Response Of Forest Ecosystem Light And Carbon Use Efficiency To Environmental Factors

Carbon Use Efficiency(CUE)reflects the ecosystem’s potential for carbon sequestration and is an important indicator in quantifying carbon allocation patterns and carbon budget in ecosystems.Light Use Efficiency(LUE)describes an ecosystem’s ability to utilize and convert light energy and is an important parameter for estimating gross primary productivity(GPP)of the ecosystem.CUE and LUE respectively provide detailed descriptions of GPP characteristics from the perspective of carbon use and light energy utilization,which can more accurately reflect the comprehensive response of GPP to factors such as climate,substrate supply,and disturbance.The two indicators,therefore,provide new perspectives and entry points for deeply studying forest carbon sequestration and storage functions.However,with climate change intensifying,the uncertainty gradually increases in understanding of the responses of the forest ecosystems functioning to environmental factors,and there is no consensus on the laws and mechanisms of the responses of CUE and LUE to environmental factors.Therefore,in-depth study of the coupling of CUE and LUE and their response to environmental factors will provide further understanding of formation of forest ecosystem productivity and provide a scientific theoretical basis and data support for further analyzing the function and mechanism of forest carbon sequestration under the background of climate change.In this study,a total of 3 628 154 sets of complete hour-scale data were selected by mining,screening and processing big data obtained from 56 forest flux observation stations worldwide over the past 24 years.The carbon sequestration capacity,expressing by LUE and CUE was investigated in various forest ecosystems at a global scale.The ensemble model was developed for describing global forest ecosystem LUE and CUE and was used to quantitatively estimate the response of forest ecosystems’LUE and CUE to the changes in environmental factors were examined,and the coupling mechanisms driving LUE and CUE in global forest ecosystems were studied.The main results showed:(1)The distribution patterns of LUE and CUE in forest ecosystems worldwide were environmentally heterogeneity.The responses of CUE/LUE of forest ecosystems to temperature factors varied at different observation stations:the highest variation(SD=0.23)in the relationship between different sites’ CUE and soil temperature(TS)was observed,followed by the relationship with air temperature(TA)(SD=0.20).For different sites’ LUE and TS relationship,the highest variation(SD=0.26)was observed,followed by the relationship with TA(SD=0.25).No obvious environmental heterogeneity was found for the relationship between CUE and LUE and photosynthetic photon flux density(PPFD)in global forests.Different sites exhibited a positive correlation between CUE and PPFD(SD=0.11),while the correlation coefficient between LUE and PPFD was negative across different sites(SD=0.11).The multi-year average temperature of each studied site could explain the variation patterns of the relationship between environmental factors and LUE well,while the multi-year average precipitation was a more suitable variable to explain the variation of the relationship between environmental factors and CUE at a global scale.Combined with the existing studies,this paper concluded that the annual mean temperature may be the dominant factor driving the variation of the relationship between LUE and environmental factors,while the water factor and temperature factor affect the change of forest ecosystem LUE in the form of changing VPD more often.The linear relationship between CUE and environmental factors is difficult to be explained by mean temperature over a long period of time.In contrast,the relationship between CUE and environmental factors will gradually change to a strong positive correlation through the significant influence of annual rainfall on net primary productivity.(2)An ensemble model was developed and established for forest ecosystem LUE and CUE driven by environmental factors.By comparing and evaluating three common-used machine learning algorithms,it was verified that random forest approach was the best machine learning algorithm for simulating carbon fluxes at various sites on an hourly scale,with r2 values of 0.88 and 0.93 for net ecosystem exchange(NEE)and GPP fitting tests,respectively.The performance parameters of the weak learner models established by using individual site observation data were also at a higher level than those of similar studies.The error brought by the two steps of integrated output was much smaller than the error of each weak learner itself.The r2 of NEE and GPP during the site integration process both reached 0.97,and the r2 between different sites exceeded 0.99.The uncertainty of the entire integrated algorithm model did not increase significantly due to the integrated output,and the final simulated values of LUE and CUE had high accuracy,with r2 values of 0.98 and 0.96,respectively.The ensemble model proposed in this study improved the integration strategy while improving the ability of weak learners to predict nonlinear problems.It was guaranteed that each simulated value was supported by a specified number of weak learner results.At the same time,the simulation results of each weak learner were supported by corresponding observation data ensuring that each simulated value had a specified number of weak learner results as support,and each simulated result of weak learners had corresponding observed data as support as well.