Flowers for bees,autumn colours for whom? An experimental test with autumn colouration in mountain birches (Betula pubescens ssp. czerepanovii) as a signal against the moth Epirrita autumnata

In 2001, Hamilton and Brown proposed a controversial hypothesis of handicap signalling to potential insect parasites as an adaptive explanation for autumn leaf colouration. In subsequent studies there has been little attention to the costs and benefits of early autumnal colour change. Yet, in an observational study by Hagen et al. (2003) it was demonstrated that birch trees [Betula pubescens ssp. czerepanovii Ehrhart (Betulaceae)] turning yellow early in autumn had less damage from insects chewing on leaves the subsequent summer. Here, two experiments are presented which test the mechanisms in this model. The first addresses the proposed defence of leaves of B. pubescens ssp. czerepanovii by letting caterpillars of Epirrita autumnata Borkhausen (Lepidoptera: Geometridae), the birches’ most common insect parasites, choose between leaves from trees that either turned yellow late or early the foregoing autumn. The second experiment addresses whether adult female E. autumnata choose between early or late senescent (i.e., yellow or green) ‘twigs’ when ovipositing in autumn. We could not find evidence of preferences in either larvae or females, suggesting that timing of colour change in B. pubescens ssp. czerepanovii is not a warning signal that elicits a response in E. autumnata.     

Effects of ultraviolet radiation on productivity and nitrogen fixation in the Cyanobacterium, Anabaena sp. (Newton’s strain)

The biological effects of ultraviolet radiation (UVR; 290–400 nm), especially the UV-B (320–400 nm) component of the spectrum, include both direct and indirect effects on many cellular processes. In cyanobacteria both photosynthesis and nitrogen fixation can be affected directly by UVR, and indirectly by UVR through the production of reactive oxygen species (ROS). For the heterocystous cyanobacterium, Anabaena sp. (Newton’s strain), exposure to UVR causes a significant decline in the quantum yields of photosystem II (PSII) fluorescence and maximum productivity despite an increase in UVR absorbing compounds, mycosporine-like amino acids (MAAs), in those cells exposed to UVR. Concurrent with these observations are significant increases in the activities of superoxide dismutase indicative of an increase in the level of oxidative stress in cells exposed to UVR. Additionally, measurements of nitrogenase activity (acetylene reduction) show a significant decrease in cyanobacteria exposed to UVR, which manifests itself as a decrease in cellular nitrogen and an increase in C:N ratios. These results show that these nitrogen-fixing cyanobacteria are particularly sensitive to UVR, both its direct and indirect effects. The effects of UVR reported here add to the increasing evidence that UVR effects on this important group of prokaryotes could affect the input of new nitrogen, and the biogeochemical cycling of this essential macronutrient in terrestrial, marine, and freshwater habitats. Handling editor: L. Naselli-Flores An erratum to this article can be found at