Vegetation Dynamics And Climate Change On The Loess Plateau, China: 1982-2014

Vegetation is the key factor for soil and water conservation on the Chinese Loess Plateau and has important influences on the critical ecological processes. Vegetation plays a crucial role in linking soil, atmospheric, hydrological, and other elements of the biosphere. Knowledge of the vegetation growth and its driving mechanisms at different spatial and temporal scales is essential to understand the mechanisms underlying ecosystem behaviors and activities, and thus this would provide scientific evidence for the interaction of vegetation, climate and human activities. The Loess Plateau was partitioned into three climate regions and 14 vegetation-type regions, to comprehensively understand the relationship between the vegetation growth and climatic variables. Using classical statistics, linear regression, wavelet analysis and residual analysis, this study aimed to(1)investigate the spatio-temporal changes in vegetation growth and climate change during the period of 1982-2014 over the Loess Plateau;(2)analyze the relationships between vegetation growth, precipitation, and temperature in different growing seasons, climate regions and vegetation types during the last three decades; and(3)analyze the relative role of climate variations and human activities in vegetation change in different climate regions, vegetation types and administrative area. The main results are presented as the following:(1) The LTDR NDVI dataset was reliable for investigating the vegetation growth on the Loess Plateau. At the regional scale, the correlation coefficient between LTDR NDVI and GIMMS NDVI was 0.90. At the seasonal scale, the vegetation phenology change of these two datasets was close. At the pixel scale, Most of the area(95% of the whole study area)had a positive correlation between these two datasets, which was statistically significant(P<0.05) over 65% of the area. Our results also suggested that there was a need to prudently deal with the vegetation dynamic results of LTDR NDVI for deciduous broadleaf forest and sparse shrub.(2) Multi-resolution analysis results showed that vegetation growth had obvious seasonal changes on the Loess Plateau, the maximum NDVI value was found in July and August. Semihumid region had a high level of vegetation cover and homogeneity through time, while arid region accounted for a high variability in the vegetation dynamics and thus low vegetation cover. The seasonal change of NDVI was unclear in the deciduous broadleaf forest, evergreen needleleaf forest, mixed forest and meadow. The range of inter-annual variations was obvious in deciduous broadleaf forest, evergreen needleleaf forest and mixed forest.(3) A significant increase in growing-season NDVI at a rate of 2.95×10-3 yr-1 was detected throughout the entire study period(P < 0.001). The NDVI increasing rate was highest in summer(3.37×10-3 yr-1), medium in autumn(2.58×10-3 yr-1) and spring(1.71×10-3 yr-1); and lowest in winter. The trend of the vegetation growth across the plateau was heterogeneous. Most of the area(95% of the whole study area) had an increasing trend in growing-season NDVI, which was statistically significant(P<0.05) over 84% of the area and was restricted mainly in areas where MAP was higher than 300 mm. The vegetation growth in areas(5% of the whole study area) of aeolian grassland of western Ordos, Northern Gansu and Xi’an city decreased during the last three decades. The change of growing-season NDVI varied across climate regions and vegetation types. The NDVI in the semi-humid region increased at the highest annual rate of 3.98×10-3 yr-1, followed by that in the semi-arid region, whose increasing rate was 2.90×10-3 yr-1. The arid region experienced the lowest and weakest increasing trend in NDVI during the study period(1.31×10-3 yr-1). At the biome scale, the growing-season NDVI of all vegetation types increased significantly. The increasing rate followed a descending order: deciduous broadleaf forest > Evergreen needleleaf forest > deciduous broadleaf shrub > Herbosa > Wet land > Crop > open forest > open shrub > meadow > steppe > sparse land > desert > sparse grassland.(4) Climate on the Loess Plateau tended to be warmer and wetter during last three decades. The regional mean annual temperature(MAT)was shown to significantly increase by 1.70 o C(P<0.001)and accumulated annual precipitation(AAP) increased by 14.89 mm(P>0.05). The temperature in the semi-arid region increased at a highest rate, followed by arid region. The semi-humid region experienced the lowest and weakest increasing trend in temperature during the study period. Most of the area(96% of the whole study area) had a significant increasing trend in MAT. Over 75% of the whole study area had an increasing trend in AAP, which was concentrated mainly in arid region and semi-arid region. The AAP in semi-humid region decreased during the last three decades. Wavelet analysis showed that the first main periods of temperature were 14 years, at the scale of 14 years, the temperature experienced 3 warm-cold cycles, the average change period was 9.5 years. The first main periods of precipitation were 10 years, at the scale of 10 year, the precipitation experienced 5 plentiful and withered circulation periods, and the average change cycle was 6.5 years.(5) Both temperature and precipitation drove the vegetation growth on the Loess Plateau, the degree of driving forces varied with climate regions, vegetation types and temporal scales. Temperature had significant effect on NDVI in 28% of the whole study area, precipitation had significant effect on NDVI 17% of the whole study area. Vegetation growth in 39% of the study area was sensitive to both temperature and precipitation. Temperature had a positive effect on NDVI in semi-humid region at both intra- and inter-seasonal scales. Precipitation had a positive effect on NDVI in the arid region. Both temperature and precipitation had a positive effect on NDVI in the semi-arid region. The NDVI of forest, wet land and cropland was sensitive to temperature, while grassland, sparse land and desert were sensitive to both temperature and precipitation. Spring and summer warming was beneficial for vegetation growth in spring and summer. The lag-time effects were found between climate and vegetation growth on the Loess Plateau. Vegetation NDVI had a 14-month lag response to temperature and 14- to 15-month lag response to precipitation on three climate regions and most vegetation types.(6) The relative roles of climate variations and human activities in vegetation change were estimated to be 77% and 23% on the Loess Plateau, respectively, indicating that climate change played an important role in vegetation growth during last three decades. The NDVI increases were primarily caused by climate change(relative effect > 50%) in semi-arid and it accounted for 81% of the area with increasing vegetation in the semi-arid region. The percentage areas of increasing NDVI induced by climate change and human activities were about the same in arid and semi-humid regions. Climate played a primary role in NDVI increase in most vegetation types, while climate and human activities exerted similar effects(the relative effect was both about 50%) on NDVI increase in wetlands, cropland and desert. The NDVI decrease were primarily caused by human activities in both climate regions and vegetation types. The vegetation change areas primarily caused by human activities were enlarged after 1999, indicating that climate change played an important role in vegetation growth during 1982-1999, while human activities played an important role in vegetation growth since the returning cultivated land to forest and grassland program commenced in 1999.