Applying principal components analysis (PCA) for separating wingless birch fruits a palaeoecological case study from northern Norway

Transcription

1 Ann. Bot. Fennici 44: ISSN Helsinki 20 June 2007 Finnish Zoologicl nd Botnicl Publishing Bord 2007 Applying principl components nlysis (PCA) for seprting wingless birch fruits pleoecologicl cse study from northern Norwy Minn Välirnt 1,2 & Jn Weckström 1 1) Deprtment of Biologicl nd Environmentl Sciences, P.O. Box 65, FI University of Helsinki, Finlnd (e-mils: minn.vlirnt@helsinki.fi, jn.weckstrom@helsinki.fi) 2) Deprtment of Geology, P.O. Box 64, FI University of Helsinki, Finlnd Received 9 My 2006, revised version received 1 Nov. 2006, ccepted 8 Nov Välirnt, M. & Weckström, J. 2007: Applying principl components nlysis (PCA) for seprting wingless birch fruits pleoecologicl cse study from northern Norwy. Ann. Bot. Fennici 44: We used principl components nlysis (PCA) to distinguish wingless birch fruit bodies of different birch species from ech other. Firts, we mesured seven dimension vribles from modern fruit selection nd nlysed their explntory power using PCA. Then, we included the subfossil birch fruits in PCA s pssive vribles nd compred the result with the dt gthered by visul nlysis. PCA clerly seprtes the modern fruits of Betul nn, B. pubescens ssp. czerepnovii, nd B. pendul from ech other. PCA lso provides n effective nd objective tool for producing more detiled pleoecologicl dt of the occurrences of different birch species thn does the trditionl visul nlysis only. Insted of clssifying fossil birch fruits into rough type-clsses t lest Betul nn, B. pubescens ssp. czerepnovii, nd B. pendul cn be seprted from ech other on the bsis of the morphologicl fetures. Key words: modern birch fruits, principl components nlysis (PCA), subfossil birch fruits Introduction Pleobotnicl reserch, especilly in Finlnd, is lrgely bsed on pollen nlysis (e.g. Hyvärinen 75, Tolonen & Ruuhijärvi 76, Seppä 96), wheres plnt mcrofossil nlysis is rther neglected method (however see e.g. Vsri 62, Vsri et l. 96, Seppä et l. 2004). Pollen dt cn, in generl, only provide regionl view of the pst plnt communities, wheres plnt mcrofossil nlysis llows to reconstruct locl ctchment-level plnt history (Birks 2001, Birks 2003, Seppä et l. 2004, Välirnt 2005). This pproch is especilly vluble when reconstructing pst environments in res tht nowdys re locted beyond or bove the tree line. For instnce, subfossil birch fruits re one of the most bundnt mcroscopic plnt remins preserved nd found from lke sediments. When compred with informtion derived from pst pollen ssemblges, identifiction of subfossil birch fruits could provide importnt dditionl informtion of pst environmentl conditions of the ctchment, due to the different ecologi-

2 214 Välirnt & Weckström Ann. BOT. Fennici Vol. 44 L b W g B L = length (mm) W = width of the widest prt (mm) = ngle formed by the shoulders b = ngle formed by the bse g = ngle formed by the lower side A = length from bse to widest prt (mm) B = width of bse (mm) Fig. 1. The mesured vribles on Betul fruit bodies. cl needs for growing conditions, reproducing nd germintion of different birch species (e.g. Kllio & Mäkinen 78). Finds of subfossil tree-type birch fruits from loctions bove or beyond modern tree line refer, by themselves, to better growing conditions in the pst. In ddition, however, relible nd precise identifiction of fossil birch-fruit mteril, covering the time period fter the lst deglcition, could provide n dded vlue to the debte of the history of different birch species, nd especilly of the history, origin nd txonomic sttus of the mountin birch Betul pubescens ssp. czerepnovii (synonym for B. pubescens ssp. tortuos) (Vrm & Vlnne 73, Kllio & Mäkinen 78, Hämet-Ahti 87, As & Frlund 