Tree ID by large leaf
Four examples are shown here of tropical or subtropical trees which have large leaves. The Catalpa is native to an area which includes parts of Alabama, Florida, Georgia Louisiana and Mississippi but is often grown in more northern areas. Magnolias are originally from Southern China and the Southern United States. There are about 80 species of magnolia of which roughly half are tropical. The Foxglove tree (genus Paulownia) is native to western and central China
Large leaves are found on trees in tropical rainforests where rainfall is high. Leaf surface temperature is controlled by water evaporation in a process known as transpiration cooling. If the leaf temperature gets too high, photosynthesis is impaired and the leaf can be damaged. Providing there is enough water this should not be a problem and there is theoretically no limit to the size of the leaf. Other factors may determine whether a tropical tree has a few large leaves, more small leaves or has divided leaves. In areas of lower rainfall, the rate of transpiration may become more critical. Every leaf has a boundary layer of still air which impedes transpiration and heat transfer. Small leaves have a thinner boundary layer because the edges are nearer to turbulent air. As the wind breaks up or thins this layer transpiration becomes easier. As a result they are better at losing heat than large leaves and perform better in hotter, dryer conditions. Generally leaf size decreases with latitude as other factors such as frost damage protection become more important. This theory suggests that large leaves with a thicker boundary layers will not be able to absorb heat from their surroundings at night compared with small leaves and will suffer more frost damage.