Interactive effects of elevated ozone and springtime frost on growth and physiology of birch (Betula pendula) in field conditions

We studied the impact of ozone enrichment and late frost, singly and interactively, on four birch (Betula pendula Roth) families selected from a naturally regenerated birch stand in southeastern Finland. Seedlings were exposed to 1.5× ambient ozone over one and a half growing seasons using free-air ozone enrichment system. Simulated springtime frost was implemented at the beginning of the second study year, 4 weeks after the bud burst. Plants were measured for timing of bud burst, visible ozone injuries, chlorophyll fluorescence, net photosynthesis and concentrations of photosynthetic pigments, as well as for growth and carbon allocation. Frost treatment caused a rapid 60% decline in net photosynthesis. The recovery of net photosynthesis from acute frost treatment was not complete during the subsequent 3 weeks, which led to significant growth reductions, decreased shoot/root ratio and accumulation of excess nitrogen in the leaves. Photosynthetic responses to ozone were very variable and family-specific. Concentrations of photosynthetic pigments were sensitive to both stress factors, while the maximum quantum yield of PSII was unaffected. Ozone exacerbated the effect of frost only on diameter increment. However, ozone and frost affected different seedling characters, e.g., ozone reduced pigments and frost collapsed net photosynthesis, and these effect combined appear to damage birch seedlings more than a single stress situation.