https://hdl.handle.net/1866/41822

Rapid warming and associated biotic disturbances are significantly impacting carbon cycling in Arctic and boreal ecosystems. Ecological transition zones, such as the forest-tundra ecotone, are particularly vulnerable to shifts in vegetation composition, which can have serious repercussions on land-atmosphere CO₂. The latitudinal gradient in vegetation composition and structure drives high heterogeneity in the strength of net CO₂ uptake during the growing season. Understanding these variations in productivity is crucial for determining how changes in vegetation could influence the net carbon balance of these regions. We employed paired eddy covariance measurements of CO₂ fluxes and supporting environmental data at a subarctic woodland and nearby mineral upland tundra site (50 km north) to assess seasonal and interannual variations in growing season productivity and their relationship to environmental conditions. The woodland displayed greater net productivity during the growing season (May-September) than the tundra. Significant differences in net productivity between sites occurred only in the early growing season, with no difference reported in peak or late season. In the woodland, enhanced spring air temperature were linked with drier soils and increased water-use efficiency, leading to earlier growing season onsets and greater growing season net productivity. In the tundra, photosynthetic uptake begins only after complete snowmelt, with earlier snowmelt years leading to higher early-season productivity, but reduced late-season productivity. A tundra fire event had no significant impact on productivity at either site. However, polluted half-hours were associated with a decrease in uptake rates in the forest, while potentially enhancing uptake in the tundra. Intensifying wildfire regimes are expected to increase aerosol concentrations, which, by enhancing diffuse radiation, may begin to affect productivity in these northern ecosystems. Furthermore, shifts in vegetation composition, notably by altering snowpack conditions and energy partitioning, could drastically modify spring environmental conditions. Our results indicate that these changes, by affecting phenological timings, could have substantial impacts on the productivity of ecosystems at the forest-tundra ecotone.

https://hdl.handle.net/1866/41822