Over the hundreds of millions of years trees have been present on Earth, they have survived a variety of circumstances. Changes to climate conditions such as temperature and precipitation through time caused tree species to evolve in a variety of different ways, and one of the most visible examples of these adaptations is their leaf type. The two most common categories of leaves are broad and needle, and each has specific advantages.
Universally, leaves have a few crucial responsibilities to help trees survive: they absorb carbon dioxide and sunlight to help fuel photosynthesis, release oxygen back into the atmosphere, and they provide food and homes for many kinds of animals and insects. All types of leaves contain a substance called chlorophyll, which absorbs energy from light during photosynthesis and gives leaves their green color. Other than their functions, though, types of leaves are more different than they are similar.
Broadleaf trees have wide, flat foliage, which is discarded if the weather gets colder. They are often deciduous—in autumn, colder temperatures cause the chlorophyll in broadleaf trees to break down, making them lose their green color and eventually fall. Some broadleaf trees live in temperate and tropical areas of the world, where the soil contains enough nutrients for the trees to produce new leaves each spring. This means in hot, humid environments such as the Amazon, broadleaf trees are never met with the low temperatures that cause all their leaves to fall at once, though some lose leaves in accordance with the wet-dry cycle. Broadleaf trees also improve forest growth and increase biodiversity in their ecosystems.
Needleleaf trees, also called conifers, are most prevalent in colder, high-latitude and high-altitude environments. They evolved during a period of history about 250 million years ago, when Earth was much colder and drier. Generally, their leaves remain on the tree year-round despite the changing seasons. The long, thin leaves are more adept at retaining water than broad, flat ones, and are less likely to be damaged by the wind. Since needles do not fall off the tree annually, they can also keep performing their responsibilities for the tree year-round as conditions allow. Needleleaf forests in boreal areas also act as an enormous carbon sink, absorbing greenhouse gasses from the atmosphere and storing it.
The phenology of broad and needleleaf trees, or their seasonal patterns and changes, have exceptions in certain species. Some conifers like the tamarack and larch are deciduous, and their needles turn reddish-orange and fall to the ground when temperatures drop. A number of species in the genus Quercus are referred to as “live oaks” because they remain green and alive through the winter, while other oaks lose their foliage.
This map layer shows the geographic distribution of broadleaf, needleleaf, and mixed-tree forests, and was created using a model that combined multiple datasets. The intensity of each color on the map layer represents the consensus prevalence, or the amount that all the different datasets agree―in other words, the darker the color, the more certain it is that a certain leaf type exists there. You can clearly see the broadleaf trees dominating the Amazon rainforest, as well as needleleaf forests spanning Canada, Russia, and the Scandinavian Peninsula. Compare this map layer to the Biomes, Surface Air Temperature, and Precipitation layers to see how leaf types are influenced by climate.
Forests are important parts of the natural world that protect us from more extreme effects of climate change, provide us with materials like timber, support global biodiversity, participate in the water and carbon cycles, and much more. However, things like logging, insect damage, disease, and fires threaten the health of these crucial ecosystems. There are many organizations that aim to protect forests from these hazards, from local groups to the United Nations and at every scale in between. To get involved with forest conservation, try searching the internet for opportunities near you.