2001 nd references therein). Accordingly, pleoecologicl interprettion should be bsed on trustworthy seprtion of pubescent nd white birches (B. pubescens nd B. pendul), mountin birch (B. pubescens ssp. czerepnovii), nd dwrf birch (B. nn). It is commonly recognised mong plynologists tht it is extremely difficult, if not impossible, to identify different birch species bsed on pollen nlysis only (however see Mäkelä 96). By contrst, in optiml circumstnces tree-type birch fruits re reltively esy to distinguish from dwrf birch fruits. Yet, lot of morphologicl vrition exists (Biłobrzesk & Truchnowiczówn 60) nd hybridiztion A W between different birch species complictes the visul bsed identifiction further due to the trnsmission of vrying mount of genetic informtion to fruits of new hybrid (Vlnne & Sulkinoj 91). As shown in studies of vn Dinter nd Birks (96), nd Freund et l. (2001) principl components nlysis (PCA) is useful tool to seprte wingless birch fruit bodies of different birch species from ech other. As complement to the previous studies crried out in reltively southern ltitudes, we here pply PCA to smll subrctic lke in northern Norwy (69 45 N, E) to provide new dt of the pst presence of different birch species in n re tht currently is locted t the subrctic tree line. Moreover, we will emphsize the potentil of the objective PCA method to distinguish fossil birch fruits from ech other in more relible wy thn the trditionl, more subjective, visul method does. Methods Fruit mesurements Modern fruits of five txonomic birch groups were selected: Betul pubescens, B. pendul, B. nn, B. pubescens ssp. czerepnovii, nd hybrid B. nn pubescens. Fruits were collected from the botnicl grden of the University of Turku, Finlnd. The fruits represent selection of birches tht originte from northern provennces but hve been trnsplnted/germinted relting to propgtion studies crried out t the University of Turku. Nomenclture follows tht of Hämet-Ahti et l. (98). Thirty fruits of ech group were copy-drwn using binoculr nd cmer Lucid. Ech fruit ws drwn three times nd seven different size nd shpe vribles were mesured from ech drwing ccording to vn Dinter nd Birks (96) (Fig. 1). The verge of three seprte mesurements ws used for sttisticl nlysis. The mesured vribles were: length (L, mm), width of the widest prt (W, mm), ngle formed by the shoulders (α), ngle formed by the bse (β), ngle formed by the lower side (γ), length from bse to the widest prt (A, mm), nd width of bse (B, mm). Insted

3 Ann. BOT. Fennici Vol. 44 Applying PCA for seprting wingless birch fruits 215 of using only one morphometricl rtio (cf. vn Dinter & Birks 96, Freund et l. 2001) we used three different rtios, A:L, A:W nd B:W ( 100), to mesure the symmetry of the fruit body. The men, medin, minimum, nd mximum vlues of the mesurements re shown in Tble 1. All ten morphologicl vribles were used in the PCA. Subfossil fruits (0 individuls) were t first identified visully to s low txonomic level s possible from 185-cm-long sediment core t 5 cm intervls. Their dimensions were then mesured nd compred with the modern birch fruit dt using PCA. Principl components nlysis Principl components nlysis (PCA) ws used to interpret nd summrise the mjor ptterns of vrition within the birch fruit dt. PCA is n indirect ordintion technique for obtining low-dimensionl representtion of multivrite dt so tht the dt cn be explored visully in two dimensionl PCA correltion biplot nd ny structure in the dt identified (Everitt 78). In the correltion biplot, vribles with high positive correltions hve cute ngles between their vectors. The length indictes the strength of the vrible in reltion to the displyed ordintion (ter Brk 94). The fossil birch fruits re included in the PCA s supplementry (pssive) vribles. These