Báo cáo khoa học: "Stem cracks in Norway spruce in southern Scandinavia: causes and consequences"

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Original article Stem cracks in Norway spruce in southern Scandinavia: causes and consequences A Persson Swedish University of Agricultural Sciences, Department of Forest Yield Research, S-77698 Garpenberg, Sweden 17 June 1993) 1st September 1992; accepted (Received Summary — Stem cracks in Norway spruce (Picea abies L, Karst) have been recognized as a prob- lem in southern Sweden since 1980. Stands 15-20 years of age that had been planted at a wide spacing on fertile sites were mostly unaffected. Damage was most severe on the larger trees with wide annual rings and a low basic density. Cracking frequency, which is partly under genetic control, varied between provenances as well as clones. Microscopy revealed that most cracks developed during the period of latewood formation. Hot, dry weather, which started in July, promoted cracking. To prevent cracking, it is recommended that suitable provenances be planted on fertile sites at square spacings not more than 1.5 m. Furthermore, clones prone to cracking should be excluded from breed- ing programmes. density / annual ring / stress-grading / Picea abies stem crack / basic Résumé— Les fentes du tronc chez l’épicéa commun en Scandinavie méridionale : causes et En Suède méridionale, les fentes du tronc d’épicéa commun (Picea abies (L) Karst) conséquences. ont été identifiées pour la première fois comme un problème technologique vers 1980. Les peuplements âgés de 15-20 ans plantés à grands espacements sur des sols fertiles ont été les plus abîmés, et c’est surtout sur les plus gros arbres présentant des cernes larges et une densité de bois faible que l’on a trouvé les plus gros dégâts. La fréquence des fentes du tronc peut être contrôlée génétiquement, au moins en partie. Des variations ont été trouvées entre différentes provenances et clones. Des études au microscope ont montré que la plupart des fentes sont apparues pendant la saison de formation de bois final. Une période de temps chaud et sec au mois de juillet a également influencé l’apparition de fentes du tronc. Les conseils suivants sont donnés aux sylviculteurs : plantations de provenances convenables sur des sols fertiles en carrés ne dépassant pas 1,5 m ; les clones présentant une ten- dance à se fendre doivent être éliminés. bois / cerne / «stress-grading» /Picea abies fente du tronc / densité du
INTRODUCTION Appearance usually 0.5-6 m long, occa- The cracks were Background and frequently extended sionally longer, from the cambium into the pith. Most cracks showed a slight bend to the left, although During this century, the widespread crack- virtually straight cracks also occurred. Dur- ing of Norway spruce (Picea abies L Karst) ing dry periods cracks reached a width of has occurred on a number of occasions. up to 20 mm, whereas during moist periods Flander (1913) reported cracks occurring they closed again. Most cracks that were in 1911 in Germany, and cracks developing wide during the dry summers of 1982 and the same year were also discussed by 1983 remained closed during the moist sum- Knuchel (1947). Cracking in 1947 was mer of 1984. The cracks eventually closed reported from Sweden by Anon (1948), and were only visible as a scar in the bark in from Denmark by Buchwald (1948) and early 1992. from Britain by Day (1954). Between 1980 and 1983 a marked increase in stem crack- Internal cracks, not visible on the surface ing in Norway spruce was observed in of the stems, were observed sporadically. southern Sweden. During the same period, In most cases they were found on trees that stem cracking was also observed in Den- had been felled and crosscut because they mark and southern Norway. However, since also had visible cracks. Figures 1 and 2 1983 hardly any stem cracking has been show logs with external cracks and a cross- section of a stem with both external and noted. internal cracks. problem became apparent when The selecting candidates for plus trees as a base for further tree breeding. Stem cracks often Earlier studies led to the rejection of otherwise well-suited trees, and already selected candidates had to be rejected when cracks were found dur- Various attempts have been made to deter- ing later inspections. mine why cracking occurs. Flander (1913) and Knuchel (1947) described stem cracks formed in connection with the dry year of Stands affected 1911 and ascribed cracking to drought and heat respectively. A thorough analysis of stem cracking in Most commonly, cracked trees were found conifers was carried out by Day (1954). His in plantations on abandoned pasture/agri- study was based on the numerous cracks cultural land in southern Sweden, up to that appeared in various parts of Britain dur- about latitude 60° N, planted with a spac- ing the latter part of 1947. The author con- ing of 2 m or wider. The extensive areas of cluded that cracking was caused by an abandoned agricultural land planted with abnormal drought, which was most severe spruce in the early 1960s were often during August 1947. affected. Although less common and severe, cracking also occurred in naturally regen- Eriksson et al (1975) studied a combined erated stands, especially where early and clonal and seedling seed orchard in Swe- heavy precommercial thinnings had been den. About 4% of the stems showed cracks, and no difference in cracking frequency was carried out.
