The ponderosa pine forests that remain Rim Country’s biggest draw could fade away as a result of rising temperatures, deepening drought and spreading wildfires, according to a recent study published in Forest Ecology and Management.
The study found that famously drought-tolerant ponderosa pines have increasingly failed to regenerate after fires, especially in lower, hotter sites like the great swath of forest below the Mogollon Rim around Payson.
Many forests that burn in the face of rising temperatures and hotter wildfires may simply never come back, converting the area to grass or shrub lands, the study authors concluded.
Moisture stress in the struggling saplings apparently plays the key role concluded the researchers from Oregon State University after a stand-replacing wildfire in both ponderosa pine and Douglas fir, which grows at higher elevations than the ponderosa.
The trend will likely accelerate if the steady rise in average temperatures called for in most climate prediction models continue.
The researchers mostly focused on the effect of a 2002 fire along the Metolius River watershed in the eastern Cascade Range of Oregon. The ponderosa pine and Douglas fir forest burned there, without salvage logging or replanting afterwards.
After a decade, the researchers found virtually no regeneration in the hotter, drier sites, said Erich Dodson, a researcher with the OSU Department of Forest Ecosystems and Society. The study documented only modest regeneration in the higher, wetter sites.
Several studies have suggested that ponderosa pine forests may face hard times in coming decades that will dramatically reduce their range. Mature ponderosas have a formidable capacity to withstand drought, but saplings have a much harder time. Moreover, the dense, overgrown conditions of the forest have put an additional strain on old growth and young tree alike.
The study noted that ponderosas have not yet grown into burns on hot, south-facing slopes that took place a century ago, even before the pronounced warming trend that has affected the entire southwest.
The researchers suggested that restoration treatments that include thinning, controlled burns and plantings could change the emerging pattern, both by reducing competition between trees in overcrowded stands and by providing trees that will reseed burn areas. Such treatment seems particularly necessary for ponderosa pine forests at lower elevations and less essential for higher altitude mixed conifer forests, the researchers concluded.