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A comparison of three age determination methods for suppressed Norway spruce: implications for age structure analysis
Forest Ecology and Management 161 (2002) 279±288
A comparison of three age determination methods for suppressed Norway spruce: implications for age structure analysis Mats Niklasson* Department of Forest Vegetation Ecology, Faculty of Forestry, Swedish University of Agricultural Sciences, S-901 83 UmeaÊ, Sweden Received 12 May 2000; received in revised form 24 January 2001; accepted 5 February 2001
Abstract Three methods for age determination of suppressed Norway spruce were compared: (1) tree-ring counting in core-like strips; (2) pith node counting and (3) a method based on dendrochronological cross-dating (named within-tree cross-dating). The methods were tested on 20 suppressed spruces (age 100±140 years) from a boreal forest in Northern Sweden with known disturbance history. In all studied trees, the oldest pith dates were found below the root collar, the point which is normally aimed for when coring trees in ®eld, thus, ring counts in the root collar level gave on average 20±26 years younger ages than pith node counting and within-tree cross-dating. Pith node counting and within-tree cross-dating gave largely identical pith dates but when top breaks had occurred at the seething stage, within-tree cross-dating gave better results. Cross-dating was achieved between short stem sections from the base of the tree with the aid of conspicuous rings that were followed down through the lower stem, section by section. This enabled more accurate dating of the pith in the lower portions of the stem where many rings are missing in the outer parts due to early bending and formation of adventitious roots which allocate stern growth above the point of origin. The results have wide implications for studies of Norway spruce stand age structures. When analysing tree age in Norway spruce stands and stands of other species that are capable to extremely slow juvenile growth (leading to initiation of adventitious roots and missing rings), ages may be systematically underestimated, and short periods of successful regeneration may ``disappear'' in age±class diagrams when age data are derived from counted tree rings in cores extracted at root collar level. The within-tree cross-dating method and pith node counting are two methods that (1) provide a precise age determination for small suppressed Norway spruce and (2) can give a better age estimation of the dating error for bigger trees where the oldest part (juvenile wood) of the tree is not possible to date accurately with normal coring and ring counting. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Age determination; Cross-dating; Dendrochronology; Picea abies; Suppressed trees
1. Introduction In forest ecology and forest history studies, an accurate determination of tree ages is crucial for the * Present address: Southern Swedish Forest Research Centre, P.O. Box 49, SE-230 53 ALNARP. Tel.: 46-40-41-51-99; fax: 46-40-46-23-25. E-mail address: [email protected] (M. Niklasson).
interpretation of age distributions and their relation to the disturbance regime (Payette et al., 1985; Fastie, 1995; HoÈrnberg et al., 1995). Usually, age determination is accomplished by counting tree rings in crosssections or in cores extracted from the base of the tree by an increment borer (Henry and Swan, 1974; Lorimer, 1985). Apart from problems with hollow trees and technical coring problems (diameter of tree too large at ground level, buttresses, etc.), the dating
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error due to missed pith or point of origin1 is relatively small when coring trees with fast initial height and diameter growth (e.g. early successional species like Pinus sp. after ®re disturbance). On the other hand, when initial growth is very slow, as is the case with suppressed individuals of shade tolerant species (e.g. Picea sp., Abies sp., Tsuga sp.), cores extracted at just short distances above the point of origin will cause large underestimations of the true age. When compiling the ages from suppressed trees into age distributions, these problems may make tree populations appear younger than they really are and may also cause short periods of successful establishment in the past to disappear or be smoothened out in age structure diagrams. Suppressed trees of Norway spruce (Picea abies [L.] Karst.) are dif®cult to age determine for other reasons, too. Extremely slow growth with very narrow rings is common and many partly or completely missing rings give underestimations of the true age when counting tree rings, even in cross-sections (SireÂn, 1951; Zackrisson, 1980). Standard dendrochronological cross-dating between different trees is dif®cult because irregular and individual growth patterns due to leaning and formation of compression wood mask the climatic signal in the tree rings (Schweingruber, 1983; Schweingruber et al., 1990b). The point of origin itself is dif®cult to locate since the root collar2 is often situated above the point where the seed once germinated due to bending of the seedling and initiation of adventitious roots (Heikinheimo, 1920; Lakari, 1921; SireÂn, 1951). The aim of this study was (1) to develop a more accurate method (within-tree cross-dating) for age determination of small suppressed Norway spruce; (2) to investigate the differences in tree age and age structures obtained by the developed method and two previously known methods for age determination: traditional ring counting of cores and the rarely practised pith node counting method (Chojnacki, 1
The term ``point origin'' refers to the oldest part of a tree, i.e. the hypocotyl of the seedling. This part is ca. 3 cm long in Norway spruce. 2 Root collar is here used to describe the widest part of the lower stem where the largest supporting roots are attached whether of adventitious origin or not. This position is usually the intuitively expected point of germination which is aimed for when coring a tree. In literature synonyms to root collar like root neck or root crown may be encountered.
1964; Pridnja, 1967; SireÂn, 1951) and (3) to provide an estimate of the error when coring trees and counting rings for age determination. 2. Material and methods 2.1. The area and the studied trees Trees were sampled from a 0.1 ha plot at Lillberget (648190 N, 198010 E), a nature reserve in Northern Sweden, chosen because of its relatively low degree of human impact and because of its known disturbance history (Niklasson, 1998). It is situated in the northern boreal region at 340 m above sea level. The climate is characterised as local continental with short, fairly warm summers and a long winter. Usually the ground is snow covered from early November to the middle of May. The mean annual temperature is 1.0 8C at Lycksele, 40 km to the NW, and annual precipitation is about 570 mm (Alexandersson et al., 1991). The soil is a gravelly-sandy moraine and the bedrock consists mainly of porphyritic Revsund granite. The ®eld layer is dominated by bilberry (Vaccinium myrtillus L.), crowberry (Empetrum hermaphroditum Hagerup) and lingonberries (Vaccinium vitis-idaea L.) while the most common species in the bottom layer are the feather mosses Hylocomium splendens (Hedw.) and Pleurozium schreberi (Brid.) Mitt. The stand is dominated by 300-year-old Scots pine (Pinus sylvestris L.) that originated after a ®re in 1693 (Niklasson, 1998). Cross-dating of ®re scars in different pines indicate that in 1850 a ®re passed over the whole plot that killed all small trees (below ca.12 cm in diameter) and scarred a few of the larger pines. Two large spruces survived on the plot, badly scarred by the ®re. Their diameter (1.3 m above ground) at the time of the ®re was about 20 cm. The dominating pines are 30±50 cm in diameter, 22±27 m in height and with a total basal area of 37.5 m2/ha. A well developed understorey of suppressed Norway spruce has developed, the majority of the trees being 1±5 m in height, and a small number reaching 10 m in height (total basal area 7.5 m2/ha). Apart from the two large and ®re-scarred spruces, no other large surviving spruces were found in the vicinity. Thus, there are very strong ground to believe that all the suppressed spruces on the plot have germinated after the ®re.