the causes and objectives of determining found between provenances. In an exten- of the stem cracking. Some sive Norwegian study (Dietrichson et al consequences of the findings of the studies involved have 1985), carried out during approximately the been reported (Persson, 1985a, 1985b; same period as the present study, late sum- mer drought was considered to be the most Persson et al, 1987). likely triggering factor for many of the cracks. MATERIALS AND METHODS Objectives Trials and stands examined The increase in the frequency of cracking during the early 1980s caused forest owners general importance of the prob- the To assess to question the suitability of the seedlings were made within the trials lem, observations recommended and supplied by the nurs- listed in table I. All these had known provenance, eries of the Swedish Board of Forestry, the origin and treatment history. Through an enquiry main supplier for private forest owners in to forest owners, 150 stands with cracked spruce Sweden. Mainly for this reason, a research were reported, out of which about 100 were programme was initiated in 1984 with the inspected during 1984.
because abnormal cell formation is initiated in the Methods cambium once the crack opens. The stage of development at that moment could be deterined, eg, earlywood in the 6th-8th cell line, initial stage Inventory of latewood formation or after the cell formation had ended. Year of cracking and time of the year Inventory methods varied, the most thorough of cracking were determined on 160 samples. being used in the experiments listed in table I. To do the inventory in the stands, temporary sam- ple plots were established in some of them. But Wood density and annual-ring width once some experience had been gained, most data were observed without using instruments. density of the 4.5-mm-thick increment The basic determined using the mercury immer- cores was Sample collection sion method (Ericson, 1959). At the same time, the widths of the annual rings were determined. In In 54 of the 100 stands discs or 12-mm-diameter some cases the basic density of complete stem increment cores were taken from cracked stems. discs was also determined. While still green, their The crack was included in the core. Samples volume was measured by submerging them into were also collected from the undamaged side of water. After drying them in a drying cabinet at cracked stems with a standard size (4.5 mm diam- 80°C to constant weight, which normally took eter) increment borer. In addition, a correspond- 48-72 h, they were weighed. ing core was taken from an equally thick nearby stem, free from external cracks. Weather Microscopic studies Weather data for the years 1975-1983 were obtained from official Swedish statistics. Maxi- Cut wood surfaces were studied under a light mum day-time temperature and daily precipita- microscope. Both transparent and non-transpar- tion for the period May to September were the ent specimens were examined. The time at which variables of greatest interest. cracking had occurred could be determined
Timber strength and fibre dimensions mation; ina few cases they developed dur- ing earlywood formation and in other cases during the period when no new cells were When studying the seed orchard described by being formed. The number of cracks per Eriksson et al (1975), test sawings and fiber-size year and their time of occurrence in relation determinations were carried out in addition to the methods used for other trials and stands. After to cell division are presented in table II. sawing and drying, the timber was machine stress-graded according to methods described by Brundin (1981). Importance of various factors RESULTS Weather conditions A study of the relationship between cracking Which trees crack? frequency and weather data was carried out between 1975 and 1983. During each of the years in which many cracks formed during Cracks were most frequent in trees between 18 and 30 years of age with a the cell division period (1975, 1982 and diameter at breast height of at least 100 1983, table II), a period with hot and dry mm. Crack width varied between 2 and 20 weather started in July. No similar hot, dry mm. The average annual diameter growth spell occurred in any of the other years dur- during the 5-yr period prior to cracking had ing the study period, nor did cracks form been at least 8 mm. Trees facing gaps and during cell division during any of these years, those along forest edge tended to crack except 1981. The hot and dry weather dur- more frequently than trees deeper in the ing 1982 and 1983 continued into August stand. and may have caused the cracks dated to the period without cell division. Occasional cracking occurred during years with no Time of cracking remarkable weather features. On the other hand, a high frequency of cracking was reported from 1969, which had a remark- The microscopic studies revealed that most cracks had developed during latewood for- ably hot summer.
