What type of biome is characterized by the absence of trees?

The tundra biome is characterized by its cold climate, short growing seasons, and soil that is often frozen (permafrost). This environment is not conducive to the growth of trees, primarily due to the harsh weather conditions and the inability of many plants to survive the extreme cold and nutrient-poor soil. Instead, vegetation in the tundra consists largely of mosses, lichens, low shrubs, and grasses that are adapted to the specific environmental stresses of this biome.

In contrast, deserts have very little vegetation due to low rainfall but can support certain drought-resistant shrubs. Forests, by definition, have a significant presence of trees. Grasslands, while lacking trees, often have a thick cover of grasses, but they are distinct from the tundra biome in terms of climate and precipitation patterns. Thus, the defining characteristic of the tundra as a biome is indeed the absence of trees, making it the best answer to the question.