Photosynthetic recovery of foliage after wind disturbance activates ecosystem CO2 uptake in cool-temperate forests in northern Japan

The effects of wind disturbance on forest dynamics and ecosystem CO2 exchange were examined in cool-temperate forests of northern Japan during 2004-2008 using eddy covariance (EC) measurements. One site was a young, even-aged, monoculture, deciduous forest (YGF); the other was an uneven-aged mixed forest of evergreen and deciduous overstory tree species, including some over 200 years old (TGF). On 8 September 2004, a strong typhoon struck the forests, after which leaf and branch amounts decreased in YGF, but foliage showed little change in TGF. By 2006, foliage at YGF had recovered to the 2004 pre-typhoon state. Average daily accumulated GPP (GPPd), terrestrial ecosystem respiration (TERd), and net ecosystem exchange (NEEd) were assessed for six growth stages annually. After the typhoon, large increases in GPPd were found during the growing stage of overstory tree species with high photosynthetic rates compared to that before the typhoon. Pronounced increases in GPPd and corresponding large reductions in NEEd were detected at YGF, indicating that NEEd was largely regulated by GPPd throughout the growing stages. Although EC measurements contain uncertainty, our continuous EC measurements revealed that annual variability in meteorological variables and structural changes in foliage have only small impacts on GPP and NEE, while photosynthetic recovery of foliage from typhoon damage has high potential to enhance annual GPP and reduce NEE as compared with those under non-damage conditions. Plant ecophysiological feedbacks from natural disturbance damage should be considered when predicting annual variability in ecosystem CO2 exchange both at the local scale and when estimating the global carbon budget. The relationship between photosynthetically active radiation (PAR) and GPP for (a1) stage C and (a2) stage D at YGF and for (b1) stage C and (b2) stage D at TGF. Data were obtained at hourly intervals and averaged using the GPP values obtained under u*c = 0.25 m s-1 and u*c = 0.55 m s-1 conditions. Lines in the figures represent the empirical relationship between PAR and GPP expressed in equation (6) for each year from 2004 to 2008.