Site All sites in the study where cracking had occurred were of high fertility. Provenance In the 3 IUFRO 1938 provenance experi- ments no tree had cracks, although all were fertile sites. The dense initial planted on 1.2 x 1.2 m, was certainly impor- spacing, ca tant in this respect. Trees in the 10 experi- ments comprising the 1969 series estab- lished by the Institute for Forest Improvement were planted with a 2.0 m spacing. Only 3 experiments had a notice- able amount of cracks, but the frequency was too low to allow detailed analyses. Two experimental sites belonging to the IUFRO 1964/68 series were more thor- oughly studied: Abild in Sweden at latitude 56° 57’ N, longitude 12° 44’ E, altitude 65 m; Bjerkøy in Norway at latitude 59° 12’ N, lon- gitude 10° 28’ E, altitude 10 m. These data were collected and used by Dietrichson et al (1985) and thereafter by courtesy made available also to this project. The results from the 2 experiments are shown in figure 3. To get enough material, the provenances have been assembled into zonal groups. Mean diameter at breast height was higher in the Bjerkøy than in the Abild experiment as was the crack frequency (about 7 and 2%, respectively). On average, the frequency of cracked higher among provenances with trees was Genotypes of individual trees rapid diameter growth than among slow- growing ones. Two zonal groups had a Studies in 2 clonal seed orchards have cracking frequency higher than expected revealed considerable differences in dispo- based on their diameter growth, namely Slo- sition to cracking between clones. Although vakia and, to a lesser extent, Romania. not an experiment by design, a seed orchard Three zonal groups had a cracking frequency has the various clones intermixed in an effi- that was lower than expected based on their cient way, thereby eliminating most site dif- diameter growth, namely Finland, northern ferences. Furthermore, conditions in seed Poland and a group containing the Baltic orchards are conducive to the formation of states, Belorussia and western Russia.
cracks since the spacing is wide and the provenance used was Rycerka, from south- soil is well fertilized. Poland, and the site index was G32-34, ern which means that the dominant height at In one of the seed orchards, Slogstorp age 100 years was assumed to be 32-34 in southern Sweden (Scania), clones of pure m. This is a high (but not an extreme) value. Swedish origin (S, 25 clones), clones from Many of the Swedish spruce plantations on Swedish stands of continental European abandoned agricultural land have a higher origin (C, 10 clones) and clones of Polish site index. origin (P, 10 clones) can be compared. From each of the 45 clones, about 150 grafts Figure 4 (modified from Persson, 1985a) (ramets) were planted. Only 5 clones had is based on the Løvenholm data and shows more than 2 cracked ramets, distributed as that cracks first appeared at 1.75 m spacing. follows: Cracking frequency then increased with spacing, reaching 17% (equalized) at the widest spacing (3.25 m). It is also evident that the mean diameter at breast height was greater for cracked stems than for the stand as a whole. In the 2 Swedish unreplicated spacing trials at Tjurvallshult (3 spacings) and at Fagerhult (5 spacings), the cracking fre- quency generally increased with increasing spacing. The major difference between the All the mentioned provenances except spacings was, however, that the yield-re- had only one clone) con- Holkastorp (which ducing gaps resulting from the removal of tained other clones in which not a single cracked stems were larger in the treatments ramet had cracked. with wider initial spacings than in those with spacings. narrower Silviculture In this context main emphasis was placed on the influence of initial spacing in planted stands. Unfortunately, no well-designed spacing trials in which cracks had occurred were available in Sweden. At Løvenholm, Denmark, stem cracking had occurred in one spacing experiment. Most of the cracks had apparently been formed during summer 1982. The assess- ments were carried out during 1984, when the trees were 24 years old. Although not replicated, the experiment had the advan- tage that as many as 11 different spacings, from 0.75 to 3.25 m, were included and thus equalizing of the trend was possible. Sum- maries of assessment data have been made available by courtesy of H Bryndum, Statens Forstlige Forsøgsvæsen, Denmark. The
