Consider an Earth like world except much hotter with extremely hot and wet tropical rain forests. Assuming 100% humidity, how hot might the alternate Earth tropics get and still support tree like organisms and vegetation? And could humans survive un-aided in such an environment?
I'm assuming that "un-aided" means no A/C. Per https://en.wikipedia.org/wiki/Wet-bulb_temperature:
Even heat-adapted people cannot carry out normal outdoor activities past a wet-bulb temperature of 32 °C (90 °F), equivalent to a heat index of 55 °C (130 °F). The theoretical limit to human survival for more than a few hours in the shade, even with unlimited water, is a wet-bulb temperature of 35 °C (95 °F) – equivalent to a heat index of 70 °C (160 °F).
With 100% humidity, 32°C will force your humans to rest somewhere cooler, and 35°C will kill them. You should move your estimate 3° cooler if they've just come down from the mountains.
As for plants, according to the introduction in this paper published by Rowan F. Sage and David S. Kubien, the upper limit for photosynthesis in heat adapted plants is 40-45°C.