My Sediments Exactly

Tree Rings to Understand Climate Extremes and Ecosystem Changes: An Interview with Prof. Zhihong Xu, Griffith University Australia

• Medha Chaturvedi • Episode 1

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Tree rings serve as a natural archive for studying past climate conditions. By analyzing the tree rings, researchers can date the year the tree was growing and reconstruct past droughts, floods, and wildfires. Nitrogen is a critical element for forest ecosystems and its availability affects forest health and productivity. Climate extremes like droughts, floods, and wildfires have a significant impact on forest ecosystems and the nitrogen cycle. The use of isotopic markers, such as N15, can help understand climate change impacts on extreme weather and future climate projections. The research aims to influence public awareness and policy regarding forest conservation and climate change mitigation. Challenges include verifying the technology globally and expanding the research to explore other isotopic markers. The goal is to develop climate mitigation strategies and achieve sustainable development goals related to climate action and life on land.

Keywords
tree rings, climate conditions, past droughts, floods, wildfires, nitrogen, forest ecosystems, climate extremes, isotopic markers, N15, climate change impacts, future climate projections, public awareness, policy, forest conservation, climate change mitigation, sustainable development goals

Takeaways
Tree rings provide a historical record of past climate conditions, including droughts, floods, and wildfires.

  • Nitrogen is a critical element for forest ecosystems and its availability affects forest health and productivity.
  • Climate extremes disrupt the nitrogen cycle and have a significant impact on forest ecosystems.
  • Isotopic markers, such as N15, can help understand climate change impacts on extreme weather and future climate projections.
  • The research aims to influence public awareness and policy regarding forest conservation and climate change mitigation.
  • Challenges include verifying the technology globally and exploring other isotopic markers.
  • The goal is to develop climate mitigation strategies and achieve sustainable development goals related to climate action and life on land.


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Medha Chaturvedi (00:00.046)
What is the similarity between Western marriage traditions and a tree? Why it's the rings!

Medha Chaturvedi (00:23.694)
And welcome to My Sediments Exactly, where we take nothing for Granite! According to latest research, last year was confirmed as the hottest recorded summer season globally since 1850. Breaking through the canopy of previous records, this is largely attributed to climate change's rising tide. Now, when I heard about this, honestly, I was stumped.

Analysis of tree rings, nature's own dendrochronological diary, stretching well beyond the roots of Victorian era records, has provided a revealing cross -section for these temperatures. This evidence suggests that last year's summer's heat was nearly 4 degrees Celsius warmer than the coldest documented summer in the last 2000 years.

This comparison highlights the unprecedented nature of our recent climate transformations vividly and alarmingly, depicting the significant alterations that Earth is currently undergoing, as clear as the growth rings on a tree. By reading between the lines, or in this case, the rings, of our planet's natural archives, we gain a deeper understanding of how our current climate crisis is off the charts, making it as plain as the rings on a tree that we are living through extraordinary times. Now, before I start getting sappy, let me welcome our guest today, Professor Xihong Xu. Professor Xu is a soil plant climate system scientist from Griffith University Australia, who has dedicated his life to studying the phenomenon of climate extremes and resulting disasters through the lens of tree rings.

With over 400 publications and a knack for unerring important climate research, Professor Shuh was currently a member of the Center for Planetary Health and Food Security and previously headed the Environmental Future Research Institute, is also the editor -in -chief of the Springer publication, Journal of Soils and Sediments. Having garnered over $24 million worth of external funding for his research,

Medha Chaturvedi (02:43.598)
and serving as an ambassador for academic collaboration between Australia and China, Professor Xu's ongoing research on the impact of climate extremes on nitrogen cycling in forest ecosystems is leading us on a path of better understanding and preparedness for climate extremes like droughts and forest fires better. His research contributes to the Sustainable Development Goal 13, which is climate action by enhancing our understanding of how climate extremes affect terrestrial ecosystems' nitrogen cycling crucial for mitigating and adapting to climate change impacts. Welcome to my sediments, exactly, Professor Xu. We are very happy to have you here with us today. 

Prof. Zhihong Xu
Thank you very much. 

Medha Chaturvedi
Could you tell me how exactly do the tree rings serve as natural archive for studying past climate conditions? 

Prof. Zhihong Xu
Thank you. You know that the tree growing every year in the growing season, it stops growing. That means every year you have a ring. And then you can go back to the history of the tree. For example, a tree is 200 years old and you can see 200 rings. And that means we can actually date the year where the tree was growing. For example, what the climate of that time and also what's wildfire may or may not happen nearby. 
So we're using our newly discovered N15 discrimination technology we call Turing Delta N15 technology, which I think is the first time in the world. Not only we can use that technology to discover and reconstruct the past drought, that can be done by other technology, but the advantage of Turing Delta N15 is that we can also reconstruct the past droughts and the flood and the wildfires and the in -space, in time. That means that we can actually really look at the climate change and the past and how they impact on the extreme weather now and also associat it to disasters like wildfires. 

Medha Chaturvedi
Thank you very much. That is really fascinating. You talked about N15. Now for people who are listening to us, could you give us a brief explanation on what that might be? 

