Effects Of Climate Change On The Net Primary Productivity Of Wetlands In Sanjiang Plain

Wetland ecosystem is one of the major carbon sink, playing important roles in global climate change. Meanwhile, wetland ecosystem is also very sensitive to climate change. In this study, we selected wetland ecosystem in the Sanjiang Plain, the most drastic region of climate change in northeastern China, to estimate the effect of climate change on the net primary productivity(NPP) of wetland ecosystem and forecast its possible effect and risk in the next 40 years. The methods include analyzing the response of wetland ecosystem to climate change in the past 50 years in this region, and also simulating vegetation NPP of wetland habitat.During 1961~2010, the characteristics of climate in the Sanjiang Plain showed that air temperature continuously increased, precipitation and evaporation slightly increase and presented warming trend. In the hydrothermal indexes, both warmth index and coldness index were increasing trends; temperature index and drought index had no obvious trend in long-term scale, but greatly fluctuated; days of continuous drought were slightly decreased, but that days in summer increased. The years of maximum and minimum average temperature in the Sanjiang Plain occur same to that in whole northeastern China, but the increasing extent of temperature in the Sanjiang Plain was higher than the average of northeastern China.The warming tend will kept continuously in the next 40 years. The average air temperature under the scenario of RCP2.6, RCP4.5, RCP6.0 and RCP8.5 were 3.84, 3.89, 3.72 and 4.12℃, which are higher than average air temperature in the past 50 years(3.21℃). The ratio of increasing temperatures were 0.35,0.47,0.32 and 0.63 ℃/10 a, with the small ratio in RCP6.0 and the largest ratio in RCP4.5 and RCP8.5, and also the highest fluctuation of air temperature.Under four climate change scenarios, precipitation and varporation presented slight increasing trend; warth index and coldness index continously increased; days of index in summer were longer; the range of the growing season was longer; average humidity index slightly higher than reference period, but had a decreasing trend; the continous drought days were lower than that in the past 50 years. The cliamte in the Sanjiang Plain showed the similar trend in the past years. Among four climate change scenarios, the change in RCP8.5 was highest and that in RCP2.6 are lowest.Using BIOME-BGC model and the localization and optimization, the results showed that the relationships between simulated NPP of Calamagrostis Aangustifolia wetland and Sedge wetland and observed data were 0.87 and 0.72. This means the BIOME-BGC is good applicable to simulate the change of wetland NPP in the Sanjiang Plain.The average annual NPP during 1961~2010 in the Sanjiang Plain was 326.8 g C m-2 a-1, varying from 281.2 to 365.6 g C m-2 a-1 with slight increasing trend. This indicates the warming and drying trend promoted the NPP. In spatial distribution, the difference in regional spatiality became large in different periods. High NPP regions increased and low NPP regions decreased, implying climate change could threat the regions with low wetland NPP. There were obvious differences among four wetland vegetations, with the orders of Reed wetland(395.0 g C m-2 a-1), Calamagrostis Angustifolia-Reed wetland(352.2 g C m-2 a-1), Calamagrostis Angustifolia-Sedge wetland(304.1 g C m-2 a-1) and Sedge wetland(255.9 g C m-2 a-1). Although the lowest in Sedge wetland, the fluctuation was largest, indicating the largest influence of climate change in Sedge wetland.In the future climate change scenarios, the simulated wetland NPP during 2011~2050 in the Sanjiang Plain was 329.4 g C m-2 a-1, similar with the referenced period(1981~2010), but the fluctuations intensified. The change of wetland NPP was most dramatic under the RCP8.5, with the range of 283.7~379.4 g C m-2 a-1 and standard deviation of 25.6; the following was RCP2.6, with the range of 293.4~378.2 g C m-2 a-1 and standard deviation of 20.8; the wetland NPP in RCP4.5 and RCP6.0 kept stable and fluctuation range was similar to referenced period.In the future climate change scenarios, the spatial distribution of wetland NPP in the Sanjiang Plain was consistent with that in the past 50 years, showing the low NPP in the west, east, and centre region and hihger in the east edge and west edge of the centre region. The NPP in most regions had increasing trend, with lowest change in regional NPP occured in RCP2.6and RCP6.0, and regional increasing trend occurred in RCP4.5and RCP8.5.Among four wetland types, the most dramatic fluctuation was Sedge wetland, with the range of 201.8~330.2 g C m-2 a-1 in the RCP8.5. The standard deviation was 32.8 in Sedge wetland, significantly higher than other scenarios and wetland types; the second is reed wetland, with standard deviation of 29.7. The NPPs in other two wetland types were stable.In the future climate change scenarios, there was no obvious change of wetland NPP as compared to referenced period, but inter-annual variations was large with high risk classes in some years. The risk in RCP2.6 was lowest and that in RCP8.5 was highest. In the next 40 years, 9 years were estimated to face high risk and may happen after 2040. In the spatial distribution, the high risk mainly occurred in central and north central regions(i.e., Wai Qi Xing He and eastern Xing Kai Hu). The spatial areas with high risk classes were estimated under RCP8.5, followed by RCP4.5; there was risk only in easte rn Xing Kai Hu under RCP2.6. Among all scenarios, the risks in Calamagrostis Angustifolia-Reed and Calamagrostis Angustifolia- Sedge wetland were higher than others and the lower risks in reed and Sedge wetlands.All multiple regression analyses showed the change wetland ecosystem will be mainly affected by water factor, and temperature-induced influence is relatively weak in the future. Among all climate factors, the amount of evaporation and drought index are two most important factors to affect wetland NPP, contributing 62% of NPP changes based on stepwise regression analyses. In addition, precipitation amount and humidity index also had large effect and temperature-related factors are not important for wetland NPP. With the promulgation and implementation of National wetland protection regulations and the strict control of protection redlined, climate change are suggested to the major driver in the change of wetland ecosystem in the Sanjiang Plian.