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These circumstances indicated an a priori maximum possible age of the suppressed spruces with no spruces established before 1850. Scots pine that established after this ®re have been outcompeted by the Norway spruce except for two extremely slow-growing and suppressed individuals. Twenty spruces were randomly selected in the height interval 1.35±5 m. Data on the sampled trees are given in Table 1. All spruces are thought to have established from seeds because layering was not encountered in any case on the plot or in the reserve. Each sampled tree was treated in the following way: The position of the apparent root collar was marked on the stem. Moss and the humus were removed so the roots and belowground stem could be cut and dug up from where they penetrate the mineral soil. The largest root was cut 2±5 cm behind this point. The aboveground stem was cut 10±20 cm above the root collar and the whole stump with its larger roots was brought to the lab. 2.2. Within-tree cross-dating All stumps were cut into 2 cm sections with a band saw. The sections were marked with their relative Table 1 Data from sampled trees Tree
Diameter at 1.3 m above ground (mm)
Total height (dm)
Diameter at root collar (mm)
a b c d e f g h i j k l m n o p q r s t
35 20 35 46 17 8 25 21 25 31 40 53 56 47 38 24 33 36 18 49
25 19 28 45 18 13 15 19 22 25 35 45 53 44 29 19 30 27 17 46
69 41 56 70 31 30 38 30 39 51 64 62 83 67 54 41 47 60 40 63
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Fig. 1. A schematic drawing of a stem and root section of a suppressed Norway spruce with 2 cm sections marked. MO: upper level of the forest ¯oor moss; RC: root collar; AD: adventitious roots; HY: hypocotyl, the oldest part of the tree that was often very small and located parallel to the much larger ``main'' stem, therefore, it was easily mistaken for a root and not always dug up; MI: mineral soil.
position to the root collar (Fig. 1), sanded and the whole upper surfaces cut with a scalpel to obtain a smooth surface and sharpest possible picture of the annual rings. Zinc paste was applied to the cut surface to enhance cell contrast. Sanding with gradually ®ner sand paper (up to 2000 grains/cm2) gave an equally smooth surface but was only used for photography. A dissecting microscope (6±80) was used for the examination and dating. The dating process was started with the uppermost stem section in each tree. Examination of the whole cross-section surface under high magni®cation (20± 80) often revealed many wedging rings (Fig. 2) that would have been missed if only part of the stem section had been examined. In a few cases, density ¯uctuations in rings with very wide compression wood could resemble double or false rings. These were easily discovered under high magni®cation and by following the rings around the circumference. Conspicuous rings (Schweingruber et al., 1990a) of the following kinds were noted in each cross-section: (1) start and end year of compression wood formation; (2) very narrow rings and rings with very light latewood
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individual growth patterns with long periods of compression wood rings formed, the periods with undisturbed growth were too short to permit crossdating. Although, not used for cross-dating between trees, it was noted that in some years, like in 1936, the majority of the trees had developed compression wood. 2.3. Age determination by ring counts in cores
Fig. 2. A stem section with wedging rings. A ring count from the ring marked with an arrow would give a younger date by 9 years on the left side than a ring count on the right side.
within periods of wide compression wood rings; (3) sudden mass appearance of resin canals; (4) abrupt growth changes and (5) microscars and other damages in the wood. With the aid of conspicuous rings noted above, it was possible to cross-date major event years between adjacent cross-sections and so establish the pith dates a all levels (Fig. 3a±d). As the dating proceeded downwards in the stem, section by section, new conspicuous rings appeared and were used for dating further down while events noted in the top sections could gradually disappear further down (Fig. 4). It was possible to determine if the point of origin had been recovered in the dug-up specimens on the basis of pith morphology. The arrangement of tracheids around the pith is different in roots compared to the stem. In root, pith parenchyma does not exist (Telewski, 1993) and the tracheids are instead arranged in a distinct lentil-like, oblong pattern with two resin ducts at the ends. The pith in the stem is usually circular. ®lled with brown parenchyma and the two resin ducts are lacking. In sections where stem pith was present only in the upper cross-cut and only root pith in the lower, the point of origin was located somewhere in-between. An attempt to cross-date the ring pattern of the suppressed trees with that of overstorey trees using climatically induced pointer years (mainly using latewood density/colour, see Niklasson et al. (1994), Stokes and Smiley (1968) was not successful. Because all spruces had very
In each stem section, tree-ring counts were made in 4 mm wide strips, equalling the diameter of a core taken out with a standard increment corer. The counts were made at two positions to test whether the radius between bark and pith in¯uenced age determination. One count was made over the longest radius and one over the shortest radius. 2.4. Age determination by counting pith nodes Finally, the spruces were dated with the pith node counting method described and used by SireÂn (1951), Chojnacki (1964) and Pridnja (1967). The pith date for the uppermost stem section obtained by the counting (see Section 2.2) was used as the start year. All stem sections were split vertically with a chisel into two halves to make the pith visible. Care was taken to position the chisel just outside the pith to avoid damage of pith nodes by the edge. In most cases the stem sections cleaved perfectly through the centre, making the pith and pith nodes clearly visible in most cases (Fig. 5). In a few cases (e.g. when the ®bre direction was very irregular), some parts of the pith had to be cut visible with a scalpel. The following characteristics were used for identi®cation of pith nodes: (1) change in pith colour; (2) domeshaped nodal diaphragms and (3) different shape, colour and orientation of parenchyma cells above and under a nodal diaphragm. Side buds, present under the apical bud, caused slight but clearly visible bends in the pith just below the pith node that were used as a guide to the pith node position. Side twigs and ®bre bending around them were also a good indication of the pith nod position. Some of these characteristics have been described earlier by Venn (1965). In sections with relatively large pith diameter (>0.4 mm), the pith nodes were clear and distinct but in slowgrowing periods with corresponding narrow pith
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Fig. 3. Four selected stem sections from tree l with conspicuous rings used for the within-tree cross-dating indicated. (a) 7 cm above root collar, pith year 1892; (b) at root collar, pith year 1888; (c) 4 cm below root collar, pith year 1886; (d) 8 cm below root collar, pith year 1884. The right arrow points in each photo points at a wide compression wood ring formed in 1936 that was useful also for within-tree cross-dating between stem sections in other trees. The left arrow points at a compression wood ring in 1913 followed by a 2 year release. The ring formed in the preceding year 1912 is clearly visible as a white and narrow ring within a period of compression wood. This kind of ring was named ``anti-compression'' wood during the dating process.
