The cones were collected in a 20-year-old stand of Scots pine in central Sweden. The respiration rate at 15°C showed pronounced seasonal variation and in.
ForestSci., Vol. 27, No. 2, 1981,pp. 267-276 Copyright1981,by the Societyof AmericanForesters
The SeasonalCourse of Respirationand Photosynthesisin Strobili of ScotsPine SUNE LINDER ERIK TROENG
ABSTRACT.The respirationof current and 1-year-oldconesof Scotspine was investigatedon excisedconesduring one season.The coneswere collectedin a 20-year-old stand of Scotspine in central Sweden. The respiration rate at 15øCshowed pronouncedseasonalvariation and in light some of the carbon dioxide releasedin respirationwas retixed. The average refixation capacityof 1-year-oldconeswas 60 percentand of current conelets79 percentat light saturation. The seasonalperformanceof cone respirationwas estimated, using records of air temperature and photon flux densities from the stand from which the cones were collected. The refixation of carbon dioxide reduced respiratory losses by 31 percent. Calculated on a carbon basis, the respirationcost of cone productionwas 50 percent of the final cone weight after correction for carbon refixation. Conescovered with aluminJurefoil in mid-Julyhad lower seedweight than the controls had when harvested
in late October.
FOREST SCI. 27:267-276.
ADDITIONAL KEY WORDS. Pinus sylvestris, cone respiration.
THE FEMALE STROBILUSIN SCOTSPINE (Pinus sylvestris L.) needs 2 years to develop into a mature cone. The strobili are first visible in late May or early June at the distal end of the 1-year-old shoots. Pollination occurs in the middle of June and the conelets grow to a diameter of approximately 5 mm during the first growing season. In May the following year the cone begins to increase in dry weight; the final weight is reached in late August. Fertilization occurs almost 13 months after pollination (Sarvas 1962), which in Central Sweden means late June or early July. The strobili of Scots pine are green throughout their development and turn brown during the last month of maturation. It has been reported by a number of authors that cone production depresses stem growth in conifers (cf. Els and others 1965). However, there is no agreement on the size of cone crop that causes a growth restriction, and its mechanism is not completely understood. Gas exchange studies in conifers have mainly concentrated on needle photosynthesis;much less is known about the gas exchange of non-green parts of the tree (cf. Linder 1979), only a few reports are available concerninggas exchange in conifer
strobili.
Ching and Ching (1962) followed the development of Douglas-fir strobili from April until maturity in late September. They reported an increase in respiration
The authors are with the Section of Forest Ecophysiology, Swedish University of Agricultural Sciences,S-75007 Uppsala, Sweden.This work was carriedout within the SwedishConiferousForest Project (SWECON), supportedby the SwedishNatural ResearchCouncil, the SwedishEnvironmental Protection Board, the Swedish Council of Forestry and Agricultural Research, and the Wallenberg
Foundation.The authorsthank Mr. J. Parsbyfor his skillfultechnicalassistancewhen buildingthe assimilationchamber. They are also indebtedto Mr. T. Lohammar for his help in processingthe data and calculatingthe field performanceof respiration. Manuscriptreceived 24 March 1980.
VOLUME 27, NUMBER 2, 1981 / 267
rate until pollination, after which there was a gradual decreasein the respiration rate until September, when it was so low that they could no longer measureit. Ching and Fang (1963) studiedthe utilization of labelled glucosein developing cones of Douglas-fir. They also measured the respiration rate in cone scalesand seed during the 30 days following pollination. They found a decreasingrate of respiration in both scales and seed during the period of study. On all occasions the seedhad a higher rate of respirationthan the scales. Dickmann and Kozlowski (1970) measured cone respiration in Pinus resinosa in light and darknessand found a 50 percent reduction in respiratory evolution of carbon dioxide in light. Rook and Sweet (1971) grafted strobili from Pseudotsuga menziesii onto seedlings and thus created a strongsink for currently producedphotosynthates.They could not demonstratean increase in needle photosynthesisas an effect of increased sink strength, but found a high refixation capacity in the cones. When the sink strength was increased by darkening the cone with aluminium foil, the allocationpattern of needle-producedphotosynthatewas altered. Dickmann and Kozlowski (1968) studiedthe mobilization of currently produced and stored photosynthatesin Pinus resinosa. They found that developingreproductive tissueshad the highestpriority when currently producedphotosynthates were mobilized. The bulk of carbohydrates for developing cones came from 1year-old needles, but smaller amountswere also suppliedfrom older age classes of needles.
