The term 'NATURE' is a fitting crossword solution for the phrase 'MOUNTAINS AND TREES', as it encompasses both of these elements. NATURE is a broad term that encompasses all of the living AND non-living things in the world, including MOUNTAINS AND TREES. MOUNTAINS are large landforms that are formed by tectonic plates pushing against each other, while TREES are tall plants with woody stems AND branches. Together, MOUNTAINS AND TREES form a beautiful part of NATURE that can be found all over the world.