diameter (<0.4 mm), these characteristics could be less developed. In these cases sometimes a slight widening and darkening of the pith could be visible at low magni®cation or with the naked eye. After cleaving, the exact position of origin could be determined with the aid of several morphological features. For example, the transition from the former hypocotyl to the stem was indicated by the change from dark and denser pith parenchyma to sparse and light yellowish parenchyma in the former hypocotyl. Additionally, the beginning of the roots was visible as a complete disappearance of pith parenchyma and distinct start of curled and twisted ®bres.
3. Results 3.1. Within-tree cross-dating of pith years The within-tree cross-dating method revealed that none of the 20 examined trees had their point of origin at the root collar level (Fig. 6a±t). All trees had germinated below this point and had been bent down and become overgrown by mosses for longer or shorter periods during their early development. The bending down and moss growth had caused a continuous initiation of adventitious roots along the lower part of the stem that gradually obtained the appearance of a
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Fig. 4. Schematic illustration of the within-tree cross-dating technique of tree j. Horizontal lines describe time covered by each disc. Broken lines describe period where missing rings occur. Vertical lines connect major pointer years.
large root (Figs. 1 and 8). Annual ring development was gradually reduced in the stem below root collar with many partially absent tree rings typically located near the bark (Fig. 2). In seven trees (Fig. 6a±f and h) where the point of origin could be determined on basis of pith morphology, the identi®ed years of origin were 1855, 1857, 1857, 1859, 1863, 1863 and 1868. In the remaining 13 trees, the stem section with the point of origin had not been recovered from the soil and the oldest pith years ranged between 1865 and 1893. The reconstructed initial height growth was slow: 1.3 cm/ year (range 0.4±2.3 cm/year). 3.2. Pith node counting
Fig. 5. After cleaving the stem sections, the pith nodes (arrows) were usually distinct and easily counted. Visible diagnostic features include: different colour in the nodes and internode pith, pith slightly bent at the node and slightly curved ®bres in the pith node area due to overgrown branches.
Age determination with the pith node counting method gave slightly younger ages in the lower parts of the stems but was generally in well agreement with the within-tree cross-dating method (Fig. 6a±t). In trees with extremely slow early height increment and/ or where the apical shoot had been broken (Fig. 6k and p), the dating difference was larger (maximum 9 years) while in faster-growing trees the two methods often gave the same age.
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Fig. 6. Pith dates and early height development for 20 suppressed Norway spruces according to the three methods: within-tree cross-dating (bold line), pith node counting (bold line and squares); ring counts (narrow line: long radius count; broken line: short radius count). Asterisk denotes trees for which the point of origin was found.
3.3. Ring counts in cores Age determination by tree-ring counting in corelike strips differed signi®cantly from the ages obtained
from within-tree cross-dating and pith node counting in the majority of trees (Fig. 6a±t). At the root collar (level 0 cm) a ring count in a core gave pith dates on average 6 (long radius count) or 12 (short radius count)
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years younger than cross-dated pith dates derived from within-tree cross-dating at the same level. The maximum difference in age that resulted from the two methods was 20 and 26 years (short and long radius count, respectively) at the root collar level, the point ideally aimed at when coring. The difference was even larger for those trees where the `point of origin' was found: here ring counts on the longest and shortest radius indicated germination on average 23 and 32 years earlier than was calculated from the within-tree cross-dating method. The maximum number of rings was counted just under the root collar, for the long radius count at 3.8 cm (average) and for the short radius count at 0.5 cm (average). At root collar level, the direction of coring gave different ages: long radius ring counts were on average 6 years older than short radius ring counts. At 6 cm there was no difference in age between short and long radius counts. Below root collar the difference between short and long radius count increased with increasing depth. At 6 cm the long radius counts were 6 years older and at
Fig. 7. Establishment periods for 20 suppressed spruces obtained with different methods. (a) within-tree cross-dating, black bars denote trees where the point of origin was found; (b) pith node counting; (c) long radius ring counts at root collar; (d) short radius ring counts at root collar.
