Why do larches turn yellow?

Western larch (Larix occidentalis) grow in the interior Pacific Northwest (Montana, Idaho, Oregon, and Washington) of the USA and British Columbia, Canada. They are conifer trees like pines because they have needles instead of leaves, and their seeds grow in cones. Unlike pines they are not evergreen; they are deciduous. In the autumn, the needles of larches turn golden and then drop off the branches.

The reason deciduous plants turn colors in the autumn is that they are saving nutrients to use later. As temperatures cool and days grow shorter, the chemical machinery in the needles that photosynthesize – or create sugars from carbon dioxide, water, and sunlight – start to break down, and those chemicals (mostly nitrogen) are stored elsewhere in the tree. It is during this process of break down that the needles become golden-colored. This period – starting about the second week of October - lasts for two or three weeks.

Why are larches deciduous, unlike other conifers? The ability to recycle nutrients, especially nitrogen, is an important advantage in nutrient-poor environments. Larches also grow in snowy climates where heavy snow loads are less likely to break bare branches compared to trees burdened with needles during the winter.

Its deciduous nature also makes western larches especially resistant to fire and resilient to injury. Larch trees can lose much of their canopy and still regrow needles the following year. Its bark is also thick and protects the stem from fire. All these reasons give western larch a competitive advantage over other conifers where it grows – and we can enjoy its autumn color.

References

Givnish, T. J. 2002. Adaptive significance of evergreen vs. deciduous leaves: Solving the triple paradox. Silva Fennica 36:703-743.

Rosenthal, S. I., and E. L. Camm. 1997. Photosynthetic decline and pigment loss during autumn foliar senescence in western larch (Larix occidentalis). Tree Physiology 17:767-775.