Detecting the drivers of forest change in Alaska and the Arctic
Scientists are using satellites to collect detailed data on forest change in remote parts of Alaska and the Arctic.
Rapid warming in the Arctic and boreal regions of Alaska is affecting boreal forests and tundra ecosystems in a number of ways. Higher temperatures and changes in precipitation have led to a higher incidence of wildfire and increased tree mortality from drought, insects, and disease. Increases in the length of the growing season and the amount of energy produced by vegetation have also been observed. While tracking how climate changes are affecting vegetation in these vast, remote areas with field data is a challenge, satellite measurements of vegetation density over time can be used to monitor trends of growth and decline and predict the consequences of rapid warming and environmental change in Alaska and the Arctic.
Satellite data collected over the past thirty years show that far northern North America is getting greener. Vegetation density data (1–8-kilometer resolution) from the Advanced Very High Resolution Radiometer (AVHRR) and Moderate Resolution Imaging Spectroradiometer (MODIS) instruments show overall growth (greening) trends in tundra vegetation and dieback (browning) trends in boreal forests, with areas with a greening trend dominating (see figure). NASA research has produced new information products at finer spatial scales that, alongside interagency modeling efforts, provide a more detailed understanding of the underlying causes of these trends and their impacts on ecosystem services such as carbon storage. For North America, finer-scale data collected by the Landsat satellite (30-meter resolution) and by commercial satellites (3–5-meter resolution) confirm the trends observed in the AVHRR and MODIS data and can be used to identify trends for specific vegetation types1. These new data products also show that some of the variations in vegetation cover trends are due to changes in lake cover, increases in shrub cover, and degradation of permafrost due to warming2,3.
Interagency atmospheric modeling studies supported by DOE, USGS, NSF, and NASA have provided further understanding of the drivers and impacts of these trends on the environment. Analyses of surface carbon dioxide measurements from 1986–2006 suggest that in spite of overall browning trends and changes in fire frequency, the boreal forest region stored more carbon than it released to the atmosphere over this time period. Further north, despite an observed greening trend the Arctic region has not contributed to carbon emissions overall, with the increases in carbon uptake from the atmosphere in summer balanced out by increased releases during the fall4.
1 Ju, J. and J.G. Masek. The vegetation greenness trend in Canada and US Alaska from 1984–2012 Landsat data. Remote Sensing of Environment 176: 1–16, 2016. https://doi.org/10.1016/j.rse.2016.01.001
2 Raynolds, M.K. and D.A. Walker. 2016. Increased wetness confounds Landsat-derived NDVI trends in the central Alaska North Slope region, 1985–2011. Environmental Research Letters 11 (8). https://doi.org/10.1088/1748-9326/11/8/085004
3 Carroll, M., M. Wooten, C. DiMiceli, R. Sohlberg, and M. Kelly. 2016. Quantifying Surface Water Dynamics at 30 Meter Spatial Resolution in the North American High Northern Latitudes 1991–2011. Remote Sensing 8. http://dx.doi.org/10.3390/rs8080622
4 Welp, L.R., P.K. Patra, C. Rödenbeck, R. Nemani, J. Bi, S.C. Piper, and R.F. Keeling. 2016. Increasing summer net CO2 uptake in high northern ecosystems inferred from atmospheric inversions and comparisons to remote-sensing NDVI. Atmospheric Chemistry and Physics 16: 9047–9066. https://doi.org/10.5194/acp-16-9047-2016