Climate Past as Prologue for Ponderosa Pines

Scientists from the National Park Service and the U.S. Geological Survey have reconstructed the recent migration history of ponderosa pine trees in the central Rocky Mountains. Their recently published study on the movement of this species, through centuries and across complex terrain, is unprecedented in its methodology and scope. The investigation informs an uncertain climate and ecological future.

Experts project that climate change will force many species to adjust their geographical distributions in the near future, with cascading consequences for biodiversity, conservation biology, and ecosystem services.  Important lessons can be drawn from an understanding of the movement rates and pathways of northward migrations of vegetation that followed the end of the last Ice Age, some of which are still ongoing.

Ponderosa pine (Pinus ponderosa), the most widely distributed pine in North America, experienced one of the most rapid and extensive of these post-glacial plant migrations. The eastern race of ponderosa pine (variety scopulorum) spread northward along the Rocky Mountains, starting at its northernmost known distribution in southern New Mexico and Arizona around 13,000 years ago, and reached central Montana only within the last millennium. The western race (variety ponderosa) experienced a parallel but less well-known migration along the Sierra Nevada, eventually mingling with the northernmost populations of the eastern race in the northern Rockies.

The researchers, funded in part by the National Science Foundation, focused their efforts on the northern half of the distribution in South Dakota, Wyoming, and Montana, which they assumed had experienced the most recent spread of ponderosa pine. The study targeted sites where ponderosa grows today in settings suitable for the preservation of fossil packrat middens.  

Packrat middens are rock-hard amalgamations of easily-identified plant and animal remains embedded in crystallized urine, commonly preserved in rock shelters and crevices, and readily datable to within a few decades using radiocarbon analysis. Since the 1960s, several thousand middens found in semi-arid areas from Mexico to Canada have been analyzed to reconstruct vegetation changes over the past 50,000 years. 

The team collected 90 middens spanning the last 11,000 years to pinpoint the arrival of ponderosa pine at each of 14 sites in western South Dakota, northern Wyoming, and west-central Montana. Jodi Norris, a National Park Service ecologist and senior author of the study, likened the fieldwork to “a treasure hunt where you and your science buddies clamber on cliffs looking for packrat leftovers to track the natural spread of a common conifer in the West.” 

A key finding was that the eastern race of ponderosa spread across the region by island hopping a few tens of kilometers at a time to suitable establishment sites, likely aided by seed dispersal via birds. The eastern race colonized many of its northernmost sites, including sites where it now hybridizes with the western race in West-Central Montana, only within the last two millennia.

Norris and her USGS co-authors, Julio Betancourt and Stephen Jackson, used a bioclimatic model for the modern distribution of ponderosa pine to infer that the most recent spread must have been driven by increases in July temperature and precipitation. Future expansion of the ponderosa pine range will largely depend on the nature and pace of climate change in the region (principally warming). Considering other factors such as heavy land use and invasive species, native plant migrations in the future might be more complicated than in the past.

Betancourt cautioned, “Ponderosa pine migration in the past happened sluggishly in fits and starts, tracking the pace of climate variability. But future migration will have to  march to unusually rapid warming, this time disrupted by pervasive land use.  If expansion to increasingly warmer and more suitable sites far to the north is desirable, ponderosa dispersal will have to be assisted by deliberate and strategic planting.”

The research study, authored by Jodi Norris (National Park Service-Flagstaff; Northern Arizona University), Julio Betancourt (USGS-Reston, Va.), and Stephen Jackson (USGS-Tucson), was published online in the Journal of Biogeography.