【作者】 郑元；

【导师】 赵忠；

【作者基本信息】 西北农林科技大学 ， 森林培育， 2010， 博士

【摘要】 当前,全球性环境问题,特别是气候变化与温室效应问题的形势已经十分严峻,受到国内外学者的广泛关注。森林能够通过光合作用吸收固定大气中的CO2,在减缓气候变暖过程中具有重要的独特作用。黄土高原生态环境脆弱,是全球变化响应的敏感区域之一。刺槐人工林在渭北黄土高原的植被恢复和光合固碳方面都起到了关键作用。开展刺槐光合生理特征与固碳能力的研究,可以为评估渭北地区林分光合生产力提供科学依据,同时,也对该区刺槐林的科学经营有重要的指导意义。本研究以地处渭北地区的永寿县马莲滩流域刺槐林为研究对象,于2008年8月和2009年6月、8月、10月,采用LI-6400便携式光合作用系统和WinScanopy冠层分析仪,系统研究了冠层部位、环境参数、天气条件、坡向、林龄、林分密度和疏伐等因子对刺槐林光合生理特征和固碳能力的影响。主要研究结果如下:(1)刺槐树冠光合作用的空间异质性刺槐树冠不同层次的光合作用差异性显著,净光合速率、蒸腾速率、气孔导度、羧化效率、光合有效辐射表现为:上>中>下;而水汽压亏缺、气孔限制值、空气温度表现为:下>中>上。对于阳坡刺槐,水汽压亏缺、空气温度、气孔导度、蒸腾速率是影响不同层次净光合速率的主要因子;对于阴坡刺槐,水汽压亏缺、蒸腾速率、光合有效辐射是影响不同层次净光合速率的主要因子。光合作用在刺槐树冠的不同方位没有显著差异,所有光合生理生态参数变化均很小,蒸腾速率、气孔限制值、光合有效辐射、空气温度是影响不同方位净光合速率的主要因子。阳坡刺槐树冠中层西方和阴坡中层东方的日总光合速率值,可以分别代表阳坡和阴坡刺槐整个冠层的日总光合速率。(2)刺槐光合气体交换与环境因子的关系刺槐净光合速率的日变化为单峰曲线,在10:00~14:00,刺槐净光合速率的降低主要由气孔限制引起,而14:00~18:00,净光合速率的降低由非气孔限制引起。蒸腾速率、气孔导度与净光合速率的日动态趋势相似。在刺槐叶片进行气体交换的日进程中,环境因子对刺槐不同光合生理参数的影响程度不尽相同。总体来看,光合有效辐射、空气温度和空气相对湿度是影响刺槐光合气体交换的主要环境因子。此外,分别建立了净光合速率、蒸腾速率、气孔导度与环境因子的最优回归方程。(3)刺槐光合作用在晴天和阴天的差异无论在6月还是8月,晴天刺槐的净光合速率和气孔导度在10:00以前高于阴天,而在12:00和14:00低于阴天。刺槐的胞间CO2浓度、空气CO2浓度、空气相对湿度、光能利用率在晴天的绝大多数时间显著低于阴天,而蒸腾速率、水汽压亏缺、光合有效辐射、空气温度在晴天的全天都显著高于阴天。在晴天条件下,虽然刺槐的气孔导度、胞间CO2浓度、空气CO2浓度、空气相对湿度、光能利用率的日均值比阴天低,但净光合速率、蒸腾速率、水汽压亏缺、光合有效辐射、空气温度的日均值比阴天高,从而能够积累更多的日均光合固碳量。在阴天条件下,刺槐的最大光合速率高于晴天,且暗呼吸速率和光补偿点显著低于晴天,表现出很强的光合适应能力。(4)坡向和林龄对刺槐光合生理特征的影响成熟刺槐(18 a)在阴坡(西北坡向)的最大光合速率、最大羧化效率、表观量子产率、表观羧化效率、暗呼吸速率、光呼吸速率、Rubisco含量、光饱和点显著高于阳坡(东南坡向),而最大电子传递速率与羧化效率的比值、光补偿点、CO2饱和点显著低于阳坡。刺槐的蒸腾速率和水分利用效率对光合有效辐射和CO2浓度的响应在不同坡向之间表现出很大的差异。幼龄刺槐(6 a)的最大光合速率、最大电子传递速率、最大电子传递速率与羧化效率的比值、暗呼吸速率、光补偿点、CO2补偿点、CO2饱和点、蒸腾速率显著高于成熟刺槐,而表观量子产率、水分利用效率显著低于成熟刺槐。分析认为,阳坡和阴坡上的成熟和幼龄刺槐都能对渭北地区脆弱的生境表现出有效的光合生理适应性。(5)坡向和林龄对刺槐林光合固碳能力的影响阳坡刺槐林比阴坡具有更低的日均蒸腾速率、水汽压亏缺、羧化效率、净光合速率、叶面积指数、光合固碳能力;幼龄林比成熟林具有更高的日均气孔导度、蒸腾速率、胞间CO2浓度与空气CO2浓度比值、羧化效率、净光合速率、光合固碳能力。阳坡刺槐林更低的日均净光合速率和光合固碳能力可能是由于阳坡与阴坡之间微气象条件的变化导致的。幼龄林更高的日均净光合速率和光合固碳能力可能受林龄相关的树体尺寸差异的影响。(6)林分密度和疏伐对刺槐林光合固碳能力的影响低密度刺槐林的光合固碳能力显著高于高密度林分,主要归因于前者林分内部更加适宜的营养空间和生态环境。在对刺槐林进行疏伐后的第1年,低强度的疏伐(伐除1/3林木)可以增强刺槐林的光合固碳能力,而高强度的疏伐(伐除1/2林木)反而降低了刺槐林的光合固碳能力,这可能与疏伐相关的叶面积指数和郁闭度的变化有关。研究认为,通过对黄土高原上的刺槐林进行合理的经营管理,它们将为中国的碳固定做出积极的贡献。更多还原

【Abstract】 Nowadays, global environmental problems, especially the global climatic change and greenhouse effect problems have been more and more serious. Forest can absorb and fix atmospheric CO2 through tree photosynthesis, as a result forest has critical and special effect to slow up climatic warming. Loess Plateau is one of the most sensitive regions to global change, which is due to its fragile ecological environment. Black locust (Robinia pseudoacacia L.) is a key tree species not only for the vegetation rehabilitation but also for the photosynthetic carbon dynamics on the Loess Plateau. Hence, a study on photosynthetic characteristics and carbon fixation capacity of black locust can provide information for the evaluation of forest photosynthetic productivity in Weibei area, and also has important significance for the forest management of black locust plantations.This study was done to investigate the effects of canopy positions, environmental parameters, climatic conditions, slope aspects, stand ages, stand densities and thinning intensities on the photosynthetic and physiological characteristics and carbon fixation capacities of black locust plantations at the Maliantan valley of Yongshou county in Weibei area in August 2008 and June, August, October 2009, respectively, by using the LI-6400 portable gas exchange system and WinScanopy canopy analyzer. The main results are as