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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?

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Can't help with citations. For temperate climates there are several critical temps that I know of:

First one is 0 or possibly -2. The freezing point of water. Most plant cells have enough dissolved material in their sap/protoplasm that there is 1-2 degrees of freezing point reduction.

Next one down seems to be somewhere around -10 C. There is a big reduction in plant species counts around this temp.

Not sure where the next one is. Jack Pine can tolerate temps down to -60C Apparently their protoplasm has enough sugars and alcohols that it becomes a glass -- no expansion.

Climate imposes other limits.

  • I bought 500 mountain hemlocks one year. Everything I read said zone 3. Maybe for the tops. Where they grow, there is typically several feet of snow in winter. Roots never got below freezing.

  • In our region of central Alberta Crimson King Norway maple is hardy -- most of the time. If it gets cold early, say November, it kills the buds. It's not just how cold it gets, but when and how fast it gets cold.

  • Near Edmonton we can grow a variety of birches. Calgary 300 km south is a growing zone warmer than us, but birches don't do well there. The chinooks are warm enough that they break bud, then the buds die when it gets cold again. Other plants avoid this by having a chilling requirement matched to our winters.

  • Our winters are very long. Some of the problem isn't cold as much as desiccation. In some cases a warmer winter will be more fatal -- warmer air can draw off more moisture from buds and twigs.

  • I think there are temps above freezing that are important: When trying to grow eggplant, any day that got below +10C seem to make the eggplants sulk and do nothing for about 3 days while they were recovering.

  • Another critical temp is in the high 80's/low 90's. (Farenheit) Many plants shut down photosynthesis to conserve water. Greenhouse operators go to great lengths to keep their houses below 90 F

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