Photosynthesis of boreal ground vegetation after a forest clear-cut

Abstract. Heather (Calluna vulgaris), rosebay willowherb (Epilobium angustifolium), wavy hair-grass (Deschampsia flexuosa) and raspberry (Rubus idaeus) are typical species at boreal clear-cut sites. In this study, we measured their photosynthesis separately in the growing season of 2005 using a manual chamber. All measured species showed clear and species-specific seasonal cycles of photosynthetic activity (P_max). The maxima of C. vulgaris and E. angustifolium occurred around June and July, while that of R. idaeus occurred as late as August. A simple model of photosynthetic activity is presented, addressing the photosynthesis of C. vulgaris was mainly explained by temperature history when the soil moisture is high. The activity of deciduous D. flexuosa also followed the temperature history, unlike the activities of E. angustifolium and R. idaeus. During a short drought, some shoots decreased their P_max levels but none of the species showed similar reactions between individuals. We also observed that the comparison of the whole-plant P_max or respiration of different-sized individuals were less scattered than the results based on full-grown leaf mass, implying that species-specific rates of photosynthesis at ground level are rather similar regardless of the plant size. Using species composition and continuous temperature and light measurements, we upscaled the species-specific process rates and integrated fixed and respired CO₂ of ground vegetation for the entire 2005 growing season. The photosynthetic production per surface area of soil was 760 g C m⁻² y⁻¹ at the fertile site and 300 g C m⁻² y⁻¹ at the infertile site. During the snow-free period (18 April–21 November), the above ground parts of measured species released 75 g C m⁻² y⁻¹ at the infertile site. At the fertile site, E. angustifolium and R. idaeus respired 22 and 12 g C m⁻² y⁻¹, respectively.