Carbon uptake of American chestnut in current and future climates

Background/Question/Methods

The American chestnut (Castanea dentata) was extirpated as a canopy tree in eastern deciduous forests by the chestnut blight fungus(Cryphonectria parasitica), which was introduced in horticultural imports from Asia. The restoration of American chestnut as a canopy dominant tree is important for ecological, economic and aesthetic reasons. The American chestnut was a foundation species reportedly accounting for 50% or more of the basal area in forest stands and affecting population, community and ecosystem processes. The American Chestnut Foundation has created trees that are both blight resistant like Chinese chestnut and morphologically similar to the American chestnut. Chestnut restoration efforts aim to introduce millions of blight resistant trees back into eastern deciduous forests. Over the long term, the reintroduction of this previous dominant tree has the potential to alter forest carbon uptake and nutrient cycling. However more quantitative physiological data are needed to accurately predict seasonal carbon uptake for American chestnut or blight resistant hybrids under current or future climate. Future predicted increases in global atmospheric CO₂ concentration ([CO₂]) will stimulate carbon uptake of most deciduous trees and thus forests as whole. Increasing temperatures may lengthen growing seasons and increase forest carbon storage, but the coupled impact of increasing atmospheric [CO₂] and increases in temperature in forests are not fully resolved because different species respond in different ways.

Results/Conclusions

We address this knowledge gap by quantifying structural traits related to carbon assimilation, measuring gas exchange, and estimating photosynthetic parameters in a diverse array of American x Chinese chestnut hybrids and 5 to 6 year old American chestnut saplings. We further assessed the response of selected third generation backcross hybrid American x Chinese chestnuts (BC3F3) to future predicted elevated carbon dioxide concentration ([CO₂] + 200 ppm) and temperature (+3.5 ^oC) in controlled environment chambers. We found that hybrids possess a suite of leaf structural traits consistent with higher carbon uptake than American chestnuts. While photosynthetic rates were similar photosynthetic capacity tended to be higher in hybrids. In the chamber experiment, American chestnuts had greater stimulation of photosynthesis but hybrids had higher biomass after one growing season. We hypothesize that early germination and higher photosynthetic capacity in hybrids led to greater biomass. We conclude that under future climate change hybrid seedlings may show greater growth and biomass accumulation during establishment than American chestnuts. However, longer term experiments are needed to determine if initial establishment advantages will carry forward to subsequent growing seasons.