vribles do not ffect the nlysis but re positioned subsequently of the ordintion biplot s if they hd tken prt in it (ter Brk & Šmiluer 98). PCA is performed on untrnsformed nd centred nd stndrdised species dt using the progrm CANOCO 4.5. (ter Brk & Šmiluer 2002). Results nd discussion PCA ordintion is shown s two-dimensionl correltion biplot (Fig. 2). The eigenvlues for PCA xes 1 nd 2 re 0.44 nd 0., respec- Tble 1. Summry of mesurements of modern birch fruits. L W A B α β γ A:L A:W B:W (mm) (mm) (mm) (mm) (%) (%) (%) B. nn (number on fruits = 30) men medin min mx B. pubescens ssp. czerepnovii (number on fruits = 30) men medin min mx B. pubescens (number on fruits = 30) men medin min mx B. nn pubescens (number on fruits = 30) men medin min mx B. pendul (number on fruits = 30) men medin min mx

4 216 Välirnt & Weckström Ann. BOT. Fennici Vol. 44 A A:W 1.0 L A:L 1.0 W g b B B:W 1.0 b Betul nn Betul pubescens Betul pubescen spp. czerepnovii Betul nn x Betul pubescens Betul pendul Fig. 2. Principl components nlysis (PCA) ordintion digrm showing the position of () modern Betul fruits nd (b) subfossil Betul fruits in reltion to the mesured morphologicl vribles. The numbers in pnel b correspond to the depths in Fig tively, thus cpturing 62.7% of the totl vrince in the dt. The third (λ 3 = 0.13) nd fourth (λ 4 = 0.10) PCA xes show low eigenvlues nd re not discussed further. Three groups with positive correltions cn be identified. The first group consists of vribles indicting the shpe of the fruit bse (β, B, γ nd B:W). Vribles describing the length chrcteristics (A, L, nd A:W) form the second group. These re strongly correlted to PCA xis 1. The third group is highly correlted with PCA xis 2 nd comprises vribles describing the width of the fruit (W, α). Smll nd round fruits of B. nn (minly upper right qurter) nd longer, slimmer nd more steep-sided B. pubescens ssp. czerepnovii fruits (upper left qurter) re distinctively seprted by PCA xis 1 with only two fruits of B. pubescens ssp. czerepnovii being plotted mong the fruits of B. nn. These outliers my be the result of nturl vrition in shpe (Biłobrzesk & Truchnowiczówn 60). Width (W) nd α seem not to be importnt vribles to seprte dwrf birch from mountin birch. Insted, these fetures seprte white birch, B. pendul, from the other birch tx. B. pendul fruits seem to, in generl, hve distinctly nrrow shoulders nd re thus negtively correlted with the PCA xis 2. Betul pubescens nd the hybrid B. nn pubescens re plotted round the origo nd cnnot relibly be seprted from ech other bsed on the wingless fruits only. The fossil Betul fruits re plotted s numbers (Fig. 2b), which correspond to the depths shown in Fig. 3. At first sight it ppers tht the fossil fruits re scttered ll round the PCA biplot. However, this exercise provides interesting nd useful informtion when converted s strtigrphicl digrm nd compred with the results of visul birch-fruit identifiction (Fig. 3). Bsed on visul nlysis (Fig. 3), subfossil fruits re ctegorized into three groups: Betul nn, tree-type Betul, nd Betul sp. Most of the fruits could not be identified to species level nd re thus mrked s Betul sp. Moreover, it is lso impossible to seprte the different tree-type birch fruits (B. pubescens, B. pendul, nd B. pubescens ssp. czerepnovii) from ech other. The PCA nlysis, insted, seprtes four groups of birch fruits (Fig. 3b) providing more detiled informtion bout the occurrences nd mounts of different birch species. Despite the ssumption tht pure B. nn fruits should esily be distinguishble, the mount of B. nn clerly increses s result of PCA (Fig. 3). The high nd frequent bundnce is in ccordnce with the fct tht dwrf birch is one of the most dominnt

