How Deciduous Trees Change Leaf Color in Autumn

The Trigger: Photoperiod and Temperature

Autumn leaf coloration in deciduous trees is not caused by cold temperatures alone. The primary trigger is photoperiod — the shortening of daylight hours as the sun moves southward after the September equinox. Specialised photoreceptors in leaves detect the decrease in light exposure, initiating the senescence programme regardless of whether temperatures have dropped significantly.

In Poland, this signal is detectable as early as mid-August at higher latitudes in Podlaskie and Warmia-Masuria voivodeships, where trees such as silver birch (Betula pendula) may begin showing early yellowing by late September — two to three weeks before the same species in the Silesian lowlands.

Chlorophyll Breakdown

Leaves appear green during the growing season because chlorophyll — the pigment responsible for photosynthesis — absorbs red and blue light while reflecting green. Chlorophyll is continuously degraded and resynthesised in active leaves. In autumn, the resynthesis stops while breakdown continues, unmasking pigments that were present all along.

Carotenoids (yellow and orange pigments including lutein and beta-carotene) are stable throughout the growing season but masked by the dominant green. As chlorophyll degrades, these pigments become visible, producing the characteristic yellow-to-orange palette of birch, hornbeam (Carpinus betulus) and linden (Tilia cordata).

Anthocyanin: The Red Pigment

Red and purple colours require a different mechanism. Anthocyanins are not unmasked — they are actively synthesised in the leaf during senescence. This synthesis is promoted by high sugar concentrations trapped in the leaf after the abscission layer forms at the leaf base, cutting off the pathway to the rest of the tree.

Species that produce vivid reds in Poland include Norway maple, wild service tree (Sorbus torminalis) and, in planted parkland, various North American oak species. European native oaks (Quercus robur, Quercus petraea) tend toward brown-amber rather than red, retaining some leaves through winter in a condition called marcescence.

The Abscission Layer

At the base of each leaf stalk, a zone of specialised cells — the abscission layer — gradually disconnects the leaf from the twig. This process is regulated by a declining ratio of auxin (produced in the leaf) to ethylene (produced in response to senescence signals). As auxin levels fall, ethylene dominance triggers the development of a protective callus and the weakening of the cell layer that holds the leaf.

The leaf does not simply fall when abscission is complete; it is held by a thin thread of vascular tissue until mechanical force — wind, rain or frost — breaks the final connection.

Species-by-Species in Polish Forests

Temperature as a Modifying Factor

While photoperiod initiates senescence, temperature modifies its speed and intensity. Warm days combined with cool nights favour anthocyanin production, because daytime warmth maintains high sugar concentrations while cool nights slow sugar transport. This pattern — characteristic of Polish September and October weather in inland areas — tends to produce more vivid reds and oranges than consistently mild or consistently cold conditions.

In years when autumn temperatures stay elevated into late October, senescence may be delayed by one to two weeks compared to the long-term average, and the colour range may be narrower, favouring yellow over red.

Further Reading

• Institute of Nature Conservation, Polish Academy of Sciences — publishes annual phenological summaries
• Lasy Państwowe (State Forests) — species distribution and forest composition data
• IMGW-PIB — historical temperature data useful for correlating with leaf timing