Nitrogen co-limitation in drought-limited boreal forests

• Applicant: Dr. Choimaa Dulamsuren
• Subject Area: Ecology and Biodiversity of Plants and Ecosystems
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 539975714

The productivity of boreal forests is generally limited by low temperatures and low nitrogen (N) availability. However, this is different at the southern edge of the boreal forest biome under continental climates, where productivity is mostly limited by drought rather than temperature. In recent years, several remote-sensing and tree-ring studies have shown that more and more boreal forests worldwide that were previously temperature-limited are switching to drought-limitation as a result of climate change. Until recently, it was unclear whether co-limitation of productivity by N known from temperature-limited boreal forests persists under drought-limited conditions or, alternatively, whether water shortage causes N limitation to become no longer relevant. In a recent paper (published in Global Change Biology in 2021), the applicant was able to show that N limitation exerted a significant impact on tree growth in addition to drought limitation in strongly drought-exposed southern boreal forests of Central Asia. This provides the opportunity to explore N relations in drought-limited boreal forests and their effect on carbon, water, and nutrient relations in more detail, because studies on N limitation in boreal forests were, so far, confined to temperature-limited ecosystems. The present project aims at analyzing the effect of N on carbon, water, and nutrient relations in drought-limited forests at the southern fringe of the boreal forest biome in northern Mongolia in a fertilization experiment. Mongolia’s boreal forests are located in close vicinity to Central Asian temperate grassland biome and are naturally exposed to frequent and intense droughts. They can thus serve as model systems (or climate analogues) for climatic conditions, which are expected to increasingly spread in currently better water-supplied parts of the boreal forest biome with progressive climate change. The N fertilization experiment (with 0, 12.5, 25, and 50 kg N ha-1 a-1) will be established in forests of Siberian larch (Larix sibirica) one year before the project term. The fate of N in the ecosystem will be tracked by analyzing plant, organic layer, and soil N contents and δ15N signatures as well as net N mineralization. Plant-ecophysiological methods (e.g. measurements of CO2/H2O gas exchange, chlorophyll fluorescence, and plant water potentials), stable isotope analysis, and biochemical analysis of non-structural carbohydrate concentrations will be combined with analyses of stemwood increment and fine-root distribution. The key target of these investigations is to test the overarching hypothesis that N availability has a significant influence on carbon and water relations of drought-limited boreal forests, affecting the ability of boreal forest trees to contribute to carbon sequestration, to produce woody biomass and to tolerate drought stress under a warming climate.

• DFG Programme: Research Grants