Recent variations in NDVI-based plant growth and their relationship with climate in boreal intact forest landscapes

Abstract:

Intact Forest Landscapes (IFLs), defined as large unbroken expanses of forest landscape without signs of significant human activity, have significant ecological values. Previous studies suggest a reversal in the greening of boreal plants was exhibited in the late 1990s. In this study, we focus on variations in plant growth of boreal IFLs from 2000 to 2014 and their correlation with local climatic factors between 45°N and 70°N. The average Normalized Difference Vegetation Index (NDVI) during the growing season (GS, which is from April to October) derived from MOD13C2, is used as a proxy of plant growth. Compared to a significant increase in GS NDVI of boreal plants during the 1980s and early 1990s, GS NDVI of ca. 85.7% of total IFLs in the study area exhibited insignificant change after 2000. About 10.2% of total boreal IFLs exhibited significant greening (an increase in GS NDVI), and only 4.1% of the total showed significant browning (a decrease in GS NDVI) during the study period. For greening boreal IFLs, ca. 46.0% of these showed a significant correlation between GS temperature and NDVI. For browning IFLs, an increase in precipitation during the non-growing season (NGS, which is from previous November to current March) and cooling in GS and NGS were the main climatic causes for decreases of GS NDVI. However, over 65% of browning boreal IFLs did not correlate with any climatic factor, and the browning may be associated with artificial activities. About 49.4% of no-change boreal IFLs showed significant correlation between GS NDVI and climatic factors, and 72.5% of these sensitive plants exhibited a significant positive correlation between GS temperature and NDVI. On the whole, an increase in GS and NGS temperature could promote plant growth of boreal IFLs, while an increase of NGS precipitation mainly inhibited plant growth. However, nearly half of total boreal IFLs displayed no sensitivity to any climatic factors chosen in our present work.