Objective The vast forest zone in Northeast China is an important ecological barrier of China. In this paper, changes of growing season NDVI and its response to climate change for forest in Northeast China (northeast forest) was studied in order to provide scientific basis and guidance for forestry establishment and protection in this region under the background of diurnal asymmetric warming.
Method Based on the SPOT Vegetation NDVI, different vegetation types’ spatial distribution, monthly precipitation and monthly temperature data from 2000 to 2019, using trend analysis, Hurst index and coefficient correlation analysis method, this paper investigates the spatial and temporal changing characteristics of growing season NDVI and its response to climate change for forest in Northeast China under the background diurnal asymmetric increase.
Result （1）In recent 20 years, growing season NDVI for forest in Northeast China generally showed an increasing trend, with the growth rate of 0.035 per ten years. The mixed forest showed the highest increase rate, reached 0.037 per ten years, the broadleaved forest showed the smallest increase rate. Northeast forests are mainly improved, accounting for 55.59% of the total area. Another 43.84% of the forests remained basically unchanged, and only 0.57% of the regional forests were degraded. （2）For the whole growing season, NDVI was significantly affected by minimum temperature and precipitation, and was weakly related to maximum temperature, at the same time, minimum temperature and precipitation in the growing season showed an obvious increasing trend, and maximum temperature had no obvious change, which contributed the increase of NDVI in northeast forest during growing season obviously under the background of diurnal asymmetric warming. Different forest types had varied responses characteristics: among the three forest types, coniferous forest growth was mainly affected by minimum temperature; broadleaved forest growth was mainly affected by precipitation and minimum temperature; mixed forest growth was most sensitive to precipitation. （3）For the monthly response characteristics, the NDVI in the early growing season (from April to May) was mainly affected by the maximum temperature and the minimum temperature, and the temperature sensitivity decreased gradually with the increase of temperature. At the end of the growing season (from September to October), it was jointly controlled by the minimum temperature and precipitation, and the precipitation sensitivity was higher in the mid growing season (from June to August). The sensitivity of NDVI to precipitation and temperature showed obvious time lag characteristics. (4) Future trend analysis indicated that the forest in Northeast China will change from improvement to degradation, around 71.94% of all forest will at risk of degradation, especially in areas with severe climate change regions.
Conclusion Diurnal asymmetric warming and differential response of NDVI to the maximum and minimum temperature lead to the asymmetric effect of daytime and nighttime temperature on the change of NDVI for forest in Northeast China. From 2000 to 2019, the growing season NDVI for forest in Northeast China has been increasing, but will be at risk of degradation in future.
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