Stem and crown growth of Japanese larch and its hybrid F grown in two soils and exposed to two free-air O regimes.

Ozone (O) pollution and soil infertility may negatively affect boreal forests across the Northern Hemisphere. Impacts to economically and ecologically important larches (Larix sp., Pinacaeae) are particularly concerning. Using a free air O enrichment (FACE) system, we investigated the effect of 2-year elevated O exposure (≈66 nmol mol) on Japanese larch (L. kaempferi) and its hybrid larch F (L. gmelinii var. japonica × L. kaempferi) planted directly into either fertile brown forest soil (BF) or BF mixed with infertile volcanic ash soil (VA). Overall, photosynthetic pigmentation and the growth performance of the stem and crown were reduced in both taxa exposed to elevated O. Furthermore, hybrid larch, in both O treatments, performed better than Japanese larch. This finding contradicts findings of prior experiments with potential experimental artifacts of O exposure facilities and root restrictions. Elevated O also disproportionately inhibited stem diameter growth and caused an imbalance in chlorophylls a/b and chlorophyll/carotenoid ratios. Hybrid and Japanese larches grown in BF and VA had a significantly lower drop of stem diameter over the run of stem height (from base to top) when exposed to elevated O, compared to ambient O. This finding indicates altered stem shape under elevated O. Among 11 response variables, there were no significant interactions between O treatment and taxa. There was also no significant interaction of soil condition and taxa, suggesting that the two larches shared a similar response to O and soil type. Understanding the performance of hybrid larch in relation to its parent species has ramifications for breeding success in a soil-degraded and O-polluted environment.