Ferns growing

A quite uncommon sound... I was walking threw the forest. My attention was drawn to a strange sound in the pines, it was a permanent and strong cracking. I through of the pines, but, going near the trunks, I heard nothing. It was coming from the ground, into the ground or perhaps near the ground. I through about insects, but digging, there were nothing special. In fact, it's the ferns growing. Wishing to understand, I put a contact microphone into the ground, crackings were here too. Also, I went back in this forest by night, and there were nothing excerpt a beautiful moon. Crackings are here only with the sun, and (almost) only in the thick and dense heaps of dead ferns. Digging into, there's small green young ferns. To record this sound, I simply put two binaural microphones in an heap of dead ferns. I guess the crackings comes from the new ferns, pushing hardly the ground into the humus. Searching on the internet about ferns sounds, I found nothing (probably because of my bad english). If somebody has an explanation about this strange happening, contact me and please help, I will complete the description. First recording, I attempted to record the crackings. There's a lot of planes. Recording made with microphones laid on the ground. Second recording without planes and microphones put in an heap of dead ferns.