Abstract
BackgroundStem CO2 efflux (ES) plays a critical role in the carbon budget of forest ecosystems. Thinning is a core practice for sustainable management of plantations. It is therefore necessary and urgent to study the effect and mechanism of thinning intensity (TI) on ES.MethodsIn this study, five TIs were applied in Larix principis-rupprechtii Mayr 21-, 25-, and 41-year-old stands in North China in 2010. Portable infrared gas analyzer (Li-8100 A) was used to measure ES and its association with environmental factors at monthly intervals from May to October in 2013 to 2015. In addition, nutrients, wood structure and nonstructural carbon (NSC) data were measured in August 2016.ResultsThe results show that ES increased with increasing TI. The maximum ES values occurred at a TI of 35 % (3.29, 4.57 and 2.98 µmol∙m-2∙s-1) and were 1.54-, 1.94- and 2.89-fold greater than the minimum ES value in the CK stands (2.14, 2.35 and 1.03 µmol∙m-2∙s-1) in July for the 21-, 25- and 41-year-old forests, respectively. The ES of the trees in low-density stands was more sensitive to temperature than that of the trees in high-density stands. Soluble sugars (SS) and temperature are the main factors affecting ES. When the stand density is low enough as 41-year-old L. principis-rupprechtii forests with TI 35 %, bark thickness (BT) and humidity should be considered in addition to air temperature (Ta), wood temperature (Tw), sapwood width (SW), nitrogen concentration (N) and SS in the evaluation of ES. If a change in stand density is ignored, the CO2 released from individual 21-, 25- and 41-year-old trees could be underestimated by 168.89 %, 101.94 % and 200.49 %, respectively. CO2 release was estimated based on the stem equation in combination with the factors influencing ES for reference.ConclusionsWe suggest that it is not sufficient to conventional models which quantify ES only by temperature and that incorporating the associated drivers (e.g. density, SS, SW and N) based on stand density into conventional models can improve the accuracy of ES estimates.
Highlights
Forest ecosystems play a pivotal role in the global carbon cycle, and the balance between carbon assimilation by forests and its release through respiratory processes is important in regulating the rate of CO2 accumulation in the atmosphere (Dixon et al 1994)
Thinning intensity variation in Stem CO2 efflux (ES) and influencing factors As shown in Fig. 2, the ES increased with increasing thinning intensity (TI) for the 21, 25- and 41-year-old forests
In this study, to clarify the effects of TI on ES and Q10 of Larix principis-rupprechtii and the related mechanisms and to evaluate the error in CO2 emission estimates at the annual tree scale and stand scale without considering the effects of stand density on ES, five TIs were applied to Larix principis-rupprechtii stands with three forest ages of 21, 25 and 41 years, and the ES, environmental, nutritional, wood structure and nonstructural carbon (NSC) factors were monitored
Summary
Forest ecosystems play a pivotal role in the global carbon cycle, and the balance between carbon assimilation by forests and its release through respiratory processes is important in regulating the rate of CO2 accumulation in the atmosphere (Dixon et al 1994). The principal environmental factor regulating cellular respiration is temperature because the rate of most enzyme-catalyzed biochemical reactions increases exponentially with temperature. This effect is often represented using the Q10 convention which specifies the proportional increase in reaction rate (or flux) for every 10oC increase in temperature, i.e. the temperature sensitivity. Both cell maintenance and growth respiration can contribute to the total RS, and maintenance respiration (Rm) is much more temperature sensitive than growth respiration (Rg; Adu-Bredu et al 1997). It is necessary and urgent to study the effect and mechanism of thinning intensity (TI) on ES
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