information:extraction, linguistic:paraphrasing
Botany

Leaf drop or abscission involves complex physiological signals and changes within plants. When leafout is completed (marked by the transition from bright green spring leaves to dark green summer ones) the chlorophyll level in the leaves remains stable until cool temperatures arrive in autumn. When autumn arrives and the days are shorter or when plants are drought-stressed, the chlorophyll steadily breaks down, allowing other pigments present in the leaf to become apparent and resulting in non-green colored foliage. The brightest leaf colors are produced when days grow short and nights are cool, but remain above freezing. These other pigments include carotenoids that are yellow, brown, and orange. Anthocyanin pigments produce red and purple colors, though they are not always present in the leaves. Rather, they are produced in the foliage in late summer, when sugars are trapped in the leaves after the process of abscission begins. Parts of the world that have showy displays of bright autumn colors are limited to locations where days become short and nights are cool. The New England region of the United States and southeastern Canada tend to produce particularly good autumn colors for this reason, with Europe producing generally poorer colors due to the humid maritime climate and lower overall species diversity. It is also a factor that the continental United States and southern Canada are at a lower latitude than northern Europe, so the sun during the fall months is higher and stronger. This combination of strong sun and cool temperatures leads to more intense fall colors. The Southern United States also has poor fall colors due to warm temperatures during the fall months and the Western United States as it has more evergreen and fewer deciduous plants, combined with the West Coast and its maritime climate. (See also: Autumn leaf color) Most of the Southern Hemisphere lacks deciduous plants due to its milder winters and smaller landmass, most of which is nearer the equator with only southern South America, the South island of New Zealand, and southern Australia, particularly Tasmania and Victoria, producing distinct fall colors.