What might cause plant life to slowly shift from one type to another, over many years?

 You might have seen this when you pulled a plant such as a weed or vegetable out of the ground and a clump of soil clung to its roots. Plants also help absorb some of the water in the soil. These effects make it harder for water to wash the soil away.

The truman doctrine was an american foreign policy whose stated purpose was to counter soviet geopolitical expansion during the cold war. it was announced to congress by president harry s. truman on march 12, 1947, and further developed on july 12, 1948, when he pledged to contain threats to greece and turkey