Recently, public access to Landsat imagery and the development of cloud-computing platforms have made it possible to use long-term remote sensing images to detect and monitor forest disturbances 27, 28, 29. However, these empirical models are difficult to apply globally because they are usually generated locally for certain tree species and are not applicable to crop trees 25, 26. By combining these with the tree species data, statistical relationships between these characteristics and tree age can be developed 23, 24. The biophysical characteristics of trees, such as leaf area index (LAI) and tree height, can be estimated through remote sensing observations. To forecast the supply and demand of tree crops and quantify the benefits or costs of planted forests, it is essential to generate and share global planting year maps for plantations.Ĭompared with traditional methods for estimating tree age, which are mainly based on the biophysical properties of sample trees (e.g., tree rings, height, or crown size) 21, 22, remote sensing has the advantages of high efficiency and low costs. Although regional and national tree age datasets/annual datasets for certain plantations have been developed recently 18, 19, 20, the planting years of plantations remain unknown at the global scale. The quantification of positive and negative outcomes from plantations, such as yield and aboveground biomass estimate, usually depend on the planting year or tree age because this directly determines the biophysical attributes, including canopy size, tree height, root structure, and soil properties 15, 16, 17. However, some plantations (e.g., oil palm) have negative impacts on society and the environment 10, biodiversity declines 11, loss of natural forests 12 and peatlands 13, and carbon emissions 14. Plantations can not only bring economic benefits by supplying wood, fiber, fruit, nuts and other products, but also provide important ecosystem services such as carbon sequestration and biodiversity promotion 9. Plantations, as distinct from natural forests, generally consist of tree crops and planted forests, where tree crops are planted to provide fruits, nuts or other tree products (such as coconut, coffee and oil palm), and planted forests are grown for the production of wood and wood fiber or for the protection of ecosystems. On the brink of the decrease in biodiversity, carbon storage and economic incomes caused by forest loss 5, many countries have implemented tree planting programmes, which have resulted in the expansion of plantations around the world 6, 7, 8.
Since the turn of the century, forest loss driven by human activities and natural disturbance, such as agricultural expansion, fire and pest infestation, has been a serious global environmental issue 4. Forests are the dominant terrestrial ecosystem on Earth, covering approximately one-third of the global land area, and they play an important role in the water and heat balance, global carbon cycle and climate change 1, 2, 3.
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