Reconstructing Holocene deforestation and its possible climatic feedback Hongyan Liu

1. SOFIE spring school 2014 Reconstructing Holocene deforestation and its possible climatic feedback Hongyan Liu College of Urban and Environmental Sciences, Peking University Beijing, 100871, China

2. Outline • Introduction • Approaches of reconstructing historical deforestation • Historical deforestation and possible climatic feedback in Inner Asia • Challenges of reconstructing Holocene deforestation • Palaeoecological and ecological studies of deforestation • Summary

3. I. Introduction Climate-vegetation system (Bonan et al., 2008)

4. I. Introduction Deforestation and climate feedback (Bonan et al., 2008)

5. I. Introduction Direct and indirect vegetation feedbacks (Liu et al., 2010)

6. I. Introduction Modeling vegetation feedback （Foley et al., 1998）

7. I. Introduction Simulated feedback of deforestation (Liu et al., 2009)

8. I. Introduction Simulated orbital and vegetation forcings (Foley et al., 1994)

9. I. Introduction Seasonal difference of vegetation forcing (Foley et al., 1994)

10. I. Introduction Oversimplified vegetation cover in simulations (Brovkin et al., 2002) (Liu et al., 2009)

11. I. Introduction Uneven distribution of global forest cover • Forests occupy only 1/3 of the land surface • Unclosed forest constitutes ~1/2 of total forests, mostly in semi-arid regions Tree cover (%)

12. I. Introduction Significances of deforestation in semi-arid regions • Climatic feedback Forest decline in semi-arid region can lead to climate cooling, equivalent to ~20% of the global anthropogenic CO2 effect, moderating warming trends (Rotenberg and Yakir, Nature, 2010). • Carbon pool effect Photosynthesis ability is 60% higher in the semi-arid forest than in other forests (Rotenberg and Yakir, Nature, 2010). • Threatens from fast warming Climate warming is faster in the semi-arid region than in the humid and arid regions (IPCC, 2007).

13. I. Introduction Why we reconstruct Holocene deforestation? • Forest borders have shifted for up to 1000 km in N America and Mediterranean region, but very finely in Inner Asia, during the last 6000 years ? Present Future Climate cooling since 6 ka BP (Liu & Yin, 2013) Climate cooling since 6 ka BP

14. II. Approaches of reconstructing Holocene deforestation Pollen-based vegetation cover reconstruction • Sediment pollen data is the only source for Holocene vegetation reconstruction • Previous works mostly focused on regional-scale biome distribution . • We can know the distribution of forest, but not its cover (Prentice, 1998) • To reconstruct Holocene vegetation cover, particularly deforestation processes, new approaches are required

15. II. Approaches of reconstructing Holocene deforestation Pollen deposition in sediment • Pollen productivity differs across species • Pollen source area and representation for regional vegetation strongly differ across time and space Wind

16. II. Approaches of reconstructing Holocene deforestation REVEAL model Pollen grain size Wind speed Vegetation pattern …… ERV model Relative Pollen Source Area Pollen Productivity Estimation Sediment Pollen REVEAL model Vegetation openness

17. II. Approaches of reconstructing Holocene deforestation Transfer function • Pollen-NDVI transfer function (Liu et al., 2013)

18. III. Historical deforestation and possible climatic feedback in IA Aims and methods • Most of the works on deforestation and its climatic feedback are based on short-term local observations or model simulations • Sediment pollen data provides possibility of reconstructing historical vegetation dynamics; however, studies focusing on deforestation and climatic feedback remain very few • By building a sediment pollen database, we reconstructed the Holocene vegetation cover changes in northern China and linked it to climatic feedback

19. III. Historical deforestation and possible climatic feedback in IA Regional scale reconstruction of forest decline • We selected 52 sediment sequences from N China, covering different habitats, to reconstruct the Holocene vegetation dynamics

20. 8.3 ka BP 6.2 ka BP 查干诺尔 4.8 ka BP 2.0 ka BP 宝绍岱诺尔 0 ka BP III. Historical deforestation and possible climatic feedback in IA Holocene forest border shifting in N China • ~200 km retreat of forest border since 6 ka BP in N China

