A Review of the Factors Influencing Variations in the Heartwood Proportion for Solid Wood

Using 146 journals, 6 chapters, 4 reports, 2 books, and a proceeding, this research seeks to investigate the factors that influence variations in the proportion of heartwood. The data were collected through an online search in databases such as Scopus, Science Citation Index, Science Direct, Google Scholar, and ResearchGate with seven search queries, namely heartwood variability, heartwood proportion, tree age of heartwood, growth rate of heartwood, silvicultural treatment of heartwood, as well as genetic and environmental effects of heartwood. By eliminating papers that were out of topic or title, purpose and year of publication, 67 journal articles, 4 chapters, 3 reports, a book, and a proceeding were selected. The review results showed that the amount of heartwood was influenced by its position in the tree, tree variations, such as age, growth rate, and species/genera, as well as the growing conditions, including location and environmental factors. It is also affected by various silvicultural treatments, namely spacing, thinning, pruning, fertilization, and irrigation. These findings indicate that the amount of heartwood is more controlled by the diameter of the wood than the age. Furthermore, several studies reported that the growth rate of sapwood rings was significantly higher than the heartwood rings, which can reduce the quality of solid wood used for construction. Therefore, it is necessary to control the proportion of heartwood through genetics and environmental factors, especially genetics, which has more effects on the growth rate of tree diameter than environmental factors. Previous studies revealed that tree diameter has a weak or negative correlation with other wood properties, such as basic density, color, and wood stiffness, which are generally important in global markets. It is important for breeders to concentrate on using species with superior genetic variations as well, especially the highly demanded traits in further studies.