When: Early September to mid-November (depends on the weather)
Where: Forests and woodlands all across Sweden
What: The science behind autumn’s beautiful transformation
Autumn in Sweden is a magical time of year. The days may be getting shorter and the air cooler, but nature puts on one last spectacular show before winter arrives. The trees burst into brilliant shades of yellow, orange, red, and even purple. But why do the leaves change colour, and what’s really happening? Let's explore the science of autumn colors...
THE HIDDEN COLORS IN LEAVES
During spring and summer, leaves are full of green chlorophyll, the pigment that helps trees capture sunlight and turn it into energy through photosynthesis. The reds, oranges, and yellows are there too all along—they're just hidden. In autumn, when the temperature drops and days become shorter, trees slow down their food-making process. Chlorophyll breaks down, and the green fades away, revealing the other pigments like carotenoids (yellow and orange) and anthocyanins (red and purple).
WHY SOME LEAVES TURN RED AND OTHERS YELLOW
Different species of trees have different pigments. For example, birch and aspen trees mostly turn yellow because they contain a lot of carotenoids. Meanwhile, maple trees can turn bright red, thanks to anthocyanins, which only develop in autumn under certain conditions—like sunny days and cool nights. Some trees, like oaks, turn brown as the pigments break down completely, leaving behind tannins, which give leaves their brown colour.
WEATHER PLAYS A BIG ROLE
The timing and intensity of autumn colours can vary depending on the weather. Cold, but not freezing, nights and sunny days can intensify the red hues in leaves. On the other hand, a very wet or warm autumn might delay the colour change or make the colors less vibrant. In Sweden, the peak of autumn colours usually happens between late September and mid-October, but this can shift depending on the region and yearly climate.
WHY EVERGREENS KEEP THEIR NEEDLES
Ever wonder why spruces, pines, and other evergreen trees don’t drop their needles in autumn like deciduous trees? It’s all about survival strategy. These trees are specially adapted to survive harsh winters. Their needles are covered in a waxy coating that helps retain moisture, and they have fewer stomata (tiny openings used for gas exchange), reducing water loss. These adaptations allow evergreens to hold onto their needles through winter, while deciduous trees shed theirs to avoid losing water and becoming damaged by freezing temperatures.
Another interesting fact: because evergreen trees keep their needles, they can photosynthesize year-round, even in winter, when temperatures are warmer. This gives them an advantage in colder climates like Sweden, where the growing season is short.
THE TREE'S INTERNAL CLOCK
Trees are also in tune with the seasons, not just through temperature but through changes in light. The shortening days signal the tree to stop producing chlorophyll and prepare for rest. This internal clock is critical for trees to survive harsh winters in Sweden. Without this natural timing, trees might face frost damage.
PREPARING FOR WINTER
While we enjoy the beautiful colours, trees are busy preparing for winter. Shedding their leaves is a survival strategy. Without leaves, trees lose less water and become more resistant to cold temperatures. The fallen leaves also decompose, enriching the soil with nutrients. So, while autumn may feel like the end of the growing season, it's actually the start of the tree’s preparation for the next spring.
HELPFUL LINKS:
A simple guide to Tree Identification in Sweden:
The Science Behind Leaf Pigments (video):
Best Autumn Hiking Trails in Sweden:
Comments