不同退化程度小叶杨人工林冠层气孔导度特征及其环境响应

Characteristics of canopy stomatal conductance of Populus simonii stands with different degradation degrees and its responses to environmental factors

冠层气孔导度(g_(s))是衡量冠层-大气水汽交换的重要指标,研究其特征及对环境因子的响应,有助于深入了解冠层水汽交换的过程及环境因子对林木冠层控制的综合机制,目前针对不同退化程度的同一树种冠层气孔导度特征及对环境因子响应的差异性仍不清楚。该研究于2019年生长季(5-9月)以坝上张北县4种退化程度(无退化、轻度退化、中度退化、重度退化)小叶杨(Populus simonii)人工林为研究对象,利用热扩散技术对林中小叶杨树干液流进行连续监测,同步监测气温(T)、光合有效辐射(PAR)、饱和水汽压差(VPD)、空气相对湿度(RH)、土壤含水率(SWC)等环境因子,分析小叶杨的液流速率、g_(s)特征及其对环境因子的响应。结果显示:(1)小叶杨液流速率日变化总体呈单峰曲线,g_(s)呈双峰曲线,月尺度上两者均呈先升高后降低趋势,7月两者值最大,在5月和9月4种退化程度人工林中小叶杨液流速率及g_(s)均差异显著。(2)4种退化程度人工林中小叶杨g_(s)均受T、VPD及SWC的影响,但由于退化有一定的差异性,无退化人工林中小叶杨g_(s)主要受T、VPD和80-160 cm SWC影响,轻度退化人工林中小叶杨g_(s)主要受T、0-80 cm SWC和VPD影响,中度退化人工林中小叶杨g_(s)主要受0-80 cm SWC、T、VPD影响,重度退化人工林中小叶杨g_(s)主要受T、PAR、VPD的影响。(3)对于4种退化程度人工林中小叶杨,T在18℃以上时g_(s)与其呈正相关关系,VPD在0.6-2.2 kPa时g_(s)与其呈正相关关系,g_(s)与SWC呈负相关关系;当PAR小于250μmol·m^(-2)·s^(-1)时,重度退化小叶杨g_(s)与其呈正相关关系。(4)重度退化程度人工林中小叶杨与另3种退化程度人工林中小叶杨相比,对T变化的响应更积极,对其他环境因子响应的敏感性降低,且因形态学上的退化,其对冠层蒸腾的控制会更加严格,以避免进一步退化或死亡。

Aims Canopy stomatal conductance(g_(s))is an important indicator for measuring canopy atmospheric water vapor exchange.Studying its characteristics and responses to environmental factors could enhance the understanding of canopy water vapor exchange process and the comprehensive mechanism of environmental factors controlling forest canopy.Currently,differences in canopy stomatal conductance characteristics and its responses to environmental factors of the same tree species with different degradation degrees are still unclear.Methods In the growing-season of 2019(May-September),four types of Populus simonii stands with different degrees of degradation(non-degraded,mildly degraded,moderately degraded and severely degraded)in Zhangbei County of Bashang Plateau were selected,and thermal diffusion technology(TDP)probes were set to monitor trunk sap flow continuously.Environmental factors such as photosynthetically active radiation(PAR),vapor pressure deficit(VPD),air temperature(T),relative air humidity(RH)and soil water content(SWC)were monitored synchronously to analyze the sap flow rate and canopy stomatal conductance characteristics of P.simonii and their responses to environmental factors.Important findings(1)The daily variation of sap flow rate in P.simonii showed a single-peak curve,while the g_(s)showed a double-peak curve.On the monthly scale,both sap flow rate and g_(s)showed a trend of increasing at first and declining subsequently,and then reaching the highest values in July.The differences in sap flow rate and g_(s)of P.simonii among the four stands of degradation degrees were significant in May and September.(2)The g_(s)was all affected by T,VPD and SWC.However,due to the differences in degradation,g_(s)in the non-degraded stands was mainly affected by T,VPD and 80-160 cm SWC.In the mildly degraded stands g_(s)was mainly affected by T,0-80 cm SWC and VPD,and that in the moderately degraded was mainly affected by 0-80 cm SWC,T,VPD,while it in the severely degraded was mainly affected by T,PAR and VPD.(3)For the all P.simonii of different degradation degrees,the g_(s)above 18℃was positively correlated with T.At 0.6-2.2 kPa,it was positively correlated with VPD,while negatively correlated with SWC.The g_(s)of the severely degraded P.simonii was positively correlated PAR when it was below 250μmol·m^(-2)·s^(-1).(4)Compared to the other three types of P.simonii,the severely degraded one had a more positive response to T changes and a lower sensitivity to other environmental factors.Due to morphological degradation,control of canopy transpiration of the severely degraded P.simonii will be even stricter to avoid further degradation or dieback.