Báo cáo khoa học: "Gas exchange in young Scots pine following pruning of current shoots"

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Original article Gas exchange in young Scots pine following pruning of current shoots B Langström E Troeng 1 University of Agricultural Sciences, Department of Ecology and Environmental Research, Swedish Box 7072, S-750 07 Uppsala; 2 Swedish University of Agricultural Sciences, Division of Forest Entomology, S-770 73 Garpenberg, Sweden (Received 8 November 1990; accepted 12 February 1991) Summary — A pine shoot beetle attack was simulated by cutting all current shoots in the upper of 2 20-yr-old Scots pines (Pinus sylvestris L) while gas exchange was followed continuously crown before and after the shoot removal. Net photosynthetic rate and mesophyll conductance of the 2-yr- old shoots decreased by 50% immediately after pruning but recovered within 10 d after the shoot = removal. The quantum yield was also much lower after cutting. Water status and transpiration rate were not systematically affected by the treatment. Possible explanations of the photosynthetic decrease could be an accumulation of assimilation prod- ucts decreasing the mesophyll conductance of carbon dioxide to the chloroplasts or changes of the carbon allocation pattern. Scots pine / shoot-pruning / photosynthesis / quantum yield / field performance Résumé — Échanges gazeux chez des jeunes pins sylvestres après élagage des pousses de l’année. Une attaque du scolytide Tomicus piniperda a été simulée en coupant toutes les pousses de l’année dans la partie supérieure de la couronne de 2 pins sylvestres (Pinus sylvestris L) de 20 ans. Pendant le même temps, les échanges gazeux ont été suivis de façon continue avant et après l’opération. Le taux net de photosynthèse et la conductance du mésophylle des pousses de un an décroît d’environ 50% immédiatement après l’élagage, mais récupère dans les 10 jours suivant le re- trait des pousses. Le rendement quantique diminue aussi beaucoup après la coupe des pousses. L’état hydrique et le taux de transpiration ne sont pas affectés systématiquement par le traitement. La diminution de la photosynthèse pourrait être expliquée par une accumulation d’assimilats ayant pour effet de diminuer la conductance du mésophylle pour le dioxyde de carbone vers les chloro- plastes ou par des modifications dans le shéma d’allocation du carbone. pin sylvestre / élagage des pousses / / rendement photosynthèse quantique / performance le terrain sur
increased photosynthetic capacity of re- INTRODUCTION maining needles was hypothetically con- sidered to be one likely explanation for the It is known that insects can cause growth observed results. Thus, combining such a losses in forest trees, eg by consuming fol- simulated attack with continuous gas ex- iage and buds (for references, see Kulman change measurements in trees in situ 1971). Besides quantitative losses in could improve the understanding of carbo- terms of reduced stem growth, insect ac- hydrate dynamics and growth responses of tivity also has qualitative effects on physio- attacked trees. logical processes in the tree. Such report- To test this hypothesis, we simulated a ed effects include increased nitrogen shoot beetle attack by cutting all cur- pine (Piene and Percy, 1984; Ericsson et al, rent shoots in the upper crown of 2 20-yr- 1985) and reduced carbohydrate levels old Scots pines while in situ gas exchange (Ericsson et al 1980a, 1985) in remaining of 1-yr-old needles was followed continu- needles, increased photosynthetic capaci- ously before and after the "attack". ty in regrowth foliage (Heichel and Turner, 1983), and increased amounts of defen- sive chemicals such as tannins and phe- MATERIAL AND METHODS nolics (eg Wagner, 1988). It has also been suggested that water and light conditions for remaining foliage may at least tempo- The experiment was carried out in a 20-yr-old rarily be improved as a result of defoliaton stand of Scots pine (Pinus sylvestris L) situated at Jädraas, Sweden (60° 48’ N, 16° 30’ E, alti- (Ericsson et al, 1980b). Altogether, some tude 180 m). The stand, growing on glacifluvial of these physiological changes counteract sand, has been described by Flower-Ellis et al, the detrimental effects of the foliage loss, (1976) and details of the site have been given and may be important in the recovery pro- by Axelsson and Brakenhielm (1980). cess of the defoliated tree. Gas exchange was measured in situ by an In a field experiment comparing induced open system with cylindrical assimilation cham- bers made of thin perspex. The chamber was at- beetle attacks on caged trees and artificial- tached to a base-plate fixed in a bifurcated hold- ly pruned trees, both treatments produced er, which enabled the chamber to be mounted in unexpectedly small growth losses in young the natural position of the shoot to be studied. Scots pine (Pinus sylvestris L) trees as The chamber temperature followed ambient compared to previously reported natural temperature by an air-cooling system. Carbon attacks by the pine shoot beetle, Tomicus dioxide was measured by