Circadian rhythms are highly conserved across the tree of life, with light serving as the primary cue for the circadian timer. Photoperiod variation follows a latitudinal gradient, reflecting fluctuations in seasonality that increases towards the poles. As photoperiod variability intensifies at higher latitudes, organisms must develop mechanisms to maintain circadian timing during prolonged periods of light or darkness. Consequently, latitudinal gradients in photic sensitivity occur, as populations in northern latitudes have physiologically adapted to significant seasonal variations in day length. However, the rise of urbanization has introduced artificial light on a global scale, creating new photoperiods for organisms to navigate. This artificial extension of the natural photoperiod is now interacting with evolutionarily established circadian rhythms. Because photic sensitivity follows a latitudinal gradient, this may also inform biogeographic patterns of vulnerability to light pollution. We introduce the term "circadian histories" to describe an organism's evolutionary history of circadian entrainment, which may influence its response to novel light pollution.

Secondary forests represent significant proportions of global forest cover, with over 65% of forests in Asia classified as regenerating. While succession has been studied extensively in temperate systems, trajectories of sub-tropical forest succession remain poorly characterized in highly disturbed, urban-adjacent forests. Investigating the additive beta diversity subcomponents of Turnover and Nestedness may reveal community assembly mechanisms driving secondary succession. The present study investigated plant community assembly along a successional gradient from 7 to 70 years following the onset of succession in secondary sub-tropical forests in Hong Kong. Plant survey data for 28 plots were analysed, generating additive Simpsons and Nestedness beta diversity metrics as subcomponents of Sorenson Beta Diversity. Dissimilarity matrices were generated and modelled as a function of transformed environmental matrices of forest plant community age (years following onset of secondary succession), elevation (metres), inter-community distance (metres), soil moisture saturation (%) and soil organic carbon (g kg-1). Generalized dissimilarity models were generated for plant species Turnover and Nestedness. Nonmetric Multidimensional Scaling of plant communities was conducted with Bray-Curtis dissimilarity matrices. Our findings indicate that elevation was the primary driver of plant species Turnover, while age and inter-community distance played less prominent roles. Models of Nestedness found that plot age and soil moisture saturation were the sole drivers of Nestedness patterns in plant communities. While models of Turnover were robust, the low explanatory power of Nestedness models suggest additional unobserved factors driving patterns of plant community Nestedness during secondary succession. Turnover patterns suggest a deterministic model of community assembly, with strong patterns of species replacement between Lowland and Montaine forest types, as well as between successional age classes.