Effects Of Poplar Plantation Clear-cutting On Ecosystem CH₄,CO₂, And Water Vapor Fluxes

Harvest and regeneration are important steps for forest plantation management, but it always leads to huge changes of ecosystem carbon and water vapor, which enlarge the uncertainty of the function of forest plantation on climate change mitigation. In order to discover the clear-cutting effects and mechanisms on ecosystem CO2, CH4, and water vapor fluxes in the subtropical region of China, and to offer a scientific support for management and ecological value estimation of poplar plantation. This research did a consecutive more than four years(2010-2013) observations on CO2, CH4, and water vapor fluxes of a floodplain Populus deltoides plantation during its clear-cutting and regeneration.The result shows that clear-cutting of a floodplain poplar plantation had significant effects on ecosystem CO2, CH4, and water vapor fluxes, and the details are as follows:1) Clear-cutting turned the ecosystem from a CH4 sink to a source. Clear-cutting first decreased the CH4 absorption in the first three months after clear-cutting, and finially turned it to be a source. The mean rate of CH4 release was 0.11±0.08 mmolm-2d-1 in non-inundation period, while 2.17±1.16 mmolm-2d-1 during inundation with a maximum of ~4.4 mmolm-2d-1.2) CH4 flux in non-inundation period and inundataion period had different major controllers. Ground water table, soil water content, friction velocity were the most possible controllers in non-inundation periods while inundtation depth was the main controller during inundation. The exhausting rate reached the peak(~40 nmol m-2 s-1) when inundation depth was ~1.2 m. As a result, no obvious diurnal and seasonal dynamic pattern was observed in non-inundtion period, and a large inter-annual different caused by inundation situation.3) Clear-cutting turned the ecosystem from a CO2 sink to a source immediately after clear-cutting and lasted for seven months as a source untill the next spring arrival. The overall released CO2 was 209 g-Cm-2. However, CO2 yearly uptake by the clearcut was only 5.5% of that by the mature forest(-83.12±8.24 mol m-2). The fast growth of understory herbaceous species was the main reason why the clearcut turn back to be a net sink of CO2 in such a short time.4) Clear-cutting increased the temperature sensitivity of ER and changed its components ratio though the ER only increased slightly. The Q10 increased from 2.01 to 2.12 and 2.34 in the following two years. What’s more, the ratio of soil respiration to ER increased to 94%, and microbial respiration was the majority of soil respiration. Eliminate the cross effect of temperature, ER showed an exponential relationship with Normalized Difference Vegetation Index(NDVI).5) Clear-cutting greatly decreased GEP to 58.2% of the mature forest(2010), because clear-cutting decrease the ecosystem light use efficience and maximum GEP by changing the plant composition. Both ecosystem light use efficience and maximum GEP before and after clear-cutting showed an linear relationship with NDVI.6) Clear-cutting decreased ET and water use efficience to 70.0% and 85.6% of those of mature plantaion(2010), respectively. Moreover, ET showed an exponential relationship with NDVI.7) The following formulas coupled with NDVI and microclimate factors performed well in modelling the dynamic of daily ER, GEP and ET under various interruption:Where is the ER under the reference temperature and NDVI, while andare the reference temperature and NDVI, set to 15℃ and 0.4, respectively. B1 and B2 are trial constants; is the ecosystem light use efficience(mol-CO2 mol-1 light quantum) while is the maximum gross ecosystem production(mol-1 m-2), and Par is the light quantum(mol). ETo is the grass reference PET caculated by FAO Penman-Monteith model(mm d-1). a, b, and c are trial constants。This research indicated that, the poplar plantaion in this region does not only has large carbon sequestration during its growth, but also has a very small carbon release when harvest and regeneration, so the poplar plantation may has a large carbon sequestration during the whole life cycle. This research also revealed that CH4 only took a very small portion in the overall carbon flux of a floodplain poplar plantation in this region, but it needs to be further proved by longer observations.