I was asked that question. At first I thought of the color, contrast, and temperature change. All that is true. What is also true is creating a three dimension on a flat plane. There must be distance. Usually the background is greyer. Not gray like the color, but not as an intense color. The foreground is where the darkest darks and the most intense color are painted. The most contrast too. Remember the details in the foreground. You cannot see every branch far away.
The focal point is best placed in one of the thirds. I looked at my landscapes and tried to look at them with a critical eye. The thing that excited me the most was the atmosphere. I felt that I was there. That is it!! The very most important thing that makes a great landscape is being there. All of my landscapes were painted “En Plein Air” a fancy French term meaning painting out in the air or outside. Painting from life. That is definitely what makes the painting real. The viewer can feel as if they are there. Hopefully bringing a feeling of peace and tranquility. (Mine that is) Every one of my landscapes I painted on location is mainly in pastel. You must paint fast. This is where I use watercolor washes. “ Real fast and loose.” Try to get close the right value and color. I mainly use Kitty Wallis Paper It has great adhesion and has a backing that will withstand the watercolor. It is acid free. I have found this works best because the weather can change rapidly. The sun is constantly moving your shadows. So that is what I try to paint first. I try to get on the paper what excited me. Usually it is color and contrast. The painter is not only fighting all the challenges of painting, but also the wind, heat, bugs, rain, cold. And that can happen all in one day. That is what is so exciting. The elements dictate your speed. I remember painting one my paintings where I bungee corded my easel to a tree so the painting and I wouldn’t blow away, with sticky tree sap running down my hand, all the while red ants were crawling all over my pastel box on the ground. I didn’t know they liked pastels! It was well worth it. I sold that painting on site.
Sometimes I am not able to finish the entire painting but I try to get the colors, shadows and get the composition right. I do take a picture of the place but it is amazing that I do not even need it. (I was there).
Also something very important and this is for any painting are shapes. Interesting shapes. Not all the same not all small, not all large. Never taking up the same amount of space.
I have heard several artists mention that landscapes are the easiest. Well you might be able to move things like a tree or bush, but you then have to make sure you do not add the shadows. Really all paintings are just shapes. Not unlike a puzzle. If the shapes are right it is right.
The answer is paint from life. Paint what you see! In order for a landscape to look vibrant the painter must be there.