10 cm the long radius counts were 14 years older at average. The compiling of maximum ages from within-tree cross-dating. pith node counting and ring counted cores at root collar resulted in apparently different periods of establishments. The tree ages obtained from the two corings at root collar level resulted in a clearly later and more prolonged period of establishment than tree ages obtained by pith node counting and withintree cross-dating (Fig. 7). In the latter case, tree establishment was con®ned to a shorter period that followed closer in time after the ®re in 1850. 4. Discussion Within-tree cross-dating substantially improves age determination of suppressed spruce undergrowth compared to ring counts in cores on root collar level. This is because dating is possible of the early growth in the lower part of the stem that often has been bent down by snow, overgrown by moss and ``disappeared'' in humus under a later-formed root collar (Heikinheimo, 1920; Lakari, 1921: SireÂn, 1951). As annual rings are incompletely developed especially below the root collar, within-tree cross-dating or pith node counting are the only possible methods to determine (or approximate) the year of germination. The reason for not ®nding the point of origin in all trees was because the earliest developed main stem and the former hypocotyl often was very thin and mistaken for an adventitious root and, therefore, easily was cut too high resulting in an inevitable age underestimation (Fig. 8). This was not possible to foresee before having dated the trees. Pith node counting is an age determination method that deserves renewed attention. The possibility to loose nodes due to the cross-cut is low, especially since ®bre bending around the node can help identify such cases. To combine pith node counting and withintree cross-dating is practical because they provide supporting evidence for the dating and facilitate the dating of critical sections. The cleaving is also necessary if the exact point of origin is searched for. Although the number of spruce trees was small in this study, the compilation of tree ages gave markedly different age distributions, depending on the dating method used. A similar result was obtained by Parent
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Fig. 8. Dug-up bottom part of a suppressed Norway spruce tree l showing the narrow 20 oldest part O at 19 cm below the not collar RC that was mistaken for a root and cut above the point of origin. Adventitious roots have developed early and grown much wider than the original stem. At 7 cm the pith date was 1892, at root collar 1888 and at the oldest section ( 19 cm) the pith date was 1872.
et al. (2000) when comparing bud scar counting with ring counts in seedlings of Abies balsamea. The age structures from within-tree cross-dating and pith node counting indicate that establishment of the suppressed spruces in fact peaked shortly after the ®re in 1850 (Fig. 7). This is not re¯ected in the age structure inferred from the tree-ring counted cores. instead it could easily be interpreted as a continuous establishment under a canopy. The differences between age structures from ring-counted cores and age structures from the two more precise methods may in practise become even larger because the real date of origin is seldom found (here in 7 of 20 trees), the pith is often missed and coring is typically done quite far above the level of the mineral soil. Whenever a more accurate age determination of Norway spruces is needed, the within-tree crossdating and pith node counting are two more accurate alternatives to coring. Some obvious limitations of the methods should, however, be mentioned. First, the inherent destructive nature of the two methods makes it dif®cult to use them where n must be kept low (e.g.
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reserves). In such cases, a small subsample of trees may be sampled and accurately dated to produce a local correction factor for age determination when counting rings from cores of older and younger trees. Secondly, the methods are quite laborious, and for larger trees (>10 cm in breast height) the methods have large practical constraints, although, dating may be fully accomplished (Niklasson, unpublished data). When time and funding is restricted and the highest precision in age is not crucial, the pith node counting method is decidedly faster and produces almost identical datings as within-tree cross-dating. Working with Picea mariana in Canada, DesRochers and Gagnon (1997) were able to improve the age by crossdating of discs from dug-up stumps. Pith nodes were not analysed. For ageing suppressed individuals of other shade tolerant species with adventitious root formation like Abies and Tsuga species the methods here presented should be useful but this has not been tested yet. Pinus sp. do not form pith nodes (Venn, 1965), so only within-tree cross-dating should then remain for accurate dating but this needs further investigations. Many of the deciduous trees are possible to age with the aid of pith nodes (Chojnacki, 1964) but their suitability for within-tree cross-dating is largely unknown. 5. Conclusions Alternative age determination methods may strongly improve the analysis of age structure in small, suppressed Norway spruce. The pith node counting and within-tree cross-dating give more accurate tree ages than ring counts in cores. By accurately assessing the juvenile growth of suppressed trees it is possible to get a better estimation of the error that is made by using ring counts from cores. This permits a higher precision when interpreting age data gathered and compiled from many trees into age structure diagrams. Acknowledgements I thank Anders GranstroÈm, Per Linder and Olle Zackrisson for critically reviewing the manuscript. Greger HoÈrnberg and Ove Nystrand gave valuable comments on earlier drafts of the manuscripts.
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Morgan Karlsson gave technical assistance. The study was funded in part by a WWF grant, Hierta-Retzius Research Foundation and Oscar and Lili Lamms foundation.
References Alexandersson, H., KarlstioÈm, C., Larsson-McCann, S., 1991. Temperature and Precipitation in Sweden 1961±1990: Reference Normals. Report 81, Swedish Meteorological and Hydrological Institute (SMHI). Chojnacki, W., 1964. O metodzie dokladnego okreslenia wieku podrostow i rocznych przyrostow u niektorych gatunkow drzew lesnych (About the method to determine accurately the age of saplings and the annual height growth of some forest tree species). Sylwan 1, 71±76. DesRochers, A., Gagnon, R., 1997. Is ring count at ground level a good estimation of black spruce age? Can. J. For. Res. 27, l263±1267. Fastie, C.L., 1995. Causes and ecosystem consequences of multiple pathways of primary succession at Glacier Bay. Alaska Ecol. 76, 1899±1916. Heikinheimo, O., 1920. Kuusen iaÈn maÈaÈraÈaÈmisesta ja kuusen myoÈhaÈisjuurista. MetsaÈtieteelisen koelaitoksen julkaisuja 2, 129. Henry, J.D., Swan, J.M.A., 1974. Reconstructing forest history from live and dead plant materialÐan approach to the study of forest succession in Southwest New Hampshire. Ecology 55, 772±773. HoÈrnberg, C., Ohlson, M., Zackrisson, O., 1995. Stand dynamics, regeneration patterns and long-term continuity in boreal oldgrowth Picea abies swamp-forests. J. Veg. Sci. 6, 291±298. Lakari, O.J., 1921. Tutkimuksi kuusimetsien uudistumisvuosista EtelaÈ- ja Keski-Suomessa. MetsaÈtieteelisen koelaitoksen julkaisuja 4, 1±54. Lorimer, C.C., 1985. Methodological considerations in the analysis of forest disturbance history. Can. J. For. Res. 15, 200± 213.
