Effects of elevated ultraviolet radiation on Quercus robur and its insect and ectomycorrhizal associates

Saplings of pedunculate oak (Quercus robur L.) were exposed at an outdoor f acility to modulated levels of elevated UV-B radiation (280-315 nm) under t reatment arrays of cellulose diacetate-filtered fluorescent lamps which als o produced UV-A radiation (315-400 nm). Saplings were also exposed to UV-A radiation alone under control arrays of polyester-filtered lamps and to amb ient levels of solar radiation under arrays of unenergized lamps. The UV-B treatment corresponded to a 30% elevation above the ambient level of erythe mally weighted UV-B radiation. Sapling growth and the occurrence of associa ted organisms were examined over two years. In both years, leaves of saplin gs exposed to UV-B treatment were thicker and smaller in area relative to l eaves exposed to ambient and control levels of radiation. UV-B treatment al so retarded bud burst at one sampling in the first year of the study. Some responses were recorded which were common to both treatment and control arr ays, implying that UV-A radiation, or some other factor associated with ene rgized lamps, was responsible for the observed effects. Saplings under trea tment and control arrays were taller in the first year of the study, suffer ed greater herbivory from chewing insects, and had lower root dry weights a nd greater insertion heights of secondary branches than saplings exposed to ambient levels of radiation. Exposure of saplings to elevated UV-A radiati on alone under control arrays increased estimated leaf volumes in the secon d year of the study and reduced the number of secondary branches and the to tal number of branches per sapling after two years, relative to both treatm ent and ambient arrays. There were no effects of elevated ultraviolet radia tion on shoot or total plant weight, root/shoot ratios, stem diameter, the numbers or insertion heights of primary or tertiary branches, total leaf nu mber, timing of leaf fall or frequency of ectomycorrhizas. Our study sugges ts that any increases in UV-B radiation as a result of stratospheric ozone depletion will influence the growth of Q. robur primarily through effects o n leaf morphology.