Mapping global forest age from forest inventories, biomass and climate data

[摘要] Forest age can determine the capacity of a forest to uptake carbon from the atmosphere. However, a lack of global diagnostics that reflect theforest stage and associated disturbance regimes hampers the quantification of age-related differences in forest carbon dynamics. This study providesa new global distribution of forest age circa 2010, estimated using a machine learning approach trained with more than 40 000 plots using forestinventory, biomass and climate data. First, an evaluation against the plot-level measurements of forest age reveals that the data-driven method hasa relatively good predictive capacity of classifying old-growth vs. non-old-growth (precision  =  0.81 and 0.99 for old-growth and non-old-growth,respectively) forests and estimating corresponding forest age estimates (NSE  =  0.6 – Nash–Sutcliffe efficiency – and RMSE  =  50 years – root-mean-square error). However, there are systematicbiases of overestimation in young- and underestimation in old-forest stands, respectively. Globally, we find a large variability in forest age withthe old-growth forests in the tropical regions of Amazon and Congo, young forests in China, and intermediate stands in Europe. Furthermore, we findthat the regions with high rates of deforestation or forest degradation (e.g. the arc of deforestation in the Amazon) are composed mainly ofyounger stands. Assessment of forest age in the climate space shows that the old forests are either in cold and dry regions or warm and wet regions,while young–intermediate forests span a large climatic gradient. Finally, comparing the presented forest age estimates with a series of regionalproducts reveals differences rooted in different approaches and different in situ observations and global-scale products. Despite showingrobustness in cross-validation results, additional methodological insights on further developments should as much as possible harmonize data acrossthe different approaches. The forest age dataset presented here provides additional insights into the global distribution of forest age to betterunderstand the global dynamics in the forest water and carbon cycles. The forest age datasets are openly available at https://doi.org/10.17871/ForestAgeBGI.2021 (Besnard et al., 2021).

[学科分类] 眼科学