Boreal and subarctic regions in the northern hemisphere are warming faster than any other parts on Earth and experiencing greatest ecological change. Better understandings of how the high-latitude ecosystems respond to past warming have substantial implications for predicting future changes. The Holocene Thermal Maximum (HTM) is a relatively warm period between 11 and 9 kyr BP. Detailed C accumulation data from peat core and pollen show dramatic changes in vegetation composition and rapid peatland expansion during the HTM, suggesting that higher summer temperature and stronger seasonality may play a major role in high C accumulation and possible high CH4 emissions. Here we use an integrated approach of paleoecological records and ecosystem modeling to examine the impacts of enhanced seasonality on vegetation productivity, CH4 emissions and the subsequent hydrological and biogeochemical processes of Alaskan ecosystems during HTM. We compiled stratigraphic pollen data from Alaska to generate biome maps of Alaska from three time slices: 15-13 kyr BP (Bølling-Allerød Warming), 11-9 kyr BP (early HTM), and 7-5 kyr BP (cooler, stable Holocene climate). Climate data was downscaled and bias corrected of ECBilt-Cli model output. The simulation was carried out with the Terrestrial Ecosystem Model (TEM) incorporated with Methane Dynamic Module.
Results/Conclusions
The vegetation composition shifted from tundra-dominated ecosystems in 15-13 kyr BP to boreal forest-dominated ecosystems in 7-5 kyr BP. The increased proportion of deciduous forest during HTM indicates warmer and moisture climate. The annual average temperature and total precipitation as well as their seasonal amplitude (summer (Jun-Aug) – winter (Dec-Feb)) were the highest during HTM. Growing season was lengthened during HTM as well. In response to the changes in climate, regional average NPP exhibited the highest average value (~140 g C m-2 yr-1; P<0.005) as well as seasonal amplitude of NPP during HTM, indicating enhanced photosynthesis during summer and stronger autotrophic respiration during winter, thus strengthened seasonality in vegetation productivity. Regional average net CH4 emission was the highest during 15-13 kyr BP (~2.84 g CH4 m-2yr-1), yet HTM also exhibits high CH4 emission with about 2.23 g CH4 m-2yr-1 possibly due to rapid peatland expansion reported in previous studies. The post HTM period was cooler and low in both productivity and CH4 emissions, indicating a more stable period of peatland development. Regional total net CH4 emission contributed 3.3 Tg CH4 per year during HTM.