A Study On Photosynthetic And Physiological Characteristics And Carbon Fixation Capacity Of Black Locust (Robinia Pseudoacacia L.)

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.