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At what warm-month temperature does the tree line usually occur?

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Mountain ecosystem Figure 1: Worldwide distribution of mountain lands.

In general, the altitude at which the tree line occurs is determined by the point where the mean temperature in the warmest month is approximately 10 °C (50 °F), provided that moisture is not a limiting factor.1

While this value holds true in most regions—particularly in temperate zones—it is not a universal rule. For instance, some tropical regions with a yearlong growing season may support forests in conditions slightly cooler than this threshold. Additionally, some researchers suggest that a more precise fit can be obtained by identifying the point where the daily maximum temperature is greater than or equal to 11.1 °C (52 °F) during the growing season.12