Background
For plants that appear to be adapted to cool temperatures, and which typically exhibit reduced growth rates in warmer environments, it has long been believed (as only seems logical) that global warming would be bad for them. However, as with many things, appearances can be deceiving.

What was done
Using climate-controlled mini-greenhouses, the authors examined the interacting effects of air temperature and vapor pressure deficit (VPD) on the growth of tall fescue (Festuca arundinacea Schreb), a cool-season grass that from past studies was expected to show declining growth with warmer temperatures over the range of 18.5 to 27°C. This they did by growing well watered and fertilized plants in two sets of six-week-long experiments, one in which air VPD was held constant at 1.2 kPa while air temperature was maintained at either 18.5, 21, 24 or 27°C, and one in which air temperature was held constant at 22°C while air VPD was maintained at either 0.9, 1.2, 1.4 or 1.7 kPa.

What was learned
In the experiment where the air VPD was held constant, the authors report that "in direct contrast to the anticipated results, the weekly growth of the tall fescue was substantially increased [our italics] with increased temperature," noting that "growth at 24 and 27°C was about 2.3 times that at 18.5°C and 1.4 times that at 21°C," while in the experiment where air temperature was held constant, they say "there was a strong, negative influence of increasing VPD on plant growth." In addition, they observed that "transpiration rates were similar across treatments," indicating that "water movement through the plants did not increase in response to increasing VPD," which led them to conclude that limitation of water movement through the plant "is likely a result of stomatal closure in response to elevated VPD (Bunce, 2006)." This phenomenon would also restrict the CO2 diffusion pathway into the plants and result in a decrease in photosynthesis, which is likely what caused the decreased growth at increased VPD. Be that as it may, their results indicated that as long as the air VPD does not rise concurrently, increasing temperatures do not lead to growth reductions in this supposedly cool-season plant. In fact, they found just the opposite was true -- that warming dramatically increased tall fescue growth.

What it means
Sinclair et al. note that "during the past 50 years, VPD has remained virtually constant (Szilagyi et al., 2001) due to an increase in atmospheric dew point temperature (Gaffen and Ross, 1999; Robinson, 2000)," even in the face of what climate alarmists describe as unprecedented global warming; and because of this fact, they conclude that, in a future warmer world, "tall fescue, and perhaps other cool season species, could experience a substantial benefit with temperature increases expected in temperate zones if VPD were to remain unchanged," which indeed appears to be what will happen in light of the real-world behavior of the air's VPD over the past half-century of warming.

References
Bunce, J.A. 2006. How do leaf hydraulics limit stomatal conductance at high water vapour pressure deficits? Plant, Cell and Environment 29: 1644-1650.

Gaffen, D.J. and Ross, R.J. 1999. Climatology and trends of U.S. surface humidity and temperature. Journal of Climate 12: 811-828.

Szilagyi, J., Katul, G.G. and Parlange, M.B. 2001. Evapotranspiration intensifies over the conterminous United States. Journal of Water Resources Planning and Management 127: 354-362.