Stomata

Stomata (singular: stoma) are microscopic openings found predominantly on the lower surfaces of leaves, though some species have them on both surfaces. Each stoma is bounded by a pair of specialized guard cells that can change shape to open or close the pore. When open, stomata allow carbon dioxide to enter for photosynthesis and oxygen to exit; simultaneously, water vapor escapes in a process called transpiration.

The regulation of stomatal opening is a critical balancing act for trees. Open stomata allow the CO2 intake needed for photosynthesis and growth, but also result in water loss through transpiration. Under drought conditions, trees close their stomata to conserve water, but this also reduces photosynthesis and can lead to overheating of leaf tissue. This stomatal trade-off between carbon gain and water loss is a fundamental constraint on tree productivity.

Stomata are relevant to tree health in several ways. Many foliar pathogens enter leaves through open stomata, which is why some diseases are more severe during wet conditions when stomata are open. Air pollutants like ozone damage the guard cells and leaf interior when they enter through stomata. The density and behavior of stomata also determine a tree's drought tolerance; species adapted to dry conditions often have fewer, smaller, or more deeply recessed stomata to reduce water loss.