不同坡位沙棘光合日变化及其主要环境因子

Diurnal changes of photosynthetic characteristics of Hippophae rhamnoides and the relevant environment factors at different slope locations

沙棘是我国干旱半干旱地区重要的生态经济树种,被广泛应用于黄土高原造林活动中。为探明不同坡位沙棘光合特征及影响沙棘光合作用的主要环境因子,在沙棘生长旺盛期分坡位测定光合特性及环境因子日变化进程,并采用通径分析法分析各环境因子对沙棘净光合速率的直接和间接影响。结果表明:①各环境因子之间存在相互作用,光合有效辐射是空气温度变化的主要驱动因子,而空气温度变化引起的空气密度变化对空气相对湿度和CO2浓度有显著影响。在上、中和下坡位环境变化可以分别解释85.6%、86.0%和55.22%净光合速率的变异。②下坡位沙棘净光合速率最小,日均值为9.38μmol.m-.2s-1,上、中坡位分别为14.22μmol.m-.2s-1,15.94μmol.m-.2s-1。③10:00之前中上坡位沙棘水分利用效率明显高于下坡位,但3坡位沙棘水分利用效率日均值无明显差异。④上坡位净光合速率主要受到光合有效辐射、空气相对湿度的影响;中坡位光合有效辐射仍为主要环境因子,此外空气温度和空气湿度之间的相互作用对净光合速率也有一定影响,为主要限制因子;下坡位影响沙棘光合速率主导因素为光合有效辐射。⑤沙棘在10:00之前光合有效辐射和空气湿度较高而空气温度较低的环境下,净光合速率和水分利用效率最高。作为阳生物种,沙棘需要足够的光照维持生长,而以往研究表明适度水分胁迫可以提高沙棘光合过程中对水的利用效率。所选流域年降水量535mm,中上坡位水分胁迫并非十分严重。因此,种植于中上坡位的沙棘即可接受足够光照(特别是10:00以前的光照)进行光合保证生长速率,又能在光合过程中保持较高水分利用效率,符合干旱半干旱地区生态经济发展的需求。

The Loess Plateau has been suffered from severe ecological degradations induced by the harsh environment and intensive human disturbances.Artificial vegetation rehabilitation,widely employed in the arid and semiarid regions,has been considered as an effective approach to restore the degraded ecosystems（e.g.to reduce water loss and soil erosion,land degradation and desertification）.In view of the rigorous natural environment,the adaptability of plant species plays a vital role in the functioning of restored vegetation.Hippophae rhamnoides,an important eco-economic species in arid and semiarid regions,was wildly used in afforestation on the Loess Plateau.Despite its strong adaptability to drought condition and wide distribution,however,the low productivity is still a problem restricting its application.The micro-topography is the most important factor driving the reallocation of water and light,therefore,has a significant effect on the growth of plants.In this study we investigated the growth of Hippophae rhamnoides and distinguished the main environmental factors at different slope locations.Net photosynthesis rate（Pn） was selected as an indicator of growth status.Diurnal changes of photosynthetic and environmental parameters of the species were determined by the Li-Cor 6400 at one-hour interval from 8：00am to 17：00pm in a typical watershed of loess hilly gully region.Direct and indirect effects of environmental factors on Pn were calculated through the path analysis.The results showed that： 1） There were significant interactions among four environmental factors： variation of photosynthetically active radiation（PAR） triggered the fluctuations of air temperature（Ta）.Ta was closely related to the air density,which had significant impacts on the air CO2 concentration（Ca） and air relative humidity（RH）.Environmental factors driven intrinsically by PAR superposed an integrative effect on Pn.The environmental changes at the upper,middle and lower slopes could explain 85.6%,86.0% and 55.22% of the total variation of Pn,respectively.2） Pn at the lower slope was the smallest with a daily average of 9.38μmol·m-2·s-1,and those were 14.22 and 15.94μmol·m-2·s-1 for the upper and middle slopes,respectively.3） Water use efficiency（WUE） of the upper and middle slopes was higher than that of the lower slope before 10：00am,although no significant differences were found in daily mean values.4） At the upper slope,PAR and RH were the main environmental factors affecting Pn.In addition to PAR,interactions between Ta and RH had an important restrictively impact on Pn regardless of their small comprehensive effects at the middle slope.At the lower slope the Pn of Hippophae rhamnoides was constrained mainly by the PAR,and the direct path coefficient was 0.833,which substantially exceeded the direct path coefficients of other environmental factors.5） Pn and WUE were the highest before 10：00 O′clock owing to high PAR,high RH and low Ta at that time.As a heliophyte species,Hippophae rhamnoides requires sufficient radiation to survive.The moderate water deficit was proved to be favorable for its effective use of water during photosynthesis.In this study,water stress at the upper and middle slopes was not supposed to be very severe in the study watershed where the mean annual precipitation is 535mm.The upper and middle slopes were therefore the optimum locations for Hippophae rhamnoides.This ensures Hippophae rhamnoides to make the utmost use of the high PAR and RH before 10：00 O′clock to conduct photosynthesis with high WUE.The results of this study provided scientific foundations for Hippophae rhamnoides plantation.