More on fragrant orchids and their identification

Back in July I put down some thoughts on identifying the three fragrant orchids following a visit to Waitby Greenriggs in Cumbria; distinguishing between them is not easy. 

I have now done some further analysis because  over the summer I gathered data from another 7 locations where fragrant orchids occur, although not sadly from Clay Lane Meadow which is only 2 km away.  Frustratingly it  was closed off because of damage cause by irresponsible dog walkers. 

Marsh fragrant (Gymnadenia densiflora)

Chalk fragrant (Gymnadenia conopsea s.s.)

 

Heath fragrant (Gymnadenia borealis)

                                                                   The locations were:

• Helsington, Cumbria - Heath fragrant
• Rampsons Farm, South Stainmore - Heath and Marsh fragrant
• Garrow, Perthshire - Heath fragrant
• Keltneyburn, Perthshire - Heath fragrant
• Wye Down, Kent - Chalk fragrant
• Eades Meadow, Worcestershire - Chalk fragrant
• Sheepscombe, Gloucestershire - Chalk fragrant
• Waitby Greenriggs, Cumbria - Chalk, Marsh and Heath fragrant.

In all I made measurements on 40 plants: 

• Spur length (in most cases)
• Sepal length and width
• Angle of the sepal above or below the horizontal (+ or -)
• Number of flowers (in most cases)
• Leaf width (in most cases)
• Width and length of the labellum
• Width and length of the central lobe (except in most of the Heath fragrants where it is not readily distinguishable).

I then looked for measures that can be used for differentiation.  The first step was to decide what a particular plant was which immediately introduces bias because I relied mainly on the floral parts, such as the size of the labellum lobes.  The gold standard, DNA analysis, was not a possibility.  The keys in Stace 3rd edition were not wholly reliable, and with a revised edition of that reference on the way, hopefully there will  be more insights. The results in summary were as follows.

There was significant variation and the boundaries between species on a particular characteristic overlapped.   Nevertheless it does seem possible to separate out the three species.  Easiest is to pick out Heath fragrant.  I found statistically significant differences (using a T-test) between it and the others on labellum width and length, spur length, and the two ratios which I think are the most useful because they avoid absolutes which might be affected by environment and latitude, viz. labellum width to length and sepal length to width.   The former is not a surprising result because my initial allocation was probably weighted  towards this characteristic.

The other two were harder to separate, but on three parameters there was a statistically significant difference (using a T-test):  angle of sepals to the horizontal, labellum width to length and sepal length to width.  Given the variability and species overlap, I would look for best fit against each of them.

I did not measure the height of each plant nor the height of just the inflorescence.  A pity because it is probably worth looking at further, though I suspect there will be latitudinal effects, with northern plants on average shorter than those further south.   It was also noticeable that the inflorescence of Chalk fragrant plants had a mor lax appearance, and what might be very useful is the ratio of the number of flowers to the length of the inflorescence.  

I should also mention habitat and the common names define what is likely in marshes, downland and essentially north and west Scotland, but it is not 100% predictive.  The Helsington plants of heath fragrant for instance are overlying a base of limestone, so there is probably a cap of more peaty soil over it.

Looking forward there could be some significant  advances in fragrant orchid disaggregation.  I have mentioned the new edition of Stace which comes out in late January,  but Richard Bateman and Ian Denholm, BSBI's joint orchid referees, will publish a  note on identification in the January edition of BSBI News, to be followed later in the year by an academic paper on the subject.   So next summer fragrant orchids will become easy peasy, at last!