Technically you are able to argue both ends. Biotic factors are living things while abiotic factors are no living things. since the plant was once living, you can argue that it is a biotic factor, however, I would say the dead plant decay would be abiotic because it is not living anymore. I hope that helped!
Relative Humidity is considered as the most important factor in regulating the closing and opening of stomata. Since it indirectly controls transpiration and stomata's main function is transpiration.
Hope this helps!
