How many trees are there in the world?

When Thomas Crowther and Henry Glick used geospatial data in 2015 to estimate the number of trees in the whole world there was good news and bad news. Good news that there were several times as many trees as anyone had previously realised (total is about 3 trillion) and bad news that the number was declining faster than expected.  The calculation was done partly to establish a baseline so that tree-planting efforts could be put into perspective - the UN's Billion Tree Campaign managed to plant about 15 billion trees over 10 years in virtually all countries across the world (193).  But whilst these numbers sound large this has only added 0.5% of trees so much more needs to be done if we want to reduce the damage humans have done to tree numbers - before humans emerged there were about twice as many trees as there are now.  Perhaps that's why the Trillion Tree Campaign was launched in 2018 in Monaco - a principality covering only two square kilometres and almost bereft of trees.

More locally, the UK has a tree count of about 3 billion trees - about 45 trees per person - calculated by analysing aerial photos and estimating tree numbers as was done under the UN's Plant for the Planet project.  There is some flexibility on the definition of what counts as a tree - and this assumes you don't count all the self-sown seedlings or bushes, which some might consider as trees.  This number is dominated by a few species - of the commercially grown plantations in Scotland, for example, 60% of the trees are Sitka Spruce so they don't add as much to biodiversity as a wider species mix would do.  And those 45 trees per person is a bit misleading in that there may be nearer to  400 trees per person in Scotland  - it is more sparsely populated with less than a tenth of the UK's population but almost half the trees.

The UK's tree-planting is put into perspective by comparing the relative numbers - the UK's 3 billion trees amounts to one thousandth of the global total whereas nearer to 1% of the world's population is British - so by comparison with the world's average, on a per-person-basis a Briton has a tenth as many trees as the average citizen of the world.  The most densely tree-ed parts of the world are the tropics where 43% of the world's trees are growing.

At a recent general election (2019) the political parties were competing to promise how many new trees they would ensure were planted - the Liberals and SNP each promised 60 million a year, whereas Labour said they would do 100 million every year.  A more realistic Conservative party promised 30 million trees every year which would equate to about 15,000 hectares (or 6,000 acres).  Despite this lower manifesto promise, the government is really struggling to achieve even that amount - but if they did reach that target every year for 10 years it would add 10% to the UK's tree cover.

There is ample scope for more trees with the UK having only 13% tree cover, but are there are other ways to increase the numbers beyond tree planting? Trees will naturally establish themselves if they are not cut or grazed - perhaps one thing the government could do would be to reduce the number of wild grazers - mostly deer and sheep.