Article from Springer-Nature Blogs, written by postdoc Mukund Rao:

https://earthenvironmentcommunity.nature.com/posts/the-eurasian-boreal-forest-may-not-be-ready-for-future-heat-waves 

Climate change and a “tipping point” in the boreal biome
The circumpolar boreal region is the most extensive biome on the planet (Fig. 1). Found mainly in the northern hemisphere at high latitudes greater than 50°N, they play a major role in the global carbon cycle. They store approximately one-third of the 450 Gt terrestrial carbon stock and account for about a third of global annual forest productivity1. Climate change is causing rapid increases in temperatures across the boreal domain. Boreal regions, including Northern Mongolia, have warmed two to three times faster than the rest of the planet2 (Fig. 1). This warming threatens the ability of the biome to continue to act as a carbon sink3. 

As climate change progresses, there is a risk that earth’s climate system might exceed certain critical thresholds known as “tipping points” where a small perturbation can dramatically and irreversibly change its state4,5. Some tipping points in the climate system include melting of the Greenland and West Antarctic ice sheets, death of low-latitude coral reefs, and dieback in the Amazon and boreal forests5. Earth System Models suggest that regional scale forest dieback in the southern margin of the boreal biome may become likely at the end of this century (i.e., year 2100) under high emission scenarios6. Further, they predict that mortality will occur relatively slowly over fifty to hundred year timescales and eventually lead to an ecosystem transition from forest to a grassland-steppe biome6.

Fig. 1 Climate trends and projections in North Mongolia and the globe. a. Distribution of Larix sibirica (black hatching) in the context of North Mongolia (red rectangle), our study site (103.17°E, 49.92°N, red star), and the circumpolar boreal biome (olive shading). b. Mean summer June-July-August (JJA) temperature for North Mongolia and the globe (area-weighted) between 1950–2021, along with CMIP6 projected ensemble median ‘historical’ (1950–2014) and ‘future’ (2015–2100) temperature for both regions (22 models, 54 ensemble members) relative to their 1961–1990 mean (dashed zero line). Projections are derived using four shared socioeconomic pathways (SSP1-2.6; SSP2-4.5; SSP3-7.0; and SSP5-8.5).

Impact of extreme heat on plant photosynthesis
As global temperatures have warmed, we have observed and already experienced notable increases in the intensity, duration, and frequency of extreme heat waves7. Warm temperatures, particularly during heat waves can impact human, animal, insect, and plant health. If plant leaves warm too much during extreme heat conditions, protein complexes that conduct photosynthesis (known as photosystems) can begin to break down7. Without healthy photosystems plants are unable to assimilate carbon-dioxide (CO2). The temperatures at which leaves suffer such damage is generally between 40 to 50°C8. While these temperatures may seem high, it is worth noting that leaf temperatures often exceed air temperatures by 5 to 20°C9. 

In a study recently published by our group in Communications Earth & Environment10, we apply a trait-based vulnerability assessment approach to evaluate whether climate change may cause temperature extremes in excess of the critical temperature threshold of plant leaves at the southern margin of the Eurasian boreal forest. To do so we compared field based plant ecophysiological measurements of leaf temperature tolerance (Tcrit) against predictions of leaf temperature derived from Earth System Models. Our field experiments were conducted on five tree species at a forested site in the Tarvagatai River valley located in the Bulgan aimag of Northerm Mongolia (red star in Fig. 1). 

We found that Earth System Models suggest that Siberian larch (Larix sibirica) trees in the region could experience lethally high temperatures in excess of its critical temperature threshold (Tcrit) by two to three days a year by 2050 under high emission scenarios SSP3-7.0 and SSP5-8.5 (Fig. 2). These Shared Socioeconomic Pathways (SSPs), served as a basis for the sixth assessment report of the Intergovernmental Panel on Climate Change (IPCC AR6), and represent middle to upper range scenarios of human emissions of CO2 into the atmosphere through this century. Siberian larch is a foundation species across boreal Eurasia where it accounts for 30% of the Eurasian forest biomass, 80% of Mongolian biomass, and provides important ecosystem services such as grazing habitat for livestock and timber. Importantly, exposure to extreme heat in excess of plant critical temperature thresholds in co-occurrence with stress factors such as fires, insects, drought, and pathogens, can potentially cause forest mortality over timescales from days to years. Thus, our study indicates the possibility that the southern boreal forest tipping point could be exceeded sooner than previous ESMs estimates that did not include plant thermal tolerance traits (2050 cf. 2100), albeit at one site in Northern Mongolia. Additionally, our work highlights the potential for the boreal forest mortality to occur over faster timescales (from days to years) than predicted previously by ESMs where forest mortality and subsequent ecosystem transition to grasslands occurs relatively slowly (decades to centuries). While more plant ecophysiological studies are needed to fully characterise the risks to forests under climate change, the ultimate fate of the boreal forest in the region lies in our hands and will depend on our decisions with respect to global greenhouse gas emissions and local land management. 

Fig. 2 Range of variability in Earth System Model (ESM) projected hottest leaf temperature (Tleaf-xx) between 2050-2100 under different SSPs compared against Siberian larch’s (Larix sibirica) critical temperature for photosynthesis (Tcrit) in 2019 and between 2050-2100 under SSP5-8.5 assuming thermal acclimation.

References

1. Pan, Y. et al. A Large and Persistent Carbon Sink in the World Forests. Science333, 988-993 (2011). https://doi.org/10.1126/science.1201609
2. Gauthier, S. et al. Boreal forest health and global change. Science 349, 819-822 (2015).      https://doi.org/10.1126/science.aaa9092
3. Davi, N. K. et al. Accelerated Recent Warming and Temperature Variability Over the Past Eight Centuries in the Central Asian Altai From Blue Intensity in Tree Rings. Geophys. Res. Lett. 48, e2021GL092933 (2021). https://doi.org/10.1029/2021GL092933
4. Broecker, W. Unpleasant surprises in the greenhouse? Nature 328, 123–126 (1987). https://doi.org/10.1038/328123a0
5. Lenton, T. M. et al. Tipping elements in the Earth's climate system. Proceedings of the National Academy of Sciences 105, 1786-1793 (2008). https://doi.org/10.1073/pnas.0705414105
6. Armstrong McKay, D. I. et al. Exceeding 1.5C global warming could trigger multiple climate tipping points. Science 377, eabn7950 (2022). https://doi.org/10.1126/science.abn7950
7. Seneviratne, S. I. et al. in The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (eds V. Masson-Delmotte et al.) Ch. Weather and Climate Extreme Events in a Changing Climate Supplementary Material. In Climate Change 2021, (2021). https://www.ipcc.ch/report/sixth-assessment-report-working-group-i/
8. O'sullivan, O. S. et al. Thermal limits of leaf metabolism across biomes. Global Change Biol. 23, 209-223 (2017). https://doi.org/10.1111/gcb.13477
9. Still, C. J. et al. No evidence of canopy-scale leaf thermoregulation to cool leaves below air temperature across a range of forest ecosystems. Proceedings of the National Academy of Sciences 119, e2205682119 (2022). https://doi.org/10.1073/pnas.2205682119
10. Rao, M. P. et al. Approaching a thermal tipping point in the Eurasian boreal forest at its southern margins. Communications Earth & Environment (2023). https://doi.org/10.1038/s43247-023-00910-6