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Posts tagged ‘silviculture’

Basal Areas for common walnut

December 27, 2011

Gabriel Hemery

common walnut basal areas

There are no published Yield Class tables for common walnut Juglans regia – at least that I am aware of. A search on the European Forest Yield Tables Database reveals that data are only available for black walnut Juglans nigra in Hungary.

I wrote previously about research that I undertook exploring the crown sizes of major hardwood species – Estimating tree crown size. This work provides the next best available data on managing a stand of common walnut, in the form of basal areas for common walnut ref.

The table below shows the stem diameter (dbh), crown diameter (cd), crown/stem ratio (cd/dbh), number of trees per hectare (Nha) and acre (Nac), and Basal Areas (G) in m2 per hectare. These data were collected from trees grown in open conditions, and calculated for stand densities with zero crown overlap.




N trees per ha

N trees per acre

Basal Area m2 per ha















































































common walnut basal areas

Common walnut Juglans regia basal areas with dbh.

A growth rate of 1cm per year in stem diameter can be presumed, permitting this graph and data to be used in estimating suitable basal areas at different stand ages. If real dbh data is available, then the accurate growth rates will provide accurate basal area increase projections for a given site.

Gabriel Hemery


Hemery, G.E., Savill, P. & Pryor, S.N. (2005). Applications of the crown diameter – stem diameter relationship for different species of broadleaved trees. Forest Ecology and Management 215, 285-294. View abstract

Christmas trees

November 21, 2011

Gabriel Hemery

Christmas tree guide

The Christmas tree – or albero di natale, arbre de noël or sapin de noël, árbol de Navidad, árvore de natal, coeden nadolig, crann Nollag, jólatré, joulukuusi, juletre, julgran, kerstboom, Tannenbaum or Weihnachtsbaum, новогодняя елка and χριστουγεννιάτικο δέντρο – is a celebrated icon of Christmas around the world. It is perhaps the only time that most people really hug a tree as they struggle to get one through the front door!

A brief history of the Christmas tree

According to Wikipedia the first records of Christmas trees are from Northern Germany in the 15th Century. Trees were initially decorated with sweets (candies) and red paper.  Tinsel was invented in Germany around 1610 using real silver.  The first record of Christmas trees in America was in 1747, where a tree was provided for children in a Moravian Church settlement in Bethlehem, Pennsylvania. Prince Albert is credited with bringing the first Christmas Tree to the British royal family; an article appeared in Illustrated London News in 1848.

Top selling trees in North America

Results of one online poll, reveals that the top five ranking Christmas trees in the USA are led by Fraser Fir (33%), followed by Douglas Fir, Balsam Fir, Colorado Blue Spruce and, in fifth place, Scots Pine. The National Christmas Tree Association sell 25-30 million Christmas trees every year in North America. The trees are grown in Fifty states and in Canada, offering a green alternative to the plastic variety (of which 80% are made in China).

Perhaps the most famous Christmas tree in the US was harvested recently on 17th November, to be delivered to the White House.  The US President will be receiving a 5.8m (19 foot) balsam fir, harvested from a farm near Neshkoro in Wisconsin (read story).

Find Christmas tree retailers in Canada

Find Christmas tree retailers in the USA by state

Top selling trees in Europe

Christmas tree guide

Christmas tree guide from the BCTGA. Click for pdf version

In Europe the Nordman Fir is the most popular Christmas tree although recently the Fraser Fir has been increasingly available and is proving ever more popular. Denmark is the biggest European exporter of Christmas trees, and their association was famously fined in 2007 for operating a cartel (read more).

In Britain alone there are many hundreds of growers who specialise in growing Christmas trees. There are 320 members of the British Christmas Tree Growers association, who together sell 8 million trees a year (BCTGA website).

British Christmas tree growers who sell online

How to grow Christmas trees

Christmas tree growing can be a lucrative business but is unforgiving on quality, as second grade trees will not sell. Growing the trees is more like a slow agricultural crop than a fast forestry crop. A typical rotation is 5-7 years for standard sized trees, although some growers specialise in the tall trees sold for town centres and large buildings.

Site selection is important, particularly avoiding frost prone sites as buds can be damaged by severe late frosts. Soils should not be too acid, or heavy clay, chalk or very sandy. If too fertile the trees will grow too fast and be ‘leggy’, requiring expensive efforts to trim and encourage suitable form; a technique called “shearing”.

The crop must be protected from browsing mammals, whose activities will adversely affect tree form; rabbit and deer proof fencing is normally required. Sometimes insect pests will need to be controlled (Blue spruce is very prone to aphids) and specialist advice should be sought by growers.

Christmas trees are normally planted at 1x1m (10,000 per ha / 4200 per acre) or 1.2×1.2m (6900 per ha / 2900 per acre). Two or three year old trees are normally planted when dormant. Weed control is extremely important in following years, not only to ensure good tree survival and growth, but also to minimise competition that could lead to poor form on the tree’s lower branches.

