The last stand? Using tree-rings and downscaled climate models to predict future ponderosa pine growth and distribution near its southern range limit
Ponderosa pine (Pinus ponderosa) is one of the most widespread tree species in western North America, providing valuable timber, wildlife habitat and ecosystem services. However, a combination of climate change, shifting disturbance regimes and legacies of land use and fire management pose potential threats to ponderosa pine throughout its range in coming decades. I strategically investigated the effects of climate change on ponderosa pine growth in the Tehachapi Mountains, CA (Tejon Ranch), near the southern edge of the species’ range where exposure to climate change is currently high. This setting is an ideal natural laboratory for global change: the current climate may be similar to the climate eventually experienced in the remainder of the species’ range farther north, providing a possible window into future forest ecosystems. I used tree-rings and local, statistically downscaled models of historic precipitation, air temperature and climatic water deficit to construct a climate response function, which is a statistical relationship between tree growth and climate. This function was then applied to predict future ponderosa pine growth across climate change projections, including business-as-usual and mitigated emissions projections under phase 5 of the Coupled Model Intercomparison Project (CMIP5).
Climatic water deficit accumulated over the current and preceding water-years and January minimum temperature, July maximum temperature and March precipitation of the current year combined to explain 54% of the variance in ponderosa pine growth between 1950-2010. Due to expected increases in droughts, water deficits and temperatures, ponderosa pine is likely to experience growth declines and heightened risk of mortality in its current habitat in the Tehachapi Mountains, especially during dry years and under warmer and drier future projections. Climate-related stress may also leave ponderosa pine more susceptible to insect attacks and fire damage. However, more favorable growing conditions will likely exist at higher elevations where ponderosa pine currently does not occur, offering an opportunity for local redistribution despite regionally unfavorable climate. Although ponderosa pine is currently a species of “least concern”, its high ecological and economic value indicate that we could stand to lose a great deal in terms of ecosystem structure, function and services if the species were to go into decline in western North America. The potential changes in tree growth and distribution in the Tehachapi Mountains represent an intriguing case study for understanding the effects of climate change on future forest ecosystems.