Monitoring Butterfly Orchids in the Chilterns - 2018 Summary

During October I have been trying to catch up on analysing data and other information I collected on a few orchid projects during the summer.  

One of the more interesting has been the study of a population of Greater Butterfly orchids, made more enjoyable because the landowner is a keen and enthusiastic participant. 

I have now finished interpreting this year's data, and while the population is one of the biggest, if not the biggest in Oxfordshire, it is in quite a confined area of woodland in the Chilterns, shaded by beech and ash.  All the plants above ground in one half of this orchid patch have numbered markers so we can look at their fate year by year.

Here are a  a few highlights from this year (and some photos from the count of flowering plants in June):

There were around 500 plants in 2018 which produced 196 flowering spikes in an area of around 500 m2. The number of plants increased by at least 40% on last year, and the number of flowering spikes by 58%. This year's flowering plants were taller, had larger leaves than those last year, and the fruit set was higher. We are looking at climate influences.

Around 5% of the plants there last year did not appear this year, maybe dead or just dormant. 44% of the plants that flowered in 2017 did so again this year; they were more robust than those that did not repeat flower.

The population was made up by flowering plants (26%), what I term maturing plants with two leaves (26%) and juvenile plants with 1 leaf (48%). The pictures are of the same plant in full flower (June 6), showing the first leaves (April 10), in bud (May 8) and in fruit (Aug 10). Seed dispersal began end August.

Fruit set was more likely for flowers in the middle of the inflorescence rather than the top or bottom.

Much more detail is available in a progress report which can be accessed by following this Dropbox link:

2018 Progress Report A Study of a Population of Greater Butterfly Orchids in the Chilterns

Identification of orchids in fruit

Orchids flower in most cases for only 2 to 3 weeks, and most of them can be identified with certainty, though some experience is required for three genera, the fragrant orchids, the marsh orchids and the helleborines.   Hybrids are a further complication.  

Before flowering reliance on the leaf shape and arrangement can work; Poland and Clement's 'Vegetative Guide to the British Flora' is the best guide and Sean Cole and Mike Waller are working on a photographic guide to be published next year.   But post flowering, what then?  

Over the last few years, but particularly 2017 and 2018, I have been trying to develop a key for orchids in fruit, at a point where they have seed pods with viable seeds, and for most species leaves have withered away.   I have examined 30 species (out of a possible 54) and a further species where I only had a seed pods.  Of the remaining 23 species, 9 are protected and it is illegal to remove any part.  In any case they are pretty much site specific anyway as are another 5.  One has not been seen for years, 3 because of their diminutive size will be impossible to locate in fruit, so that I am left with 5 to look for in 2019, and one more needs further data.

I have measured from photos and direct measurement the angle of the fruits to the stem, the length and width of seed pods.   Using a microscope (x40 and x100) I have measured the length and width of seeds (actually the outer casing, the testa, provided that it contains an embryo.   Habitat, and extant leaves are also useful.  

Seed pod measures

Angle to stem

Seed measures

It does seem possible to discriminate, especially where for example the length / width ratio of seed pods is used rather than absolutes, and provided a reasonably large random sample has been used to determine parameters (5 seems to be enough).  

These are a couple of examples of filtering the results.

I am planning to have a poster on this at the upcoming BSBI meeting in November at Edge Hill.