Trees in rainforests have broad leaves. The advantage is

Plants are adapted to the climate they live in for the most part. Trees in rainforests have broad leaves, shaped to encourage the heavy rain to run off the leaves. The large leaves encourage transpiration so the plants don’t overheat. In very dry climates plants may develop an ability to store water, such as the cactus, which also has its leaves reduced into tiny spines which reduce transpiration to almost nothing so its precious water is conserved. At the first sign of drought, plants close their leaf pores to prevent wilting and slowing down growth so that they need less water. Coniferous plants also have small spiky leaves so that they don’t lose too much water during the frozen winters. Grasses roll their leaves into tubes to protect their leaf pores from the drying effect of the wind. Tropical air plants have moisture sensitive plugs attached to their leaf pores which are pulled down over the pores, sealing them to hold in moisture, by the contraction of the stalks in dry weather.
    Plants are affected by strong winds which make them grow thicker and more stunted stems to strengthen and prevent themselves from being blown over. A special sort of strong wood called "reaction wood" grows on the leeward side of the wind. Plants are also susceptible to waterlogging and freezing and many plants have developed complex mechanisms and adaptations to protect themselves from disasters.