It isn't the temperature per se, it is the rate of temperature increase:

"Leemans and Eickhout (2004) found that adaptive capacity decreases rapidly with an increasing rate of climate change.

Their study finds that five percent of all ecosystems cannot adapt more quickly than 0.1 C per decade over time. Forests will be among the ecosystems to experience problems first because their ability to migrate to stay within the climate zone they are adapted to is limited.

If the rate is 0.3 C per decade, 15 percent of ecosystems will not be able to adapt. If the rate should exceed 0.4 C per decade, all ecosystems will be quickly destroyed, opportunistic species will dominate, and the breakdown of biological material will lead to even greater emissions of CO2. This will in turn increase the rate of warming" --Leemans and Eickhout (2004), "Another reason for concern: regional and global impacts on ecosystems for different levels of climate change," Global Environmental Change 14, 219–228

In case you missed it: "Forests will be among the ecosystems to experience problems first because their ability to migrate to stay within the climate zone they are adapted to is limited."