Vegetation Cover Change And Its Response To Different Time Scales Of The Climate Characteristics

The vegetation is important to maintain the ecological environment in the earth, and it isaffected by climate factors significantly. Dynamic monitoring of the vegetation variationcould reflect the trend of climate change to some extent. Vegetation cover change is one of thecores of global environmental change research, but also the front and hot field of modernphysical geography. SPOT AVHRR NDVI (Normalized Difference Vegetation Index) in theSouthwest during the period from1999to2012was selected to get the vegetation coverchange information based on GIS and RS technology. The temporal and spatial vegetationcover change were analyzed by using spatial data covering1999to2012of the Southwest,and explored the spatio-temporal variation of major climatic factors (temperature andprecipitation), discussed the relationship between its and the vegetation. Secondly, theseasonal changes of vegetation and their spatial-temporal patterns were explored, and monitorvariable rules for the growing season of vegetation in the Southwest; in addition, at the sametime to explore its possible causes. It also analysis the six categories of vegetation typesresponse to different time scales of climatic characteristics.The results show that:(1)In the past25years, NDVI vegetation of the Southwest has an entirely rising trend,with the growth of6.9%/10a. This was faster than average of China. There were somesignificant spatial differences of the vegetation cover change. The NDVI of the sevenvegetation types increased significantly; in addition shrubbery has the maximum value ofstandard deviation, and broad-leaved forest has the minimum value of standard deviation.Meanwhile, from1999to2012the vegetation coverage appeared improvement as a whole areabut degeneration in some local areas. The NDVI decreased area covered only about1.18%ofthe whole study area. The decreased area was mainly distributed on the north of HengduanMountains, the east of Sichuan Basin and the central part of Yungui Plateau. While the meanof Hurst indices of NDVI time series in various areas is above0.6, and changes in vegetationcover in most parts of the Southwest would maintain the current trend. Comprehensiveanalysis of variation tendency and persistence, showed that the area of vegetation coverchange to healthy development, but the NDVI decreased areas with strong persistence and theNDVI increased areas with weak persistence might be indicative(5.61%of the total area).Grassland degradation&strong sustainability area is the largest, as well as the improvement of the mixed needle&strong sustainability area.(2)The annual average temperature raised overall in the Southwest, while overall yearaccumulative precipitation and drought indices appeared to decline. In the past30years, theclimate of the Southwest presented a process of distinct warming and drying. Analysis thespace distribution principle of temperature, the annual average temperature raised overall inthe Southwest, Meanwhile west of the Southwest has an entirely rising trend and east wasopposite. Precipitation and drought trend was decreased in South and the North of the area isincreased. There was a significantly positive correlation between NDVI and climatic factors(temperature and precipitation), but correlation between NDVI and precipitation was complexin space. NDVI had a closer correlation with temperature than with precipitation for the area.In addition, no significant correlation between SPEI and NDVI could be found in theSouthwest.(3)According to the spatial distribution of the linear trend, we can see that the lineartrend of annual average temperature are more obvious upward trend in most areas, with themost obvious change in summer, while that in winter is least. In addition, precipitationshowed a decreasing tendency in three seasons (spring, autumn and winter), while that insummer was opposite. Spatially averaged seasonal NDVI in the Southwest showed apronounced increase in all four seasons (spring, summer, autumn and winter) during the past14years. The average spring NDVI indicated the most significant increase, while the averagewinter NDVI showed the least increase. There are different the linear trend of temperature andprecipitation in all regions, and the seasonal average temperature raised overall in theSouthwest, while precipitation appeared to decline. Analyzing inter annual changes inseasonal mean NDVI by vegetation type, it was found that Beginning of growing season ofvegetation an exhibition of the trend of the advance, as well as the trend of the postponed, andlength has prolonged. The results show that about75%of the study of region was covered bythe advance of growing season and the trend of the postponed. The advance of growingseason was a major way for response of vegetation activity to climate changes and that theway exhibited a large regional heterogeneity. All the vegetation types’NDVI are also reflectedin a clear upward trend, and forestry staggered vegetation has the maximum rate of increase inautumn, but others is in spring, indicated that the postponed of forestry staggered vegetationhas prolonged and advance of growing season of other vegetation. (4) Most of the vegetations showed the highest responses to SPI time-scales of5–9months while grassland did respond more to SPI time scales longer than9–12months. Thevegetations growth variability in the Southwest is determined by the precipitation recordedduring the year of tree-ring formation but also by the precipitation that fell in the previousyear. The response of vegetations growth to drought greatly varied among species, being themaximum growth-drought correlation very high for broad-leaved forest(r=0.60–0.70),moderate for coniferous forest(r=0.50–0.60), and low for grassland (r=0.20–0.35). However,the results suggest that grassland have showed a higher growth plasticity in response todrought, broad-leaved forest appeared to be affected moderately by drought. All the studiedvegetations found in this area showed the strongest growth response to drought during springand summer months, which indicates that vegetation growth in the area is very dependent onspring and summer cumulative water deficit. In addition, in this area we found that thegrowth-drought correlation decreased at time scales lower than6months, showing that thesevegetations may show certain adaptive capacity in response to short droughts.