Nervous Pine

A ponderosa pine tree is framed by a group of crosscut saws in a storage rack Wednesday afternoon during the 78th Annual AWFC Logging Sports Conclave in the Centennial forest off Woody Mountain Road.

Jake Bacon, Arizona Daily Sun

As I was driving home two days ago after seeing the latest weather forecast (with no precipitation), I actually began to feel sorry for the forest. I know that ponderosa pines are a drought-adapted species, but we've been in drought for so long, and now we've had no precipitation for months. I wasn't sure how trees responded to extended drought, so I did some research.

We know that trees utilize carbon dioxide (CO2) and water in the presence of light, in order to create carbohydrates (their food) through the process of photosynthesis. Trees use stomata (microscopic pores on the surface of leaves or needles) to absorb CO2 and then release oxygen and water vapor. Stomata are usually arranged in rows on the underside of the needles, where they appear as white lines. Needles of ponderosa pines, like those of other conifers, are well-adapted to dry environments. The outer surface of the needles has a thick waxy layer, called a cuticle, which reduces evaporative water loss. And in times of drought, trees can close stomata to prevent dehydration. However, there's a catch-22: without access to CO2, they can't photosynthesize--and thus begin to starve. Simply put, without adequate water and food they will die of thirst and hunger.

This has been happening across the west, but also in our backyard. Between 1997 and 2007 scientists recorded accelerating mortality in southwestern ponderosa pine forests due to warming temperatures and extended drought.

Other than watering the conifers in our yards, what are we doing to help preserve our forests?

Forest thinning helps. Our forests today are unnaturally thick with trees--something that causes competition for what little precipitation we get. Experiments conducted at the Fort Valley Experimental Forest show positive effects of thinning, particularly during drought conditions

Scientists are also utilizing experimental gardens and other settings to identify genetic differences within plant species that indicate greater ability to adapt to climate change. Just as no two coyotes are exactly the same genetically, neither are ponderosa pine trees, or native bunch grasses. And when you look at the wide geographic distribution of ponderosa pine trees, it only makes sense that those in Washington State will be different than those in Sonora, Mexico. Which will adapt more easily to our changing climate?

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The Southwestern Experimental Garden Array (SEGA), jointly developed by NAU and the Arboretum, is providing the setting for just such research.

SEGA is an array of highly instrumented field sites spanning a large elevation gradient; ten different sites at various elevations essentially provide different climates within which to test, they include for example, white pine trees from different geographic locations.

Growing the same plant species and genotypes (plants with the same genetic makeup) at SEGA sites across this elevation gradient allows scientists to discover which species and genes are most likely to survive and reproduce during drought and predicted climate warming.

What does this all mean? It means that we may find trees and shrubs and grasses that will survive climate change: ponderosa pines, and pinyon pines, and Colorado spruce that won't starve or die of thirst. This work may just keep our forests alive and thriving.

Lynne Nemeth is Executive Director of The Arboretum at Flagstaff. To reach her with articles, ideas, or comments, please email


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