an infra-red gas ana- lyzer (UNOR 2, Maihak, Hamburg, Germany) piniperda (L) (Coleoptera, Scolytidae) and water vapour concentration by dew point (Ericsson et al, 1985). The pruning pattern mirrors (Walz Mess- und Regeltechnik, Effel- well as the induced attacks were very as trich, Germany). Air temperature was measured similar to the attack pattern observed un- by thermocouples (copper/constantan) and pho- der natural conditions. The discrepancy in ton flux density was recorded by quantum me- growth reduction as compared to earlier ters (LI-190 Lambda Instruments, USA). Each studies was thought to be a result of chamber was provided with a separate quantum meter placed horizontally outside the chamber. younger trees being used in the experi- Rates of photosynthesis and transpiration were ment. A later study confirmed that older expressed on a projected needle area basis, the comparatively strongly to trees react more needle area of each branch being determined at needle loss than young trees (Langström the end of the experiment by a leaf area meter et al, 1990). Among different compensato- (LI-300, Lambda Instruments, USA). Stomatal ry mechanisms discussed in these studies, conductance for the flux of carbon dioxide
by Langstr6m et al (1990). Gas exchange was the stomata and mesophyll conduc- through then followed continuously to September 4th. To tance for the flux from the stomatal cavity to the chloroplasts were calculated according to Lud- evaluate gas exchange data, the average diur- low and Jarvis (1971).Further details of the gas nal course of 4 periods of 10 d before and after measurement system are given by pruning as well as daily photosynthetic input exchange Linder et al (1980). Gas exchange data together were calculated for each branch. The apparent with climatic data were collected, stored and quantum yield was calculated from the linear processed by a computer (PDP 11/40) at the part of the photon flux density response curve field site (cf Engelbrecht et al, 1980). obtained from field data. The daily light use effi- ciency was calculated from average daily values Needle water potential was measured occa- of photon flux density and the net CO uptake 2 sionally (table I) with pressure chamber tech- during the 24-h period, thus including the respi- nique based on the design of Waring and Cleary ration losses during the night hours (cf Troeng (1967) and further developed by Hellkvist et al and Linder, 1982). (1980). For these measurements, 1-yr-old nee- dles from the third whorl from the top on the The total number of records from each as- south side of the trees were collected in the mid- similation chamber varied between 3 100 and dle of the day. 3 335. Separate data from each of the cham- bers were collected at least every 45 min In 1982 budbreak occurred at the end of throughout the measurement period and later May. Shoot growth finished at the beginning of condensed to average values. By comparing July while needles reached their final length in gas exchange data between trees and between the middle of August. Three trees of height 2.5- periods before and after the pruning treatment, 2.8 m standing close to each other were used some general information was obtained con- for the study. Assimilation chambers were cerning the reactions of gas exchange to a large mounted on last year’s needles on the main reduction of needle biomass. branch axis of south-facing branches of the third whorl from the top. Gas exchange measure- ments started on June 16, 1982 and on July 27, most current shoots of the 6 uppper whorls, in- RESULTS cluding the branches with the assimilation cham- bers, were pruned from 2 of the trees while the third tree was kept intact as a control. The total The period before shoot pruning was char- needle biomass was thus reduced by 25%, = acterized by low precipitation and above and that of the current shoots by 70%, simulat- = average air temperature (cf Lindroth 1985). ing a heavy attack of the pine shoot beetle. The Water stress in the stand increased slowly clipping procedure has been described in detail
during July and reached a maximum at the the first after pruning (fig 1) and the days daily quantum yield was considerably low- beginning of August, as indicated by par- er for the cut trees during the first 10 d af- tial stomatal closure and midday depres- ter shoot pruning (fig 2). The decrease af- sion (Troeng, 1985). Precipitation during ter pruning was primarily caused by low the period July 5-August 5 was < 5 mm photosynthetic rates towards the end of and soil water potential (Jansson and Hall- the day while photosynthesis during morn- din, 1979) at the end of July was ≈ 0.1 ing hours was similar to that of the control MPa in the layer 0-30 cm (Lindroth, 1985). (fig 3). Transpiration data were analysed The water stress conditions disappeared for the 3 trees before and after pruning, but after 40 mm rainfall on August 6. since no drastic changes nor any syste- Daily net photosynthetic performance matic trends in transpiration were found, was similar for the 3 shoots during the pe- data are not reported in detail