analysed. The arithmetic mean DBH for the Basic density sample trees was 239 mm ob, and the top diameter ub of the logs was 206 and 211 In stands where trees had cracked during from cracked and uncracked trees, 1981-1983, increment cores were taken mm respectively. Results from the in autumn 1984 from 30 cracked trees and measure- ments are shown in table III. 30 uncracked trees with the same diame- ter. Basic density was determined and was The results agree fairly well with studies compared for the 5 annual rings formed from ordinary stands with cracked trees, ie during 1980-1984. On average the basic at breast height cracked trees had about density for all trees was 275 kg per m, 3 5% wider annual rings and a 5% lower basic and the width of the corresponding annual density than uncracked ones. It is worth not- rings 5.2 mm. Although the variation was ing that at 4 m height there was no differ- large, on average, the cracked stems had ence in ring width, but the cracked trees 7% wider annual rings and a 6% lower had a 7% lower basic density. basic density than their uncracked coun- terparts. Quality assessment A study including test-sawing (see below) was carried out in the combined Rot is a serious defect for sawn timber as clonal and seedling seed orchard described well as for pulpwood. An open crack auto- by Eriksson et al (1975) (Persson et al, matically provides an entrance court for 1987). It is situated at Marma in Uppsala spores of rot fungi. Rot in connection with County, latitude 60° 28’, altitude 35 m; thus cracks was registered frequently, although it is considerably more northerly than most never in annual rings formed after the last of the other sites at which cracks have been cracking. registered. The orchard was established Test-sawing and subsequent determi- on abandoned agricultural land in spring nation of strength by stress-grading of 138 1958, and by summer 1974 it was found central yields from bottom logs of the seed that 4.4% of the trees had developed stem orchard at Marma, as described in the pre- cracks. A renewed crack assessment in vious section, resulted in the following. autumn 1985 revealed that 11.7% of the Among yields from cracked trees, 21% qual- trees had cracks. ified for the lowest grade of structural timber Various provenances from Poland, (T 18), whereas the corresponding value Czechoslovakia, Austria and Germany were for yields from uncracked trees was 24%. included. Since none of these provenances On average, yields from the cracked trees are recommended today in Swedish were estimated to have 6% less strength forestry, for reasons unrelated to their than those from uncracked trees when the propensity for cracking, and since no prove- comparison was based on unclassified nance-related differences in cracking fre- machine values. quency were found, the orchard is treated as one stand in this analysis. The grafts had Fibre dimensions either died or were hopelessly suppressed. The spacing of the seedlings was 1.6 x Cracked and uncracked test-sawn trees 8.0 m, corresponding to a square spacing of from the Marma seed orchard were similar 3.6 m. The site index was very high (G 37). in terms of their measured fibre dimensions, In late 1985, at an age of 32 yr, 4-m-long ie length and width (total, cell-wall thickness bottom logs from each of 51 trees with vis- and diameter of lumen). ible cracks and 25 trees without cracks were
DISCUSSION density and latewood content Wood These 2 traits are related and difficult to dis- tinguish from each other. It is well known Causes that annual ring width in Norway spruce is negatively correlated with basic density. Drought or frost? This is a natural result of the fact that an increase in annual-ring width normally is Drought and frost have been proposed to associated with an increase in the amount of be the 2 most important direct causes of the less dense earlywood, whereas the stem cracking. Day (1954) based his study amount of the denser latewood is less on the assumption that cracking was caused affected. by frost or lightning, but ended up conclud- Day (1954) showed that cracked trees ing that drought was the main reason. had light wood of poor structure that lacked Kubler (1983) evaluated the possibility that well-thickened latewood, whereas uncracked water movement in the stem in connection trees had denser, more structurally sound with frost will cause cracking, which also wood with a larger latewood component. led Hellström et al (1984) to suggest frost Dietrichson et al (1985) concluded that the as the most likely reason for cracking in Nor- latewood content of cracked wood is very way spruce. Dietrichson et al (1985) con- low, its cell walls thin and its lignification cluded that many of the cracks had devel- delayed in the autumn. These conclusions oped in the late part of the growing season, are in accordance with findings in the pre- which would exclude frost as a triggering sent study. Also Caspari (1990) showed that factor. This result is well in line with the find- cracked trees had wider annual rings than ings of the present study. The importance of had uncracked ones, in spite of having nar- water stress and high temperature was also rower latewood. Studies in which cracked recognized by Monchaux and Nepveu trees and uncracked control trees of approx- imately the same diameter have been com- (1986).