The presentreport is part of a researchprogrammeconcerningthe gasexchange in a 20-year-old stand of Scots pine in central Sweden (cf. Linder and Troeng 1980).The programmehas been carried out within the SwedishConiferousForest Project (SWECON). MATERIALS
AND METHODS
The study was performed at J•idrafisEcologicalResearchStation in central Sweden (60ø48'N.; 16ø30'E.;alt. 180 m). J•idrafisis the main researchsite within the Swedish Coniferous Forest Project (SWECON), and consists of a number of standsin differing age classes(cf. Axelsson and Brfikenhielm 1980). During 1978 current and 1-year-old cones were collected frequently from the middle of March until the 1-year-old cones were mature at the end of October. The cones were collected from trees on untreated plots in a 20-year-old stand of Scots pine. A detailed descriptionof the stand can be found in Flower-Ellis and others (1976). The respiration rate of the cones was determined in an open gas exchange systemby using an infrared gas analyser and a temperature-controlledassimilation chamber in a laboratory adjacent to the stand. The respiration rate was determined in light and darknessat 15øC.The assimilationchamber in the laboratory is part of a large gas exchangesystem for field measurementsof photosynthesisand transpirationin the 20-year-old stand of Scots pine. The systemis described in detail in Linder and others (1980). The cylindrical assimilation chamber consisted of a brass tube (height 25 cm, diameter 17.5 cm) with a "perspex" window at the upper end. The chamberwall was cooled by circulating water in a tube soldered to the outside of the brass tube. The water was cooled in a temperature-controlledwater bath (Type SB 2, 1500 W; Grant Instruments, U.K.). The cooling coil was surroundedby heating foil (hotfoil, Wolverhampton, U.K.) and the preset chamber temperature was obtainedby intermittent heatingby the foil. The heatingfoil was controlledby a proportionaltemperaturecontroller(NE 221, Chino Works Ltd, Japan). The photonflux densityin the chamberwas regulatedby raisingand lowering
268 / FOREST SCIENCE
a metal halogenlamp (Osram HQI TS-400 W) above the chamberand the photon flux densities within the chamber were measured by a quantum sensor (LI-190, Lambda Instruments, USA). Air and cone temperatureswere measuredby thertoocouples(coppar/constantan). Immediatelyafter collection,coneswere placedin the assimilationand allowed to equilibratein darknessat 15øC.The coneswere placed in the chamberwithout any specialorientationto the incominglight. The numberof conesin each sample varied over the investigatedperiod, becauseof the increasingsize of the cones. At the beginningof the year, 100 cones per sample were used; at the end of the seasonthere were 15 cones per sample. When a constantrespirationrate was obtained in darkness, the light responsecurve of refixation was determined by measuringthe respirationat different levels of light, startingwith the light saturating levels > 1300/xE m-2 s-L The respirationrate was recorded as soon as the rate had been constant during 15 minutes. During measurements,cone temperaturewas recorded by insertinga thermocoupleinto one of the cones. Cone temperaturewas always kept as closeto 15øC as possible and the temperature in the cones never deviated from the preset temperature more than _+0.2øC.In spite of the small temperature variations all respirationrates were recalculatedas the respirationrates at 15.0øCusing a Q•0 of 2, a value that was found to be very constant during the period of study. The fresh weight of the cones was determined before measurement of respiration. Afterwards they were dried for 24 hours at 85øCfor dry weight determination.
In mid-July, a number of 1-year-oldconeswere darkened, in situ, by wrapping them in aluminium foil. The foil remained on the cones until they were mature in the end of October. control.
