Browsing by Subject "NDVI"

As a response to Arctic climate warming, Arctic vegetation is changing. Studies have shown a greening trend both in satellite-derived data (spectral greening) and in field measurements (vegetation greening). Some studies indicate that the increase in vegetation productivity in the Arctic is one of the drivers of the increase in organic carbon in lakes, lake brownification. The aim of this thesis is to bring more knowledge about this possible connection between vegetation change and lake brownification in a circumpolar context. My two main research questions are: 1) How has the summer terrestrial NDVI value, in short, the normalized vegetation index to estimate the amount of green vegetation, changed during the past 20 years in the study areas and is there variation between lake groups? and 2) How has the amount of organic carbon changed in the studied lakes? These questions are complemented by the following sub-questions: Have summer temperature or precipitation affected NDVI change? What is the relationship between summer NDVI and lake organic carbon, and between the change in temperature/precipitation and the change in lake organic carbon? I conducted a circumpolar study about multi-decadal greening and climate trends around 15 Arctic and subarctic lakes divided into five groups, using satellite-derived vegetation data (NDVI). I then compared these data with sediment data to study if potential greening has a connection to the lakes’ total organic carbon load. The study areas are located in South-Central Canada, Alaska, North Finland, Russia – Yakutia and Russia – Chukotka. I found that summer surface air temperature has increased significantly in all of the study areas during 1961-2018. Total summer precipitation has not significantly changed during the study period, but the trend has been positive in all study areas except for Russia, Chukotka. Winter precipitation has significantly decreased in Alaska, and in both of the Russian areas, and significantly increased in Jänkäjärvi, Finland during 1961-2018. NDVI has increased significantly in Jänkäjärvi (Finland), Rauchuagytgyn and Illerney (Russia, Chukotka) during 2000-2021. In these lakes the environmental variables were affecting NDVI. The connection between NDVI in the study areas and lake TOC was positive, but not statistically significant. The situation was the same for the comparison between temperature and TOC for most of the lakes. When comparing the change in summer and winter precipitation and the change in TOC, for most of the lakes, precipitation had decreased while TOC increased. In conclusion, the results indicate that environmental conditions are changing in the study areas and that in some areas that has led to an increase in summer terrestrial NDVI values. It may also be that catchment area greening can increase lake TOC. However, this connection needs more research, for instance with a larger sample size. This study with its varying results also supports the notion of heterogeneous Arctic environments.