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The term "tree" has no precise definition that's commonly recognized, either in common language or botanically. The broadest definition is any plant with an elongated stem and leaves that perform photosynthesis. Botanists typically include a minimum height requirement to distinguish them from shrubs. However, this value can vary from 1.5 feet to over 30 feet, depending on the researcher. This broad definition includes large herbaceous plants like bananas and papayas. This inclusion is significant when consider tree populations because rainforests are often dominated by plants without woody stems.
The evolution of trees has been strongly driven by the need to compete for sunlight. They also tend to be long-lived, and some are several thousand years old. As a result, many species of trees are extremely tall and grow thick trunks to add structural stability.
Due to their height, these specimens develop a single main stem giving a distinctly tree-like form as compared to shrubs. However, many species grow as both trees and shrubs, depending on environmental conditions. For example, a plant that grows as a tree in a forest or open woodlands may grow as a shrub in the mountains or tundra. As a result, the classification of plants into trees and shrubs can't be consistent.
The ability to derive energy directly from the sun provides a strong evolutionary advantage, resulting in a greater diversity of trees than any other form of life. Botanists estimate that the total number of species of trees is between 60,000 and 100,000, which could account for one in four of all living species of plants. The greatest biodiversity of trees is found in tropical regions, where botanists have difficulty conducting detailed surveys. Overall, the diversity and ranges of trees on Earth are therefore poorly understood.
A 2015 study estimates there are about 3.04 trillion trees in the world, with 46 percent in tropical and subtropical zones, 20 percent in temperate zones and 24 percent in sub-artic zones. While population densities are higher in tropical areas, countries tend to be larger in cooler areas. As a result, only one of the top five countries with the most trees is in a predominately tropical region.