The
same number
of uncovered
cones was used as a
CALCULATIONS
The refixationcapacitywas expressedas the reductionin respirationrate, at light saturation, as percent of the rate in darkness. An estimateof respirationduringthe year was obtainedby usingthe respiration rates and refixation capacitiesfound in the laboratory, and air temperaturesand photon flux densities recorded with 10-minuteintervals in the stand where the cones were collected. The meteorologicalmeasurementsused are included in the biometeorological programme carried out at the station (cf. Perttu and others 1977).
Respiration under field conditionswas calculated for the period January-October, using the following formulae: T-
15
Re = Rl•(t)'2 •0
(1)
where Ra = dark respiration, Ri, = dark respirationat 15øC,t = time, T = air temperature in the stand.
The net respirationwas calculatedas dark respirationminusrefixation: T-
15
where R• = net respiration, co= refixation capacity at light saturation, I = photon flux density, I• = photon flux density at 50 percent of maximum refixation; the value used was 160/xE m-2 s-1 (cf. Fig. 3). Daily values for R1, and cowere obtained by linear interpolation from Figure
VOLUME 27, NUMBER 2, 1981 / 269
M
A
M
J
J
A
S
0
FIGURE 1. The dry weight of 1-year-old cones of Scots pine from March to October, 1978. Moving average n = 3.
2 and Figure 4, respectively. For each day the integrals of equation (1) and (2) were calculated using the records of air temperature and photon flux densities. Based on profile measurementsof photon flux density within the canopy, the photon flux densitiesrecorded above the stand were reduced by 25 percent in the calculations to give a reasonable estimate of the light conditions at the level at which the cones are found in the canopy. RESULTS
During the summer the current strobili grew to a weight of approximately 0.08 g, which is in good agreement with the weight of 1-year-old conelets in early spring (cf, Fig. 1). The development of the 1-year-old conelets started in midMay, and final size was reached in late August. The highest relative growth rate of the conelets was found in June, but the greatest increase in dry weight was 1 month later (Table 1). From March to June there was a decrease in the ratio of dry weight:fresh weight, but this ratio increased again from July until the cones were mature
in late October.
There was a pronounced seasonalvariation in respiration rate, both when ex-
TABLE 1. Mean relative growth rates (mg g-• day-•), increase in dry weight (mg month-•), and the ratio dry weight.fresh weightfor 1-year-old cones of Scots pine during the period of April-October, 1978. Initial cone weight in March, 0.08 g; final weight 2.60 g. Relative
growth rate Period
Growth
Dryweight
(mg g-• day-•)
(mg month •)
Fresh weight
April May
6.6 33.3
19 193
0.58 0.36
June
43.3
800
0.21
July August September
20.1 6.1 1.6
950 430 120
0.26 0.45 0.54
--
0.68
October
270 / FOREST SCIENCE
--
I I I I I I I J 1
o
M
A
M
J
J
A
S
0
FIGURE 2. The seasonalcourse of respiration of 1-year-old cones of Scots pine. Open symbols: respirationrate per unit dry weight (mg g-• h-l), filled symbols:respirationrate per cone(mg cone-• h-l). The measurementswere made at 15øCin darkness.
pressedon a dry weightbasisor whencalculatedper cone(Fig. 2). The respiration rate increased during the early stage of cone development and had maximum values during June and early July. From the middle of July until October there was a gradual decreasein respirationrate when expressedon a dry weight basis. This is to be seenas an effect of the increasingamount of structuraltissue. When respirationrate was expressedper cone, the peak value was obtainedin early July. On the last sampling, 15 October, the respirationrate was very low, 0.004 mg g-• h-• or 0.009 mg cone-• h-•. At that time of the year the current conelets had a respirationrate of 0.91 mg g-• h-• or 0.065 mg cone-• h-•.
100
I
!
I
o
.50
.x_
o
o
I
soo
I
•ooo
I
•soo
2000
FIGURE3. The relationshipbetweenrefixationof carbondioxide releasedby dark respirationand photonflux density.The valuesare givenas the percentageof the respirationrate in darkness.The example is for 1-year-old cones in July 1978.
