Study On Photosynthetic Eco-physiology In The Canopy Of Castanopsis Fargesii In Subtropical Evergreen Broad-leaved Forest

Subtropical evergreen broad-leaved forest (EBLF) is the largest forest vegetation types in distribution area in China, and plays an important role in the world forest vegetation. It is full of great significance to estimate the primary productivity of EBLF and evaluate its contribution to global carbon sequestrations. This has become the priority research field in current global climate change research. The study on spatial distribution of daily dynamics of photosynthesis, respiration at night and photosynthetic characteristics in the forest canopy is very helpful in forest primary productivity estimation and model up-scaling from leaf level to community level and further to landscape level. In this sduty, we select Castanopsis fargesii, one of the dominant species in the EBLF climax community in the Tiantong National Forest Park, to study the plant eco-physiological characteristics in the forest canopy.The field observations of the canopy photosynthetic characteristics were conducted on a 30-meter-height tower using Li-6400 Portable Photosynthesis System in typical weather conditions in March, April, September 2006 and January 2007, respectively. Our results are as followed:(1) The dynamic of daily photosynthesis show the various patterns e.g. bell-shape curve, bimodal curve, or irregular curve in the different position of the canopy.From spring to winter, the seasonal net photosynthetic rate (Pn) in the top of canopy was significantly higher than that of middle and bottom of canopy, and was 1.26-4.49 times higher than that at the bottom and mid-canopy,respectively. but the Pn in the sunlit leaves was higher than that of shaded leaves by 5.3%, 5.1%, 30% and 43.8% in March, April, September 2006 and January 2007, respectively. Temporally, the Pn showed that April>March>September>January, and the Pn in April was 1.60, 2.99 and 3.54 times than that in March, September and January respectively. The spatio-temporal distribution of photosynthesis has significant relationship with micro-environmental and physiological factors in the canopy of C.fargesii. the Pn was extreme significantly correlated with intercellular CO₂ concentration (Ci), stomatal conductance (Gs) and transpiration rate (Tr) (r=0.91, P<0.01), and was also correlated with photosynthetically active radiation (Par), atmospheric temperature (Tair), and vapor pressuredeficit (Vpdl) (r>0.60, P<0.05). Especially the Pn was in napierian logarithm (R²=0.9, P<0.05) relationship with Par, and in quadratic law (R²=0.87. P<0.05) with nitrogen use efficiency (NUE_ph) .(2) The respiration rate at night present the pattern as bell-shape curve, bimodal curve and monotonous curve in the rangeof 0.2576μmolCO₂/m²s to 1.410μmolCO₂/m²s from 19:00 PM to 5:00AM. The respiration rate demonstrate that the pattern of top of canopy >mid-canopy>bottom of canopy, and the mean value decreased by 18.3%, 25.4%, 7.9% and 17.8%. the respiration rate at night of sunlit leaves is higher than that of shading leaves in March and April, but those of September and January are opposite. Temporally, the respiration rate at night demonstrated the pattern of September>April>January>March, and the value declined by 28.0%, 36.8% and 37.3%. The respiration rate at night had the significant spatial and temporal heterogeneity in the canopy of C.fargesii. Respiration rate at night correlated significantly with Ci, atmospheric CO₂ concentrations (Ca) and stomatal conductance (Gs), respectively. The respiration rate was in linear relationship with Ci(R²=0.82. P<0.01) and Ca (R²=0.63. P <0.01).The vertical and horizontal differences in energy transmission, photosynthetically active radiation, and micrometeorological factors in the forest canopy have led to photosynthetic heterogeneity in the canopy of C.fargesii. Photosynthetic indicies such as maximum photosynthetic rate (Amax), light saturation point (LSP) and carboxylation efficiency (CCE) of the shading leaves showed the pattern of the top > the middle > the bottom of the canopy. Mean values of light compensation point (LCP), respiration in light (Rd), and Amax reduced from top to bottom of the canopy in 19.4%, 18.1%, and 37.1%, respectively. LSP and CCE of the sunlit leaves in the canopy demonstrated that the top of canopy>bottom of canopy>mid-canopy pattern. These indexes decreased by 12.3 % in the bottom and by 71.4% in middle. Apparent quantum yield (AQY) of leaves presented the pattern of the bottom>the top>the middle. It is 1.1 and 1.3 times at the bottom of the canopy higher than at the top and middle canopy, respectively. But in horizontal, Amax, LSP and CCE of sunlit leaves at the top and bottom of the canopy are higher than those of the shading leaves in the canopy in the range of 0.9% to 31.5%. In the middle canopy, however values of the six indexes in the shading leaves are higher than those of sunlit leaves in the canopy in the range of 9.6% to63.2%. Apparent quantum yield negatively correlated with leaf nitrogen, content and specific leaf weight, but the left 5 indexes is in positive correlations.