pared in terms of basic density, ie Monchaux values with the 1 100 prove- zonal on mean and grouped into 20 zones, whereas it Nepveu (1986), Boulet-Gercourt and nances Nepveu (1988, refers to Abies grandis) and was weaker when based on provenances, the present study, yielded similar results. In and nonexistent when based on individual all 3 studies the average basic density of trees. They concluded that "winter dam- cracked trees was 5-6% lower than that of age and stem cracks have acted indepen- uncracked trees. It is worth noting that even dently on individual trees within prove- uncracked trees can have a basic density nances, even though similar provenance so low that the wood is unsuitable for many effects are observed for the 2 types of dam- ages". uses. The most severly affected trees were those from the central European zones, Physiology which tend to harden late (Dietrichson, 1964). It has been shown previously that Both Day (1954) and Dietrichson et al (1985) late hardening can lead to needle dis- could only speculate as to the physiologi- colouration and subsequent needle loss cal basis of cracking, as was also the case (Skrøppa and Dietrichson, 1986). Further- in the present study. more, in the same study they showed that In Caspari (1990) and Caspari and such damage reduced height increment. Sachsse (1990), descriptions and analyses Evidence that needle loss also causes of the formation of stem cracks are given. reduction in diameter growth was demon- The explanation was that during dry and hot strated by Björkdahl and Eriksson (1989). periods the normal sucking tension between the crown and the root increases to such The apparent contradiction between an extent that cell collapse occurs. This results obtained with provenances and those develops into xylem cracks and when wood obtained with individual trees can be strength proves be insufficient, fully devel- attributed to the fact that cracks and dis- oped stem cracks are formed. It was shown colouration have a common cause. Late that this was not due to tension through hardening results in needles that are sen- shrinkage, because spruces under water sitive to harsh winter conditions and poorly stress crack although the wood moisture developed latewood. Individuals subjected content is higher than the fibre saturation to heavy needle damage, which may have point. been repeated, show reduced diameter growth, which in turn makes them less prone to stem cracking. Furthermore sucking ten- Winter damage sion between the crown and the root is prob- ably lower, owing to the reduced needle An interesting comparison that leads to fur- mass. ther conclusions was made by Skrøppa In conclusion, hardiness must be given and Dietrichson (1986). In their study of when recommending Norway high priority discolouration and needle loss in various spruce provenances. If insufficiently hardy provenances in the IUFRO 1964/68 prove- provenances are used, needle loss and/or nance experiment, they attributed these stem cracking may result, depending on the problems to winter damage that occurred site. This is one of the major reasons why because the shoots had not become suffi- Fottland and Skrøppa (1989) and Persson ciently winter-hardy the preceding autumn. and Persson (1992) warn against using The correlation between degree of winter Romanian and Slovakian provenances in damage and frequency of cracked trees Norway and Sweden. was high when the comparison was based
Consequences than 1.5 high yield; the wood to give m a is of resonable produced quality; Clones used in clonal forestry should be Some consequences of cracking have - tested with respect to disposition to cracking; already been mentioned, eg, the crack pro- however, methods for such testing must be vides an entry point for rot fungi, and developed first; removal of cracked trees may create unwanted openings in the stand. Cracking Clones with a high predisposition to crack- - constitutes a very serious problem when it ing should be removed from seed orchards; affects a large proportion of the stand. For Spruce plantations between 15 and 30 yr - instance, in some cases up to 60% of the of age should be inspected in connection number of stems can be cracked, and in with prolonged dry spells, thereby permit- terms of basal area, the proportion is even ting crackedtrees to be removed without higher since mainly the large trees crack. unnecessary loss of value because of rot. The economic consequences of rot has become more serious during recent years as the pulp industry has shown increasing ACKNOWLEDGMENTS reluctance to accept wood with even the least sign of rot. I wish to thank Forest Engineer J Axelson, who However, it should be kept in mind that responsible for and carried out most of the was low basic density rather than cracking itself field and laboratory work during the study. This is the major cause of economic losses in investigation was financed by the Swedish Board cracking-susceptible stands. Thus, even if of Forestry. cracking does not occur in such stands (because of no dry spell