Niklasson, M., 1998. Dendroecological Studies in Forest and Fire History. Doctoral Thesis, Silvestria 52, Department of Forest Vegetation Ecology, SLU UmeaÈ. È stlund, L., 1994. A dendroecoloNiklasson, M., Zackrisson, O., O gical reconstruction of saami use of Scots pine Pinus sylvestris L., inner bark over the last 350 years at SaÈdvajaure. N. Sweden Veget. Hist. Archaeobot. 3, 183±190. Parent, S., Morin, H., Messier, C., 2000. Effects of adventitious determination in Balsam ®r Abies balsamea) regeneration. Can. J. For. Res. 30, 200±213. Payette, S., Filion, L., Gauthier, I., Boutin, Y., 1985. Secular climate change in old-growth tree-line vegetation of Northern Quebec. Nature 315, 135±138. Pridnja, M.V., 1967. A trial in determining the age of Picea obovata advance growth with the help of pith nodes. Lesovedenie Moscow 5, 72±78. Schweingruber, F.H., 1983. Tree rings. Basics and Applications of Dendrochronology. Kluwer Academic Publishers, Dordrecht. Schweingruber, F.H., Eckstein, D., Serre-Bachet, F., BraÈker, O.U., 1990a. Identi®cation, presentation and interpretation of event years and pointer years in dendrochronology. Dendrochronologia 8, 9±37. Schweingruber, F.H., Kairiukstis, V., Shiyatov, S., 1990b. Sample selection. In: Cook, E.R., Kairiukstis, L.A. (Eds.), Methods of Dendrochronology. International Institute for Applied Systems Analysis, Kluwer Academic Publishers, Dordrecht, pp. 23±35. SireÂn, G., 1951. On the biology of undergrown spruce. Acta For. Fenn. 58, 1±90. Stokes, M.A., Smiley, T.L., 1968. An Introduction to Tree-Ring Dating, University of Chicago Press, Chicago. Telewski, F.W., 1993. Determining the germination date of woody plants: a proposed method for locating the root/shoot interface. Tree-Ring Bull. 53, 13±16. Venn, K., 1965. Nodal diaphragms in Picea abies L. Karst., and other conifers. Meddelelser fra det norske forsùksvesen 72, 93± 114. Zackrisson, O., 1980. Forest ®re history: ecological signi®cance and dating problems in the North Swedish boreal forest, In: Proceedings of the Fire History Workshop. General Technical Report RM-81, Forest Service USDA, Rocky Mountain Forest and Range Experiment Station, Tucson, Arizona, 20±24 October.
A comparison of three age determination methods for suppressed Norway spruce: implications for age structure analysis Mats Niklasson* Department of Forest Vegetation Ecology, Faculty of Forestry, Swedish University of Agricultural Sciences, S-901 83 UmeaÊ, Sweden Received 12 May 2000; received in revised form 24 January 2001; accepted 5 February 2001
Abstract Three methods for age determination of suppressed Norway spruce were compared: (1) tree-ring counting in core-like strips; (2) pith node counting and (3) a method based on dendrochronological cross-dating (named within-tree cross-dating). The methods were tested on 20 suppressed spruces (age 100±140 years) from a boreal forest in Northern Sweden with known disturbance history. In all studied trees, the oldest pith dates were found below the root collar, the point which is normally aimed for when coring trees in ®eld, thus, ring counts in the root collar level gave on average 20±26 years younger ages than pith node counting and within-tree cross-dating. Pith node counting and within-tree cross-dating gave largely identical pith dates but when top breaks had occurred at the seething stage, within-tree cross-dating gave better results. Cross-dating was achieved between short stem sections from the base of the tree with the aid of conspicuous rings that were followed down through the lower stem, section by section. This enabled more accurate dating of the pith in the lower portions of the stem where many rings are missing in the outer parts due to early bending and formation of adventitious roots which allocate stern growth above the point of origin. The results have wide implications for studies of Norway spruce stand age structures. When analysing tree age in Norway spruce stands and stands of other species that are capable to extremely slow juvenile growth (leading to initiation of adventitious roots and missing rings), ages may be systematically underestimated, and short periods of successful regeneration may ``disappear'' in age±class diagrams when age data are derived from counted tree rings in cores extracted at root collar level. The within-tree cross-dating method and pith node counting are two methods that (1) provide a precise age determination for small suppressed Norway spruce and (2) can give a better age estimation of the dating error for bigger trees where the oldest part (juvenile wood) of the tree is not possible to date accurately with normal coring and ring counting. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Age determination; Cross-dating; Dendrochronology; Picea abies; Suppressed trees
1. Introduction In forest ecology and forest history studies, an accurate determination of tree ages is crucial for the * Present address: Southern Swedish Forest Research Centre, P.O. Box 49, SE-230 53 ALNARP. Tel.: 46-40-41-51-99; fax: 46-40-46-23-25. E-mail address: [email protected] (M. Niklasson).
interpretation of age distributions and their relation to the disturbance regime (Payette et al., 1985; Fastie, 1995; HoÈrnberg et al., 1995). Usually, age determination is accomplished by counting tree rings in crosssections or in cores extracted from the base of the tree by an increment borer (Henry and Swan, 1974; Lorimer, 1985). Apart from problems with hollow trees and technical coring problems (diameter of tree too large at ground level, buttresses, etc.), the dating
0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 1 ) 0 0 5 0 0 - X
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error due to missed pith or point of origin1 is relatively small when coring trees with fast initial height and diameter growth (e.g. early successional species like Pinus sp. after ®re disturbance). On the other hand, when initial growth is very slow, as is the case with suppressed individuals of shade tolerant species (e.g. Picea sp., Abies sp., Tsuga sp.), cores extracted at just short distances above the point of origin will cause large underestimations of the true age. When compiling the ages from suppressed trees into age distributions, these problems may make tree populations appear younger than they really are and may also cause short periods of successful establishment in the past to disappear or be smoothened out in age structure diagrams. Suppressed trees of Norway spruce (Picea abies [L.] Karst.) are dif®cult to age determine for other reasons, too. Extremely slow growth with very narrow rings is common and many partly or completely missing rings give underestimations of the true age when counting tree rings, even in cross-sections (SireÂn, 1951; Zackrisson, 1980). Standard dendrochronological cross-dating between different trees is dif®cult because irregular and individual growth patterns due to leaning and formation of compression wood mask the climatic signal in the tree rings (Schweingruber, 1983; Schweingruber et al., 1990b). The point of origin itself is dif®cult to locate since the root collar2 is often situated above the point where the seed once germinated due to bending of the seedling and initiation of adventitious roots (Heikinheimo, 1920; Lakari, 1921; SireÂn, 1951). The aim of this study was (1) to develop a more accurate method (within-tree cross-dating) for age determination of small suppressed Norway spruce; (2) to investigate the differences in tree age and age structures obtained by the developed method and two previously known methods for age determination: traditional ring counting of cores and the rarely practised pith node counting method (Chojnacki, 1
The term ``point origin'' refers to the oldest part of a tree, i.e. the hypocotyl of the seedling. This part is ca. 3 cm long in Norway spruce. 2 Root collar is here used to describe the widest part of the lower stem where the largest supporting roots are attached whether of adventitious origin or not. This position is usually the intuitively expected point of germination which is aimed for when coring a tree. In literature synonyms to root collar like root neck or root crown may be encountered.
