What climatic factor primarily determines a tree's hardiness across different zones?

The primary climatic factor that determines a tree's hardiness across different zones is low temperature extremes. Tree hardiness is fundamentally linked to a species' ability to endure cold temperatures, which can influence not only survival but also growth and overall health.

In different regions, plants and trees are categorized into hardiness zones that reflect their resilience to the coldest temperatures they are likely to encounter in winter. Understanding these temperature extremes helps in determining which species are suitable for planting in specific areas, as some trees are adapted to survive harsh winters while others can only thrive in milder conditions.

Other factors, such as soil moisture, humidity levels, and sunlight availability, are important for tree growth and health but do not primarily define hardiness across different zones. Soil moisture impacts the availability of water, humidity affects transpiration rates, and sunlight availability influences photosynthesis—these factors can all influence a tree's growth pattern and stress levels but are secondary in establishing a tree's capability to withstand low temperature extremes that can lead to frost damage or death.