Monday, 5 June 2017

Fruiting of Early Purple Orchids at Westwell Gorse

The Early purple orchids (Orchis mascula) are now in fruit at Westwell Gorse. 

In flower 4 May
In fruit 2 June

In flower 4 May
In fruit 2 June (top chewed)
In early May we measured the height and counted the number of flowers on 83 plants in a survey area we designated.  We went back on Friday to count the number of seed pods in order to estimate the fruiting success.  We found 63 plants; once the sward is higher it is  challenging to find just the fruiting plants.   For each we counted both the number of seed pods and the number of dead flowers that had not fruited.   From this we estimated the fruiting success at 36%, but it needs correcting.   In May the average number of flowers per plant was 9.8, whereas last Friday we got a lower figure of  8.6.   Either we missed some of the larger plants with more flowers, or more likely some of the buds that we originally counted as flowers did not develop, but just fell away.  If we apply an adjustment of 8.6/9.8 for the number of flowers then the fruiting success falls to 31%, which by coincidence is the same figure as in 2016.

The figure compares with references given for some German studies by Claessens and Kleynen in The Flower of the European Orchid ranging from 7 to 67% with an average of 34%.  Results from Sweden quoted in the same text were lower.   A Norfolk study (Hardy Orchid Society: April 2013) got a much higher result, 84%.  That looks atypical.

The range across individual plants was from zero to uncommonly 100%, with no real pattern.   The chart below plots Fruiting success (Y-axis) against Number of flowers per individual plant (X-axis)

It was noticeable that flowers at the top of the spike - those flowering later - were less likely to be pollinated, though we made no attempt to quantify the effect.

Pyramidal orchids were just coming in to flower, whilst there were 10 plants of Purple Milk Vetch, which is on the Red Data List.
Pyramidal Orchid

Purple Milk Vetch

No comments:

Post a Comment