Spatio-temporal variation in vegetation net primary productivity and its relationship with climatic factors in the Shule River basin from 2001 to 2010
Vegetation net primary productivity (NPP) is the solar energy fixed by vegetation during photosynthesis,which can be described as the net increase in biomass per unit time per unit area.The NPP,which plays an important part in the terrestrial carbon cycle,directly reflects the production capacity of terrestrial vegetation under natural conditions,and determines the ability of ecosystems to store carbon.The Shule River basin,located in northwest Gansu Province,is an important water source and ecological division within the Hexi Corridor.Increasing human populations and regional economic development have resulted in severe environmental problems within the study area,including natural disasters,vegetation degradation,and soil erosion.The study of NPP in the Shule River basin can provide a scientific basis not only for the prevention and control of natural disasters but also for the improvement of the ecological environment.This is important for local sustainable ecological development in the context of global climate change.The NPP in the Shule River basin was calculated using Moderate Resolution Imaging Spectroradiometer (MODIS) images,meteorological data,and vegetation type maps,along with an improved Carnegie-Ames-Stanford Approach (CASA) NPP model.The spatio-temporal characteristics of NPP in the Shule River basin were analyzed for the period 2001-2010,using linear trend analysis,the coefficient of variation,and the Hurst index.The relationship between NPP and climatic factors was investigated.The results showed that:(1) NPP in the Shule River basin had considerable regional differences.NPP gradually decreased from southeast to northwest in the Shule River basin,and presented a noticeable linear pattern in a longitudinal direction.The inter-annual variability showed overall growth from 2001 to 2010.The annual NPP value ranged from 99.97 to 127.2 gC m-2 a-1,with the minimum value occurring in 2001 and the maximum in 2010.The average annual NPP was 125.08 gC m-2 a-1 over the period 2001-2010;(2) however,within this overall increase,the annual NPP across the Shule River basin varied from 2001 to 2010.Of the total area of the basin,25.15％ showed a significant increase in NPP,whereas 11.93％ showed a significant decrease;(3) the Hurst index analysis indicated that the majority of changes in NPP followed the same direction;78.3％ of the entire study area was predicted to follow a continuously increasing trend,while 21.7％ of the total area will decrease in future;and (4) there was a weak relationship between NPP and annual precipitation,and no significant relationship between NPP and average temperature at an annual scale,indicating that precipitation was the key influencing factor.However,at a monthly scale,temperature was the dominant climatic factor affecting NPP in the study region.Analysis of the relationships between NPP,precipitation,and temperature showed that correlations at an annual scale were lower than those at a monthly scale.Precipitation and temperature exerted a variety of influences on different parts of vegetation NPP.The relationship between NPP and climatic factors suggested that NPP displayed no obvious time lag or cumulative effect due to climatic factors.
net primary productivity (NPP)CASA modeltemporal and spatial variationMODISShule River basin
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