Measuring the Number of Trees in the World

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A recent estimate of tree counts performed by academics at Nature, an international science journal, found there to be three trillion trees across the globe. With a ratio of trees to humans at 61:1, this optimistic number has been called into question by a previous assessment of  around 400 billion trees. This method, however, seemed to rely heavily on satellite imagery as a basis for estimation. Nature’s article shows a more comprehensive model, accounting for additional environmental attributes and using a combination of satellite imagery, forest inventories, and supercomputer technologies.  The updated methodology helped to alleviate the issue of missing data from particular tree-covered regions and produced a dataset at the resolution of one square kilometer.

Researchers were able to initially map about 430,000 hectares of surveyed forest data to provide an initial coverage of tree density throughout the globe. Once these densities were plotted, the remaining unobserved regions would then use a spatial mean of adjacent tree plots to best estimate the amount of trees in these areas. For example, more remote parts of the world like Canada and Northern Europe had their model–generated estimates revised by survey data. In order to account for the disparity between datasets, two models were generated: a global level and biome-level model. The biome-level metric generates a tree density for that biome type and this then gets applied to the regions with no observations. Using the biome data allows for a higher accuracy in mean tree densities, but remains limited on the individual hectare level.

Incorporating GIS data layers for environmental attributes, like moisture level and average temperatures, the model better displays the differences between the global mean tree density and the biome-specific trends. The model showed that higher tree density tends to correlate with high temperature and moisture availability. Moreover, the estimates derived can offer more informed strategies surrounding the management of global forests and ecosystems. It also helps to explain the particularities of environmental processes like the carbon storage model across regions. As a result, this study estimates that humans are destroying nearly 15 billion trees per year and that we have used up nearly 46% of all trees since the Agricultural Revolution began around the year 10,000 BCE. Environmental researchers are hopeful still that the findings from these direct quantifications of trees will produce further insights and offer new workable data for similar scientific studies and policymaking.

Map of the world's forests.  Source: Crowther et al, 2015.

Map of the world’s forests. Source: Crowther et al, 2015.

References

Crowther, T. W. et alNature http://dx.doi.org/10.1038/nature14967 (2015).

Ehrenburg, R. (September 2, 2015). Global count reaches 3 trillion trees. Nature News. Retrieved from http://www.nature.com/news/global-count-reaches-3-trillion-trees-1.18287

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