Gradual warming over the southwestern United States in recent decades is causing a decline in pinyon pine reproduction, with implications for other species sharing their ecosystem, according to a new study led by University of Colorado researchers.
In a paper appearing recently in the journal Ecosphere, published by the Ecological Society of America, the study's co-authors linked a 2.3 degree Fahrenheit temperature rise over four decades recorded at nine research sites to a decline in pinyon pine seed cone reproduction.
Leading the study were CU doctoral student Miranda Redmond and assistant professor Nichole Barger. Joining them as co-author on the study was Frank Forcella of the U.S. Department of Agriculture, based in Morris, Minn. Their work was mostly funded through a National Science Foundation graduate research fellowship to Redmond.
The researchers recorded temperatures and precipitation at official long-term weather stations near each of the nine sites, in New Mexico and northwestern Oklahoma, over the past four decades.
It was discovered that pinyon pine seed cone production was reduced at many of the study sites, and the highest reduction was in high-elevation areas that were experiencing more dramatic warming than the lower elevations during the March-to-October growing season.
While some of the study sites showed very little decline in pinyon pine seed cone production, others showed more than a 50 percent drop-off. The average reduction over the entire study area was about 40 percent.
"I was surprised by these findings," Redmond said. "I wasn't expecting to see the amount of declines we saw, and I also wasn't expecting to see that the areas with the greater temperature increase would have the greatest decline."
Redmond added that pine seed cone production decreases "could have significant impacts on the wildlife that rely on the pinyon pine cone," and could pose a "bottleneck" to future successful regeneration of the pinyon pine.
Pinyon pine is a dominant tree throughout the Southwest and provides critical habitat for a wide range of wildlife species, she said. Pinyon pine seeds are eaten by numerous wildlife species, including many birds and small mammals. For that reason, declines in cone production may not only affect future regeneration of pinyon pine but may also affect the numerous species that rely on the trees' seeds.
Redmond's study piggybacks on work done by Forcella, who conducted research using the same sites in New Mexico and Oklahoma, with data collected in 1978 and published in 1981.
"I read his paper and then realized I could go back and use the same methods to look at the cone production for the last 10 years," Redmond said.
The study utilizes pine cone production from two 10-year intervals, 1969-1978 and 2003-2012.
Pinyon pines are a tree species that, instead of reproducing annually, shed large quantities of cones every few years in what scientists call "masting" events. Pinyon pine masting occurs every three to seven years, producing bumper crops of nut-carrying cones.
Those nuts -- which, if uneaten, eventually develop into seedlings -- are too numerous in masting years for wildlife to consume in their entirety. Instead, birds such as crossbills and Clark's nutcrackers will "cache," or store, excess pinyon pine nuts as an available future food supply.
A University of Northern Arizona study in 2007 estimated that 150 Clark's nutcrackers stored roughly five million pinyon pine nuts in one season.
The new CU study shows that not only did total pine cone production in the 2003-12 decade decrease, but the production of cones during "masting" events in that time frame also declined from the earlier decade.
Looking forward, Redmond said, "Given the patterns we saw, our results suggest that with increasing temperatures we may see more declines in cone production in the future. However, I view our research as a platform for asking future questions.
"We saw significant declines in cone production but are unclear of the mechanisms of these changes. More research is needed to further understand how climate affects cone production."
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