Impacts of enhanced climate seasonality on productivity and CH4 emissions of Alaskan ecosystems during the HTM

Background/Question/Methods

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.