Light And Water-Use Efficiency Of Subtropical Evergreen Coniferous Forest In Qianyanzhou Experimental Station Of Jiangxi Province

Light-use efficiency (LUE) is used commonly with remotely sensed and meteorological data to monitor terrestrial ecosystem primary production (NPP). Water use efficiency (WUE) is an important indicator to in-depth understanding the coupling relationship between the ecosystem water and carbon cycles. Study on the different levels of forest LUE and WUE provides the oretical basis for scaling up as well as testing the accuracy of scaling up results. Studying the variation in space and time of LUE and WUE are helpful to obtain a deep understanding of the water and carbon cycles.In this investigation, we use Pinus elliottii, P. massoniana and Cunninghamia lanceolata, the main afforestation tree species in Subtropics of China as the research object. At leave level, during the middle growth stage (in July) and late growth stage (in October), gas change and environment parameter were estimated with Li-6400 portable CO2/H2O infrared gas analysis. The variation in time and space of LUE and WUE in different postion and ages needles was studied. At stand level, the Effects of species and soil characterstics on LUE were studied; at local level, based on the data published in peer-reviewed literature with data collected from nearby weather stations, LUE and WUE of subtropics evergreen coniferous forests were estimated, the spatial patterns were explored as well. The main findings of this study are as follows:(1) At leaf level, LUE and WUE in different depths (the upper and lower), orientations (north-facing and south-facing) and ages (the current year and one year old) needles within canopy scale were caculated based on gas change. Results showed that, the photosynthesis and transpiration of the whole canopy was affected by the interaction of light availability and leaf ages. Forest assimilates carbon from atmosphere by adjusting LUE of leaves to adapt the light environment within canopy; The variation of time and space of WUE in different position and aged needles were studied. Results showed that the WUE of P. elliottii, P. massoniana, C. lanceolata in late growth stage decreased to 81%,60% and 40% when compared with that in middle growth stage. The lower canopy WUE of P. massoniana, C. lanceolata were 1.1-1.2 times of upper canopy, the upper canopy WUE was 80% of lower canopy in C. lanceolata in middle growth stage. In later growth stage, the upper canopy WUE of P. massoniana, C. lanceolata were 77% and 63% of that in lower canopy because photosynthetic carbon assimilation was limited chronically by low light levels in the lower canopy. However, as for C. lanceolata, there was no significant difference in different depths of canopy. Higher WUE in the north facing leaves were noted. No significant differences of WUE in different year old needles were observed.Results from light response curve model showed that, the needles of Maximum photochemical efficiency (Amax), Dark respiratory rate (Rd), Light saturated point (LSP), and Light compensated point (LCP) in upper canopy and current year needles were higher than that in lower canopy of the three species. The needles in weak light could adjust the content of chlorophyll, chlorophyll a/b, soluble protein and Fv/Fm to increase the carbon acquisition. Nevertheless, the Amax in weaker light was still lower than that in strong light.(2) At stand level, the LUE of three forest species was estimated based on biomass, and the impacts of soil nutrient supplying to forest LUE were analyzed and discussed. Results showed that, when LUE was expressed as energy percentage, the LUE of P. elliottii, P. massoniana and C. lanceolata was 0.36%, 0.44% and 0.76%, respectively, whereas those of the three plantations was 0.18,0.20, and 0.34 g·MJ-1 respectively when LUE was expressed as biomass transformed from per unit of energy. LUE of C. lanceolata forest was significantly higher than P. elliottii and P. massoniana forests. Our data did not support the functional convergence hypothesis. Therefore, different tree species should be distinguished to accurately estimate NPP of evergreen coniferous forest by light use efficiency models. LUE was in positive relationship with soil organic matter, but in negative relationship with pH.(3) At local level, variation of time and space of LUE and WUE in subtropics evergreen coniferous forests based on NPP were studied. The results showed that, the average of LUE and WUE was 0.71 g·MJ-1 and 1.12 g·mm-1, respectively. LUE tracked closely with NPP, NPP gradually decreased from southeast of China to northwest of China. However, the WUE performed disciplinarian is not evidence. LUE of P. massoniana and C. lanceolata were higher than that of Abies-Picea, P. tabulaeformis, P. armandii, P. taiwanensis, P. densata, P.yunnanensi, P. khasya, P. taiwanensis, P. densata, P. khasya Cupressus. There are no signifanct differences among the WUE of tree species. In a linear regression, longitude, latitude, and preciptation accounted for approximately 50.5% of the variability in LUE, longitude, latitude, and PAR accounted for approximately 1.4% of the variability in WUE. WUE showed the functional convergence characterstics.Based on the measurement and analysis from different levels, the following results are obtained:at leaf level, LUE and WUE were mainly controlled by PAR; at stand level, species and LUE were controlled mainly by soil organic matter and pH; at local level, LUE was controlled mainly by precipitation. Understanding the different levels on spatiotemporal dynamics of LUE and WUE can provide basis for scaling up and estimating NPP of forest ecosystem.