## 6 Discussion
### Surface Characteristics of Modern and Radiocarbon-Dated DTLBs (0-10,000 Years since Drainage)  
We attribute the logarithmic relationships of basin age with mean basin backscatter and mean basin NDVI in the summer season to post-drainage change in vegetation, soil formation and peat accumulation, and permafrost aggradation. Our analysis also showed seasonal differences in backscatter between the 27 August and 9 September scenes and the lack of a clear relationship in the September scene backscatter with basin age. A plausible explanation could be that seasonal changes in vegetation phenology and soil moisture properties of basin floors obscure the link between backscatter and basin age later in the fall. This suggests that the time of image acquisition should be taken into consideration when investigating post-drainage succession pattern of thermokarst basins using radar datasets. [PERSON] _et al._[52] have already demonstrated variations in NDVI throughout the season and its implications for estimating plant productivity more accurately for the tundra ecosystem. To fully understand the seasonal behavior of basin backscatter and the basin age relationship, further analysis of multi-temporal data is required. Diverse surface properties among modern DTLBs, as well as within a modern drained basin, affected the SAR signal in separating modern basins from young and medium-aged basins. This is because of complex and non-linear interaction between radar backscatter and diverse biophysical parameters [41, 57] that dominated basin floors during early successional stages after lake drainage. Since modern DTLBs exhibited relatively high NDVI due to mostly dense graminoid vegetation compared to older DTLBs, NDVI was a good indicator of basin age over a period of 0-10,000 years. However, TSX data performed much better when used to discriminate basins older than modern basins (50-10,000 years BP), likely due to the better sensitivity of soil moisture. This shows that the usefulness of NDVI for discrimination of DTLB successional stage diminishes when the rate of vegetation succession slows down in older and less productive basins. A decreasing trend in basin backscatter with increasing basin age can be explained by the decrease in soil moisture and presence of patterned ground with the increasing number of small ponded low-center polygons in medium-aged, old and ancient DTLBs. It is likely that the reduced specular property resulted in a lower backscatter signal as soil moisture decreased [57] with increased basin age. In the case of some older radiocarbon-dated DTLBs that were dominated by fully developed polygonal networks with a significant number of small ponds, ponded polygons contributed the most to the overall very low backscatter value. Even though a water mask was created to exclude backscatter of larger water bodies from our study, microscale ice wedge polygonal ponds below TSX image resolution had an impact on backscatter properties. However, we found vegetation growing in some inundated areas exhibited high backscatter due to the double bounce effect of the radar signal [56, 58]. Despite an overall decrease in productivity of old basins, the waterlogged conditions in low-center ponds in older DTLBs could have created preferential conditions for growth of productive wetland plant communities [32]. The relationship of decreasing NDVI with increasing basin age agrees with findings by [PERSON] _et al._[12], [PERSON] _et al._[32] and [PERSON] _et al._[36] from their study on DTLBs on the Arctic Coastal Plain of Alaska. [PERSON] _et al._[10] also highlighted a significant relation between Landsat-derived NDVI and basin peat thickness on the northern Seward Peninsula, Alaska. They observed a decrease in basin scale NDVI as basin peat depth increased. Decreasing basin productivity, as indicated by decreasing NDVI, is caused by a lack of nutrient availability in older DTLBs as nutrients become sequestered in the peat as the basin ages [32].