Mitigation of Drought Limitation in Photosynthesis of Larix gmelinii Saplings by Clear-cutting in East-Siberian Taiga

Although Larix gmelinii forest ecosystem on permafrost is frequently disturbed by human activity like logging, regeneration system is not developed because of little understanding of the eco-physiological process. In the east-Siberian ecosystem, water cycle plays an important role in the regeneration, namely the annual precipitation is only 200-250mm approximately, and the melting of permafrost after the logging changes forest to wet land called as Alas where regeneration of tree is impossible. Thus we focus the drought stress in CO2 assimilation rate in the regenerated L. gmelinii sapling under canopy, and the effects of clear-cutting. In order to evaluate the drought limitation in the CO2 assimilation rate, in this study, irrigation treatment was conduced for 10 days in understory and clear-cut area. The clear-cutting was carried out in 2000, and this experiment was carried out in 2002. The light saturated CO2 assimilation rate (Amax) was measured in the morning and the afternoon. Volumetric soil moisture content at the 15 cm of the depth was approximately 10% under canopy and 30% in clear-cut area. The midday depression of Amax was larger in understory than that in clear-cut area, especially the understoried Amax was near 0 μmol m-2 s-1. During the irrigation treatment, i.e. 10 days, the volumetric soil moisture content was kept over 30% in the both areas. After the irrigation, the Amax increased to 3.2-fold in understoried and 1.3-fold in clear-cut area in morning. The Amax in afternoon increased to 50-fold in understory and 2.5-fold in clear-cut area after the irrigation. Thus the drought limitation of Amax was larger in understory than clear-cutting area. These results suggest that (1) drought stress is major factor to limit the CO2 assimilation of the regenerated L. gmelinii saplings under canopy, and (2) clear-cutting mitigates the drought limitation in the CO2 assimilation rate during the early stage of the disturbance of clear-cutting.