[saalweachter]

3: increase in days over 100F. Being hotter isn't just an inconvenience: photosynthesis stops working efficiently when temperatures are over 100F (fun chemistry fact: chemical reactions are temperature sensitive; this is why your body tries to maintain a very narrow temperature range. Otherwise, the chemical reactions all run at the wrong speed (or not at all), and the delicate balance of chemical pathways is disrupted). So when temperatures get too hot, plants switch from photosynthesizing and storing energy to burning their stored sugars and respiring. Forests begin emitting CO2 instead of consuming it; crops do not have the extra sugars to store as fruits and grains for us to harvest and eat.

[marcosdumay]

That's less than 38°C. At Brazil, we have huge areas of florest that rarely see a temperature smaller than that during the day.

Are plant leaves cooler than the environment?

[saalweachter]

The rain forests themselves will be quite a bit cooler -- they tend to average 80F/27C, with temperatures rarely much higher than 92F/33C. I think it's a side effect of the constant rain.

I'm not sure about your non-rain forests. It's possible they are seasonally operating at a loss -- storing sugars during the cool seasons and burning them during the hot seasons.

[marcosdumay]

Averaging the florests do not mean much.

As I've said, there are big areas of rain-florest in Brazil that rarely see less than 38°C during the day. There are also big areas of savanna that stay around that temperature for half a year (the half that rains). The fact that there are also cooler areas does nothing to explain how plants survive on those both.

I do think it's quite likely that plants keep leaves bellow the environment temperature, I'd just like somebody that knows it to confirm it or not.