Just like people, trees are all different. Some people are tall, some are short, some have straight hair and some have curly hair. Some trees have leaves and others have needles. But what are the benefits of leaves vs needles? What mechanisms remain the same? What makes them different?
Well, we will start with what is the same. All tree foliage, whether its leaves or needles, are needed for photosynthesis. Photosynthesis uses chlorophyll cells in the foliage to convert sunlight and carbon dioxide to oxygen and carbohydrates for tree food.
The most obvious difference between trees with leaves (deciduous, angiosperms) and trees with needles (coniferous, gymnosperms) is the foliage shape. Deciduous leaves are broad and flat, while needles are long and thin (fig. 2).
Another fairly obvious difference is the defoliation cycles. Everyone knows that leafy trees lose their leaves every fall in preparation for winter dormancy. Evergreen trees are known for maintaining their foliage all year round. Most people will use the terms evergreen and conifer synonymously. However, not all conifers (needle trees) are evergreens. There are leafy evergreens, such as a Holly Plant or a Boxwood, as well as deciduous conifers that defoliate every year, like tamarack or larch trees (fig. 3).
The less obvious differences are in the physiology of each tree species. Each species has evolved features to ensure the tree functions at its best. For example, evergreen conifers such as Pine trees and Spruce trees, have adaptations to ensure they can withstand the harsh winter conditions while maintaining foliage year round. A needle has much less surface area compared to a leaf, making it much more wind and snow resistant (fig. 4). There is also a waxy coating on needles that reduces water loss and acts as a layer of protection during winter.
Even the cells have adapted to better perform during winter. The trunk wood of conifers have only one layer of parenchyma cells, which act as carbohydrates storage. Since these species maintain their foliage during winter, they can photosynthesize their carbohydrates rather than depend on stored carbohydrates in the dead of winter like deciduous trees (fig. 5). Stomata are specialized cells that open and close to let water into the leaf in a process called transpiration. Evergreens have extra special guard cells for their stomata that are not found in deciduous leaves (fig. 5). These guard cells protect the water supply within evergreen trees during the dry winter months.
Essentially, everything is the way it is thanks to evolution. Every species is able to perform its best because of how it has adapted to function in its environment. Scientists have spent decades studying photosynthesis and they have determined that while both broad leaves and needles photosynthesize, needles are much more efficient at photosynthesis compared to leaves. This would suggest that needle trees rely more on photosynthetic energy than energy absorbed through the soil. If we look at the root systems of deciduous vs evergreen trees, this statement would make sense (fig. 6). Deciduous trees have very deep root systems to optimize its ability to absorb the most nutrients from the soil. Conifers on the other hand have very shallow root systems, often only reaching the first few inches of the soil. Very few nutrients reside in shallow soils, so the conifer would have to get their energy from other features as well (fig. 7).
Even the different defoliation cycles have been adapted as a result of evolutionary need. The term “evergreen” may insinuate that the tree never loses its foliage, but that would be incorrect. All trees have a defoliation cycle where their foliage eventually loses its ability to preform photosynthesis. Leaves have a defoliation cycle of 1 year, so they use a lot of energy every year to regrow all new foliage. Needle trees also have a defoliation cycle, it's just a much longer cycle. It will take a mature pine tree about 2 years to fully replenish their foliage, while some spruces will have a cycle as long as 7 years! Older needles will gradually die back rather than all at once. This is why you will see isolated browning sections on a perfectly healthy cedar tree, or how a pine tree sheds its bottom branches as it ages and more new foliage grows in the crown. This long cycle preserves energy by not having to produce foliage yearly, and that extra energy is helpful come cold, harsh winter weather!
So there you have it. All you need to know about needles vs leaves. While both are very important, they have big differences. But nothing is solely defined by its differences, because it is those differences that make something special!!
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