follows:(1) Spatial heterogeneity of canopy photosynthesis in black locust plantationsPhotosynthetic parameters at different canopy levels of black locust plantations have showed significantly different values. Irrespective of slope aspects, values for net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), carboxylation efficiency (CE), and photosynthetic active radiation (PAR) follow the pattern: the upper canopy is more than the middle canopy which is more than the lower canopy. In contrast, values for vapor pressure deficit (VPD), stomatal limitation (Ls), and air temperature (Ta) show the pattern: the lower canopy is more than the middle canopy which is more than the upper canopy. The contributions of main photosynthetic parameters to Pn at different canopy levels or orientations of black locust plantations were determined using path analysis. For the black locust plantation on the sunny slope, VPD, Ta, Gs, Tr are the main factors to impact Pn at different canopy levels. For the black locust plantation on the shady slope, VPD, Tr, PAR are the main factors to impact Pn at different canopy levels. No marked difference was observed in photosynthesis at different orientations of black locust canopies both on sunny and shady slopes. Values of most photosynthetic parameters change little and Tr, Ls, PAR, Ta are the main factors to influence Pn at different orientations of the canopies. The daily total Pn of black locust plantation at the west middle canopy level on the sunny slope and at the east middle level on the shady slope can represent the daily total Pn of the whole canopies on the sunny and shady slopes, respectively.(2) Relationships between photosynthetic gas exchange of black locust and environmental factorsDiurnal dynamics of the net photosynthetic rate (Pn) show a single peak curve, the decline of Pn at 10: 00-14: 00 is associated with stomatal limitation, while the decline of Pn at 14: 00-18: 00 is associated with non-stomatal limitation. The trends of dynamics of transpiration rate (Tr) and stomatal conductance (Gs) are similar with Pn. In the diurnal dynamics of gas exchange, the effects of environmental factors on photosynthetic and physiological parameters of black locust are different. In conclusion, the photosynthetic active radiation (PAR), air temperature (Ta) and relative humidity (RH) are main environmental factors to influence photosynthetic gas exchange of black locust. Moreover, the optimal regression equation among environmental factors and Pn, Tr, Gs is built, respectively.(3) Differences in photosynthesis of black locust under sunny and cloudy daysWhether in June or August, the net photosynthetic rate (Pn) and stomatal conductance (Gs) of black locust under the sunny days are higher than those under the cloudy days before 10: 00, while both of them are lower at 12: 00 and 14: 00. The intercellular CO2 concentration (Ci), ambient CO2 concentration (Ca), relative humidity (RH), light use efficiency (LUE) across most times of the day are significantly lower under the sunny days than those under the cloudy days. In contrast, the transpiration rate (Tr), vapor pressure deficit (VPD), photosynthetic active radiation (PAR), air temperature (Ta) in the whole day are markedly higher under the sunny days compared to those under the cloudy days. Under the sunny day conditions, although black locust has lower daily mean values of Gs, Ci, Ca, RH, LUE, they show higher daily mean values of Pn, Tr, VPD, Ta and can in turn accumulate more average daily photosynthetic carbon fixation amounts. Under the cloudy day conditions, black locust presents the greater maximum photosynthetic rate (Pmax) and the pronouncedly lower dark respiration rate (Rd) and light compensation point (LCP), which indicates that black locust has a strong photosynthetic acclimation capacity.