1964; Pridnja, 1967; SireÂn, 1951) and (3) to provide an estimate of the error when coring trees and counting rings for age determination. 2. Material and methods 2.1. The area and the studied trees Trees were sampled from a 0.1 ha plot at Lillberget (648190 N, 198010 E), a nature reserve in Northern Sweden, chosen because of its relatively low degree of human impact and because of its known disturbance history (Niklasson, 1998). It is situated in the northern boreal region at 340 m above sea level. The climate is characterised as local continental with short, fairly warm summers and a long winter. Usually the ground is snow covered from early November to the middle of May. The mean annual temperature is 1.0 8C at Lycksele, 40 km to the NW, and annual precipitation is about 570 mm (Alexandersson et al., 1991). The soil is a gravelly-sandy moraine and the bedrock consists mainly of porphyritic Revsund granite. The ®eld layer is dominated by bilberry (Vaccinium myrtillus L.), crowberry (Empetrum hermaphroditum Hagerup) and lingonberries (Vaccinium vitis-idaea L.) while the most common species in the bottom layer are the feather mosses Hylocomium splendens (Hedw.) and Pleurozium schreberi (Brid.) Mitt. The stand is dominated by 300-year-old Scots pine (Pinus sylvestris L.) that originated after a ®re in 1693 (Niklasson, 1998). Cross-dating of ®re scars in different pines indicate that in 1850 a ®re passed over the whole plot that killed all small trees (below ca.12 cm in diameter) and scarred a few of the larger pines. Two large spruces survived on the plot, badly scarred by the ®re. Their diameter (1.3 m above ground) at the time of the ®re was about 20 cm. The dominating pines are 30±50 cm in diameter, 22±27 m in height and with a total basal area of 37.5 m2/ha. A well developed understorey of suppressed Norway spruce has developed, the majority of the trees being 1±5 m in height, and a small number reaching 10 m in height (total basal area 7.5 m2/ha). Apart from the two large and ®re-scarred spruces, no other large surviving spruces were found in the vicinity. Thus, there are very strong ground to believe that all the suppressed spruces on the plot have germinated after the ®re.
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These circumstances indicated an a priori maximum possible age of the suppressed spruces with no spruces established before 1850. Scots pine that established after this ®re have been outcompeted by the Norway spruce except for two extremely slow-growing and suppressed individuals. Twenty spruces were randomly selected in the height interval 1.35±5 m. Data on the sampled trees are given in Table 1. All spruces are thought to have established from seeds because layering was not encountered in any case on the plot or in the reserve. Each sampled tree was treated in the following way: The position of the apparent root collar was marked on the stem. Moss and the humus were removed so the roots and belowground stem could be cut and dug up from where they penetrate the mineral soil. The largest root was cut 2±5 cm behind this point. The aboveground stem was cut 10±20 cm above the root collar and the whole stump with its larger roots was brought to the lab. 2.2. Within-tree cross-dating All stumps were cut into 2 cm sections with a band saw. The sections were marked with their relative Table 1 Data from sampled trees Tree
Diameter at 1.3 m above ground (mm)
Total height (dm)
Diameter at root collar (mm)
a b c d e f g h i j k l m n o p q r s t
35 20 35 46 17 8 25 21 25 31 40 53 56 47 38 24 33 36 18 49
25 19 28 45 18 13 15 19 22 25 35 45 53 44 29 19 30 27 17 46
69 41 56 70 31 30 38 30 39 51 64 62 83 67 54 41 47 60 40 63
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Fig. 1. A schematic drawing of a stem and root section of a suppressed Norway spruce with 2 cm sections marked. MO: upper level of the forest ¯oor moss; RC: root collar; AD: adventitious roots; HY: hypocotyl, the oldest part of the tree that was often very small and located parallel to the much larger ``main'' stem, therefore, it was easily mistaken for a root and not always dug up; MI: mineral soil.
position to the root collar (Fig. 1), sanded and the whole upper surfaces cut with a scalpel to obtain a smooth surface and sharpest possible picture of the annual rings. Zinc paste was applied to the cut surface to enhance cell contrast. Sanding with gradually ®ner sand paper (up to 2000 grains/cm2) gave an equally smooth surface but was only used for photography. A dissecting microscope (6±80) was used for the examination and dating. The dating process was started with the uppermost stem section in each tree. Examination of the whole cross-section surface under high magni®cation (20± 80) often revealed many wedging rings (Fig. 2) that would have been missed if only part of the stem section had been examined. In a few cases, density ¯uctuations in rings with very wide compression wood could resemble double or false rings. These were easily discovered under high magni®cation and by following the rings around the circumference. Conspicuous rings (Schweingruber et al., 1990a) of the following kinds were noted in each cross-section: (1) start and end year of compression wood formation; (2) very narrow rings and rings with very light latewood
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individual growth patterns with long periods of compression wood rings formed, the periods with undisturbed growth were too short to permit crossdating. Although, not used for cross-dating between trees, it was noted that in some years, like in 1936, the majority of the trees had developed compression wood. 2.3. Age determination by ring counts in cores
Fig. 2. A stem section with wedging rings. A ring count from the ring marked with an arrow would give a younger date by 9 years on the left side than a ring count on the right side.
