Over the past decade, researchers have documented the increased vulnerability of large stands of a Southwestern forest icon – the pinyon pine – to the dangers associated with a warming climate: drought, insects, and wildfires.
Now, it appears that rising temperatures could also put a damper on pinyon reproduction, potentially limiting the ability of trees that survive the other scourges to recolonize disturbed areas, a recent study says.
Across nine stands of pinyon – two at the western tip of Oklahoma's panhandle and seven throughout New Mexico – the production of seed-bearing cones dropped 40 percent from a 10-year period centered on 1974 to another centered on 2008.
Looking only at years of exceptional seed-cone production, known as masting years, cone production fell 43 percent from the earlier decade to the recent one.
Meanwhile, the average temperature during the growing season in the two periods increased by 1.3 degrees Celsius (2.3 degrees Fahrenheit).
The study represents a first look at the potential impact of a warming climate on the production of pinyon seeds, which, in addition to being vital to repopulating pinyon forests, are also an important source of food for wildlife, notes Miranda Redmond, a University of Colorado PhD student who led the study.
The work doesn't attempt to address potential mechanisms that would tie seed production to temperature changes. Nor does it examine the potential impact on pinyon-forest regeneration, she adds.
But it does show a broad pattern over a wide expanse of the pinyon's range that is suggestive enough to warrant closer looks at these issues, she suggests.
The results come at a time of rising concern over the future of the West's forest in general and the Southwest's in particular.
For instance, a new study of a recent widespread die-off of aspens in Colorado identified a three-year drought and high summer temperatures as the trigger for the die-off, which affected some 17 percent of the state's aspen forests. The work, published online Jan. 25 by the journal Global Change Biology, was led by researchers at the Carnegie Institution for Science's Department of Global Ecology. The results suggest an increasing vulnerability of aspens to future large-scale die-offs if climate projections pan out for the area.
In addition, researchers are concerned about the future of the Southwest's Ponderosa forests, especially after record-breaking fires in Arizona and New Mexico in 2011 torched hundreds of thousands of acres of the trees.
Ponderosa also experience masting events, in which seed-cone production in a single masting year can rival the total of all the seed cones produced between masting years.
Even without climate change, some of the burn scars are so extensive that some researchers say some burned-out areas may come back as oak and brush, rather than forests. Anywhere from three to seven years can pass between masting years. This can give more aggressive plant species time to recolonize large burned areas and out-compete any Ponderosa seedlings that trickle in from unburned spots.
So far, the only study looking at the potential impact of temperatures on Ponderosa seed-cone production hasn't found any, Ms. Redmond says.
But that study looked at only two stands, while the study she and her colleagues – Frank Forcella with the US Department of Agriculture and University of Colorado ecologist Nichole Barger – examined covered nine stands over a wide geographic range.
"We're getting increasingly confident that these types of woodlands are very sensitive to a warmer, drier climate," he says.
"The way you would compensate for that, if it's possible, is to have additional regeneration. It's like a bank account – trees in and trees out. Now what we're seeing is, wow, both of those are going in the wrong direction," says Dr. Breshears, who did not take part in the study.
Buried in the results are hints that the situation could be a bit less dire than the overall numbers suggest. The two Oklahoma plots showed no significant change between the two periods the data cover. And where the decrease in seed-cone production was significant, the pinyons that were most vulnerable were those at higher elevations, where warming is most pronounced.
That suggests that the trees at lower elevations may be better adapted to warming so far, Redmond says.
Still, "the results were not what I was expecting at all," Redmond says. The magnitude of warming's apparent effect overall, and the trends showing the highest rates of decline among the stands experiencing the largest increases in temperatures came as a surprise, she says.
Concern over the future of the Southwest’s forests extends beyond the obvious – lumber, changes in the behavior of the region's fragile watersheds, and the region's overall biodiversity. A large-scale change in plant types also could reinforce the warming the region is projected to experience with climate change.
"The climate modelers will tell us that one of the first variables they have in terms of the land surface is how much tree cover there is," Breshears says. "With some of these die-off events, we're seeing a huge change in tree cover."
Experiments have shown on relatively tiny plots that when trees are removed, temperatures increase, he says.
Much remains to be done to see whether the large die-offs the region has experienced also reinforce warming, he continues.
"But based on other things we know, it's certainly reasonable to expect that here could be feedbacks to the climate system," he says.
"We're losing a lot of woodlands quickly, and we have a lot of questions about what will come back and how fast it will come back" to areas hit by extensive wildfires or die-backs, he says. "To what extent within the course of our lifetime might we be losing some of these forests and woodlands?"