(4) Effects of slope aspect and stand age on the photosynthetic and physiological characteristics of black locust Mature black locusts (18-year-old) on the shady slopes (northwest-facing) present significantly higher maximum photosynthetic rate (Pmax), maximum carboxylation efficiency (Vcmax), apparent quantum yield (AQY), apparent carboxylation efficiency (ACE), dark respiration rate (Rd), light respiration rate (Rp), Rubisco (rubisco content), light saturation point (LSP) but lower ratio of Jmax to Vcmax (Jmax/Vcmax), light compensation point (LCP), CO2 saturation point (CSP) than those on the sunny slopes (southeast-facing). The responses of transpiration rate (Tr) and water use efficiency (WUE) to photosynthetic active radiation (PAR) and intercellular CO2 concentration (Ci) show considerable discrepancies at different slope aspects. Juvenile black locusts (6-year-old) have significantly greater Pmax, Jmax, Jmax/Vcmax, Rd, LCP, CCP, CSP, Tr but lower AQY, WUE compared to mature trees. It is concluded that the mature and/or juvenile black locusts on the sunny and shady slopes perform various effective acclimations of photosynthetic physiology to different slope conditions in Weibei area.(5) Influences of slope aspect and stand age on the photosynthetic carbon fixation capacities of black locust plantationsMature plantations on the sunny slopes have lower average daily transpiration rate (Tr), vapor pressure deficit (VPD), carboxylation efficiency (CE), net photosynthetic rate (Pn), leaf area index (LAI) and photosynthetic carbon fixation capacity (PCFC) than those on the shady slopes. Juvenile plantations have higher average daily (Gs), transpiration rate (Tr), ratio of intercellular to ambient CO2 concentrations (Ci/Ca), carboxylation efficiency (CE), net photosynthetic rate (Pn) and photosynthetic carbon fixation capacity (PCFC) compared to the mature plantations. It is concluded that the lower average daily Pn and PCFC of the mature black locust plantations on the sunny slopes may be due to variations in the microclimatic conditions between sunny and shady slope aspects. The higher average daily Pn and PCFC of the juvenile black locust plantations are likely associated with stand age-related differences in tree sizes.(6) Effects of stand density and thinning on the photosynthetic carbon fixation capacities of black locust plantationsBlack locust plantations at lower stand densities present a significantly greater photosynthetic carbon fixation capacity (PCFC) than those at higher stand densities, mainly due to more suitable nutrition space and environmental condition of the former. In the first year after thinning, low intensity thinning (1/3 trees removed) increases the PCFC in black locust plantations, while high intensity thinning (1/2 trees removed) decreases the PCFC in black locust plantations, which is likely associated with a variation in thinning-related leaf area index (LAI) and/or canopy density. Our findings suggest that black locust plantations on the Loess Plateau may make a marked contribution to the carbon fixation in China if these plantations are well managed.更多还原