The forms seem veiled, and the values are relatively light in key, with just that dark tree mass anchoring the tones.
The values in this painting are high key, with the warm bank at left nearly the same tone as the cool cliff in the distance. The top of the blue cliff is lost in fog. This fog has a granular effect of sedimentary pigments in watercolor.
The tree masses are greatly softened, blending into the sky and ground, and they're composed of variegated hues that seem layered over each other. He eliminates any detail that's not essential.
I'm not sure exactly how he accomplished these effects. I'm guessing that there's a lot of big wet washes, maybe some scrubbing out. He may also have painted over a surface primed with white gouache. It's hard to tell without seeing the originals.
Does anyone have any insights into his method—or his bio? Please share them in the comments. I couldn't find out very much.
John William Tristram on Wikipedia