In this paper lead by Sarah Elmendorf, we compared three methods of estimating warming effects on plant community composition: 1) manipulative warming experiments, 2) repeat sampling under ambient temperature change (monitoring), and 3) space-for-time substitution.

The three approaches showed agreement in the increase in the relative abundance of species with a warmer thermal niche, but differed in the magnitude of change estimated. Experimental and monitoring approaches were similar in magnitude, whereas space-for-time comparisons indicated a much stronger response.

These results suggest that all three approaches are valid, but experimental warming and long-term monitoring are best suited for forecasting impacts over the coming decades.

Check out the paper here:

http://www.pnas.org/content/early/2014/12/25/1410088112.abstract