trees are the main producer/way in which energy enters the ecosystem;
there is a variety of nutrient and energy pathways / allows for varied food webs;
tree layers create a variety of habitats so increasing the diversity of species;
trees protect each other from high winds;
trees protect the soil from heavy precipitation/erosion;
interdependence of species means if the main plant types are gone then many other organisms cannot survive there either;
increased genetic diversity allows species to adapt to change;
the roots hold the soil together keeping the nutrient cycle going;
no trees increases the risk of soil erosion and loss of nutrients and a downward spiral of degradation and instability;