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I used to think exactly like you did till I did more research on the topic and talked to botany folks from there, etc. You have to remember that even though the SE gets some bad cold waves in winter, the averages are still quite warm compared to say SE China. Much warmer latitude for latitude.Those warm averages allow a lot of subtropical flora to survive cold waves since soon after it warms back up. I have seen CIDP at a very dirunal location outside Mobile, AL survive 14F and in a year or two later look like nothing ever happened to them. I have pics of all of these things such as before and after. Once you travel down there you notice far more broadleaf evergreens and subtropical foliage in peoples landscaped and garden areas. Even SE Florida where I have a house, I have 10 large coconut palms on my property which have been there for years, and originally the forests in that area were coniferous and scrubby type vegetation. Now it is all tropical.
Also, China has a land link to tropical jungles in Vietnam, the SE US has a seed block from the Gulf of Mexico and the Caribbean Sea. Also the desert SW is a seed block from Mexico on South. That being said most agree the broadleaf evergreens are making their way back to their pre ice age locations. But even since the last ice age the forests of the SE were different than what you see today.
The forests you see throughout the majority of the SE US is not the original forest. I read Bartrams book Travels where he toured throughout the SE and wrote a journal. The original forests were mostly Longleaf Pine, a subtropical conifer, with an undergrowth of broadleaf evergreen shrubs. Those forests were entirely clearcut just as PA and NJ were. That's why the eastern US has really no substantial old growth forests. When you clear cut the fastest growing crap trees move in quickly to richer soils like riverbanks, deltas, etc. When you travel throughout the SE in winter as I have four times, you quickly notice that in poor soil areas like sandy soils in midlands of SC say, you see lots of broadleaf evergreens. Augusta, GA was very green in winter, and when I drove from Augusta down to Savannah it was very green for the most part. Beaufort SC all kinds of flowers blooming and broadleaf evergreens and huge citrus trees. Literally looked like summer to me.
I recommend you do some traveling down in the lower South in winter. Where you live is just as cold or maybe even colder than the coastal Mid Atlantic. I don't consider TN one iota subtropical btw.
Not sure what the OP meant by saying a place like Charleston regularly goes into the low teens, but it is not correct. The last time Charleston got that cold was in the 1990's. I'm not talking the airport ten miles inland but the city. Even the airport doesn't get that cold regularly. The zone is high 8b to 9a in the Charleston area. Charleston itself is very green in winter.
Does it have anything to do with the winter lows in the coldest days of the year? Even South Florida, which has a true tropical climate, gets awfully cold compared to any other tropical climate. Even places like Charleston (SC), Birmingham, Dallas and Atlanta get into the lower teens regularly, while places like Washington D.C., Nashville/Murfreesboro and Oklahoma City often see positive single-digits and places like Charleston (WV) even negative single-digits. It gets even worse for places out west with the same winter averages, unless you're in very low valley floors (Phoenix, Yuma, Las Vegas, etc.) or right by the coast.
Is it the soil type? The soil closer to the coast is awfully waterlogged, and it's notoriously bad (rocky, clay, highly acidic) here in Tennessee. However, I thought evergreens (whether conifers, palms or generic) were better at dealing with poor soil than deciduous trees of the same size? They don't lose their leaves (and thus nutrients) as often.
Could it have anything to do with the Glacial Periods? The temperate regions of the eastern U.S. were glaciated, which would've pushed those deciduous trees into the subtropics by force. However, much of the South (even northern Florida and central Texas) was either semi-arid or coniferous due to the cool, dry summers, and I don't know if it actually got that cold in winter even then (the polar vortex has ruined half of our winters, especially in Tennessee and the Great Plains, the more global warming has messed with the Arctic).
Or have the birds and other seed-spreading animals just missed this area by chance?
Has evolution just not kept pace in this Humid Subtropical climatic region compared to others? I doubt this though, given that there are broad-leaved evergreen plants like Mountain Laurels and Grand Rhododendrons that can survive even in brutally cold places like Vermont and Maine, and there are some subtropical indicator plants (like Southern Magnolias, Needle Palms, some Palmettos and Crepe Myrtles) that grow extremely well in Nashville, Washington D.C. and Oklahoma City (and can even survive as seedlings if you're lucky and careful) but just aren't native for some reason.
Otherwise, is it a combination of the above?
None of the above, exactly, although all the things you mentioned are factors in how plants are distributed in the world.
Life is all about energy flows.
Deciduous trees lose their leaves during winter in order to conserve energy. There are metabolic costs to keeping a leaf functional, and the larger the leaf, the more energy required. But the larger the leaf, the more energy it can produce. And also, the larger the leaf, the more vulnerable that leaf is to environmental damage. So deciduous trees produce large leaves when the photosynthesizing is good, and drop them when the metabolic cost becomes greater than what the leaves can produce. Although they then have to incur the metabolic costs of developing a new leaf canopy every spring, the growing season is so favorable to photosynthesis that it works out over the entire year, and the leaves can produce more energy than the cost of producing the leaves.
