Final answer:
Climate is the primary determinant of plant distribution as it affects temperature and moisture, which are critical for plant growth and the characteristics of biomes.
Step-by-step explanation:
The single most important determinant of plant distribution is climate. This is because climate encompasses both temperature and moisture, fundamental elements that dictate the existence and resilience of various biomes. Terrestrial biomes are mainly determined by climate as it influences plant growth, biodiversity, and the adaptations of land organisms. Climate's impact ranges from the available energy in an ecosystem to the geographical distribution of species, affecting biodiversity, which generally increases towards the equator and in more humid environments. Factors like soil type, topography, competition, and predation also play roles but are often secondary to the overarching influence of climate.