occurred, etc) the timber may nevertheless be unsuitable for REFERENCES structural purposes, and contain too little dry matter to be marketable as raw mate- (1948) Sprickbildning genom torka? Sko- Anon rial for chemical pulp. gen 35, 2 Björkdahl G, Eriksson H (1989) Effects of crown decline on increment in Norway spruce (Picea MANAGEMENT IMPLICATIONS abies L, Karst) in southern Sweden. Commun Norw For Res Inst 42, 1 Boulet-Gercourt B, Nepveu G (1988) Relations From the above discussion it becomes clear entre certaines propriétés du bois et les fentes that the major task of foresters is not to avoid des arbres sur pied chez Abies grandis (Lindl). the cracking as such, but to avoid producing Ann Sci For 45, 33-52 trees with cracking-susceptible wood. Thus Brundin J (1981) Maskinhållfasthetssortering - one basic rule should be to avoid planting at Sambandet mellan hållfasthet vid böjning på a wide spacing on very fertile sites. högkant och böjstyvhet för svenskt furu- och Based on the knowledge gained, more granvirke. Principer för maskinprogrammer- ing. Svenska Träforskiningsinstitutet, STFI- recommendations and generaliza- specific meddelande Serie A 543, 1-35 tions can be made with regard to Swedish Buchwald NF (1948) Tørkespalter i staaende rød- conditions: gran, Dansk Skovforenings Tidskrift 196-199 Avoid planting Romanian and Slovakian - Caspari CO (1990) Untersuchung über Auftreten, provenances on fertile sites; Ursache und Genese von Rißschäden im North Polish and Belorussian provenances Schaft lebender Fichten (Picea abies L, Karst) - should be planted at a spacing of not more unter besonderer Berücksichtigung der
Krutzsch P (1975) Zwei Herkunftversuche mit Dissertation from Holzqualitätsminderung. Forstwissenschaftlichen Fachbereichs der Fichte in Schweden (IUFRO 1938). Universität Göttingen Skogshögskolan, Institutionen för skogs- genetik, Rapporter och Uppsatser 14 Caspari CO, Sachsse H (1990) Rißschäden an Fichte. Forst und Holz 685-688 Kubler H (1983) Mechanism of frost crack for- mation in trees - a review and synthesis. For WR (1954) Drought crack of conifers. For Day Sci 29, 559-568 Comm For Rec 26, 1-40 Monchaux P, Nepveu G (1986) Fentes d’arbres Dietrichson J (1964) Proveniensproblemet belyst sur pied dans deux plantations d’épicéa com- ved studier av vekstrytme og klima. Summary: mun - Influence de la densité du bois. Ann the provenance problem illustrated by stud- Rech Sylv 305-321 ies of growth-rhythm and climate. Meddelelser fra det Norske Skogforsøksvesen 19, 499- (1985a) Granens kvaliteti södra Persson A 656 Sverige. Summary: The quality of Norway spruce in southern Sweden. Sveriges Dietrichson J, Rognerud PA, Haveraaen O, Skogsvårdsförbunds Tidskrift 3, 35-40 Skrøppa T (1985) Stem cracks in Norway spruce (Picea abies L Karst). Reports of the Persson A (1985b) Sprickbildning hos gran. In: Norwegian Forest Research Institute 38, 21, Skogsfakta Konferens, Sveriges lantbruk- 1-32 suniversitet, Skogsvetenskapliga fakulteten 7, 126-131 Ericson B (1959) A mercury immersion method for determining the wood density of incre- Persson A, Persson B (1992) Survival, growth ment core sections. Statens skogsforskn- and quality of Norway spruce (Picea abies, ingsinstitut, avdelningen för skogsproduktion, Karst) provenances at the three Swedish sites 1 Report of the IUFRO 1964/68 provenance experi- ment. Swedish University of Agricultural Sci- Eriksson G, Andersson S, Schelander B (1975) ences, Department of Forest Yield Research, Lovande tillväxt hos introducerade granprove- nienser i en kombinerad klon- och fröplants- Report 29 plantagei norra Uppland. Summary: Promis- Persson A, Ganered N, Ståhl EG (1987) Hur ing growth of introduced provenances of starkt blir virket från våra granplanteringar på Norway spruce in a combined clonal and åkermark? En preliminärstudie. Summary: seedling seed orchard in the northern part of How strong timber will we get from Norway the county of Uppsala, Sweden. Sveriges spruce plantations on agriculture land - a pilot Skogsvårdsförbunds Tidskrift3, 277-286 study. Sveriges Skogsvårdsförbunds Tidskrift (1913) Hitzerisse in Fichten. Forstwiss Flander A 2, 25-33 Centralbl 35, 124-127 Skrøppa T, Dietrichson J (1986) Winter damage Fottland H, Skrøppa T (1989) The IUFRO in the IUFRO 1964/68 provenance experi- 1964/68 provenance experiment with Norway ment with Norway spruce (Picea abies L, spruce (Picea abies) in Norway. Variation in Karst). Commun Nor For Res Inst 39, 10, Commun Norw and mortality height growth. 161-183 For Res Inst 43, 1, 1-30 Werner M, Karlsson B (1983) Resultat från 1969 Hellström C, Karlsson B, Werner M (1984) års granproveniensserie i syd- och mel- Aktuella skadetyper på gran. Summary in lansverige. Summary: Results from a series of Institutet för skogsförbättring, Infor- English. Norway spruce provenance trials within south- mation, Skogsträdsförädling8 1983/84 ern and central Sweden, established in 1969. Föreningen skogsträdsförädling, Institutet för Knuchel H (1947) Holzfehler. Werner Classen skogsförbättring, Årsbok 1982, 90-158 Verlag, Zürich, p 59