Pinyon Pines Facing Dark Future As Climate Shifts

Climate shift threatens wholesale slaughter of tree that dominates Rim Country

The projected rise in the length and severity of drought could devastate the pinyon pine forests that surround Payson, according to a growing body of research.

Photo by Pete Aleshire. |

The projected rise in the length and severity of drought could devastate the pinyon pine forests that surround Payson, according to a growing body of research.


The projected rise in the length and severity of drought could devastate the pinyon pine forests that surround Payson, according to a growing body of research.

The efforts to understand the massive die-offs of the hardy, low-elevation pines that mingle with junipers to cover hillsides across Rim Country has revealed fascinating — but alarming — details of their response to climate shifts predicted to transform the forests of central Arizona in coming decades.

One recent study concluded that a 2.3-degree average temperature rise in the past 40 years in the Southwest has already cut pinyon pine seed production by 40 percent.

Another study found that drought conditions have killed more than half of the pinyons in many areas — especially big, reproducing trees. As a consequence, mixed stands of pinyon and juniper will wind up almost entirely juniper in coming years. That could affect more than 1,000 species that depend on the pines.

A third study found that in drought conditions shrubs drastically reduce the growth rates of pinyon pines — all the way down to the fungus that helps the roots absorb water. Shrubs have grown thickly on millions of acres as a result of an all-out effort to stop wildfires in the past century, which has made the pinyons much more vulnerable to mass die-offs in a drought.

Reproduction rate plunges

The sharp reduction in cool summers has apparently drastically reduced the pine cone crops for pinyon pines throughout the West, according to a study published in Ecosphere and conducted by researchers from the University of Colorado at Boulder and the United States Department of Agriculture.

The researchers concluded that unusually low temperatures in the late summer periodically trigger a huge increase in pine cone production and therefore reproduction. A cold snap in the late summer triggers changes in the trees that 26 months later produce a “masting” events and a bumper crop of seeds. Normally, masting events take place every two to 10 years.

No one knows for sure why the trees produce so many cones in such a year and very few in normal years. Most ecologists think that the trees effectively overwhelm all the birds, rodents and other animals that eat the seeds. Such seed-eaters like pinyon jays, Clark’s nutcrackers, mice and squirrels would build up into huge populations if the pines produced large numbers of seeds every year. Instead, the bumper crop masting years produce more than the seed-eaters can consume.

For instance, just 150 Clark’s nutcrackers can gather up and stash 5 million pinyon nuts in a single season, according to a 2007 study by researchers from the University of Northern Arizona. The massive outpouring of cones in rare years might also have something to do with climatic factors and interactions with the insect pollinators.

However, the number of cool, late summers has declined in the past 40 years as average temperatures have risen, the researchers concluded.

The study compared pine cone production in two periods — a warm, drought-plagued period from 2003-2012 and another period of more normal rainfall from 1969-1978. The research found that the higher average temperatures reduced pine cone production by 40 percent.

Researchers included Jeffry Mitton, Miranda Redmond, Nichole Barger and Frank Forcella.


The Pinyon Pines that mingle with juniper to dominate the scenery of Rim Country have suffered mass die-offs and a drop in cone production.

Die-offs in a drought

Recent droughts have dramatically increased mass die-offs among pinyon pines, especially in lower-elevation areas like Rim Country, concluded researchers from the University of Northern Arizona, the University of California at Irvine, who published their work in the Journal of Ecology.

The study of mixed stands of pinyons and junipers near Sunset Crater National Monument found that two periods of extreme drought between 1996 and 2002 killed almost 60 percent of the pinyons.

The junipers also suffered — but had a much lower death rate. In fact, the death rate among the pinyon pines was seven times greater than the junipers. Moreover, to the surprise of the researchers, the biggest pinyons had the highest death rate — about five times greater than small pinyon seedlings, especially those shaded by other plants.

Normally, biologists expect big trees can withstand harsh conditions better due to their greater reserves of moisture and nutrients.

Researchers had hoped that they would find that the loss of so many trees in the first few drought years would at least help the surviving trees during the second drought period — since they would face less competition for the scarce water. However, the lack of water and heat seemed to have a cumulative effect and the trees couldn’t fully recover between episodes.

The junipers fared much better. As a result, the predicted onset of drought and rising temperatures could eliminate the pinyons from the mixed forests. That could have a huge impact on the roughly 1,000 species of birds, reptiles, rodents, mammals, insects and fungi that depend critically on the pinyons.

Researchers conducting the study included Rebecca Mueller, Crescent Scudder, Marianne Porter, Talbot Trotter, Catherine Gehring and Thomas Whitham.

Shrubs offer tough competition

Shrubs that fill in the spaces between crowded stands of pinyons and junipers across a vast swath of Rim Country can become deadly competitors in time of drought, according to a study by researchers Theresa McHugh and Catherine Gehring from Northern Arizona University.

The researchers wanted to know how shrubs affect the growth of pinyon pines and the fungi that grow on their roots to help them absorb water. Most plants have such root fungi, which get nutrients from the plants and in return greatly increase the ability of the roots to take in water.

The researchers studied several stands of pinyons with few competing junipers near Sunset Crater, north of Flagstaff. First, they assessed the root and needle growth together with the diversity and density of fungi on the roots in two patches. Then they removed almost all of the shrubs from one patch, mostly Apache plume, skunkbush sumac and desert olive. Then they measured the differences in tree and root growth for several years, much of it during a drought.

They found that that the pines freed from the competition with the shrubs had 150 percent longer needles and three times the root mass. Moreover, the roots had about four times as much of the beneficial fungus growing on them as the pinyons still competing for water and nutrients with the grasses and shrubs.

The findings provide a fresh glimpse at the impact of the near exclusion from periodic, low-intensity wildfires from most of Rim Country in the past century. The frequent, low-intensity fires controlled the growth of shrubs.


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