Stem and crown growth of Japanese larch and its hybrid F1 grown in two soils and exposed to two free-air O3 regimes.

Ozone (O3) 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 O3 enrichment (FACE) system, we investigated the effect of 2-year elevated O3 exposure (≈66 nmol mol-1) on Japanese larch (L. kaempferi) and its hybrid larch F1 (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 O3. Furthermore, hybrid larch, in both O3 treatments, performed better than Japanese larch. This finding contradicts findings of prior experiments with potential experimental artifacts of O3 exposure facilities and root restrictions. Elevated O3 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 O3, compared to ambient O3. This finding indicates altered stem shape under elevated O3. Among 11 response variables, there were no significant interactions between O3 treatment and taxa. There was also no significant interaction of soil condition and taxa, suggesting that the two larches shared a similar response to O3 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 O3-polluted environment.