article id 301,
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                        Research article
                    
        
                                    
                                    
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                            Our purpose was to develop a FTIR spectroscopic method to be used to  determine the lignin content in a large number of samples and to apply  this method studying variation in sapwood and heartwood lignin content  between three fast-growing cutting clones grown in three sites. Models  were estimated with 18 samples and tested with 6 samples for which the  Klason lignin + acid soluble lignin content had been determined.  Altogether 272 candidate models were built with all-subset regressions  from the principal components estimated from differently treated  transmission spectra of the samples; the spectra were recorded on KBr  pellets of sieved and unsieved unextracted wood powder and subjected to  four different preprocessings and two different wavenumber selection  schemes. The final model showed an adequate fit in the estimation data  (R2 = 0.74) as well as a good prediction performance in the test data (R2P = 0.90). This model was based on the wavenumber range of 1850–500 cm–1 of the line-subtraction-normalised spectra recorded from sieved  samples. The model was used to predict lignin content in 64 samples of  the same material. One of the clones had a slightly lower sapwood lignin  content than the two other clones. The fertile growing site with fast  growing trees showed slightly higher sapwood lignin content compared  with the other two sites. The model was also used to predict the lignin  content in the earlywood of 45 individual annual rings. Variation  between individual stems and between annual rings was found to be large.  No correlation was found between the lignin content and density of  earlywood.
                        
                
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                            Raiskila,
                            Finnish Forest Research Institute, Vantaa Research Unit, P.O. Box 18, FI-01301 Vantaa, Finland
                                                        E-mail:
                                                            sr@nn.fi
                                                                                
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                            Pulkkinen,
                            Department of Forest Ecology, P.O. Box 27, FI-00014 University of Helsinki, Finland
                                                        E-mail:
                                                            mp@nn.fi
                                                                                
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                            Laakso,
                            Finnish Forest Research Institute, Vantaa Research Unit, P.O. Box 18, FI-01301 Vantaa, Finland
                                                        E-mail:
                                                            tl@nn.fi
                                                                                
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                            Fagerstedt,
                            Department of Biological and Environmental Sciences, Plant Biology, P.O. Box 65, FI-00014 University of Helsinki, Finland
                                                        E-mail:
                                                            kf@nn.fi
                                                                                
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                            Löija,
                            VTT Building and Transport, P.O. Box 1806, FI-02044 VTT, Finland
                                                        E-mail:
                                                            ml@nn.fi
                                                                                
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                            Mahlberg,
                            VTT Building and Transport, P.O. Box 1806, FI-02044 VTT, Finland
                                                        E-mail:
                                                            rm@nn.fi
                                                                                
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                            Paajanen,
                            VTT Building and Transport, P.O. Box 1806, FI-02044 VTT, Finland
                                                        E-mail:
                                                            lp@nn.fi
                                                                                
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                            Ritschkoff,
                            VTT Building and Transport, P.O. Box 1806, FI-02044 VTT, Finland
                                                        E-mail:
                                                            acr@nn.fi
                                                                                
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                            Saranpää,
                            Finnish Forest Research Institute, Vantaa Research Unit, P.O. Box 18, FI-01301 Vantaa, Finland
                                                        E-mail:
                                                            pekka.saranpaa@metla.fi
                                                                                        