Effects of solar UV radiation on birch and pine seedlings in the sub-Arctic

The responses of Betula pubescens Ehr. (European white birch), B. pendula Roth (silver birch) and two provenances of Pinus sylvestris L. (Scots pine) to solar ultraviolet (UV < 400 nm) radiation were investigated in a UV-exclusion field experiment during the 1997-99 growing seasons in Finnish Lapland (68°N). The seedlings were grown from seed under UV-B exclusion (a clear polyester filter) and UV-B/UV-A exclusion (a clear acrylic plate) as compared to control treatment (a polyethene filter) and ambient plants (no plastic filter). The mean daily maximum solar biologically effective UV-B irradiance (UV-B_BE) was 88 mW m^-2, 68 mW m^-₂, and 91 mW m^-2 for 1997, 1998, and 1999. A number of growth and biomass variables, PSII (Photosystem II) efficiency, and total concentration of nitrogen were recorded during and/or at the end of the experiment. Exposure (191 d) to solar UV radiation over three growing seasons did not cause many statistically significant UV effects in the growth or biomass of the seedlings. The only significant impacts of UV exclusion were found in P. sylvestris provenance Enontekiö. During the first growing season, the UV-B/UV-A exclusion treatment significantly accelerated the height increment (18-20%) of P. sylvestris, and in the same seedlings, the UV-B exclusion treatment resulted in significantly increased dry weight of one-year-old needles (45-57%) after the second growing season. These UV impacts could not be seen at the end of the experiment or in any other species. The low concentration of N in current foliage was related to increased dry weight, but not to solar UV radiation (control vs UV exclusion). The present study indicated that solar UV radiation had limited, but sometimes transient, impacts on the growth of tree seedlings in the sub-Arctic. Longer-term field studies are needed, however, in order to detect the cumulative characteristics of the UV responses.