21. III. Historical deforestation and possible climatic feedback in IA Reconstructed vegetation cover in N China • Even if forest existed, their covers differed • Validated by modern NDVI values (Liu et al., 2013)

22. II. Holocene deforestation and possible climatic feedback in IA Forest decline and climatic feedback in N China • Three curves (vegetation cover, aerosol intensity, monsoon intensity) well matched • Our results validated the scenario simulation of deforestation-induced cooling by Dallymeyer et al. (2011) Vegetation cover Aerosol intensity Monsoon intensity (Liu et al., 2013)

23. III. Holocene deforestation and possible climatic feedback in IA Suggested climate-vegetation interactions • Precipitation brought by the monsoon determined changes in vegetation cover (deforestation, savannification, grassland degradation) • Decline in vegetation cover contributed to climate cooling through: (1) increasing albedo; (2) increasing aerosol caused by windy erosion of soil (Liu et al., 2013)

24. III. Holocene deforestation and possible climatic feedback in IA Vegetation-climate interaction in China • The higher cooling velocity in N China than in S China is hypothesized to be caused by the substantial decline of vegetation cover in N China

25. IV. Challenges of reconstructing Holocene deforestation An example of birch abundance in East Asia 11.5 ka BP 10.0 ka BP 1.0 ka BP 6.0 ka BP （Cao et al., 2013)

26. IV. Challenges of reconstructing Holocene deforestation How many data are available? • In East Asia, 271 sediment cores were published, but only 122 of them with >4 radiocarbon-dated ages (Cao et al., 2013). • Previous efforts of building sediment pollen database commonly adopted Bayesian analysis to uniformly interpolated sediment age from selected radiocarbon dating in original publications (Blaauw and Christen, 2011). • Sediment cores available for Holocene vegetation cover reconstruction are much less than those seen in publications.

27. IV. Challenges of reconstructing Holocene deforestation Criteria for selecting cores • There are only 22 from 371 sites fit for benchmark sites in North America (Blois, 2011). • Factors include valid dating, date precision and accuracy, age range, temporal resolution

28. IV. Challenges of reconstructing Holocene deforestation Upscaling reconstructed vegetation cover • Sediment cores are unevenly distributed

29. IV. Challenges of reconstructing Holocene deforestation Climate drove forest vs. forest drove climate (Herzschuh et al., 2009)

30. V. Palaeoecological and ecological process of deforestation Different responses of forest to drought (Hao et al., 3P in press)

31. V. Palaeoecological and ecological process of deforestation Climate-fire-vegetation system (Yin et al., in preparation)

32. V. Palaeoecological and ecological process of deforestation Modern Analog of Holocene deforestation • Conifers Picea meyeri Larix sibirica Pinus sylvestris • Broadleaf trees Betula platyphylla Populus davidiana Ulmus pumila Tolerance to drought

33. Tree growth Temp. Airhumid. Soil temp. Field observation in Duolun, Inner Mongolia Soil humid. V. Palaeoecological and ecological process of deforestation Field observations on forest under drought • Our field observation showed highly resistance of tree growth to drought due to water storage in stem (Hu et al., in preparation)

34. VI. Summary Take home message • Our palaeoecological study validated a simulation of the mid-Holocene climatic cooling caused by deforestation in N China • Available data for reconstructing the Holocene deforestation are much less than expected • Palaeoecological and modern ecological studies will help to understand climate-driven deforestation and climatic feedback of deforestation

35. Thank you for your attention! FASTER (Forest And STeppe Ecotone Research) An International Joint Working Team on Inner Asian Vegetation Peking University Saihanba Station Acknowledgements: Dali Guo, Xiuchen Wu, Yi Yin, Guozheng Hu (Peking University) Oleg Anenkhonove, Natalia Badmaeva, Denis Sandonov, Andrey Korolyuk (Siberian Branch, Russian Academy of Sciences Eryuan Liang (Inst. of Tibet Plateau Research, CAS)