5 Ann. BOT. Fennici Vol. 44 Applying PCA for seprting wingless birch fruits 217 vsculr plnt inhbiting northern fjeld res (Kllio & Mäkinen 78). In ddition, PCA lso suggests tht B. pubescens ssp. czerepnovii hs been present in the ctchment lmost throughout the time under exmintion. This fits the conception of Hämet-Ahti (87), who suggests tht mountin birch ctully origintes from isolted B. pubescens popultions tht survived in northwestern Europe through the lst glcition. PCA lso suggests tht B. pendul hs been present in the re. This is interesting, since ccording to Kllio nd Mäkinen (78) in Finlnd the modern northern limit of B. pendul is c. 69 N. The fruits minly occur t the lower prt of the core tht most probbly represents the time period of erly to mid Holocene. The sediment core is not dted, but bsed on 14 C rdiocrbon dtes derived from three lkes being situted in close vicinity, with sediment thicknesses of 209 cm, 240 cm nd 140 cm, respectively, nd representing t lest the lst 7000 yers (R. Brdshw pers. comm.) this sediment sequence (185 cm) likely covers t lest the sme time spn. Furthermore, since the long-term verge sedimenttion rte in subrctic/rctic lkes is shown to be c mm 1 (e.g. Brnekow 99, Sorvri 2001, Korhol & Weckström 2004), the sedimenttion probbly strted during the erly Holocene. Another evidence for erly Holocene inception is the finds of Nuphr sp. from the bottom prt of the core. This species cnnot survive in subrctic lkes, thus, its presence indictes wrmer thn present climte conditions. According to Mäkelä nd Hyvärinen (2000), B. pendul hd much wider northern distribution during the Holocene therml optimum yers go but the distribution decresed towrds the present due to the slowly cooling climte. Thus, in theory it is possible tht B. pendul colonised the ctchment fter deglcition. Alterntively, the fruits might represent the northern form of the silver birch B. pendul vr. lpponic. Unfortuntely, this could not be verified, since no modern reference fruit-mteril ws vilble. A possible misinterprettion of PCA, resulting from the internl vrition in fruit morphology reported e.g. by Biłobrzesk nd Truchnowiczówn (60) must lso be considered. However, PCA seems to seprte the modern B. pendul fruits firly well from the other birch fruits (Fig. 2). Depth code Depth (cm) Betul nn Tree-type Betul Betul sp Number of finds Betul nn Betul (nn x) pubescens Betul pendul Betul pubescens ssp. czerepnovii Number of finds Fig. 3. The strtigrphicl distribution of subfossil Betul fruit finds from northern Norwegin lke bsed on () visul interprettion nd (b) PCA. PCA proves to be helpful tool to seprte subfossil birch fruits from ech other nd is recommendble id to use together with the visul nlysis. In this study, the drwings of fruits s well s the vrible mesurements were done mnully but modern computer-ided drwing nd mesuring techniques cn provide less time-consuming wy to copy-drw fossil fruits. In order to verify the possible occurrence of B. pendul in northern Norwy during the Holocene therml optimum more studies re needed. In the ner future DNA nlysis might provide new insights of the phytogeogrphicl history of different birch species in Fennoscndi. b

6 218 Välirnt & Weckström Ann. BOT. Fennici Vol. 44 Acknowledgements This reserch ws finnced by the DART-project (ENV4- CT ), the Acdemy of Finlnd (Grnt ) nd HERC (Helsinki University Environmentl Reserch Centre) (Grnt ). We thnk two nonymous reviewers for vluble comments. References As, B. & Frlund, T. 2001: The Holocene history of the Nordic mountin birch belt. In: Wielgolski, F. E. (ed.), Nordic mountin birch ecosystems: 5. The Prthenon Publ. Group, New York, London. Brnekow, L. 99: Holocene tree-line dynmics nd inferred climtic chnges in the Abisko re, northern Sweden, bsed on mcrofossil nd pollen records. Holocene 9: Biłobrzesk, M. & Truchnowiczówn, J. 