Thread by @crawley_mick

As with the oaks, the birches have two native species and a hybrid. But Betula is even trickier, because the genetics are more complicated.

Linnaeus didn’t make very many mistakes, and he considered both our two birches to represent extreme forms of one variable species, which he called Betula alba. What he couldn’t know is that this lumped together sexually incompatible plants with different chromosome counts.

We now know that there are two distinct species, with different chromosome numbers (ploidies). Betula pendula has 2n = 28 (a diploid, Fig. A) and Betula pubescens has 2n = 56 (a tetraploid, Fig. C). The image in the middle is a triploid hybrid (2n = 14 + 28 = 42, Fig. B)

The two species have different ecological niches: Betula pendula in warmer, drier habitats and B. pubescens in colder, wetter sites. This is reflected in their geographic distributions: in Scotland, for instance, B. pubescens is the only species you’ll find in the north and west

We’ll start with the easiest feature, but Sod’s Law is at work here, because it’s not unambiguous. Look at the young twigs (x10). If they are densely velvety with very short, fine (puberulent) hairs, then you have Betula pubescens (hence the name, left).

Unfortunately, you can’t conclude that hairless twigs means B. pendula (right), because some forms of B. pubescens (despite their name) have hairless twigs.

So how to proceed? A two-stage process is most effective, starting with easy, accessible characters. Find a mature individual (more than, say, 6m tall) and observe it from a distance. Does the canopy have slender twigs, many of which hang almost vertically downwards? Or not.

Next, look at the base of the trunk of a mature individual. Is the bark broken up by deep, corky fissures into large, dark grey blocks. Or is the trunk smooth right down to the ground.

A big difference between birch and oak is that birch produces new leaves from the shoot tip throughout the growing season (oak leaves appear in just one or two (occasionally 3) discrete bursts). Terminal birch leaves differ in both size and shape as the summer progresses.

It is important, therefore to use the less variable leaves from the (lateral) short-shoots rather than the terminal leaf. There are two things we need to note. First, is the leaf tip acuminate (left) or not (right) ?

Now look at the marginal leaf-teeth. Do the big teeth have smaller teeth upon them (left), or are all the teeth un-toothed (right, even though the teeth may differ somewhat in size )?

OK. So here’s the initial, rough-and-ready scoring scheme:

Slender twigs hanging almost vertically down = 1;
not so = 2
Basal bark black and chunky = 1; not so = 2
Leaf tip acuminate = 1; not so = 2.
Leaf teeth with teeth = 1; not so =2

Add your 4 scores together.

Total = 4= Betula pendula,

Total = 8 = B. pubescens,

Total = 5, 6 or 7 = something intermediate (details later)

You may want to stop at this point, but there are more useful features to investigate. These have to do with the female catkins and with more detailed measurements on the leaves. Break open a female catkin (or find an old but intact one on the ground). Look closely (x10).

Take time to distinguish two kinds of things: winged fruits (left) and reproductive bracts (right).

Start with the fruits: there is a pair of papery wings and a nut in the middle. On the top of the nut look very closely at the 2 stigmas (x10). Do the wings clearly over-top the stigmas (left) or not (right) ?

Now concentrate on a reproductive bract, and see what is meant by ‘the lateral lobes of the fruiting bracts’. The outlines look a bit like birds in flight. Are the wings outstretched (like a gliding fulmar, left) or is the leading edge lobed (like a stooping peregrine, right) ?

Go to the bottom of the leaf, next to the petiole. Measure (in mm) the distance from the top of the petiole along the bottom margin of the leaf to the first (i.e. lowest) tooth. This is a big distance in B. pendula (>10mm, left), and a short one in B. pubescens (right)

There is a toothiness index for the leaf margin. From the base (top of the petiole) count the main veins upwards to find the 3rd and 4th veins. Count the teeth between the two end-vein-teeth. Multiple teeth is B. pendula (left = 3); 1 or 0 is B. pubescens (right)

Last is a way to quantify the degree to which the leaf tip is acuminate. Measure the leaf from tip to petiole. Divide by 4. Check your arithmetic (!). Make a mark ¼ of the way down from the tip. Measure (mm) the width of the leaf at right angels to the central vein.

Narrow (less than 13mm) means acuminate (left), wider (14mm or more) means not acuminate (right). Now score 1 for the first question or 2 for the second (as on the next tweet) and add up the scores

Fruit wings above stigma = 1, not so = 2.
Floral bract wings fulmar = 1, peregrine = 2
Length to 1st tooth long (>9mm) = 1, short (<9) = 2
Toothiness index of multiple = 1, none or 1= 2
Acuminate width small (<14mm) = 1, large (>14mm) = 2

Add all the 5 scores together:

5 = B. pendula

10 = B. pubescens

6-9 = something else

Now let’s get down to the serious business of thinking about the “something else”. First, a brief genetic refresher: sex involves halving the number of chromosomes (meiosis); male and female haploid gametes unite at fertilization to restore the original diploid chromosome number

Sex only works when both parents have the same chromosome count. But the two Betula species have different counts. Betula pendula has 2n = 28 (diploid) and B. pubescens has 2n = 56 (tetraploid), so this can’t work.

So how can a functioning hybrid chromosome-count come about? One way is if B. pendula could produce un-reduced gametes (meiosis failure, so 2n=28 instead of n = 14). Then sex with B. pubescens could work (28+28 = 56).

Another possibility is the formation of triploid hybrids between reduced diploid and tetraploid parents (14+28 = 42); these would normally be infertile, but not always.

We really need some new vocabulary here. Hybridization is a cross where half of an individual’s chromosomes come from one species, and half from another. The two parents belong to separate, distinct species.

Introgression is more subtle: it is the movement of a gene from one species into the gene pool of another. It comes about as a result of repeated back-crossing of an interspecific hybrid with one of its parent species, resulting in a complex mixture of parental genes.

Birch hybrids are so very variable, because of different ways of combining the chromosomes. Betula pendula is diploid (2n=28) and B. pubescens is tetraploid (2n=56). Hybrids can be 2n=56 or anything between 2n=30 and 2n=48. They may be fully fertile or completely sterile.

Current thinking is that most of the hybrids involve gene movement from Betula pendula into B. pubescens. The classic hybrid Betula x aurata is close to 50-50 from each genome. But the complexities mean that the whole space from 50% B. pubescens up to 99% is possible in theory

Using the total scores (above) you have the tools to measure how far off from either parent species is the sample in hand. What you then do with this, is a moot point. Very few people record B. x aurata. You might want to become one of this select band.

The fact that you have got this far shows that you are interested. If you want more detail, I strongly recommend that you read "The Genus Betula" by Ashburner and McAllister (2013, Kew Publishing).

Just to finish off, you might have been wondering why I said we have 2 native birch species. The answer is that the third is not a tree a tall, but rather a diminutive shrub, found on peat bogs in remote parts of the Scottish Highlands. This is Dwarf Birch (Betula nana).

Tailpiece. Stace (4th Ed) has two sub-species of Betula pubescens. The Scottish plant with tiny leaves (<3cm) is subsp. tortuosa. The other is contentious. B. celtiberica is a rare plant of the Cantabrian mountains in NW Spain. Stace's subspecies is probably just a hybrid.