VOLUME 27, NUMBER 2, 1981 / 271
lOO
I
I
I
I
I
I
I
5o
M
A
M
J
J
A
S
0
FIGURE4. The seasonalvariation in refixation capacity(•o)in 1-year-oldcones(filled symbols),and current conelets (open symbols)of Scots Pine. The values shown are the refixation capacitiesat light saturationon different occasionsduring the season.The measurementswere made at 15øC and a photonflux densityof >1300/aE m 2 s-•.
When respirationwas measuredin light, the effiux of carbon dioxide decreased (Fig. 3). The relationship between respiration rate and photon flux density was similarto a "normal" light responsecurve of leaf photosynthesis.Light saturation occurred at photon flux densitiesabove 900/•E m-2 s-•. The shape of the curve was very constantduring the season,but the rate of refixation at light-saturating photon flux densitiesexhibited some variation (Fig. 4). In springthe percentage
refixation was approximately30 percent, but this increasedduring May to 75 percentand was thereafterrather constantuntil the end of September,when there was a sudden decrease. At the same time as the fall in refixation capacity occurred, the coneslost their greencolourand turnedbrown. The averagerefixation capacity from May to September was 61 percent. The current conelets had a muchhigherrefixationcapacity(• 79 percent)and showedno decreasein October (Fig. 4). Cones covered with aluminium foil had a lower weight and lower weight per 1000 seeds than the controls had when harvested in late October (Table 2).
The seasonalperformance of cone respiration was estimated, using the results obtainedin the laboratory (Figs. 2-4) in combinationwith recordsof light and air temperaturesfrom the stand. Both dark respiration, Ra, and net respiration, were calculated (Fig. 5A) as well as the daily refixation in percent of the dark respiration (Fig. 5B). The resultsare expressedper cone; therefore the respiratory losses are low from January to May, in consequence of the low weight of the cones. During this period, the daily refixation was equivalent to 5-10 percent of the carbon released by dark respiration. From mid-May to mid-June, there was a steadyincreasein both dark and net respiration,causedby the increasein cone weight and by higher air temperatures.A dark respirationrate of approximately 40 mg cone-• day-• was maintaineduntil the end of July. The sharppeak in daily respirationin late July was causedby a spellof warm weather. From the beginning of August there was a constant decreasein daily respiratory losses, and respiration ceasedcompletely in late October. The daily refixation was more than 20 percentfrom May to September,with the highestvaluesduringJune, when daily refixation was close to 40 percent (Fig. 5B). The total estimated dark respiration, Ra, from January to October was 3.43 g CO2 cone-•. The net respiration, Rn, during the same period was 2.38 g CO2 cone-1. Assuminga carboncontentof 50 percentin the cones(cf. Larcher 1969), 272 / FOREST SCIENCE
TABLE2. Thedry weightof conesand seedof 1-year-oldconesin late October. Someof the coneswerecoveredwithaluminiumfoil on 19July to avoidrefixation of carbon dioxidereleasedby dark respiration.8-25 conesper sample. Tree
number
Weight per
and cone cover Tree
Weight/cone (g)
S.E. (percent)
Percent of control
1,000seeds (g)
Percent of control
1
Control Foil
2.57 2.82
5 6
-110
6.46 4.87
-75
2.94 2.57
11 7
-87
4.51 4.04
-90
2.38 2.07
11 10
-87
4. I 1 3.71
-90
Tree 2
Control Foil Tree 3
Control Foil Tree 4
• Control Foil
4.09
10
--
4.72
--
3.70
8
90
4.46
94
2.67 2.60
10 14
-97
5.03 4.51
-90
Tree 5
Control Foil
• 30 E
15
I
•
I
I
I
I
I
I
•
I
80
"' 6C o
J
F
M
A
M
J
J
A
S
O
FIGURœ5. The calculatedseasonalcourseoœrespirationin 1-year-oldconesoœScots pine. A: Dark •esp[ration,•=, (solid line) and net respiration,•,, (brokenl•ne) durin• 1978.The respirationrate
expressedper coneand day. B: The daily percentageof refixafiono• carbondioxidereleasedby d•k respiration.
