for carbon water relation of the species in proper balance. These morphological features allocation between leaves (which increase evaporative demand) and stems (which include: leaf area supported per unit stem area, stomatal diameter, stomatal frequen- supply the demand for water evaporated cy (number per unit area) and xylem struc- from the leaves). ture (small versus large conduits). One can speculate that xylem structure (tra- cheids versus vessels) might be much References less genetically mutable than the genetics that determine leaf size and number, sto- Bailey I.W. (1916) The structure of the bordered matal size, frequency and physiology. A pits of conifers and its bearing upon the tension tree cannot improve its competitive status hypothesis of the ascent of sap in plants. Bot. with regard to competition for light and net Gaz. 62, 133-142 assimilation through any process that Dixon H.H. (1914) In: Transpiration and the would increase E without changing the Ascent of Sap in Plants. MacMillan, London less mutable xylem morphology. Typical Ewers F.W. & Zimmermann M.H. (1984a) The hydraulic architecture of balsam fir (Abies bal- field values of E for cedar are about one samea). Physiol. Plant. 60, 453-458 tenth that of broadleaf species. Also, Ewers F.W. & Zimmermann M.H. (1984b) The cedar supports slightly larger leaf areas hydraulic architecture of eastern hemlock per unit stem area than do broadleaf spe- (Tsuga canadensis). Can. J. Bot. 62, 940-946 cies. This can be explained in terms of 2 Sperry J.S., Donnelly J.R. & Tyree M.T (1987) factors that make cedar sapwood less A method for measuring hydraulic conductivity hydraulically sufficient than that of broad- and embolism in xylem. Plant. Cell Environ. 11, leaf species: 1) the vulnerability of cedar 35-40 to cavitation is higher than that of many Tyree M.T. (1988) A dynamic model for water broadleaf species (Tyree and Sperry, 7 flow in a single tree. Tree Physiol. 4, 195-217 1989); and 2) the hydraulic conductance M.T. & Sperry J.S. (1988) Do woody Tyree plants operate near the point of catastrophic per unit sapwood area in cedar is much xylem dysfunction caused by dynamic water less than that of broadleaf species. model. Plant stress? Answers from Physiol. a The of trees may hydraulic sufficiency 88, 574-580 into the evolution of provide new insights Tyree M.T. & Sperry J.S. (1989) Vulnerability of xylem to cavitation and embolism. Annu. Rev. the morphology, physiology and ecophy- Plant Physiol. 40, 19-38 siology of woody plants. For example, up Tyree M.T, Graham M.E.D., Cooper K.E. & until now, it had been presumed that sto- Bazos L.J. (1983) The hydraulic architecture of matal closure under water stress occurred Thuja occidentalis L. Can. J. Bot. 61, 2105-2111 1 primarily to prevent desiccation damage to Zimmermann M.H. (1978) Hydraulic architec- the biochemical machinery of the photo- ture of some diffuse porous trees. Can. J. Bot synthetic system. It is now clear that an- 56, 2286-2295 other important role of stomatal regulation Zimmermann M.H. (1983) In: Xylem Structure is to prevent runaway embolism, while and the Ascent of Sap. Springer-Verlag, Berlin, pressing water conduction through stems pp. 143