Effects of increasing UV-B radiation and atmospheric CO2 on photosynthesis and growth: implications for terrestrial ecosystems

Increases in UV-B radiation reaching the earth as a result of stratospheric ozone depletion will most likely accompany increases in atmospheric CO₂ concentrations. Many studies have examined the effects of each factor independently, but few have evaluated the combined effects of both UV-B radiation and elevated CO₂. In general the results of such studies have shown independent effects on growth or seed yield. Although interspecific variation is large, high levels of UV-B radiation tends to reduce plant growth in sensitive species, while CO₂ enrichment tends to promote growth in most C₃ species. However, most previous studies have not looked at temporal effects or at the relationship between photosynthetic acclimation to CO₂ and possible photosynthetic limitations imposed by UV-B radiation. Elevated CO₂ may provide some protection against UV-B for some species. In contrast, UV-B radiation may limit the ability to exploit elevated CO₂ in other species. Interactions between the effects of CO₂ enrichment and UV-B radiation exposure have also been shown for biomass allocation. Effects on both biomass allocation and photosynthetic acclimation may be important to ecosystem structure in terms of seedling establishment, competition and reproductive output. Few studies have evaluated ecosystem processes such as decomposition or nutrient cycling. Interactive effects may be subtle and species specific but should not be ignored in the assessment of the potential impacts of increases in CO₂ and UV-B radiation on plants.