VOLUME 27, NUMBER 2, 1981 / 273
and ignoring the respiratory lossesduring the first year of development, the carbon demand for the development of one cone would be: cone biomass 1.3 g C + 0.94 g C (Ra) = 2.24 g C. From this, the refixation should be subtracted;thus, 2.24 g C - 0.29 g C = 1.95 g C. The cost of producing one cone in terms of Ra was 72 percent of the final weight; in consequenceof refixation, this cost was reduced to 50 percent. DISCUSSION
The relative growth rate of 1-year-old cones during the period May to July was similar to the growth rates reportedfor Pinus resinosa(Dickmann and Kozlowski 1969), Pinus radiata (Sweet and Bollmann 1971) and Pseudotsuga rnenziesii (Ching and Ching 1962) for the same stage of development (cf. Fig. 1, Table 1). The rapid increase in dry weight of the cones coincides with the period for the development of current shootsand stem growth, and will therefore influence the distributionof current and storedphotosynthates.In Pinus resinosait was shown that currently produced photosynthates was preferentially mobilised by reproductive tissues, and that l-year-old needles contributed the major part of carbohydratesto cones and current shoots(Dickmann and Kozlowski 1968). Minor amounts of current photosynthateswere supplied from 2-year-old, and from 3-year-old needles. In the stand where the present study was performed, it was shown by Ericsson (1978) that the l-year-old needles were the only needles supplying the current shootsduring their initial development. The effect of the competition between developingcones, needle and stem growth for the current photosynthateshas been shown for other conifers to result in reduced needle growth (Tappeiner 1969)and decreasedannual ring width (Eis and others 1965). Grafting strobili from Pseudotsuga menziesii onto seedlingsreduced vegetative growth of the seedlings,especially root growth (Rook and Sweet 1971). The respiration rates found (Fig. 2) were lower than those reported for Pseudotsugamenziesii (Ching and Ching 1962, Ching and Fang 1963), but could be an effect of the differences in the methods used. Ching and Ching (1962) and Ching and Fang (1963) investigated the respiration of Douglas-fir cones, using Warburg respirometry, on two to four scalesthat had been dissectedfrom the cone. By doing so they altered the partial pressureof carbondioxide and oxygen, compared with the situation in the intact cone, thus probably increasingthe respiration rate more than the wound
effect alone.
The maximum rate of respiration at the beginning of July coincides with the time of fertilization of Scots pine strobili (Sarvas 1962). The decrease in respiration rate during maturation was in Pinus resinosa reported to be accompanied by transformationof reserve materialsinto structuralcomponentsin the form of cellulose and lignin (Dickmann and Kozlowski 1969). The relationship between photon flux density and cone respiration was similar to the results for bark photosynthesisin aspen (Schaedle and Foote 1971, Foote and Schaedle 1976) and Scots pine (Linder and Troeng 1980). The average retix-
ation capacity was higher than that reportedfor Pinus resinosa(Dickmann and Kozlowski 1970) but when calculated as rate of photosynthesis,much lower than the values given for cone photosynthesisin Pseudotsugamenziesii(Rook and Sweet 1971).
The increasein refixation capacity in early summer (Fig. 4) was similar to the increasein needlephotosynthesisthat takesplaceduring early summer(cf. Linder and Troeng 1980). The fall in cone photosynthesisin October does not follow needle photosynthesis,but is an effect of the final phaseof cone maturation. The refixation capacity in current coneletswas unaffectedduring this period. The cost of cone respirationwas estimatedindirectly in earlier studiesby anal-
274 / FOREST SCIENCE
ysis of ring width (Eis and others 1965, Tappeiner 1969), and reduction in needle biomass (Tappeiner 1969). In the present study it was possible to obtain an estimate of the respiratory lossesduring cone development,as well as an estimate of the gain by refixation of carbon dioxide releasedby dark respiration(Fig. 5).
