I am working on a climate classification system. Some previous works like Koppen and Trewartha use temperature breakpoints of 18℃ monthly minimum of daily average to delimit tropical climates, and 0℃ monthly minimum of daily average to distinguish subtropical from continental (Koppen only).
The first question, is there any scientific basis for these specific temperature breakpoints?
As an example of what I am looking for, I want to point out something in plant distributions in the area of the world I'm most familiar with; the South of the US. In Atlanta, where January averages are 6.3℃ and lows are 1.3℃, most of the natural trees you see around are either conifers or deciduous hardwoods (Oaks, hickories, yellow poplar (Liriodendron tulipifera) and sweetgum (Liquidambar styraciflua). However, towards the coast in New Orleans 11.9℃/7.1℃ or Savannah 9.8℃/3.7℃, there are a lot more evergreen non-coniferous trees. If you look at the (very similiar) distributions from Wikipedia of evergreens such as southern magnolia, live oak, or swamp laurel oak, you can see that there is a cline where the coolest month-averaged daily temperature goes below around 6-8℃ where these evergreen plant no longer grow.
So the general second question is, are there temperature breakpoints that are significant in controlling what sorts of plants can actually grow in an area?