Beyond blight: Phytophthora root rot under climate change limits populations of reintroduced American chestnut

Abstract

Abstract American chestnut (Castanea dentata) was functionally extirpated from eastern US forests by chestnut blight, caused by a fungus from Asia. As efforts to produce blight‐resistant American chestnut germplasm advance, approaches to reintroduce chestnut throughout its former range are being developed. However, chestnut is also quite susceptible to a root disease in the southern half of its former range, and the pathogen that causes the disease (Phytophthora cinnamomi) is expected to move northward as climate warms. Genetic resistance to root rot appears to vary among individual chestnut trees, and the prevalence of resistance is highly uncertain. Because restoration of a self‐sustaining chestnut population is ultimately a landscape‐scale problem, we used a process‐based forest landscape model (LANDIS‐II) to conduct experiments to quantify the effects of root rot on the effectiveness of chestnut population restoration efforts in the center of the former range of chestnut under various climate scenarios. We developed a new LANDIS‐II extension to simulate root rot‐induced tree mortality as a function of temperature and soil moisture. We conducted a factorial simulation experiment with climate and resistance to root rot as factors and found that root rot greatly reduced chestnut biomass on the landscape, even when resistance to root rot infection was at the highest levels currently observed in published studies. Warming climate enhanced the virulence of the pathogen and resulted in a greater reduction in chestnut biomass. Results indicate that root rot has the potential to seriously hamper chestnut restoration efforts if resistance of chestnut is not enhanced through breeding and biotechnology, suggesting restoration efforts will be more successful if targeted to latitudes, elevations, and site conditions where root rot is not expected to be present well into the future, including areas north of the historical chestnut range (Canada). These results demonstrate the vital importance of incorporating root rot resistance into the larger blight resistance breeding program.

Keywords

• American chestnut restoration
• Castanea dentata
• climate change
• elevated CO2
• LANDIS‐II
• Phytophthora cinnamomi