here. Water riod before pruning (fig 1).The control potential measurements (table I) indicated shoot had slightly higher photosynthetic similar water stress situation for all trees a performance during all periods before July before shoot-pruning. After pruning, the 27. This was due to the fact that photon control tree showed slightly lower water flux density was 10% higher for the con- = potential records while the general water trol branch due to less shading from sur- stress disappeared after the heavy rain fall rounding trees. Since calculated quantum August 6. on yields from photosynthetic light response Calculated conductances showed that from each tree significant- not curves were the mesophyll conductance was generally ly different, net photosynthetic efficiency considered to be equal before shoot was pruning. The records of daily photosynthet- ic input showed a drastic decrease during
thesis of current shoots under attack by a beetle declined drastically as a result of lower than stomatal conductance and was wilting of the damaged shoot, whereas the the limiting factor for photosynthesis. Aver- 1-yr-old needles on the same axis reacted age stomatal conductance during daytime with reduced photosynthesis soon after the was between 2 and 4 mm.s while meso- -1 collapse of the current shoot (Troeng et al, phyll conductance seldom exceeded . -1 1.5 mm.s This was in accordance with 1979). earlier results obtained on the same stand In the present study, where we cut the (Troeng and Linder, 1982). Even during observed a similar dras- shoots, current we the water stress period in the end of July tic but transient drop in photosynthesis and mesophyll conductance was lower than mesophyll conductance. Apart from cutting stomatal conductance. Daily mean values the shoots instead of using beetles, the of mesophyll conductance decreased con- main difference was the whole-tree- siderably after the shoot pruning on July approach in the present study as com- 27, but increased again to levels before pared to one single attack in the previous pruning within 10 d (fig 4). The photosyn- study. However, knowing the indepen- thetic decrease after pruning was in good dence of individual branches (Langström agreement with the decrease of mesophyll et al, 1990), the similar outcome of the 2 conductance (cf figs 1, 4). studies is not surprising. Simulating a pine shoot beetle attack by artificial pruning has been shown earlier to be a good mimic of DISCUSSION natural attacks (Ericsson et al, 1985; Langström et al, 1990). similar experiment in the One possible explanation for the de- Previously, a stand demonstrated that photosyn- crease in photosynthesis after pruning same
could be the carbon balance of the tree. It the results being similar for all pruning, trees. This observation is supported by the is known from other investigations in the data which did not indicate same stand (Ericsson, 1978; 1979) and transpiration any differences in stomatal conductance elsewhere (Senser et al, 1975) that current between the trees either before or after needles accumulate starch as long as they pruning. Also, it is known from the same are still growing. As needle growth finished stand that transpiration and stomatal con- in the middle of August, current needles ductance between individuals vary more were still a strong sink at the time of prun- than photosynthesis (Troeng and Linder, ing at the end of July. Consequently, when 1982). Thus the most likely conclusion is the current shoots were cut off, the main that the marked decrease in net photosyn- sink for photosynthetic products of 1-yr-old thesis of the experimental trees just after needles disappeared and an accumulation cutting was not caused by water stress, of carbohydrates may have taken place, but merely by the shoot pruning treatment. thereby inhibiting photosynthesis (cf net Similar results have been obtained for de- Neales and Incoll, 1968). Lake (1967) sug- foliated trees in the same stand (Ericsson gested that assimilate accumulation may et al, 1980b). decrease mesophyll conductance and Rackham (1966) has stated that such a After partial defoliation, remaining decrease could be an effect of accumulat- leaves needles may increase their pho- or ed starch grains in the chloroplasts. Thus, tosynthesis (Maggs, 1965; Sweet and Wa- this source-sink relationship could explain reing, 1966). Heichel and Turner (1983) the decrease of photosynthesis and meso- found that regrowth foliage of defoliated phyll conductance immediately after prun- maples displayed increased photosynthe- ing. Also, the diurnal pattern of photosyn- sis capacity. On the other hand, King et al thesis during the days after pruning (fig 3) (1967) has suggested that photosynthesis supports this hypothesis. During the sec- of a specific wheat leaf can be regulated ond 10-d period after cutting photosynthet- by the demand for assimilates from that ic rates and mesophyll conductances were leaf. In such a case a low demand due to back to normal. This could be due to car- loss of a strong sink would decrease pho- bohydrate transport to new sinks (eg the