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Posts tagged ‘birch’

Birch in the Scottish Highlands

December 7, 2012

Gabriel Hemery

Day two of our Scottish drawing expedition took us to the southern shore of Loch Rannoch. We were in search of a treescape that would enable us to feature birch and water together. We had a specific place in mind for where the drawing will feature in the book.

The weather was cold, as it was yesterday when we visited the Black Wood of Rannoch, but it was sleeting too and the sun didn’t appear all day. Sarah worked from the shelter of a tent, as the paper had to be kept dry at all times. She developed the drawing first with pencil, allowing the various elements of the treescape to be brought together.

Sarah Simblet working in pencil

Sarah Simblet working on the pencil under-drawing before adding ink

Gabriel Hemery working on The New Sylva

Gabriel Hemery working on The New Sylva on the shore of Loch Rannoch

Sarah Simblet working on the drawing of birch

Sarah Simblet working on the drawing of birch and the loch shore from the shelter of a tent during a bitterly cold December day.

Scots Pine in the Black Wood of Rannoch

December 6, 2012

Gabriel Hemery

Yesterday our Scottish Drawing Expedition for The New Sylva got underway. In search of Scots Pine (Pinus sylvestris) we had travelled to one of the last remaining and best examples of Caledonian Pinewood: the Black Wood of Rannoch, in Central Scotland.

High above Loch Rannoch, on an undulating heather-clad ridge, we found the perfect subject; an ancient ‘granny pine’ set amongst a backdrop of younger pine, downy birch (Betula pubescens) and rowan (Sorbus aucuparia).

Brilliant Winter sun added a glowing aura to the red bark of the pine stems and branches. The pinewood was carpeted in freshly-fallen snow and perfectly quiet. Temperatures remained below freezing throughout the short day, dropping as low as -5°C, but multiple layers of clothing just about kept us warm (see the photo of Sarah three-hoods Simblet below).

Sarah Simblet drawing Scots Pine in Black Wood, Rannoch

Sarah Simblet drawing Scots Pine in Black Wood, Rannoch

Black Wood is a precious and unique habitat. We were not fortunate enough to see Scottish Wildcat, Crossbill or Red Squirrel on this occasion but were accompanied by troops of Tits and Goldcrests all day, while a lone Robin kept watch for our lunch crumbs.

Tomorrow we are in the Caledonian Pinewoods again; this time in search of birch.

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

Birch pollen allergy

March 28, 2011

Gabriel Hemery

birch pollen

Silver birch pollen. Each is about 20 microns in diameter

Trees provide us with many miracle cures, most famously aspirin from the willow and the cancer-busting drug taxol from the yew. However, they can also affect human health negatively. Pollen from the silver birch, Betula pendula, is the second most severe allergen for people in the UK. Late March is the usual peak in birch pollen levels so many people with ‘hay fever’ are likely to be suffering at the moment.

Birch pollen grains are tiny, typically 20 microns (0.02mm) across, and can travel hundreds, if not thousands, of miles in the atmosphere. A recent study that assessed birch pollen levels for Londoners concluded that their concentrations were likely to be heavily influenced by birch forests in southern England and continental Europe, not only local street trees.

Birch pollen is also strongly associated with allergic reactions in the mouth to certain foods. This is termed pollen-related oral allergy syndrome (OAS). Sensitivity to birch pollen is related to OAS when eating almonds, apples, apricots, avocados, bananas, carrots, celery, cherries, chicory coriander, fennel, fig, hazelnuts, kiwi fruit, nectarines, parsley, parsnips, peaches, pears, peppers, plums, potatoes, prunes, soy, strawberries, wheat, and potentially walnuts. As many as 75% of birch pollen-allergic people may be affected by the mildest form of OAS, a sensation in the lips or tongue after eating raw apples. However, many sufferers do not realise that they have an allergic problem.

I came across an interesting account by a Swede who reports his own solution to battling a birch pollen allergy by drinking birch tree sap, including instructions on how to tap a tree for its sap: read more. I’m not a medical man so I can’t comment on this except to say that this definitely intrigued me.

Gabriel Hemery

Climate change, timber quality and carbon

March 11, 2011

Gabriel Hemery

Changes projected in the climate will affect tree growth in most parts of the world. Warmer and wetter conditions in many temperate regions, at certain times of the year, will favour many trees.  But if the trees grow faster will this be good for sequestering carbon and for producing more home-grown timber?

hardwood typesFor conifers that produce softwood timber, their accelerated growth will lead to a fall in timber density and therefore strength.  As a large part of the market for softwood is the construction industry then the consequences may be quite serious.

Broadleaved trees producing hardwood timber on the other hand are likely to be affected positively by a warming climate, as a longer growing season will lead to increased yields without loss of strength. Why?

Broadleaved trees can be split into two types in terms of their hardwood timber character.
1. Diffuse-porous wood type: timber quality is independent of growth rate. Species include beech, birch, wild cherry, maple and sycamore.
2. Ring-porous wood type: these act the opposite of softwoods, becoming denser, harder and stronger with increased growth rate.  Species include ash, oak, sweet chestnut and walnut.

So what does this mean for carbon management in our woodlands? Simply, the better the quality of timber produced the greater the carbon sequestration; as the carbon is locked into solid wood products for longer.  The long rotation time (time between planting, felling and restocking) and the high long-term yields are also factors in favour of hardwood forestry for carbon sequestration.

Gabriel Hemery

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