Epsilon Open Archive

Abstract

Biomass production in boreal forests is mainly nitrogen (N) limited, so alleviating this limitation can improve productivity. As the climate warms, N limitation is expected to be reduced, which, in turn, could result in enhanced biomass productivity. However, empirical evidence from long-term studies is scarce. In addition, although water availability may constrain biomass production once N limitation has been partly or fully alleviated, little is known about the effect of the interaction between N and water availabilities on biomass production.
In this research, I first examined the interactive effect on biomass production between N and water availabilities, based on field experiments in a Norway spruce forest and a Scots pine forest, supplementing these results with additional data from a literature survey. Nitrogen additions enhanced biomass production in both types of forest, while water availability only affected production in the pine forests in which N limitation had been partly or fully alleviated. In Scots pine forests, biomass production increased with increasing precipitation as the rate of N addition also increased. These forests are N limited, but the sigmoidal response to N additions indicates that even under moderate N supply, N availability meets their demand if precipitation is near average, and N limitation increases with increasing precipitation.
Second, I examined the effect of soil warming on biomass production in the Norway spruce forest. The treatments comprised fertilization and soil warming (+5°C) at a plot scale of 100 m2 for 18 years. Increased biomass production in association with soil warming was only observed in the unfertilized plots, suggesting that the enhanced biomass production was mediated by increased N availability. However, the enhancement was ephemeral and, therefore, not of sufficiently long duration to significantly enhance biomass accumulation. Foliar nutrient analyses together with the findings from earlier studies of the same plots, suggest that soil warming shifted N uptake to deeper soil and may increase C stock in the mineral soil.
Synthesizing the above results, I conclude that 1) biomass production of N limited boreal forests is strongly responsive to N additions following a sigmoidal curve, but the magnitude of response may depend on soil water availability, and 2) a warmer climate may not alleviate N limitation and thus will not increase biomass accumulation.