within periods of wide compression wood rings; (3) sudden mass appearance of resin canals; (4) abrupt growth changes and (5) microscars and other damages in the wood. With the aid of conspicuous rings noted above, it was possible to cross-date major event years between adjacent cross-sections and so establish the pith dates a all levels (Fig. 3a±d). As the dating proceeded downwards in the stem, section by section, new conspicuous rings appeared and were used for dating further down while events noted in the top sections could gradually disappear further down (Fig. 4). It was possible to determine if the point of origin had been recovered in the dug-up specimens on the basis of pith morphology. The arrangement of tracheids around the pith is different in roots compared to the stem. In root, pith parenchyma does not exist (Telewski, 1993) and the tracheids are instead arranged in a distinct lentil-like, oblong pattern with two resin ducts at the ends. The pith in the stem is usually circular. ®lled with brown parenchyma and the two resin ducts are lacking. In sections where stem pith was present only in the upper cross-cut and only root pith in the lower, the point of origin was located somewhere in-between. An attempt to cross-date the ring pattern of the suppressed trees with that of overstorey trees using climatically induced pointer years (mainly using latewood density/colour, see Niklasson et al. (1994), Stokes and Smiley (1968) was not successful. Because all spruces had very
In each stem section, tree-ring counts were made in 4 mm wide strips, equalling the diameter of a core taken out with a standard increment corer. The counts were made at two positions to test whether the radius between bark and pith in¯uenced age determination. One count was made over the longest radius and one over the shortest radius. 2.4. Age determination by counting pith nodes Finally, the spruces were dated with the pith node counting method described and used by SireÂn (1951), Chojnacki (1964) and Pridnja (1967). The pith date for the uppermost stem section obtained by the counting (see Section 2.2) was used as the start year. All stem sections were split vertically with a chisel into two halves to make the pith visible. Care was taken to position the chisel just outside the pith to avoid damage of pith nodes by the edge. In most cases the stem sections cleaved perfectly through the centre, making the pith and pith nodes clearly visible in most cases (Fig. 5). In a few cases (e.g. when the ®bre direction was very irregular), some parts of the pith had to be cut visible with a scalpel. The following characteristics were used for identi®cation of pith nodes: (1) change in pith colour; (2) domeshaped nodal diaphragms and (3) different shape, colour and orientation of parenchyma cells above and under a nodal diaphragm. Side buds, present under the apical bud, caused slight but clearly visible bends in the pith just below the pith node that were used as a guide to the pith node position. Side twigs and ®bre bending around them were also a good indication of the pith nod position. Some of these characteristics have been described earlier by Venn (1965). In sections with relatively large pith diameter (>0.4 mm), the pith nodes were clear and distinct but in slowgrowing periods with corresponding narrow pith
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Fig. 3. Four selected stem sections from tree l with conspicuous rings used for the within-tree cross-dating indicated. (a) 7 cm above root collar, pith year 1892; (b) at root collar, pith year 1888; (c) 4 cm below root collar, pith year 1886; (d) 8 cm below root collar, pith year 1884. The right arrow points in each photo points at a wide compression wood ring formed in 1936 that was useful also for within-tree cross-dating between stem sections in other trees. The left arrow points at a compression wood ring in 1913 followed by a 2 year release. The ring formed in the preceding year 1912 is clearly visible as a white and narrow ring within a period of compression wood. This kind of ring was named ``anti-compression'' wood during the dating process.
diameter (<0.4 mm), these characteristics could be less developed. In these cases sometimes a slight widening and darkening of the pith could be visible at low magni®cation or with the naked eye. After cleaving, the exact position of origin could be determined with the aid of several morphological features. For example, the transition from the former hypocotyl to the stem was indicated by the change from dark and denser pith parenchyma to sparse and light yellowish parenchyma in the former hypocotyl. Additionally, the beginning of the roots was visible as a complete disappearance of pith parenchyma and distinct start of curled and twisted ®bres.
3. Results 3.1. Within-tree cross-dating of pith years The within-tree cross-dating method revealed that none of the 20 examined trees had their point of origin at the root collar level (Fig. 6a±t). All trees had germinated below this point and had been bent down and become overgrown by mosses for longer or shorter periods during their early development. The bending down and moss growth had caused a continuous initiation of adventitious roots along the lower part of the stem that gradually obtained the appearance of a
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Fig. 4. Schematic illustration of the within-tree cross-dating technique of tree j. Horizontal lines describe time covered by each disc. Broken lines describe period where missing rings occur. Vertical lines connect major pointer years.
large root (Figs. 1 and 8). Annual ring development was gradually reduced in the stem below root collar with many partially absent tree rings typically located near the bark (Fig. 2). In seven trees (Fig. 6a±f and h) where the point of origin could be determined on basis of pith morphology, the identi®ed years of origin were 1855, 1857, 1857, 1859, 1863, 1863 and 1868. In the remaining 13 trees, the stem section with the point of origin had not been recovered from the soil and the oldest pith years ranged between 1865 and 1893. The reconstructed initial height growth was slow: 1.3 cm/ year (range 0.4±2.3 cm/year). 3.2. Pith node counting
Fig. 5. After cleaving the stem sections, the pith nodes (arrows) were usually distinct and easily counted. Visible diagnostic features include: different colour in the nodes and internode pith, pith slightly bent at the node and slightly curved ®bres in the pith node area due to overgrown branches.
Age determination with the pith node counting method gave slightly younger ages in the lower parts of the stems but was generally in well agreement with the within-tree cross-dating method (Fig. 6a±t). In trees with extremely slow early height increment and/ or where the apical shoot had been broken (Fig. 6k and p), the dating difference was larger (maximum 9 years) while in faster-growing trees the two methods often gave the same age.
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Fig. 6. Pith dates and early height development for 20 suppressed Norway spruces according to the three methods: within-tree cross-dating (bold line), pith node counting (bold line and squares); ring counts (narrow line: long radius count; broken line: short radius count). Asterisk denotes trees for which the point of origin was found.