tosynthesis. root system), thus decreasing accumulat- Our results support the latter view. Also, ed carbohydrates in the source needles. the results of a previous study in the same However, no attempt was made to follow stand support the same hypothesis the carbohydrate flow or carbohydrate (Troeng et al, 1979), where net photosyn- content in different organs of the trees. thesis of 1-yr-old needles decreased simul- taneously with the photosynthetic collapse Removing most current shoots de- of the beetle-attacked current shoot. The creased the transpiring needle area con- observed decrease in photosynthesis was, siderably and improved the water balance however, transient and may well have of the experimental trees as shown by been compensated for by a possible later needle water potential measurements car- increase in net photosynthesis due to an ried out 2 d after cutting. Midday values of increase in needle nitrogen of remaining needle water potential of 1-yr-old needles needles. The fertilizing effect of shoot- were then ≈ 0.2-0.3 MPa lower for the pruning (ie, a clear and lasting increase in control tree than for the experimental needle nitrogen) has been documented in Unfortunately, measurements were trees. the same stand (Ericsson et al, 1985; carried out only once before shoot-
handling and simulating technique used in Langström et al, 1990), as has the relation- the Swedish Coniferous Forest Project. In: ship between needle nitrogen concentra- Structure and Function of Northern Conifer- tion and photosynthetic performance (Lind- ous Forests-An Ecosystem Study (Persson er and Ingestad, 1977). Since no increase T, ed) Ecol Bull 32, 65-71 of net photosynthesis was observed in the Ericsson A (1978) Seasonal changes in translo- cut branches towards the end of the study cation of C-14 from different age-classes of period we postulate that the possible com- needles on 20-year-old Scots pine trees (Pi- nus sylvestris). Physiol Plant 43, 351-358 pensatory photosynthetic effect develops more slowly than the increase in needle ni- Ericsson A (1979) Effects of fertilization and irri- the seasonal changes of carbohy- gation on trogen (cf Langström et al, 1990). This is in drate reserves in different age-classes of contrast to defoliation experiments where needles on 20-year-old Scots pine trees (Pi- remaining needles reacted with an in- nus sylvestris). Physiol Plant 45, 270-280 creased uptake of 14 within a week after C Ericsson A, Larsson S, Tenow O (1980a) Ef- needle removal (Ericsson et al, 1980b). fects of early and late season defoliation on Hence, the different photosynthetic reac- growth and carbohydrate dynamics in Scots tion to defoliation as compared to shoot pine. J Appl Ecol 17, 747-769 pruning may be caused by other reasons. Ericsson A, Hellkvist J, Hillerdal-Hagströmer K, Larsson S, Mattsson-Djos E, Tenow O In conclusion, we found a drastic but (1980b) Consumption and pine growth - hy- transient decrease in photosynthetic per- potheses on effects on growth processes by formance and a minor improvement in wa- needle-eating insects. In: Structure and ter status of the shoot-pruned trees. No ev- Function of Northern Coniferous Forests-An Ecosystem Study (Persson T, ed) Ecol Bull idence of increased compensatory 32, 537-545 photosynthesis due to improved nitrogen, Ericsson A, Hellqvist C, Langström B, Larsson water or radiation status was found during S, Tenow O (1985) Effects on growth of sim- the months following shoot pruning. Such ulated and induced shoot pruning by Tomi- a development may well have occurred cus piniperda as related to carbohydrate and during the next growing season, but stud- nitrogen dynamics in Scots pine. J Appl Ecol ies did not cover that period. 22, 105-124 Flower-Ellis JGK, Albrektsson A, Olsson L (1976) Structure and growth of some young Scots pine stands: dimensional and numeri- ACKNOWLEDGMENTS cal relationships. Swed Conif For Project, Tech Rep 3, 1-98 We would like to thank C Hellqvist for field assis- Heichel GH, Turner NC (1983) CO assimilation 2 tance, A Lindroth for computer assistance and F of primary and regrowth foliage of red maple Lieutier for with the French summary. helping (Acer rubrum L) and red oak (Quercus ru- us brum L) response of defoliation. Oecologia 57, 14-19 Hellkvist J, Hillerdal-Hagströmer K, Mattson- REFERENCES Djos E (1980) Field studies of water relations and photosynthesis in Scots pine using man- Axelsson B, Brakenhielm S (1980) Investigation ual techniques. In: Structure and Function of sites of the Swedish Coniferous Forest Pro- Northern Coniferous Forests-An Ecosystem ject-biological and physiographical features. Study (Persson T, ed) Ecol Bull 32, 183-204 In: Structure and Function of Northern Conif- Jansson PE, Halldin S (1979) Model for annual erous Forests-An Ecosystem Study (Pers- water and energy flow in a layered soil. In: son T, ed) Ecol Bull 32, 25-64 Comparison of Forest Water and Energy Ex- B, Lohammar T, Pettersson change Models (Halldin S, ed) Elsevier, Am- Engelbrecht L,Sundström KB, Svensson J (1980) Data sterdam, 145-163
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