Why do you think this way?
Colonists on Mars would have to build almost everything. But is there anything stopping them from building parks and forests? It will be tremendously difficult but they absolutely will do it, it's human nature. What they miss, they'll build. They'll spend much of the first years building up the rudiments of industry. Methane and Oxygen production will be there from day one. They'll start by mining water ice to allow them to produce arbitrary amounts of methane, oxygen, water, Hydrogen, and carbon monoxide. They'll use that to kickstart other industrial infrastructure like production of iron/steel, glass, concrete, plastic, etc. All of which can begin in only a handful of years past the initial colonization and get going with a minimal amount of capital equipment. They'll move on to expanding their habitat, building greenhouses, growing food. And from there they will increase the carrying capacity of their habitat(s) and the capabilities of their industrial base continually. Then it's just a matter of time until they have the ability to build parks and forests, things they'll want to get started on sooner rather than later.
They won't have the redwoods or the amazon rainforest but they'll have lakes to swim in and trees to walk under.