Key Vocabulary: deciduous, surface area, volume, photosynthesis, chlorophyll, adaptation, biome, ecosystem, temperate forest, nutrient, pigment, equinox
Next Generation Science Standards:
- MS-ESS2-6. Develop and use a model to describe how unequal heating and rotation of the Earth cause patterns of atmospheric and oceanic circulation that determine regional climates.
- MS-LS4-4. Construct an explanation based on evidence that describes how genetic variations of traits in a population increase some individuals’ probability of surviving and reproducing in a specific environment.
- MS-ESS3-3. Apply scientific principles to design a method for monitoring and minimizing a human impact on the environment.
As summer transitions into autumn, the days get shorter and cooler. Football season begins in America and you may even indulge in drinking coffee flavored like a large, orange gourd. But perhaps the most obvious sign of the seasons changing is on deciduous trees. During the autumn months, their leaves change color from vibrant green to yellows, oranges, and reds.
However, rising global temperatures could have a significant impact on the trees and plants in temperate forests. There is increasing evidence that climate change is causing deciduous trees to change their leaves later in autumn or not at all, which would be extremely bad for trees and the ecosystems in which they live.
Why Leaves Change Color
Deciduous trees are the dominant plants in temperate forests, a biome characterized by having four distinct seasons; winter, spring, summer, and autumn. These trees have adapted thin, broad leaves, which sprout each spring and are great for capturing the Sun’s light for photosynthesis during the long, warm days of summer.
However, the high surface area to volume ratio of the leaves would cause the trees to lose a lot of heat and moisture if they were to keep them during the winter. The same principle applies to cooking, food with more surface area per unit volume heats up faster (a steak or ground meat cooks much faster than a roast). Winterizing their leaves would take a lot of energy and since there isn’t much sunlight during the winter months anyway, the trees drop the leaves before it gets too cold out. Once spring comes and the weather starts to get warmer, the trees make new leaves and the process starts all over again.
But making new leaves each year is a lot of work, the trees have to extract a lot of nutrients like nitrogen and phosphorus from the soil. Dropping the leaves straight from the tree would be a waste of valuable resources. To conserve these nutrients, the trees extract them from the leaves and store them in their twigs and branches to be recycled for other needs.
This process is a lot easier than it sounds. The leaves contain chlorophyll, the green pigment in leaves that captures the Sun’s light for photosynthesis. If the plant starts harvesting the nutrients from the leaves with the chlorophyll still intact, the pigment will continue to absorb light even though other parts of the photosynthetic process are not there, making oxygen molecules in the leaves reactive and cause damage to the tree.
To solve this problem and recycle the nutrients safely, plants take apart their chlorophyll molecules. Once dismantled, the tree can then harvest nutrients from their leaves without any of the damaging chemical reactions. This makes the chlorophyll essentially transparent, letting other pigments which have always been there to show through. These pigments are called xanthopylls and they give the leaves their yellow and orange colors. Some trees take an extra precaution and produce a pigment called anthocyanin to shade chlorophyll from the Sun’s light, turning the leaves red or purple.
Why climate change is bad news for deciduous trees
Trees take their cues to change their leaves mainly from two prompts: cooler temperatures and shorter days. After the autumnal equinox, the Northern Hemisphere has angled away from the Earth and receives less sunlight. This causes autumn days to get cooler and shorter. Even accounting for outlier years with warmer autumns, this has been a predictable cycle for millions of years and has given plants reliable signals on when to start changing their leaves.
But the number of greenhouse gases in the air have significantly increased over the last 200 years, allowing the atmosphere to hold on to more heat. Even as the days get shorter toward the end of the year, temperatures might not necessarily fall with them. This could send mixed messages to the trees, causing them to change their leaves later in the year or turn brown and drop them without harvesting the nutrients.
The other issue is that as temperatures across the globe go up, trees may begin to migrate northward. New saplings may find it more habitable climates north of their traditional range, which would mean two serious problems. First, trees tend to be picky about the kind of soil they live in. If tree species begin to move north, they could encounter soil with which they are not adapted to, causing more stress on the plants. Second, any migrating trees would be an invasive species, which have no natural threats and can out-compete native plants and trees for resources. Since deciduous trees are the most important producers in the temperate forests, this could completely disrupt local ecosystems.
There are also direct impacts on humans. The local economies of many cities and states in the Northeast United States make a lot of money on tourism in the autumn. Visitors come to see the leaves change into their beautiful colors and bring about $3 billion with them. If the trees start to turn their leaves brown or drop them altogether, many small towns, whose existence is sustained by tourism, could lose a lot of money.
Maples syrup is another multimillion-dollar industry that could be affected. Maple trees make up most of the forests in New England. As temperatures across the world rise, those maple trees could not only be stressed but may migrate north into Canada. It could also mean less sap with which to make syrup. In the spring, the maple trees store extra energy as sugar in their sap, but in the summer, they store it as starch. As the transition between seasons becomes shorter, it could shrink the number of sugaring days for the maple trees, drastically reducing the amount of maple syrup that could be produced.
However, the biggest impact of climate change would be felt on the trees and the ecosystems in which they live. A warmer planet would couple not only mean trees delaying change their leaves but devastating effects on the trees and the species which rely on them.
- Vox has an interactive map which shows when you can expect the autumn foliage to change color
- The Globe Program’s Phenology and Climate Project is a citizen science research effort aimed at understanding of how climate relates to the cycles of living things (phenology). Students make observations of the timing of budburst, green-up, and green-down of native plants near their school. Students enter the data in a national database to answer key questions about how the growing season is affected by climate.