Conifers must also produce more energy than their leaves cost over the course of a year. They do it by having leaves that can photosynthesize intermittently during the winter, during those sunny breaks. But in order for those leaves to survive the cold and desiccation of winter, the leaves have extra epidermal layers and other adaptations that limit their ability to produce.
So what you see in nature is that conifers predominate in areas with harsh conditions / short growing seasons, because their leaves can keep on ticking however slowly under harsh conditions all year-round, and that broad-leaved trees predominate where the growing seasons are more favorable and they can produce the energy they need to survive for 12 months in a few months.
BTW, conifers shed and replace their leaves too. Usually they drop them in the spring or early summer. You don't notice it as much because they don't shed all of them (typically around a third) and because the trees don't bother to resorb all the chlorophyll pigments as deciduous trees do.
Evolution being evolution, there are plenty of species of both conifers and broad-leaved trees "encroaching" on the others' territories, but this is the standard explanation for the general pattern you have noted. And it explains why you see non-coniferous species that have evolved small, tough leaves, for the same reasons that coniferous species have them.
And, evolution being evolution, humans have been carting plants around for a very long time, because our evolution responded to the advantages of being able to move our food sources with us. We keep many plants alive in our gardens and farms that would not be able to survive if we weren't taking care of them. Of course, in some cases, we move a plant to a place far from where it evolved, and it thanks us for the opportunity to colonize into a favorable habitat by spreading - like a weed.
Conifers came first, and they have stuck with the pattern of photosynthesizing year-round. And they kept that habit as they spread across the planet and into cooler regions than the warmer areas where they first evolved, adding additional protections against harsh weather.
When non-coniferous plants arose, one of the ways they first competed with the dominant coniferous plants was by out-photosynthesizing them in regions with more favorable growing seasons.
There's an awful lot more (i.e., entire libraries of books and journal articles) that can be said regarding this, but this is the short explanation.
I recommend you do some traveling down in the lower South in winter. Where you live is just as cold or maybe even colder than the coastal Mid Atlantic. I don't consider TN one iota subtropical btw.
I get that POV, but it is warm enough to grow subtropical indicator plants (magnolias and crepe myrtles are very commonly cultivated here, Needle Palms and Dwarf Palmettos typically do well if you bother watering the first few months and plant early in the growing season, etc.). Combined with the fact that the winters are almost 10F too warm to maintain any snowpack (even snow on the ground lasting slightly less than a week is extreme), it's certainly not a hot-summer continental climate.
I wouldn't even consider staying if it was any type of continental. Even in Cookeville (over 1,000ft above sea level), the average in the coldest month is 36F, which is generally the lowest the most cold-hardy palms' seedlings can survive long enough to reproduce in that species' preferred summer conditions. All six major cities are even warmer. They're just not native to here for some other reason, along with other equally hardy subtropical foliage, so it puzzles me either why more evergreens haven't evolved or those haven't spread as far as the climate would allow.
I tend to take Koppen's system strongly into consideration, but my slight weighing in Trewartha's system prompts me to avoid considering moving to places in KS, KY, MO, etc. (which get colder than Nashville, Oklahoma City and the mid-Atlantic do but are still marginally considered "subtropical" in Koppen's system). I just thought I'd clear this up.
EDIT: As for the thing about Charleston, Atlanta and Dallas, I would remove the world lower right before where it says teens. Sadly, it won't let me edit my original post now, so I'm afraid my current clarification will have to do; Atlanta and Dallas do get into the lower teens regularly though, due to elevation in Atlanta's case and the volatile weather of the Great Plains in Dallas's case; and Charleston does still get upper teens at least inland, although maybe not on the waterfront.
Last edited by Sun Belt-lover L.A.M.; 08-07-2019 at 11:20 AM..
Apart from the lower light levels in winter, no doubt. However, some hardier broad-leaved evergreens (magnolias, a few cold-hardy palms, hollies, yews, mountain laurels, rhododendrons and river canes just to name several) CAN be grown in these "Upland South" regions, and in a place as warm as Arkansas, most of Oklahoma and parts of Tennessee, Virginia and the mid-Atlantic, should theoretically be able to survive as seedlings (An average of 36F in the coldest month is generally enough for the cold-hardiest palms to survive like that, given the species's proper summer conditions). On the other hand, icehouse cities like Chicago, Boston, St. Louis or Denver would probably not be able to maintain a lush landscape year-round, even if they tried. So what puzzles me is what the deal with all these deciduous trees here is when so many evergreens can survive and are in Deep South regions without a whole continent to cross.
Also, one of those pictures was taken in February. Although it takes until late March/early April for most of our deciduous trees to get their leaves back, the second half of February can still feel quite nice during the afternoon and does still have some flowers (which is a sure sign of winters being shorter and milder than in a true temperate climate, even though February is usually warmer than December and almost as warm as November). As for the one taken in January, though, it definitely is even more different than the NATIVE (though not necessarily lushest possible here, as I've said) foliage at that time.
The live oak seen in those pictures can survive z8 easily. They are everywhere in North Texas as well. The winter scenery of North Texas is definitely not green or lush.
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