Contrôles environnementaux de la variabilité interannuelle de la reprise et de la fin de la photosynthèse au sein de la forêt boréale nord-américaine

The boreal forest, storing large amounts of carbon in its soil and covering a majority of the Alaskan, Canadian,  Fennoscandian and Russian territory, is an integral part of the climate system. However, climatic variability and ecosystem  properties, particularly with regards to the presence or absence of permafrost, limits our understanding of the carbon  balance variability in the boreal biome, which comprises forest, lake and wetland ecosystems. The boreal carbon sink-source strength is greatly influenced by phenological events, including the start and end of the photosynthetic growing  season, which are themselves dependent on several environmental variables such as air and soil temperature, soil water content, vegetation development stages, etc. This research aims to provide new insights on the influence of environmental  variability on the variability in the timing of the photosynthetic growing season, by broadly distinguishing between boreal  forests with and without permafrost. The photosynthetic growing season is characterized using gross primary productivity derived from eddy covariance measurements of net ecosystem carbon dioxide exchange. Data from 40 black spruce- dominated site-years of observation across the North American boreal forest are used. The considered environmental  predictors were air and soil temperatures, vegetation development stages, snow cover, photosynthetically active radiation  and soil water content. The statistical framework included the calculation of Pearson correlation coefficients, commonality  analyses and structural equation modeling. This study shows that the variability in the start of the growing season in  permafrost-free sites is directly controlled by the variability in vegetation development stage as well as by the thawing of  seasonally frozen ground. This result thus emphasizes the importance of access to liquid soil water for the vegetation to  initiate photosynthesis. No environmental variable could significantly explain photosynthesis recovery in sites with  permafrost. In fall, the soil water content as well as the start of snow cover directly influence the variability in the end of the  photosynthetic growing season. These results suggest that the availability of water can limit photosynthesis in the fall. The  effect of snow cover is opposite in sites with and without permafrost. A delay in the appearance of continuous snow cover in sites without permafrost indicates that the air temperature is high enough for precipitation to fall in liquid form and for  photosynthesis to continue. In contrast, its delay in sites with permafrost indicates less snowfall, thus delaying the  appearance of an insulating layer for the soil, which could have lengthened the photosynthetic growing season. This study  sheds light on the controls of the annual variation of the timing of the photosynthetic growing season and will help  understanding of the effects of climate change on the strength of the North American boreal forest carbon sink.

Keyphrases: Plant phenology index, Carbon dioxide, Growing season, Photosynthesis, Boreal forest, Permafrost, Air temperature, Soil temperature, Snow cover, Soil water content