Gabriel Hemery

Popular Christmas tree species

Prune your walnut trees on Saint Swithin’s day

July 15, 2011

Gabriel Hemery

Saint Swithin window at York cathedral

Saint Swithin window at York cathedral

It is Saint Swithin’s Day today, 15th July: the day on which people traditionally watch the weather.  Tradition says that whatever the weather is like on St. Swithin’s Day, it will continue the same for the next forty days.

There is a well-known weather-rhyme in Britain:

St. Swithin’s day if thou dost rain
For forty days it will remain
St. Swithin’s day if thou be fair
For forty days ’twill rain nae mair.

St. Swithin’s Day is also the ideal time of the year to prune walnut trees.  Pruning walnut trees in the dormant season (i.e. when the leaves are absent) is not recommended as it is for nearly all other broadleaved tree species.  If a walnut is pruned when the tree is dormant, or even worse when it is actively growing, the pruned wound will bleed profusely which will weaken growth the following season, and possibly increase risk of infection.  Pruning is undertaken ideally near the end of the walnut’s active growing season and St. Swithin’s Day, in mid July, provides a perfect marker.

Gabriel Hemery

Coppice lazarus

May 30, 2011

Gabriel Hemery

Ancient ash coppice stool
Ancient ash coppice stool

Ancient ash coppice stool

I featured this ancient ash coppice stool in January.

It had been coppiced to regenerate new growth; a cycle of management that this tree may have been through perhaps a half a dozen times in its life.  It attracted my interest because on one of the freshly cut faces of the stump, a member of the public had written with a marker pen:

“This was one our best loved trees.  We are sad that you have cut it down” Read more …

I returned to the coppice stool last week, to search for the new growth that I had predicted so confidently in my defense of woodland management.

I was pleased, and secretly relieved, to discover its restoration to life: several tiny sprouts were emerging from the gnarled and hollow coppice stool.  See the photo below.  Let’s pray that the deer allow these shoots to grow.

Ancient ash coppice stool regenerates

Gabriel Hemery

Read more about the story of this ancient ash coppice stool

Estimating tree crown size

May 23, 2011

Gabriel Hemery

I co-authored an academic paper in 2005 that summarised research undertaken to explore the relationship between a tree’s stem diameter and its crown (or canopy) diameter 1.  Out of my 60 or so publications, it has been one of the most popular among forest scientists (e.g. Google Scholar citations).

It was fascinating to discover that statistically there was a very good relationship (scientists would refer to a correlation from a regression analysis) between stem diameter and crown diameter.  We decided to explore this further by calculating the ratio between the two, we called it the z ratio (= crown diameter ÷ stem diameter).  We then plotted this z ratio against stem size.  You can see the result on the graph below for nine common European broadleaved trees.

Tree crown and stem diameter ratio graph

Crown diameter: stem diameter relationship for nine broadleaved tree species. The z ratio (y axis) is crown diameter divided by stem diameter; the dbh (x axis) is stem diameter at breast height (measured at 1.3m). Click to enlarge graph

The graph highlights some very interesting growth patterns and difference between different species:

  • Common walnut (Juglans regia) has the largest crown diameter at any given stage in its stem size.  When a walnut stem is 15 cm in diameter its crown can be estimated to be 5m wide.  Foresters can use that knowledge to design walnut plantations: e.g. if they plant their walnut trees 5m apart, their crowns will not compete until their stem diameter is 15 cm (which will take about 15 years from planting in the UK).
  • Sweet chestnut (Castanea sativa), like walnut, has a very large crown while it is young (with a small stem size).  Unlike walnut however, as its stem size increases, the ratio with its crown diameter decreases rapidly to the point after 35cm in diameter, when it has the smallest crown diameter for any of the nine tree species assessed.
  • Sycamore (Acer pseudoplatanus) has the most consistent crown to stem ratio while it grows.

basal area per hectare (G, m2 ha-1)The data can be used to plan tree spacings and to calculate basal area.  For example: for walnut with a stem diameter of 0.60m, its crown diameter is 13.27m, and its z ratio is 22.12.   Using the equation (left) for estimating basal area per hectare (G, m2 ha-1) tells us that there would be 57 trees per hectare with a basal area of 16.1 m2 ha-1.

These findings can be used beyond tree spacings and calculating basal area; they can also be used to help in:

  • planning thinning regimes (how many trees to remove in a growing plantation and when)
  • planning stand density (how many trees to retain in a forest stand at any given size)
  • assisting in managing mixed conifer-broadleaved stands
  • estimating branchwood  and woodfuel volumes
  • maintaining free-growth silvicultural systems, and
  • in urban tree planning by arboriculturists and landscape gardeners (e.g. designing and managing tree avenues).

