Artificial Light at Night Effects on Vegetation: Urban Trees Stay Green Longer

Urban streetlights are tricking trees into extending their growing season. A comprehensive analysis of 452 cities reveals that **artificial light at night effects on vegetation** include a significant delay in foliar senescence—the biological process where leaves change colour and autumn. This delay is not merely aesthetic; it represents a fundamental decoupling of plant cycles from natural seasonal cues.

Understanding Artificial Light at Night Effects on Vegetation

Rapid urbanisation floods ecosystems with lumens. While energy waste and sleep disruption are well-documented, the ecological consequences for flora have remained obscure. Trees rely on darkness to signal the approach of winter. When nights remain bright, these biological clocks fail. The study addressed a critical gap by measuring exactly how urban radiance shifts the timing of autumn, utilising 62,994 site-year records alongside satellite data from 2001 to 2022.

Nonlinear Responses in Urban Flora

The correlation is stark. Elevated light levels push back the date of leaf autumn. However, the relationship is nonlinear. The most dramatic shifts occur at low intensities (< 15 nW cm^-2 sr^-1). Beyond this threshold, the effect saturates. Brighter does not necessarily mean later; the initial disruption causes the primary shift. Regional variability exists, with socioeconomic factors and specific vegetation traits dictating the severity of the delay. Not all trees react equally, but the trend across urban centres is undeniable.

Mechanism: Carbon Assimilation and Climate Response

Why does this happen? The researchers suggest the delay may result from enhanced carbon assimilation. Trees exposed to night-time glow appear to continue photosynthesis longer than their rural counterparts. They essentially work overtime. This extended activity alters how vegetation responds to climatic cues like temperature drops. The artificial glow masks the natural signal of shortening days, confusing the plant's internal regulation systems. By incorporating these light variables into phenological modelling, the researchers improved prediction accuracy. Current models, which ignore light pollution, consistently predict leaf autumn too early.

Impact: Ecosystem Dynamics and Future Forecasting

So what? A few extra weeks of green leaves might seem benign. It is not. Delayed senescence alters the carbon cycle. While specific downstream effects on flora and fauna remain to be fully quantified, the immediate consequence is a significant blind spot in forecasting. When trees autumn out of sync with the temperature, our understanding of ecosystem dynamics becomes flawed. As cities expand, this previously underrecognised pathway becomes a critical variable. Climate change models must now account for the radiative footprint of our infrastructure to accurately predict ecosystem resilience under continued urban growth.