The most logical explanation (and use common sense) is that it's not as hot as lower latitudes. Plants adapt to changes and when they cannot produce enough because of heat, they will die but their offsprings who are luicky to ve in the higher latitude will survive thanks to cooler temperature and keep producing more.
Koko - With the increasing warming going further into the northern latitudes more plants are able to grow and prosper there in higher latitudes. A good example of how increasing warming in the higher latitudes are benefitting plant growths and survivability was the recent discussion about Greenland's plants. It was recently in the last 20 years or so that plants, trees and crops have been growing more abundantly every year because of the gradual warming over the last 20 years or so (despite the static level in the last 10 years).
Like I said, if they're dying because of drought or heat, then no amount of CO2 will revive them. It pretty defeats the "benefit" of increased CO2 if the heat is also increased and causes more droughts.
Koko - rather it's the global cooling that produces more droughts than a warming one as records have indicated. Go back in TIME and read - Another Ice Age? - TIME .
Again, it has been shown that increasing CO2 concentration and temperature resulted in more plant growths in the northern latitudes by expanding the growing season even more.
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