Practice question

Deserts are commonly defined by chronic water limitation, and many receive less than about 250 millimetres of precipitation per year. Low rainfall, often combined with high evaporation, prevents continuous tree cover and limits primary productivity. Desert organisms show water-conserving adaptations: plants may use CAM photosynthesis, reduced leaves, succulent tissues, deep or widespread roots, and rapid life cycles after rain; animals may be nocturnal, burrowing, or produce concentrated urine. Not all deserts are hot. Polar and high-altitude deserts can be cold, demonstrating that aridity rather than temperature is the essential criterion. Rainforests and temperate forests need substantially more moisture to maintain closed canopies, while grasslands generally receive intermediate precipitation, often with seasonal drought and fire. The 250-millimetre boundary is approximate because rainfall variability, soil properties, and potential evapotranspiration also determine biological water availability. Even so, among the listed ecosystems, desert most closely matches this annual precipitation threshold. Sparse vegetation then reinforces low productivity and exposes soil to wind and episodic water erosion.