Gabriel Hemery


1 Hemery, G.E., Savill, P. & Pryor, S.N. (2005).  Applications of the crown diameter – stem diameter relationship for different species of broadleaved trees. Forest Ecology and Management 215, 285-294. View abstract

How to calculate tree height using a smartphone

May 15, 2011

Gabriel Hemery

Foresters use a clinometer to calculate the height of a tree.  These can be quite expensive to buy so you’re not likely to invest in one unless you need to measure trees frequently. However, don’t despair as it is quite possible to achieve good results with a smartphone; with the advantage that you nearly always have one with you.  This is a method I’ve developed that I hope others will find useful.


Install a spirit level app

iPhone and Google Android phones (sorry, I have no working knowledge of Blackberrys) allow apps to be downloaded.  There are a number of apps available that use the smartphone’s accelerometer to provide a virtual spirit level.  Some of these include a tilt meter or level meter and it’s this kind that you will need to be able to measure an angle of elevation.

For the example below I use an iPhone and the Carpenter app from iHandy.  The full version costs under $2 and I’ve found that it works very well.  Even better the Level app, which you need for this method, is currently free as a stand-alone app.

Calibrate – the level … and yourself

  1. Zero your virtual spirit level
    If you’ve installed a good spirit level app it should allow you to calibrate your smartphone to reasonable accuracy.  On a shelf that you’ve checked to be perfectly level with a real spirit level, place your smartphone on its side , then ‘zero’ it (calibrate it to read ‘0’).
  2. Calibrate your pacing
    You will need to be able measure the distance that you are standing away from a tree.  You can of course carry a long tape measure with you when you think you’ll need one but I find it very helpful to know my pacing with reasonable accuracy.  For example I know that 21 of my strides equal 20m.
  3. Measure the height above ground of your eye
    Simply measure the distance from the ground under your feet to one of your eyes.

The data and the maths

The data required

You will need to know three things:

  1. The angle of elevation to the top of the tree from the horizontal
    As shown below the horizontal will be equivalent to a line between your eye and someone of the same height standing next to the tree.
  2. The distance from where you stand to the base of the tree
  3. The height of your eye above the ground
how to calculate tree height

The three measurements needed to calculate the height of a tree

The maths

Don’t be put off if you’re not confident about the maths as, although it may seem daunting, it’s actually quite simple.  You will use trigonometry but your smartphone’s calculator will do all the hard calculations for you.

The equation you will use is:

Tan angle of elevation   x  distance to tree


+ height of eye above ground

Step by step guide (with working example)

1.   Stand away from the tree

Stand away from the tree so that you can see its top.  The method works best if your angle of elevation is about 45o .  In other words that your distance from the tree is equivalent roughly to the height of the tree (you’ll get better at estimating this, the more trees you measure).

2.  Pace (or measure) the distance away from the tree

If you are working with someone else they can help you measure the distance from where you’re standing to the tree.  If you’re on your own, drop something where you’re standing to mark the position so that you can return once you’ve paced or measured the distance to the tree.

I was 21m away from my tree.  Write this distance down or memorise it.

3.  Measure the angle of elevation

measuring the angle of elevation with a smartphone

measuring the angle of elevation with a smartphone = 43.7 degrees

Standing at your spot (step 2) open the virtual spirit level app on your smartphone and select the angle measure.  Bring the smartphone to your eye and sight along its edge, as if you’re looking down a gunsight, aiming at the very top of the tree.  You will need to hold the phone so that your fingers are not in the way (see photo).

With the app I use there is a ‘hold’ button that freezes it when you are satisfied that you are ready to record the angle.

My angle of elevation was 43.7o.  Write this angle down or memorise it.

4.  Calculate tree height

Open the calculator on your smartphone.  You will need to access the scientific calculator.  For iPhones and HTCs you can do this by tilting your phone onto its side (landscape), where you will find the function for Tangent or Tan.

enter angle of elevation

enter the angle of elevation (43.7)

tan of angle

press tan (= 0.9556)

x distance to tree

multiply this by the distance to the tree (x 21m)

add height to eye

add height to eye (+ 1.73m)


press equals (result = 21.79m tall)

So my tree was 21.798m tall. Given that there is lots of room for error I would recommend that you round the result to the nearest whole number = 22m tall. It doesn’t matter whether you use metres or feet as long as you use consistently the same units throughout (i.e. don’t switch between m and cm, or between feet and inches).

Increasing accuracy

The method I describe above works well on level ground.  However, if you want to increase accuracy especially if the ground is not level, you can improve it quite simply.  Instead of using the height of your eye above ground, you substitute it with (Tan angle to the base of the tree x distance to tree).  So the full equation would be:

(Tan ∠ to tree top x distance to tree) + (Tan ∠ to tree base x distance to tree)

Other applications

You can use this method to calculate the height of other features on trees, such as the height of the lowest branch, a major fork, a bird nest or bat roost, or of course for any tall object such as a bridge or building.

Gabriel Hemery

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