2019-04-16_10-59-42.48_DSC8204.NEF (22.4 MB)
This file is licensed (CC BY-NC-SA 4.0)
Seeking to highlight the uncurling frond and darken the bright green foliage behind, this is what I managed in darktable myself. I’d be pleased to see what others may be able to achieve.
Moinchen, Thomas!
Much better than my attempt!
But the OP wanted the background foilage darker?
Have fun!
Claes in Lund, Sweden
You know, I always hear that but I don’t see anything from you. 
That’s simply because I am so modest.
Well, don’t be. I am always wondering what you are up to. 
Yep, I tried. But darker background came out ugly! So, I just tried to bring out the foreground a bit.
Grüße,
Thomas
“Modesty is the color of virtue” (Diogenes) 
Here is an attempt using GIMP’s foreground select tool and then modifying the resultant selection using masks. One can definitely do better at the edges. Treat this as a proof of concept rather than final result!
Thank you for the contributions (and @Thomas_Do and @Claes for the banter).
Maybe my wish for darker background is forlorn … I agree, @Thomas_Do, it can look somewhat ugly. However the combination of the intricate texturing and the high luminosity of the background seems to always draw my eye away from would should be the subject. @Nick_Auskeur, your rendering seems to do this the least … nice! Perhaps the tighter crop is a better answer than seeking to darken what is light!
Many thanks, again, to those who have contributed. But don’t let this post stop others from also applying their creative minds … 
Here is my version with darktable:
2019-04-16_10-59-42.48_DSC8204.NEF.xmp (10,2 KB)
The channel mixer is very handy here. 
Thank you to all the further contributors; @AxelG: very much like it was on the day, however my eye keeps getting drawn to the bright busyness, which was my original problem; @Thomas, the B&W looks surreal, almost infra-red syle, the tension in the way you have cropped is fab; @s7habo - thank you for the pointer to the channel mixer, which I haven’t done more than play with hitherto; @sls141 - your colour version is my absolute favourite - brilliant to rotate and crop the way you did.
