Temporal And Spatial Variation Of Stem CO₂ Efflux And Its Influence Mechanism For Larix Princicpis-rupprechtii Mayr

Since the industrial revolution,with the continuous aggravation of global warming,more and more attention has been paid to carbon emissions from forest ecosystems.Stem CO₂ efflux plays an important role in carbon balance of forest ecosystem.Stem is one of the main carbon pools in forest ecosystem.It is of great practical significance to explore the variation of stem CO₂ efflux to explain the dynamic change of forest ecosystem carbon.The study of the effects of different tree position height and thinning on the carbon budget of forest ecosystems plays a key role in exploring the carbon budget assessment method of forest ecosystems in China.However,there are few related researches,and the effect and mechanism are still not clear.Therefore,it is of great significance to focus on researching the rules and mechanism of stem CO₂ efflux change from multiple angles.Larix principis-rupprechtii is the main domestic species and major artificial afforestation tree species in North China.In this study,taking four Larix principis-rupprechtii forests at the age of 16,21,25 and 41 respectively in Saihanba area of Hebei Province as the research object,the variation of stem CO₂ efflux with time,the height of stem position and thinning intensity was studied,the stem CO₂ emission rule of Larix principis-rupprechtii forest during tending production processes was clarified,the response mechanism of stem CO₂ efflux to time,the height of stem position and thinning intensity was explored from the point of view of environmental factors,wood structure,nutrient element content and respiratory substrate content,and the main regulatory factors regulating stem CO₂ efflux were determined.These research results could provide a basis for establishing an accurate stem CO₂ efflux model,supply strong data support for further precise quantification in stem carbon efflux and evaluation on forest carbon sequestration capacity,and offer scientific data and theoretical reference supporting to relevant departments for formulating forest development plans and environmental protection strategies to improve the forestry ability when facing with climate change.The main conclusions are as follows:?1?The diurnal and seasonal variation of stem CO₂ efflux could be both described as "unimodal curve".Daily CO₂ efflux reached its peak value at 14:00 to 18:00,and the maximum seasonal variation appeared in late July or early August.During the growing season,the stem CO₂ efflux decreased with forest age.The trend was opposite in non-growing season: 41-year-old?0.38 ? mol m-2 s-1?> 25-yearold?0.32 ? mol m-2 s-1?> 16-year-old?0.29 ? mol m-2 s-1?.?2?The main environmental factors affecting the seasonal variation of Larix principis-rupprechtii are air temperature and air humidity.Stem CO₂ efflux temperature sensitivity coefficient?Q10?of Larix principis-rupprechtii decreased to the lowest point?1.34?in July.When the air temperature is above 0 ?,the temperature sensitivity of stem CO₂ efflux increases as air temperature rises.When the air temperature is at the range between 0 ? and 5 ?,the stem CO₂ efflux reach its most sensitive response to temperature,which is 9.89.?3?Though estimation on the seasonal variation of stem CO₂ release from 16-,25-and 41-yearold stands of Larix principis-rupprechtii,it was found that the maximum CO₂ release from stem,which occurred in July,was 61.68 ± 5.51,99.42 ± 17.32 and 32.93 ± 3.07 kg CO₂ ha-1 day-1,respectively;and it was in October that the lowest was 7.98 ± 3.33,10.46 ± 4.25 and 4.87 ± 1.82 kg CO₂ ha-1 day-1,respectively.The annual CO₂ release rules from stem of Larix principis-rupprechtii forest was 25-year-old?11.53 ± 2.08 t ha-1?> 16-year-old?7.45 ± 1.02 t ha-1?> 41-year-old?4.87 ± 0.66 t ha-1?.?4?The CO₂ efflux of Larix principis-rupprechtii stands at the crown and at the base of the stem was significantly higher than that in the middle of the stem,which could be demonstrated in all 3 age stands.The main environmental factors,wood structure factors,nutrient elements and respiratory substrate affecting the variation of stem CO₂ efflux with the height of stem position were air temperature and air humidity,sapwood width,nitrogen content and soluble sugar content,respectively.?5?Among the three age stands of Larix principis-rupprechtii,the stem CO₂ efflux at 4.0m?Q10 = 2.90?,0.2m?Q10 = 2.84?and 2.0m?Q10 = 3.30?showed the most sensitive response to temperature.In this study,a fast estimation model of stem CO₂ efflux was established by exponential regression for stem CO₂ efflux and air temperature.?6?Combined with stem CO₂ efflux from different height of stem position to calculate the annual stem CO₂ release of individual Larix principis-rupprechtii in 16-?3833 plant ha-1?,25-?3050 plant ha-1?and 41-year-old forest?950 plant ha-1?,estimation values by traditional methods were underestimated by 5%,6% and 24%,respectively.Therefore,in order to obtain more accurate estimates of CO₂ emission from individual tree stem,the differences of stem CO₂ effluxes at different the height of stem positions should be taken into account.?7?The stem CO₂ efflux of three age Larix principis-rupprechtii stands increased with t he increment of tending thinning intensity.The main regulatory factors affecting the change of these three Larix principis-rupprechtii forests?21-,25-and 41-year-old forest?with thinni ng intensity were respectively soluble sugar content,starch content,air temperature,sapwood width and nitrogen content;stem temperature,soluble sugar content,air humidity,relative hu midity and nitrogen content;soluble sugar content,bark thickness,air humidity,relative hum idity and air temperature.Combined with the effect of stand density on stem CO₂ efflux,ste m CO₂ efflux estimation model was established.?8?The temperature sensitivity of stem CO₂ efflux in three age stands of Larix principisrupprechtii tended to increase with thinning intensity.The temperature sensitivity of stem CO₂ efflux from 21-year-old Larix principis-rupprechtii TI 30%?thinning intensity = 30%?plots was the highest?Q10 = 3.26?,while the stem CO₂ efflux of 25-and 41-year-old forests with TI 35% were the most sensitive to temperature?Q10 = 3.33 and 3.86?.?9?The CO₂ release from the stem of 21-,25-and 41-year-old Larix principis-rupprechtii increased with the increment of thinning intensity.On the landscape scale,the stem CO₂ release from 21-and 41-year-old forest TI 35% plots of Larix principis-rupprechtii was the highest,which was 10.86 ± 0.37 and 5.87 ± 0.31 t CO₂ ha-1 yr-1 respectively,and the maximum CO₂ release of 25-yearold forests in CK plots was 15.89 ± 0.54 t CO₂ ha-1 yr-1.In summary,the variation rules of Larix principis-rupprechtii with time,the height of stem positions and thinning intensity are complex and different,and there has been some differences exist in its change mechanism and main regulatory factors.This study explored the variation rules of stem CO₂ efflux from the perspectives of time,the height of stem position and thinning intensity,and analyzed the regulatory factors including environmental factors,tree structure,nutrient elements and respiratory substrate.It provides scientific data and theoretical support for in-depth understanding of stem CO₂ release mechanism and accurate evaluation of forest carbon sequestration capacity in North China.