The respiratorylossesduring the secondyear of developmentwere 0.94 g C for a cone with a final weight of i.3 g C. The loss was reduced by 31 percent as an effect of cone photosynthesis.This did not agreewith the conclusionby Dickmann and Kozlowski (1970) that cone photosynthesisis of minor importancefor the carbon balance
in conifer strobili.
The effects of darkening the cones in mid-July to avoid refixation of carbon dioxide were small, even if there was a tendencytowards lower cone weightsand lower seed weight for cones that were darkened, as compared with the controls (Table 2). Similar results were obtained by Pearsonand Lawrence (1958), when they prevented bark photosynthesisin aspen. However, when they girdled the darkened stem sections,the effect was pronounced.Since developingconeslike growing stems are strong sinks, they will have a high priority when the photosynthatesare allocated, and the sink strengthshouldincreasewhen refixation is hindered. This was clearly shownby Rook and Sweet (1971), who could alter the translocation pattern of labelled photosynthatesby covering the cone with aluminiurn foil.
An annual carbon balancefor a tree from the investigatedstand was established
by 3,grenandothers(1980).In thisbalance, noconeswereincluded, but75cones on the 14-year-old tree in question would require as much carbon as was used in stem growth, i.e., 6 percent of the annual photosyntheticproduction. Using litterfall data from a 120-year-old stand at J•tdrafis(Flower-Ellis and Olsson 1978) the cost of cone production in a mature stand of Scots pine may be estimated. An averagelitter productionby cones of 210 kg ha-• year-• during a
4-year period was reported by these authors. This value was higher than could be expectedfrom estimatesfor Scotspine basedon data from the National Forest Survey (Hagner 1965). The discrepancy is probably caused by current conelets being included in the litterfall data, but not in the counts by the National Forest Survey. Assuming a carbon content of 50 percent, the annual cone production in the stand would be 105 kg C ha-1 year-1. The respiration costs would then be 52.5 kg C ha-• giving a total of 157.5 kg ha-• year-•. The annual production of stem wood in the present stand is 3-4 ms ha-1 year •. Using a density of 400 kg m-a and a carbon content of 50 percent, the production of stem wood would be 600800 kg C ha-• year-•. The respiration cost for that amount of wood production would be 400-550 kg C (cf. Linder and Troeng 1981) giving a total cost of 1000 to 1350kg C ha-• year-1. From this it may be seenthat duringa "normal" year, cone production equals 10-15 percent of the stem wood production. Since the growth of reproductive organsseemsto have a high priority when current photosynthates are mobilized, the effect on stem wood production is likely to be pronounced during years with large cone crops. LITERATURE
CITED
•GREN,G., B. AXELSSON, J. G. K. FLOWER-ELLIS, S. L1NDER, H. PERSSON, H. STAAF, andE. TROENG. 1980. Annual carbon budget for a young Scots pine. In Structure and function of
northern coniferousforests--an ecosystemstudy (T. Persson,ed). Ecol bull (Stockholm)32: 307-313.
AXELSSON,B. and S. BR$•KENmELM. 1980. Investigation sites of SWECON--biological and physiographicalfeatures. In Structureand function of northernconiferousforests-an ecosystemstudy (T. Persson, ed). Ecol Bull (Stockholm) 32: CnI•G, T. M., and K. K. CraiG. 1962. Physical and physiologicalchangesin maturing Douglas-fir cones and seed. Forest
Sci 8:21-31.
VOLUME 27, NUMBER 2, 1981 / 275
CHING, T. M., and S.C. FANG. 1963. Utilization of labeled glucosein developingDouglas-firseed cones. Plant Physiol 38:551-554. DICKMANN, D. I., and T. T. KOZLOWSKI. 1968. Mobilization by Pinus resinosa cones and shoots of C•4-photosynthatefrom needlesof different ages. Am J Bot 55:900-906. DICKMANN, D. I., and T. T. KOZLOWSKI. 1969. Seasonalvariations in reserve and structural componentsof Pinus resinosa cones. Am J Bot 56:515-520. DICKMANN,D. I., and T. T. KOZLOWSKI. 1970. Photosynthesisby rapidly expandinggreen strobili of Pinus resinosa.