(3)The responses of forest ecosystem LUE and CUE to the changes in environmental factors were quantitatively analyzed by using the integrated algorithm model.Overall,the net ecosystem productivity and light response parameters were positively responded to temperature factors,and parabolically responded to water factors.The apparent quantum yield,α,was not sensitive to atmosphere temperature(TA)when TA was within 0～15 ℃.NEE and light response parameters were more sensitive to soil temperature(TS)than TA.But with the increase of TS,the impact of biological factors on photosynthetic parameters gradually exceeded that of environmental factors,resulting in a trend of gradual decoupling between photosynthetic parameters and TS at a large scale.At the global scale,the responses ofα,maximum photosynthetic rate(Pmax)and daytime respiration rate(Rd)to VPD were alternated when VPD reached approximately 1 hPa、6.5 hPa and 10 hPa,respectively.Additionally,the responses of these three parameters to soil water content(SWC)changed when the SWC reached about 27%、26%and 26%.The response of CUE in forest ecosystems to temperature factors was more complex on the global scale.The relationships between CUE and temperature factors were obviously altered twice when TA reached-10℃ or TS reached 3℃,and when TA reached 9℃ or TS reached 13℃,respectively.Most of the responses of CUE to the increase of VPD were monotonous negative responses.Although the positive responses of CUE to VPD existed objectively,the positive response was difficult to be observed and captured in the field due to the limitation of observation conditions.However,the parabolic relationship between CUE and SWC was relatively easy to observe,mainly because the threshold for variation of CUE’s response to SWC was usually high.By comparing the ratio of the Rd and Pmax functions,it was found that drought stress in atmospheric and waterlogging stress in soil were the two most critical abiotic stresses affecting the ability of forest ecosystems to sequester carbon through the water factor.(4)The hypothesis that heterogeneity of LUE and CUE coupling in forest ecosystems is driven by environmental factors was proposed in the present study.This hypothesis was supported by analyzing the responses of LUE and CUE to the changes in environmental factors alone or in combination.Globally,the coupling relationships between LUE and CUE in different sites could be divided into two types.One tended to have a positive correlation between LUE and CUE,with both CUE and carbon sequestration capacity relatively low.The other tended to a negative correlation between LUE and CUE,with both CUE and carbon sequestration capacity relatively high.The main reason for this pattern was the change in the relationship between CUE and photosynthetic parameters,and its primary driving force came from the change in CUE’s response to TA in the past 30 years.Based on the conclusions drawn in this study,it is inferred that as the climate warms,the relationship between CUE and LUE in forest ecosystems with lower temperatures will gradually shift from a positive correlation to a negative correlation due to the increasing TS.This marks the end of the significant increase in carbon sequestration potential of the forest ecosystem due to photosynthesis.On the other hand,when forest ecosystems are subjected to drought stress in air or flooding stress in soil,the positive correlation between CUE and LUE may shift due to water factor driving,and both may decline simultaneously.This indicates that the productivity of this ecosystem will probably decrease significantly.In conclusion,this research comprehensively elucidates the nonlinear response patterns of forest ecosystems’ CUE and LUE to different environmental factors,and reveals the potential connection between CUE and LUE.The results of this study provide a scientific reference to better understanding of the mechanisms in regulating carbon sequestration,productivity formation,CUE and LUE and their relationship with environmental factors in global forest ecosystems.This study also provides a foundation for further research on forest carbon sequestration function under the background of climate change.Meanwhile,the correlation between LUE and CUE can be further investigated as a new characteristic to represent the potential for carbon sequestration and the productivity of forest ecosystem is influenced by environmental factors.