Trees | | DJS


This might be moved to another place, but it is numeracy devoted to trees. Essay 256 relates to some extent.

I quote (brown) several reports about forestry and then pose some questions based on the content. You should use appropriate precision (e.g. 2 sig fig)

In total, 10,860 hectares of new woodland were planted, the second highest level since 2001. That means that nearly 22 million more trees were planted in Scotland last year.  Around 9.5 million tonnes of CO2 are removed from the atmosphere from Scotland’s forests each year. Scotland’s forests cover 18.8% of the total land mass area and the ambition contained in the Scottish Government’s forestry strategy is to increase this to 21% by 2032.    source.

1. How many new trees  were planted per hectare this reported year? 

2. How many hectares to a square kilometre?

3. If the land area of Scotland is 77910km², how much is forested?

4. The new woodland mentioned represents what percentage increase in forested land?

5 Using your answers so far, does this rate of planting fit with reaching 21% by 2032, if these are 2020 figures?

6. The 21% target for Scotland requires how many additional hectares of planting?

7. The area from Q6 demands a mean tree planting across 12 years of how many hectares? This should agree with your answer to Q5.

Provisional figure at 31 March 2020: 1,311 thousand hectares (ha) of woodland in England that equates to 10.05% of the area of England. This is an increase of 2 thousand hectares over the position a year before.

Over the last 10 years the area of woodland has increased by an average of 2.1 thousand ha per year. Over the 20 year period from 1999 to 2019 the area of woodland has increased from 9.59% to 10.05% of the area of England. source.

8. Express the mean increase in English woodland per year as a percentage of the area of England.

9. If that last answer represents 2100ha per year, then the land area of England is what?

10 The land area of England is 130279km². Find the 2019 woodland area in England.

As of 31 March 2020, around 13% of the UK (3.2 million hectares) was covered by woodland. In England, this was 10%, compared with 15% in Wales, 19% in Scotland and 9% in Northern Ireland.  Forestry Commission, Forestry Statistics 2020, p10

This is relatively low compared with other developed countries: in most European countries, forest covers between 31-50% of land area, and this is also true of the US and Canada.2 For example, woodland covers 32% of France’s land area, 33% of Germany’s and 37% of Spain’s. FAO, FRA, Country reports 2020: France, Germany, Spain

Commenting on the statistics, a Defra spokesperson said: “Tree planting remains at the heart of our ambitious environmental programme which is why we have committed to increase tree planting across the UK to 30,000 hectares per year by 2025 source.

A written answer from October 2019 set out the Government’s progress:

Since 2010 the Government has planted just over 15 million trees, or nearly 13,000 hectares of new woodland in England. In this Parliament over 4.1 million trees have been planted, and we are on track to meet our targets to plant 11 million trees and a further 1 million trees in and around our towns and cities.

The Government is investing £5.7 million to kick-start the development of the Northern Forest, led by a partnership of the Community Forests and the Woodland Trust. Specifically, this will fund the planting of at least 1.8 million new trees across the Northern Forest by 2022, and help the partnership develop approaches to achieve their longer-term vision of 50 million trees planted over the next 25 years.

The Conservatives 2019 manifesto pledged to plant 30 million trees per year, or 30,000 ha (75,000 acres) annually by 2025  source

11  If England succeeded in planting 30 thousand hectares per year from 2025 to 2050, how many additional km² is that under woodland?

12. Looking at European figures, we might well expect a UK target to be 30% of land area to be woodland by 2050. If we apply that target to just England and if we think of this as a target for 30 years, how many hectares per year do we need to be considering planting?

13 The 30 thousand hectares per year of new woodland, if achieved from 2020 to 2050, would result in what percentage area of woodland in England?

Politicians prefer to announce big numbers, so while the forestry industry counts hectares under trees, the politicians would far rather talk about counting trees. There are several things wrong with this, not least that trees are not equal in terms of ground cover, nor in terms of the benefits they provide.

14. Using the figures above, show how many trees you think are planted per hectare. You may prefer to find a range of answers, but do justify your answer.

There is a dramatic difference in the cost of planting a two-year growth of tree in a forest extension and planting a semi-mature tree (defined as over 4m in height and with a girth of 20 cm or more) in a street in town. The first costs around £15 and the second more like £10,000, mostly because of having to move the services to make space for the tree root, which eventually will take up the same sort of volume as the foliage. DJS

15 Comment on how you think a local council might look at a proposal for more tree-lined avenues in town. By all means quote sources. Don't spend longer than twenty minutes on this, though!

Q1 2026 per ha, so say 2000.

Q2 100, since there are 10,000 m² per hectare and so a million m² to a square kilometre.

Q3 18.8% of 77910 is 14647km², 1,464,708ha (which will be rounded off to an answer of 1500 thousand hectares).

Q4 If the 18.8% is after the 10860ha are planted then 1,464,708 is 100% and 10,860ha represents 0.74% of the forested land, 0.14% of Scotland. 

Faulty answer: 0.74% is a lot bigger than 0.18% and at the 0.74% per year would reach the 21% figure by 2025. This is comparing the wrong figures; 0.74% applies to forested land, while 2.2 and 0,.183% apply to total Scottish land.

Q5 12 years to add (21-18.8= ) 2.2% suggests 0.183% added per year as a mean.   0.183% is bigger than 0.14%, so planting would need to rise by about 30% to meet this target.

Q6 21% of 77910km² is 1,636,110 ha. 2.2% of 77910km² is 171,402ha. 

Q7 141,402 ha across 12 years is 14,283 ha per year, quite a bit higher than the 10860 quoted for 2020. This agrees with Q5 answer.

Q8 10.05-9.59 = 0.46% across twenty years is 0.023% per year.

Q9 if 0.023% is 2100 ha, 100% is 91,304k

Q10 10.05% of 130279 is 13093km² or 1,309,300 ha, say 1.3 million hectares.

Clearly the ten year average 2009-19 and the twenty year average 1999-2019 are quite different.

Q11 25 years of 30 kha (300km² per year) is 7500km² more under trees, an additional 5.75% of England's land area.

Q12 To go from 10% to 30% of land area in 30 years is about 0.67% per year. 0.67% of England's land area is 519km² per year, more like 50 thousand hectares per year than 30 thousand.

Q13 2020 area under woodland is10.05%. Call 2020-2050 30 years at 30 kha per year, which is 900kha or 9000km² additional growth, an additional 11.55% of land area, taking us to 21.6%. whether we call this 20% or 21% or 22% depends on how much we trust the 30 thousand number to be given to one, two or three significant figures. 

We must recognise that England is much more densely packed with people than the other UK nations. That suggests that the competition for land to convert to woodland might be hard to find. That does not excuse England such that they can, for example, declare that their share of trees is in Scotland. If the UK target is indeed more like 30%, then reaching 22% by 2050 is a relatively weak target.

Q14 Any number from 1000 per hectare to 1750 per hectare fits the available information (and I've looked quite a bit wider than what is given here). 

Q15 My research is in Essay 356. I suggest that private money, possibly funded largely by the state will account for the bulk of forest planting, including all that done by the Forestry Commission  Forest Scotland, etc. Farmers and other landowners will be given incentives to plant (and maintain) woodland. We have to hope that we have enough variety for the ecological benefits and also that the many pests infecting our woodlands are given enough attention to keep them in check. At an urban level, though, there is a growing body of evidence that says that the street scene has a significant effect on public attitudes to their locality (trees being a good thing). But we've done this on the cheap and it shows, so we must learn to do it right and to successfully separate tree from underground services (cables and pipes) — all of which costs public money.

DJS 20210806

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