Photosynthesis And Water Use Of Several Common Tree Species In The Dongting Lake Basin

Forest ecosystems play an important role in water cycle and carbon balance. Their performance depends on various environmental conditions. Understanding of photosynthesis, plant water use of forest tree species and their responses to environmental variables is required to predict how forest ecosystems respond to climate variability and change. In this study, we investigated photosynthesis and water use behaviors for four selected common native tree species: Pinus massoniana of a conifer species, Cinnamomum camphora and Osmanthus fragrans of evergreen broadleaf species, Liquidambar formosana of a deciduous broadleaf species. Severaleco-hydrological variables were measured on various tree species over three years using LI-6400 XT, heat-pulse sap flow sensors, and thermocouple stem psychrometers.The measurements of microclimatic variables(solar radiation, rainfall, air temperature, relative humidity and wind speed) were also conducted. The aims of this study were to investigate variation of photosynthesis, plant water use of forest tree species and their responses to different environmental conditions(including drought stress).The mainly results are summarized as follows:(1) Overall, C. camphora is more sensitive to a change of environmental conditions(e.g., vapor pressure deficit and root zone water potential) in a drought than O. fragrans. O. fragrans appears to be more tolerance to drought, while C. camphora develops an effective survival mechanism in drought via leaf shedding and dieback of shoots. During the in-drought period, transpiration rate(Ec) of the two species declined with an enhanced VPD and reduced stem water potential. Especially when VPD> 1.75 k Pa, both Ec of the two species showed a significant reduction. But Ec of C. camphora declined dramatically in the drought. C. camphora and O. fragrans both appear to be anisohydric, but to a different degree in response to the drought.(2) At the leaf scale, net photosynthetic rate(Pn), transpiration rate(Tr) and leaf stomatal conductance(gc) for C. camphora and O. fragrans decrease with an increase in soil water deficit and leaf vapor pressure deficit(VPDL) during a drought. But the degree of this response to the drought differs between the two species. Pn, Tr and gc of O. fragrans remain relatively constant at a high VPDL(4 < VPDL <4.9 k Pa). This indicates that O. fragrans has a strong ability to adapt to changes in environment. Photosynthesis for L. formosana is not sensitive to a change in VPDL(1.5 < VPDL <4.5 k Pa).(3) Daytime variation of Pn, Tr and gc of sunlit leaves for C. camphora, O. fragrans and L. formosana are all higher than shaded leaves, while intercellular CO2 concentration(Ci) of sunlit leaves were all lower than shaded leaves. Midday depression of Pn is observed for sunlit leaves, but not on shaded leaves.(4) During leaf senescence of L. formosana, the light sensitivity of Pn, Tr, gc and water use efficiency(WUE) decreases gradually. This indicates that the photosynthetic ability of L. formosana become weaker during leaf senescence. What’s more, the light response curves of L. formosana were fitted with non-rectangle hyperbolae model, which can well fit the simulation of photo response process. The non-rectangle hyperbolae model simulated maximum photosynthetic rate(Pmax) and dark respiratory rates(Rd) how a clear decreasing trend during the leaf senescence. But Pmax decreases faster than Rd during leaf senescence.(5) Nighttime sap flow is found in all tree species: C. camphora, O. fragrans, P.massoniana, and L. formosana. Nighttime sap velocity(SFn), from nightfall to midnight, is higher than that after the midnight. For different tree species, L. formosana show the highest SFn, followed by P.massoniana and C. camphora, O. fragrans has the lowest SFn. Maximum SFn of different speices all occurs in summer, while SFn in winter is very unconspicuous.(6) Nighttime sap flow of the four species shows a remarkable relationship with nocturnal VPD. In normal years(without droughts), SFn positively responds to VPD for the four tree species. In drought condition, SFn of O. fragrans is still sensitive to VPD, while SFnof P.massoniana and C. camphora reduces with an enhanced VPD. During the post-drought period, P.massoniana and O. fragrans recovere their nighttime sap flow, but for C. camphora, SFn becomes very small.(7) Canopy stomatal conductance(Gc) is modeled following the Jarvis-Stewart approach. An optimization modeldete rmined by comparing the 16 Gc models of different combination of response functions of each environmental factor. The results indicate that thismethod is useful in determining the appropriate response function for each environmental factor in order to optimize the Gc model.The optimized model for C. camphora, O. fragrans, P.massoniana, and L. formosana show a significantly improved estimation of Gc.