Trends in tree improvement methods: from classical breeding to genomic technologies

Genetic improvement of trees is a key avenue for enhancing the productivity and quality of forest products. There is a need to improve the trees' ability to adapt to changes in the environment including increases in frequency and intensity of biotic and abiotic stress, as well as to produce more forest products such as pulp, timber and other bioproducts. For efficient tree improvement, selecting a suitable breeding or improvement method is essential. Here, we reviewed the existing literature and conducted a meta-analysis using more than 1,500 scholarly articles published between 1990-2021. We categorized the articles into three broad tree improvement categories, 1) conventional or classical breeding, 2) marker-assisted- or genomic-selection, and 3) genetic modification or editing. The results of our meta-analysis indicated that higher adaptability, productivity, and quality are the main objectives of tree improvement approaches throughout time. Growth, quality, and biotic and abiotic stress tolerance-related traits are considered the most important. In the 1990s to early 2000s, conventional breeding methods were the most used category, but with the advent and development of state-of-the-art genetics, genomics, bioinformatics, and artificial intelligence tools, we can improve trees more efficiently and affordably. Genomic selection and genetic modification of non-model tree species are becoming more popular. Our analysis provides future directions and trends for selecting more efficient tree improvement techniques in a system dependent manner.