A strange population of Paphiopedilum callosum in habitat in Southern Cambodia.

On August 2016, while I was leading a botanical survey in Southern Cambodia with friends Jeremy Holden and Reaksmey, we observed an interesting population of some identified slipper orchids. Sadly, the plants were not in flower at the time and I was not sure when I was in the field what they really were. My best bet was that they were Paphiopedilum callosum.

Indeed Paphiopedilum callosum is quite well documented from Cambodia and is also known to occur in neighboringThailand and Laos. Incidentally,I spent some time in Phnom Penh herbarium at the end of that journey in Cambodia and the only other Paphiopedilum I could find within the herbarium vouchers were some Paphiopedilum appletonianum samples collected further North in the country. It was not clear to me back there in the field or in the herbarium how to separate P. callosum from P. appletonianum. 

Luckily, I would be given some help a few months later: my initial guess turned out be accurate as my local guide provided me with the following photograph taken in the very same site:

Paphiopedilum callosum without a doubt. What a shame that I couldn't witness its peculiar beauty with my own eyes. It's just a matter of time until I can return to that location anyway and I'll do my best then to arrange a field trip during the flowering season.

Those Cambodian Paphiopedilum callosum grew in shady habitats on the the forest floor in leaf litter at about 300 masl. Some specimens that you will see at the end of this article were growing as lithophytes on moss covered rocks near some forest streams, always in the shadier spots. Both habitats are typical of the species. 

Although we were not at sea level, temperatures were really high on that day and it was even suffocating at times, for me at least. It was very humid as well as one can expect from this part of Cambodia. I managed to enjoy the day trip anyway as it was my very first occasion to observe and study Paphiopedilum in situ. A few months earlier, I did spotted some large Paphiopedilum sanderianum on Mulu National Park cliffs, in Sarawak but the plants were far away and I could only catch a fleeting glimpse with my camera lens. Again, they were not in flower. :-/

Let's start this field report with a habitat shot with a nice Paphiopedilum plant on the foreground.

Below is a better shot of the plant. Although the tesselated leaves species are numerous, I guess a seasoned hobbyist or better a taxonomist would see in the specimen below a typical P. callosum.

I noticed something strange though: some plants had green non tesselated leaves, a feature not expected for P. callosum as far as I know. As my experience with this genus is almot non existent, I wondered while I was in the field if this could be some kind of natural variation.

It even seemed to me that these green leaf plants, present in quite some numbers in the same spot that the tesselated leaf specimens, had narrower leaf lamina but I did not have the time to examine all the plants carefully so I am no too confident about this.

Even stranger: I found some plants that looked like some intermediates between the two types of plants. The ones below, although predominantly green show some of the tesselated pattern if you look closefully.

It's even more obvious with the two following photographs:

One surmise would be that all these plants represent Paphiopedilum callosum phenotypical variation but as far as I know, this Indochinese species strictly produces tesselated leaves. 

The other hypothesis would be that we have here two species growing in sympatry with some hybrids (or introgressed plants) that could be found here and there, in little numbers. This is one seducing idea but I really wonder in that case what this dark green leaf species would be as Paphiopedilum appletonianum, the other Cambodian slipper orchid has tesselated leaves as well. I can't imagine that we have found something new on that day but I have to wonder. What do you girls and guys think about this?

The last shots show the lithophyte plants I mentionned earlier in the post. What an amazing sight it was. I can just imagine how dreamy the place must be when all the orchids there are in flowers (and there were many other orchids on that site as well) including some lovely Appendicula cornuta.