60: The vribility of shpe of fruits nd scles of the Europen birches (Betul L.) nd their determintion in fossil mterils. Monogr. Botnice 9(2): Birks, H. H. 2001: Plnt mcrofossils. In: Smol, J. P., Birks, H. J. B. & Lst, W. M. (ed.), Trcking environmentl chnge using lke sediments, vol. 3: Kluwer Acd. Publ., Dordrecht. Birks, H. H. 2003: The importnce of plnt mcrofossils in the reconstruction of lte-glcil vegettion nd climte: exmples from Scotlnd, western Norwy, nd Minnesot, USA. Qut. Sci. Rev. : Everitt, B. S. 78: Grphicl techniques for multivrite dt. London, Heinemnn Educ. Books Ltd. Freund, H., Birks, H. H. & Birks, H. J. B. 2001: The identifiction of wingless Betul fruits in Weichselin sediments in the Gross Todtshorn borehole (Lower Sxony, Germny) the occurrence of Betul humilis Schrnk. Veget. Hist. Archeobot. 10: Hämet-Ahti, L. 87: Mountin birch nd mountin birch woodlnd in NW Europe. Phytocoenologi 15: Hämet-Ahti, L., Suominen, J., Ulvinen, T. & Uotil, P. (eds.) 98: Retkeilyksvio, 4th ed. Finnish Mus. Nt. Hist., Bot. Mus., Helsinki. Hyvärinen, H. 75: Absolute nd reltive pollen digrms from northernmost Fennoscndi. Fenni 142: Kllio, P. & Mäkinen, Y. 78: Vsculr flor of Inri Lplnd. 4. Betulcee. Rep. Subrctic Res. Sttion Kevo 14: Korhol, A. & Weckström, J. 2004: Pleolimnologicl studies in Arctic Fennoscndi nd the Kol Peninsul (Russi). In: Pienitz, R., Dougls, M. S. V. & Smol, J. P. (eds.), Long-term environmentl chnge in Arctic nd Antrctic lkes, vol. 8: Kluwer Acdemic Publishers, Dordrecht. Mäkelä, E. 96: Size distinctions between Betul pollen types review. Grn 35: Mäkelä, E. & Hyvärinen, H. 2000: Holocene vegettion history t Vätsäri, Inri Lplnd, northestern Finlnd, with specil reference to Betul. Holocene 10: Seppä, H. 96: Post-glcil dynmics of vegettion nd tree-lines in the fr north of Fennoscndi. Fenni 174: Seppä, H., Hnnon, G. & Brdshw R. H. W. 2004: Holocene history of lpine vegettion nd forestline on Pyhäkero Mountin, northern Finlnd. Arct. Antrc. Alp. Res. (4): Sorvri, S. 2001: Climte impcts on remote subrctic lkes in Finnish Lplnd: limnologicl nd pleolimnologicl ssessment with prticulr focus on ditoms nd Lke Snjärvi. Kilpisjärvi Notes 16, Yliopistopino, Helsinki. ter Brk, C. F. J. 94: Cnonicl community ordintion. Prt I: Bsic theory nd liner methods. Écoscience 1: ter Brk, C. J. F. & Šmiluer, P. 98: CANOCO reference mnul nd user s guide to Cnoco for Windows: softwre for cnonicl community ordintion, ver. 4. Microcomputer Power, Ithc, NY. ter Brk, C. J. F. & Šmiluer, P. 2002: CANOCO Reference mnul nd CnoDrw for Windows User s guide: Softwre for Cnonicl Community Ordintion, ver Microcomputer Power, Ithc, NY. Tolonen, K. & Ruuhijärvi, R. 76: Stndrd pollen digrms from the Slpusselkä region of southern Finlnd. Ann. Bot. Fennici 13: Vrm, A. & Vlnne, T. 73: On the txonomy, biology nd origin of Betul tortuos Ledeb. Rep. Subrctic Res. Sttion Kevo 10: Vlnne, T. & Sulkinoj, M. 91: Lpin koivujen risteytymisestä. Luonnon Tutkij 95: Välirnt, M. 2005: Plnt mcrofossil evidence of chnges in qutic nd terrestril environments in north-estern Europen Russi nd Finnish Lplnd since the lte Weichselin. Publ. Dept. Geol. 2, Univ. Helsinki, Yliopistopino, Helsinki. Vn Dienter, M. & Birks, H. H. 96: Dishtinguishing fossil Betul nn nd B. pubescens using their wingless fruits: implictions for the lte-glcil vegettionl history of western Norwy. Veget. Hist. Archeobot. 5: Vsri, Y. 62: A study of the vegettionl history of the Kuusmo district (north est Finlnd) during the lte- Quternry period. Ann. Bot. Soc. Vnmo 33(1): Vsri, Y., Tonkov, S., Vsri, A. & Nikolov, A. 96: The Lte-quternry history of the vegettion nd flor in northestern Finlnd in the light of re-investigtion of Aplmpi in Sll. Aquilo Ser. Bot. : This rticle is lso vilble in pdf formt t