Thick forest
Grassy feild
If the Sun is out, the grassy field would FEEL warmer, because you are getting warmed by the Sun. If it is Cloudy or at night, the thick forest would hold in more heat than the open Field.
I know that a thick forest would be warmer, because it is a lot more dense, and would hold any kind of heat. Also trees can hold leaves and animals may rest inside which creates more heat and more dense.
Grassy feild
The weight of the massive pines would challenge any longer. My weight sunk into the thick moss that covered the forest floor.
cannibas
Minister
Thick fur
The forest would likely be warmer on a winter day without wind because the dense evergreen trees create a barrier that helps trap heat and blocks wind. The grassy field, on the other hand, lacks this barrier and the ground can lose heat more easily, making it feel colder.
The Parque Nacional El Chico, in the Mexican state of Hidalgo would be one of them.
You would find the Black Forest in Germany.