3.3. Ring counts in cores Age determination by tree-ring counting in corelike strips differed signi®cantly from the ages obtained
from within-tree cross-dating and pith node counting in the majority of trees (Fig. 6a±t). At the root collar (level 0 cm) a ring count in a core gave pith dates on average 6 (long radius count) or 12 (short radius count)
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years younger than cross-dated pith dates derived from within-tree cross-dating at the same level. The maximum difference in age that resulted from the two methods was 20 and 26 years (short and long radius count, respectively) at the root collar level, the point ideally aimed at when coring. The difference was even larger for those trees where the `point of origin' was found: here ring counts on the longest and shortest radius indicated germination on average 23 and 32 years earlier than was calculated from the within-tree cross-dating method. The maximum number of rings was counted just under the root collar, for the long radius count at 3.8 cm (average) and for the short radius count at 0.5 cm (average). At root collar level, the direction of coring gave different ages: long radius ring counts were on average 6 years older than short radius ring counts. At 6 cm there was no difference in age between short and long radius counts. Below root collar the difference between short and long radius count increased with increasing depth. At 6 cm the long radius counts were 6 years older and at
Fig. 7. Establishment periods for 20 suppressed spruces obtained with different methods. (a) within-tree cross-dating, black bars denote trees where the point of origin was found; (b) pith node counting; (c) long radius ring counts at root collar; (d) short radius ring counts at root collar.
10 cm the long radius counts were 14 years older at average. The compiling of maximum ages from within-tree cross-dating. pith node counting and ring counted cores at root collar resulted in apparently different periods of establishments. The tree ages obtained from the two corings at root collar level resulted in a clearly later and more prolonged period of establishment than tree ages obtained by pith node counting and withintree cross-dating (Fig. 7). In the latter case, tree establishment was con®ned to a shorter period that followed closer in time after the ®re in 1850. 4. Discussion Within-tree cross-dating substantially improves age determination of suppressed spruce undergrowth compared to ring counts in cores on root collar level. This is because dating is possible of the early growth in the lower part of the stem that often has been bent down by snow, overgrown by moss and ``disappeared'' in humus under a later-formed root collar (Heikinheimo, 1920; Lakari, 1921: SireÂn, 1951). As annual rings are incompletely developed especially below the root collar, within-tree cross-dating or pith node counting are the only possible methods to determine (or approximate) the year of germination. The reason for not ®nding the point of origin in all trees was because the earliest developed main stem and the former hypocotyl often was very thin and mistaken for an adventitious root and, therefore, easily was cut too high resulting in an inevitable age underestimation (Fig. 8). This was not possible to foresee before having dated the trees. Pith node counting is an age determination method that deserves renewed attention. The possibility to loose nodes due to the cross-cut is low, especially since ®bre bending around the node can help identify such cases. To combine pith node counting and withintree cross-dating is practical because they provide supporting evidence for the dating and facilitate the dating of critical sections. The cleaving is also necessary if the exact point of origin is searched for. Although the number of spruce trees was small in this study, the compilation of tree ages gave markedly different age distributions, depending on the dating method used. A similar result was obtained by Parent
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Fig. 8. Dug-up bottom part of a suppressed Norway spruce tree l showing the narrow 20 oldest part O at 19 cm below the not collar RC that was mistaken for a root and cut above the point of origin. Adventitious roots have developed early and grown much wider than the original stem. At 7 cm the pith date was 1892, at root collar 1888 and at the oldest section ( 19 cm) the pith date was 1872.
et al. (2000) when comparing bud scar counting with ring counts in seedlings of Abies balsamea. The age structures from within-tree cross-dating and pith node counting indicate that establishment of the suppressed spruces in fact peaked shortly after the ®re in 1850 (Fig. 7). This is not re¯ected in the age structure inferred from the tree-ring counted cores. instead it could easily be interpreted as a continuous establishment under a canopy. The differences between age structures from ring-counted cores and age structures from the two more precise methods may in practise become even larger because the real date of origin is seldom found (here in 7 of 20 trees), the pith is often missed and coring is typically done quite far above the level of the mineral soil. Whenever a more accurate age determination of Norway spruces is needed, the within-tree crossdating and pith node counting are two more accurate alternatives to coring. Some obvious limitations of the methods should, however, be mentioned. First, the inherent destructive nature of the two methods makes it dif®cult to use them where n must be kept low (e.g.
287
reserves). In such cases, a small subsample of trees may be sampled and accurately dated to produce a local correction factor for age determination when counting rings from cores of older and younger trees. Secondly, the methods are quite laborious, and for larger trees (>10 cm in breast height) the methods have large practical constraints, although, dating may be fully accomplished (Niklasson, unpublished data). When time and funding is restricted and the highest precision in age is not crucial, the pith node counting method is decidedly faster and produces almost identical datings as within-tree cross-dating. Working with Picea mariana in Canada, DesRochers and Gagnon (1997) were able to improve the age by crossdating of discs from dug-up stumps. Pith nodes were not analysed. For ageing suppressed individuals of other shade tolerant species with adventitious root formation like Abies and Tsuga species the methods here presented should be useful but this has not been tested yet. Pinus sp. do not form pith nodes (Venn, 1965), so only within-tree cross-dating should then remain for accurate dating but this needs further investigations. Many of the deciduous trees are possible to age with the aid of pith nodes (Chojnacki, 1964) but their suitability for within-tree cross-dating is largely unknown. 5. Conclusions Alternative age determination methods may strongly improve the analysis of age structure in small, suppressed Norway spruce. The pith node counting and within-tree cross-dating give more accurate tree ages than ring counts in cores. By accurately assessing the juvenile growth of suppressed trees it is possible to get a better estimation of the error that is made by using ring counts from cores. This permits a higher precision when interpreting age data gathered and compiled from many trees into age structure diagrams. Acknowledgements I thank Anders GranstroÈm, Per Linder and Olle Zackrisson for critically reviewing the manuscript. Greger HoÈrnberg and Ove Nystrand gave valuable comments on earlier drafts of the manuscripts.
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Morgan Karlsson gave technical assistance. The study was funded in part by a WWF grant, Hierta-Retzius Research Foundation and Oscar and Lili Lamms foundation.
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