Carbon ^^ is the answer if I’m not mistakenn
Interspecific competition
A type of competition in which organisms belong to different species compete with each others for same resources.
Example
Leopard and lion.
intraspecific competition
A type of competition in which organism belong to same species compete with each others for same resources.
Example
birds for breeding or shelter compete with each others
Im not really sure about the water, other than the trees wouldn't be absorbing it so there would be extra? But a huge amount of trees like that produce a lot of oxygen, so there would not be less oxygen in the environment. Hope this helps!
I'm not completely sure but I came up with Carbon dioxide, methane, water vapor, and nitrous oxide. I hope this is right and maybe helps you. Good luck!!