Hello, The short answer will be that the forest balances out the global climate and if we take out forest from the equation there is bound to an imbalance to occur. For instance if we consider tropical forests with dense trees they serve as the CO2 storehouse and give out Oxygen and in turn the CO2 level in the atmosphere will decrease leading to lesser or normal temperature. If we consider mangroves their vegetation stops the storms from invading the inland and keep them on bay itself.

In terms of Ecological system, the forest will house numerous organisms in the food chain and every organism has its own value in the working of our biome. It may look insignificant in separate but consider as a link that how the higher and lower species in the food chain will survive without the middleman species. Thus forest brings out the balance in the climate and ecosystem.