What is the major factor keeping forests from growing in grassland areas?

Answer 1

Climate extremes, fire, and grazing herbivores are the major factors that discourage trees from growing in grassland areas.

Fire especially is detrimental to tree growth, because a tree's growth points are above the ground and at the mercy of the intense heat that fire is all about. Historically indigenous folk used fire to push these woody species back to encourage more grass for more hunting opportunities. Fire was also used to reduce the dead plant material load that native grazing herds couldn't utilize fast enough.

Climate extremes can be harmful to trees. When there are extended periods of dry weather with very little to no rain for months, this can cause extreme stress to a tree to the point where it will die. Rains that do come often come either as light rains, sometimes two or three-day steady heavy rains, or short bursts of torrential downpours with some hail. And the wind on the prairies will also stunt or discourage tree growth. Trees (and shrubs) are more likely to survive in a river valley or along a creek because they have easy access to water and are a little more sheltered to the extremes that makes up and defines a grassland.

Climate extremes can dry out grassland soil. This is why most all native plants growing in grasslands have deep root systems (in addition to fibrous roots close to the soil surface to catch moisture that filters through) to reach deep down to find what little water there is to grow. These plants also know to shut down growth and/or grow and spread their seeds as quickly as possible during the spring so that when the dry periods come, they are ready and will go dormant in wait for the next bit of rain available to them.

Grazing herbivores in large herds can damage woody plants more so than they can damage grass plants. Grass plants are adapted to grazing because they have their growth points close to or below the soil surface. Most woody plants do not; those that do have growth points at the soil surface still suffer from browsing and trampling by these large animals, pushing them more out of the system.

Answer 2

Fire, climatic extremes, and large herds of grazing, pooping, trampling herbivores.