September 2, 2015

expert reaction to new research on tree numbers

Dr Helen McKay, Head of the Centre for Sustainable Forestry and Climate Change at the Forestry Commission’s Forest Research agency, said:

“Trees and forests are essential to a sustainable future for the Earth, and we welcome this paper’s contribution to our developing understanding of tree density in the world’s major biomes. The findings will provide scientists, forest managers and policy-makers with valuable evidence, although additional information on the size, species and management of the trees in these biomes is needed to inform our view of the forests’ sustainability. We will study this paper with interest.”

Dr Nathalie Pettorelli, Research Fellow at the Zoological Society of London, said:

“Trees are a key component of the global carbon cycle, due to their capacity to absorb carbon from the atmosphere through photosynthesis and due to the release of dioxide carbon associated with deforestation and forest degradation: understanding the contribution made by woodlands to the global carbon balance is thus of high importance in the context of climate change mitigation.

“In a given place on Earth, both tree density and forest extent are likely to shape the rate at which dioxide carbon is removed from the atmosphere and/or the quantity of carbon retained in the woodland as a reservoir – yet, up until the study published by Crowther and colleagues, information on the distribution of tree density wasn’t available globally, meaning that that this dimension could not be considered when developing estimates of the world’s forests’ contribution to biogeochemical cycles. This has certainly limited our ability to comprehend how tree density was affecting carbon absorption and retention, and more broadly the delivery of services such as water purification, by forests in various places on Earth.

“Although this study may open new opportunities to account for variability in tree density when assessing carbon storage within ecosystems and potentially revise estimates of the world’s capacity to absorb carbon, it may be important to acknowledge that these first estimates produced by Crowther and colleagues are derived from data primarily collected in Europe and North America, with eg very little information collected in the Congo basin, China, Australia or India. As more information becomes available for these countries, it might be interesting to refine the estimates and check that key processes shaping spatial variability in tree density have not been overlooked. Similarly, it may be important to highlight that any update of the estimates presented by Crowther and colleagues will necessitate the coordinated collection of new field data at the global scale, an exercise that could be proven extremely costly and difficult to implement on a regular basis.

“This research does not directly challenge estimates of CO2 absorption and retention by the world’s trees.  Instead, it offers a layer of information that could become important in future efforts to derive these estimates.”

Dr Martin Lukac, Forest Scientist at the University of Reading, said:

“This research provides some useful new information, particularly about average tree densities in different types of forest around the world. This should help scientists improve computer models that simulate the impact of tree growth on the climate.

“However, I have several misgivings about the findings. One of my biggest concerns is the method used to calculate the total number of trees. The researchers only looked at areas categorised as ‘forest’ to calculate the total number of trees. In the UK, for example, this only accounts for 13% of the British land area and ignores the rest, even though we know that Britain has millions of trees growing in areas which would not be classified as forest.

“The study states that forests with a higher density of trees store more carbon than those with fewer trees. This is nonsense. In a normal forest, a lot of small trees will store less carbon than fewer large trees.

“The previous estimate of trees in the world was 400 billion. The new estimate is 3 trillion large trees. There are so many margins of error in this study that the real number could be anything between the two – or even 10 times higher.”

Dr Simon Lewis, Reader in Global Change Science at University College London, said:

“This is a good study, compiling already published data into a synthesis. However, it is important to note that there aren’t many times more trees than we previously thought, as there has not been a robust global estimate before. In my view this is the first. Care is required when talking about numbers of trees as they are usually not the most important attribute of an ecosystem. A plantation forest of many small trees all of the same type isn’t better than a patch of pristine Amazon rainforest with fewer very large trees of all different species. Similarly, measuring carbon storage in forests required different techniques than counting trees, as most carbon in a forest is held in a small number of large trees, not the many small trees. However, global overviews do allow us to see important new aspects of Earth, as the study shows that humans have removed 46% of Earth’s trees, an important statistic showing the heavy influence of human activity on all Earth’s ecosystems. ”

‘Mapping tree density at a global scale’ by T. W. Crowther et al. published in Nature on Wednesday 2nd September.

Declared interests

None declared