Life Sci 9:549-552.
EIS, S., E. H. GARMAN,and L. F. EBELL. 1965. Relation between cone productionand diameter incrementof Douglas-fir(Pseudotsugamenziesii(Mirb.) Franco), grandfir (Abies grandis(Dougl.) Lindl.), and western white pine (Pinus monticola Dougl.). Can J Bot 43:1553-1559. ERICSSON,A. 1978. Seasonalchangesin translocationof •4C from different age-classesof needles on 20-year-oldScots pine trees (Pinus silvestris). Physiol Plant 43:351-358. FLOWER-ELLIS,J. G. K., and L. Ol•ssoN. 1978. Litterfall in an age• seriesof Scots pine standsand
its variationby components duringthe years1973-1976.SwedConifForestProj Tech Rep 15, 62 p. FLOWER-ELLIS,J. G. K., A. ALBREKTSSON, and L. OldSSON.1976. Structure and growth of some
young Scotspine stands:(1) dimensionaland numericalrelationships.Swed Conif Forest Proj Tech Rep 3, 98 p. FOOTE, K. C. and M. SCHAEDLE. 1976. Physiologicalcharacteristicsof photosynthesisand respiration in stemsof Populus tremuloidesMichx. Plant Physiol 58:91-94. H^GNER,S. 1965. Cone crop fluctuationsin Scotspine and Norway spruce.Stud For Suec 33, 21 p. LARCHER,W. 1969. Physiologicalapproachesto the measurementof photosynthesisin relation to dry matter productionby trees. Photosynthetica3:150-166. LINDER, S. 1979. Photosynthesis and respirationin conifers.A classifiedreferencelist, 1891-1977. Stud For Suec 149, 71 p. LINDER, S., and E. TROENG.1980. Photosynthesisand transpiration of 20-year-old Scots pine. In Structureand function of northernconiferousforests•an ecosystemstudy(T. Persson,ed). Ecol Bull (Stockholm)32:165-181. LINDER, S., and E. TROENG. 1981. The seasonalvariation in stem and coarse root respirationof a 20-year-old Scots pine (Pinus sylvestrisL.). Mitteil der Forstl BundesoVersuchsanstalt, Wien. (In press.) LINDER, S., a. NORDSTR6M,J. PARSBY,E. SUNDBOMand E. TROENG. 1980. A gas exchangesystem for field measurementsof photosynthesisand transpirationin a 20-year-old standof Scots pine. Swed Conif Forest Proj Tech Rep 23, 34 p. PE^RSO•, L. C., and D. B. L^wR•c•. 1958. Photosynthesisin aspenbark. Am J Bot 45:383-387.
PERTTU,K., ]k. LINDGREN, A. LINDROTH, and B. NoRris. 1977. Micro-and biometeorological measurementsat JSdrafis.Instrumentation and measurementtechniques. Swed Conif Forest Proj Tech Rep 7, 69 p. ROOK, D. A., and G. B. SWEET. 1971. Photosynthesisand photosynthate distribution in Douglas-fir strobili grafted to young seedlings.Can J. Bot 49.'13-17. S^Rv^s, R. 1962. Investigationson the flowering and seedcrop of Pinus silvestris.Corem Inst For Fenn 53:1-198.
SCHAEDLE,M., and K. C. FOOTE. 1971. Seasonalchangesin the photosyntheticcapacityof Populus tremuloides
bark. Forest Sci 17:308-313.
SWEET,G. a., and M.P. BOLLMANN. 1971. Seasonalgrowth of the female strobilusin Pinus radiata. N Z J For Sci 1:15-21.
T^PPEINER,J. C. 1969. Effect of coneproductionon branch, needle and xylem ring growth of Sierra Nevada Douglas-fir. Forest Sci 15:171-174.
276 / FORESOt SCIENCE
