I only hear about negative results from global warming. But that does not seem to make sense. As we all know, Michael Mann wrote a report which introduced us to the "Hockey Stick" (yes it is debunked but stick with me here). He studied tree rings and used the size of the rings to estimate historic temperatures. I would have to assume that a larger tree ring implies a warmer climate. If this is the case, then does not his very report indicate that trees do in fact grow faster in a warmer climate?

yes warm weather is good, but a few cold months are essential for fruit bearing trees.

The answer to your question is both yes and no.

There are some trees and plants that do better in warmer environments, some do better in cooler ones - hence the diversity across the different climate zones.

As the planet warms some species will benefit and for the time being the overall effect is a net benefit. Crops have done better overall in recent decades but an optimum temperature has now been reached, any warmer and crop production will start to fall, slowly to start with then at a rate of 10% of global production for each 1°C temperature rise.

The things that trees need to grow are water, sunlight and carbon dioxide. There's more CO2 that they need so reducing or increasing levels of this gas will have little effect. More important is water and sunlight and levels of both of these are affected by changing temperatures.

As the world warms or cools it causes a range of shifts in weather patterns - primarily driven by ocean temperatures and wind generation radiating outwards from the Equatorial regions.

Where this leads to warm or hot regions with high rainfall then trees and plants thrive but as the temperatures rise more rainfall is needed. If it's not forthcoming you get areas of desert or scrub in which very little can grow.

In short, there's more to tree rings than just warmer or cooler temps but overall trees and plants will do better in warmer climates. I've simplified the answer but I hope it makes sense.