Are big leaves better than small leaves? It depends on where the plant is, the temperature, the amount of water, the altitude, the amount of UV light, and several other factors.
Not all plant leaves are green, but all plants have leaves. Those leaves can be big, small, long, short, fat, thin, and many other shapes as well. Each shape of leaf has evolved to suit the environment that the tree is growing in. Plants are autotrophs, which means that they are able to produce their own food using sunlight and some other chemicals. The rest of us are heterotrophs because we cannot make our own food from sunlight. In fact, there are three trophic levels. Plants are the first level and they convert the sunlight into energy. Herbivores are the second trophic level and they break down the energy in the plants. Then carnivores are the third trophic level and they take the easily accessible energy from the bodies of herbivores.
To make energy from sunlight, plants use their leaves. The chlorophyll pigment gives the leaves their color but it also absorbs the sunlight. The plant then combines that sunlight energy with carbon dioxide and water to make glucose, which it can then store in its root system. A byproduct of this process is oxygen. Because they use their leaves to make energy, it would seem logical that a bigger leaf would be able to make more energy, but that is not always true. There is a balance between how much energy the leaf can make and all of the things that can damage the leaf. Plants have to balance the amount of sunlight, the temperature, the amount of water, the amount of UV radiation, the altitude, and the amount of wind.
One of the biggest factors that dictates leaf size is the amount of sunlight that is available. If all things are equal, trees tend to grow larger leaves if there is less sunlight. This could be because they are in a usually cloudy area, or it could be because they are surrounded by other trees and they have to compete for the sunlight. A larger leaf has a larger surface area and is more able to catch sunlight.
However, things are not always equal and the amount of sunlight isn’t the only factor. Plants have to take into consideration the temperature of the environment they are living in. If the air temperature is very hot, leaves will be small because larger leaves can lose more water than smaller ones. The more surface area that a leaf has, the more space there is for water to evaporate from. Plants need sunlight, but they also need water. The leaves on cacti are very small because of this. The cactus has balanced how much sunlight it can take in with how much water it is willing to lose to evaporation. On the other hand, leaves in cold environments will also be small because it is more difficult to regulate the temperature in a large leaf and they can lose heat too easily. That is why the leaves on evergreen trees are short, thin needles. There is a risk of cold and frost damage with large leaves. This can generally be seen because the size of leaves decreases as you head north or south from the equator. Plants that live in very warm and very wet areas, like rainforests, have the largest leaves because they don’t have to worry about water loss through evaporation and they don’t have to worry about heat loss at night because the temperature doesn’t drop that far.
Plants also have to account for the amount of UV radiation coming from the sun. As altitude increases, the atmosphere gets thinner and it is less effective at stopping UV radiation. Plants want the energy that sunlight sends, but they can only deal with so much of it. That is one of the reasons why most plants are green. They absorb the light in the lower end to the middle of the visible spectrum and reflect anything higher. It has more energy, but more energy isn’t always a good thing. That is the same with UV radiation. It has a lot more energy than visible light and can be very harmful to plants. The energy can cause irreparable damage to cells and can damage proteins. If there is too much UV light, the plant could die. To counter this, plants at high altitude where there is more UV light have smaller and much thicker leaves. They need to reduce their surface area and the thickness serves as an extra layer of protection.
Plants also have to take the weather into account as well. In places with strong winds or heavy rains, plants will usually have smaller leaves because bigger leaves risk getting broken off. The same in places with lots of snow. So, it is a balancing act. Plants have to balance the optimal amount of sunlight they can get while reducing the risk of damage to themselves. And this is what I learned today.
Photo by Pixabay: https://www.pexels.com/photo/green-heart-shape-leaf-57436/
Sources
https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2019.00058/full