| In recent years,climate change and a series of ecological problems brought by it have attracted great attention.Climate change has a great impact on vegetation distribution and terrestrial carbon balance.Accordingly,ecological feedback from terrestrial ecosystems will greatly accelerate climate change in the twenty-first century.In terrestrial ecosystems,the water cycle and carbon cycle are the core of material and energy exchange in the earth's surface system,and the water-carbon exchange between soil-plant-atmosphere is critical to the water balance and carbon balance of the ecosystem.Water use efficiency which defined as the ratio of gross primary productivity and evapotranspiration in this study is an important indicator to deeply understand the coupling of water-carbon cycles in ecosystems and can reflect the response strategies of vegetation to global climate change.As a sensitive region in response to climate change,the forest and grassland ecosystems of the alpine subalpine area of southwestern China play an important role in water conservation and maintaining the ecological environment of this area.Estimating the spatio-temporal characteristics of the WUE of vegetation and its relationships with enviromental factors will be helpful to understand response characteristics of water and carbon processes of ecosystem to climate change.We studied the spatio-temporal characteristics of GPP,ET and WUE of vegetation and examined the relationships between WUE with temperature,precipitation and altitude in the alpine and subalpine area of southwestern China during the period of 2000-2014.Preliminary conclusions were drawn as follows:(1)The range of GPP was 0.2?2701.4 gC·m–2,the mean annual GPP was 553.3 gC·m–2,and the GPP of whole area had an upward trend in the past 15 years.The GPP showed a parttern of higher in the southeast region but lower in the northwest region.Among different vegetation types,the mean annal GPP of evergreen broadleleaf forest was the highest,with a value of 1541.7 gC·m–2,the mean annal GPP of grassland was the lowest,with a value of303.6 gC·m–2.The range of ET was 13.3?1275.5 mm,the mean annual ET was 583.5 mm,and the GPP of whole area had an downward trend in the past 15 years.The ET showed a pattern of higher in the east and southwest region but lower in the mid and northwest region.Among different vegetation types,The mean annual ET of evergreen broadleaf forest was the highest,with a value of 880.1mm,and the mean annual ET of closed shrublands was the lowest,with a value of 511.1mm.(2)The range of WUE was 0.01?3.96 g·m–2·mm–1,and the mean annual WUE was 0.95g·m–2·mm–1.Area of WUE with an upward and downward trend accounted for 85.84%and14.16%of the whole study area,respectively,of which the area of WUE with a significant increase accounted for 27.79%.Among different vegetation types,the mean annal WUE of evergreen needleleaf forest was the highest,with a value of 1.77 g·m–2·mm–1,and the mean annal WUE of grassland was the lowest,with a value of 0.56 g·m–2·mm–1,which was significantly lower than other vegetation types.In the past 15 years,the WUE of all vegetation types had an upward trend,indicating that the vegetation in this area had better adaptability to the current climate change.(3)The area with negative correlation coefficient and positive correlation coefficient between WUE and precipitation accounted for 92.54%and 7.46%of the whole study area,respectively,and the area that passed the significance test accounted for 43.56%and 0.21%.The correlation between WUE and precipitation of different vegetation types were both negative.Except evergreen broadleaf forest,deciduous broadleaf forest,and mixed forest,WUE of other vegetation types were significantly negatively correlated with precipitation,the correlation which was-0.732 between WUE woody savannas and precipitation was the highest,and correlation which was-0.192 between WUE of evergreen broadleaf forest was the lowest.(4)The area with positive and negative correlation coefficients between WUE and temperature accounted for 89.56%and 10.41%of the whole study area,and the area that passed the significance test accounted for 48.51%and 0.95%.The correlation between WUE of all vegetation types and temperature were both positive.Expect deciduous broadleaf forest and cropland,the WUE of other vegetation types had significant positive correlation with temperature,the correlation which was 0.815 between WUE of grassland and temperature was the highest,and the correlation which was 0.254 between WUE of evergreen broadleaf forest and temperature was the lowest.(5)The altitude of WUE's high-value area of different vegetation types were different.Closed shrubs had a higher WUE between the altitude of 3400 to 3600 meters,and the WUE of other vegetation types have higher WUE between he altitude of 1000 to 2400 meters,which indicated that this range of altitude was probably the most suitable area for plant physiological in the alpine and subaloine area of southwestern China.We found that the stability of WUE of evergreen needleleaf forest between altitude of 1200 to 4400 meters was relatively high,which showed wide altitude adaptability and toughness.This may indicate that as the regional climax vegetation type in southwestern China,the evergreen needleleaf forest's WUE has high adaptation abilities to both altitudinal change and climate change.Under the background of climate change,the temperature and precipitation in the alpine and subalpine area of southwestern China showed an upward and downward trend,and the carbon sequestration capacity of vegetation has been enhanced,and its WUE has increased significantly,indicating that the vegetation in this region had a better coping strategy for the current climate change. |
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