Henry Adams and his colleagues quantified this phenomenon in a 2009 paper in the proceedings of the National Academy of Sciences. Piñon pines are adapted to dry conditions, but prolonged drought can still kill them. A group of piñons was deprived of water and left to endure several months in a laboratory setting. Raise the temperature by just a couple of degrees, however, and things go downhill rather quickly.
The researchers analyzed 103 years of regional climate records from four southwestern states and found that the 2002 forest die-off coincided with a 26.1-week drought, the only six-month regional drought identified in their dataset. With survival time reduced by 28% in the heat, a drought lasting 18.7 weeks could have similarly devastating effects, and the records show at least five such droughts. Because stored energy is depleted more quickly at higher temperatures, only the first mechanism would depend on temperature. Because it did not account for increases in drought frequency or bark beetle activity, they considered this a conservative projection.
The researchers examined what caused higher temperatures to shorten the trees’ survival. Drought can kill a tree in two main ways. The first is that the tree can close up its leaf openings, not make its food anymore, and use up all its stored energy. The second is that it can be blocked by air bubbles from the flow of water inside. As the measures of water stress were the same in both groups, there was no reason to think the trees had become hydraulically blocked. Photosynthesis declined rapidly in both groups of unwatered trees, approaching zero by the third week. Respiration rates began to diverge during the third and fourth weeks. The warmer trees consumed their stored carbon reserves faster. Adams and his colleagues also looked at history. Based on their model, the authors projected that temperature increases alone could make droughts severe enough to cause forest dieback five times more frequent. They also cautioned that the experiment tested only one species.
Twenty mature piñon pines were transferred from a ranch in New Mexico to a climate-controlled biosphere in Arizona . Water was withheld from five trees from each of these groups. Ten trees were kept in conditions similar to their natural temperature, and the other 10 were placed in an environment that was approximately 4.3 degrees Celsius warmer than their natural condition. The unwatered trees in the warm room died after an average of 18 weeks, compared with 25.1 weeks for those in the normal room, a 28% shorter survival time, according to the study.
An April 2010 interview with Southwest Climate Outlook, published by CLIMAS, offered a comparison to explain the findings. Adams compared two marathoners who had eaten the same meal before their respective races, one in Chicago and the other in Tucson in 100-degree Fahrenheit heat. As a result, warmer trees deplete their energy reserves sooner than cooler ones.
Trees that stop taking in carbon dioxide continue consuming stored energy.
The marathoner running in high temperatures exhausts his energy reserves earlier.
