Nature is Big. Trees can be really Tall. It can be hard to see that in Fotos sometimes though. Imagining the Size of something is not easy without having some Sense of Scale.
The easiest Way to get that Sense into Fotos is to include something most would know the approximate Size of. Like Cars, or People.
Both those Things have their Ups and Downs though. For Cars, you usually have to be near Roads or such Things. And I try to not include those in my Landscape Photography to much anymore. Once, I only shot along Roads. But I was bored back then.
People are great for Scale, but only if you can actually see them. It also does not work when they are to close to the Camera or to far away. It is about the Scenery after all, but tiny Humans do not really work.
It also helps when there are Colours involved. Those always look great in Nature. Especially when they are complimentary.
Nature can provide Scale by itself as well though. Like in the form of Animals. I really would love to include some Wildlife in my Fotos someday, but I usually do not come across any.
And if I do, I am not fast enough to actually get them into Frame. And in those Times I was able to Capture some Animal, it is always really blurry. That does not help at all.
It is a lot of Fun to include Scale in Landscapes. But sometimes, Things also look great without anything Extra in them. Scale is important, but it is definitely not Paramount.
It always depends on the Scene and what Story is being told in a Foto. And sometimes nothing is better than something. Photography is as much about what is in the Foto as it is about what gets excluded.