Acer negundo, commonly known as the boxelder, is a species of tree found primarily in North America. The clusters of Acer negundo can be found in various regions across the continent, particularly in the United States. These trees are most abundant in the central and eastern parts of the country, where they thrive in a variety of habitats, including forests, woodlands, and along riverbanks. The boxelder is known for its distinctive clusters of compound leaves, which are a key feature in identifying the species.

The reason for the prevalence of Acer negundo in these regions is due to its adaptability to a wide range of soil types and environmental conditions. According to the United States Department of Agriculture (USDA), Acer negundo is hardy in USDA hardiness zones 3 through 9, which covers a significant portion of the United States. This adaptability allows the tree to grow in both urban and rural settings, making it a common sight in many areas.

Data from the USDA's Plant Hardiness Zone Map indicates that the boxelder is well-suited to the climates found in the central and eastern United States. The tree's ability to tolerate a range of soil pH levels and moisture conditions contributes to its widespread distribution. Additionally, the boxelder is known for its fast growth rate, which makes it a popular choice for reforestation projects and windbreaks.

In the central United States, particularly in states like Kansas, Nebraska, and Oklahoma, Acer negundo is often found in bottomland forests and along waterways. The tree's preference for moist, fertile soils in these areas explains its abundance in these regions. In the eastern United States, the boxelder can be found in a variety of forest types, from deciduous woodlands to mixed forests, where it serves as a valuable component of the ecosystem.

Furthermore, the boxelder's ability to fix nitrogen in the soil through its root nodules benefits the surrounding vegetation. This nitrogen fixation makes the tree an important species for improving soil fertility, especially in areas where nitrogen is a limiting factor for plant growth.

In conclusion, the clusters of Acer negundo are most commonly found in the central and eastern United States due to the tree's adaptability to a wide range of soil and environmental conditions. The USDA's Plant Hardiness Zone Map and data on soil preferences further support the prevalence of this species in these regions.