Southern U.S. Pines Can Take the Heat

Reference
Wertin, T.M., McGuire, M.A. and Teskey, R.O. 2010. The influence of elevated temperature, elevated atmospheric CO₂ concentration and water stress on net photosynthesis of loblolly pine (Pinus taeda L.) at northern, central and southern sites in its native range. Global Change Biology 16: 2089-2103.

There is a disconnect between forecasts of biospheric doom and experimental studies of how species are likely to respond to future changes in climate. This study adds to that disconnect.

The commercial forestry industry in the Southeast United States depends heavily on loblolly pine; and this part of the country is forecast to become warmer and perhaps slightly drier, as time progresses and as the air's CO₂ content continues to increase. Any climate-induced damage to this tree would thus have a huge negative impact on the region's economy; and, therefore, the three researchers set out to evaluate the effects of elevated CO₂, elevated temperature, and moisture stress on loblolly pine trees in a full-factorial experiment that they replicated at three sites spanning most of the north-south range of Georgia. Few studies have evaluated all three factors in this way; but Wertin et al. were able to do so using controlled-environment chambers that could not only keep the air's CO₂ concentration continuously elevated at 700 ppm, but could also keep its temperature a constant 2°C above ambient.

The study revealed that there was essentially no effect of temperature on net assimilation rate (A_net), which the researchers attribute to the very broad temperature optimum of loblolly pine that extends from about 20 to 35°C. Thus, there is no realistically possible global warming scenario under which a direct temperature effect would be detectable on this important commercial tree species.

The CO₂ effect, on the other hand, was substantial. A_net in the CO₂-enriched trees was 43% higher than in the ambient-air trees in June in the high-water treatment and 79% higher in the low-water treatment. The critical interaction between water stress and CO₂ also showed that the low-water with high-CO₂ treatment had equal or higher A_net than the high-water with ambient-CO₂ treatment at all sites and during all seasons, indicating that even a substantial decrease in moisture in this region would be compensated by the positive Anet response to increased atmospheric CO₂ under future warmer conditions.

This study shows that even under the worst-case predicted scenario, the dominant tree species in the Southeastern United States -- loblolly pine -- will grow as well as, or even better than, it does presently. And if the increases in precipitation that are forecast by some models come to pass, this state of climatic affairs would lead to even greater growth enhancements.

Archived 1 July 2010