Prof. Zhihong Xu (05:13.102)
Okay. Well, the N15, you know that the most of nitrogen in the atmosphere or in the soil plant system or in the nature on earth, 99 .6 % of nitrogen is N14. That means they have 14 protein in the electron layers. But the N15 is where Nitrogen has one more neutron. So that means they are heavier. And when the nitrogen isotope is leaving the system, for example, as a gas or leach through isolation, the N14 is moving faster and the N15 is slower. So by the end of the process of leaching or nitrogen gas loss, the N15 will be enriched in the soil or in the tree, in the plant. But the gas leaves the system and is lighter. So that is what we call the discrimination against heavy N15. That means the N15 is heavier while remaining in a soil plant system. This would be taken up by the trees of that year. So you know, you get a flood, for example, or get wild fire. Because when you get a flood, you get a lot of nitrogen loss. That means the N14 will leave the system more and the small amount of nitrogen that is left, will be heavier. If you get a wild fire, that means you get a lot of nitrogen deposited from these adjacent wild fires, you have a lot of nitrogen, when you have a lot of nitrogen, you're also likely to have a lot of nitrogen loss. So the N15 will be enriched too.

Medha Chaturvedi
That is so interesting that with just by measuring this N15, we are able to indicate and understand how the forest fires and droughts occurred. That's really exciting. My next question would be, why is nitrogen such a critical element for forest ecosystems and how does it affect their health and productivity? And in what ways do climate extremes disrupt this?

Prof. Zhihong Xu (07:20.718)
Yes, well first is this will be very useful to understand extreme weather, for example drought and the flood and the wildfire and the impact on forest ecosystems itself. And the most extreme weather like a drought, they have a severe impact, negative impact on the forest health and the grass. Likewise, on the flood actually, although they do not have a significant impact on the ground because they just come and go in a few days. But they have a significant impact on the nitrogen leaching process. That means that when you have a flood, they wash away a lot of nitrogen. This nitrogen is getting poorer. That means after the flood, the trees may experience severe nitrogen limitations. And the wildfires, you know, they burn a lot of forest. So to understand the climate change impact on wildfires in the past, also helps us to look ahead to management the fire risk as well. 

Medha Chaturvedi
I see. And what is the connection of this to the original question we were exploring with regard to the tree rings as environmental records?

Prof. Zhihong Xu
Well, we use tree ring as a historical archive because you can date it back 50 years or 100 years or 200 years, whatever time the tree lives through. But N15 is a very powerful technology. You can see the climate extreme, as I said, for example, drought frequency and intensity. Likewise, flood frequency intensity and the wildfire frequency intensity by understanding its climate change impact on the extreme weather and you can understand better the future climate extreme. 

Medha Chaturvedi
I see. So when you say that we can understand the future climate extremes, how do you propose that we could do that based on historical archival records of climate extremes? 

Prof. Xhihong Xu (09:35.534)
Because the trees is on continual growth, for example, from the past to present. If you can see in the past 50 years or 100 years, and the climate changing extremes are intensified or increased in frequency or intensity, and you would project that in future, that will continue to increase even more. For example, 50 years ago, in some places, the flood probably would be expected every 50 years. But now, in the last 20 or 30 years, they might have a three or four floods or two or three droughts. That means the frequency and the intensity of flood and the drought and the wildfires are significantly increased  through records of tree rings objectively. It's not just a thought, you know, it's basically objective by the same tree, which is standing there experiencing the real climate change.

Medha Chaturvedi
I see. And have you been able to use these findings to actually check, to actually test your hypothesis and to see if these are accurate? 

Prof. Xhihong Xu
Yes, we are still testing that because we have done a lot of soil work. Look at the nitrogen leaching, de-nitrification. That means nitrogen loss, its impact on N15 enrichment remaining in the soil and which is ultimately taken up by the trees after drought or after flood or after wildfires. And we have been testing that in China, in Australia, in sub-tropical forests, in boreal forests, in temperate forests, it seems the nitrogen cycle is the same everywhere. And the climate changing pattern, particularly the increasing extreme weather and the wildfire pattern, is also the same everywhere. So that means that climate change is common in many parts of the world. 

Medha Chaturvedi
Okay. Are you also planning to explore other isotopic markers in this research and this expanding field of research that you've undertaken? 

Prof. Zhihong Xu (11:52.91)
Yes. I think we're going to be using some existing methods, which is a known method, which can test the drought, like tree ring growth and carbon iso -deta C13 and the deta oxygen 18, it's mainly related to water. But the water, you know, that is clearly implicating climate change. Climate change, one of the key features of climate change is getting warmer and drier on land. That means water is becoming more limiting, but with increasing climate extremes, like drought and the flood, the intensity and the frequency are increased. So, I think that we're going to verify that with other traditional technology. But uniqueness of the Theta -6 N15 is they can pick up flood and wildfires while the others cannot.

Medha Chaturvedi
I see. And to come to you with regard to the beginning of this research, my question would be what motivated you to undertake this research because it's so fascinating and to use this kind of chemistry to indicate what kind of climate history can show us about climate futures. So what motivated you to undertake this research?

Prof. Zhihong Xu
So basically, you know, climate change is very complex. It's increasing through the greenhouse gas, which leads to increasing temperature and the temperature driver water cycle, for example, subsequently. And with more evaporation and extreme weather, that means you get more drought because once they get warm and dry, there's more vapor or moisture in the air. When they hit with the cold flood and then you have a lot of water coming down as a flood. So I think the carbon cycle, nitrogen cycle and the water cycle are closely linked through climate change in land, so in space, in time. But climate change has passed the tipping point and the carbon, particularly tree growth, benefits from increasing CO2 concentration also passed tipping point in the last 20 or 30 years. So the more CO2, the less growth (of the tree) because water is becoming more limiting. And likewise, once the carbon cycle has passed the tipping point, the nitrogen and carbon go very closely. So it's very likely in the last 20 or 30 years, the nitrogen is also becoming more limiting. So that means the carbon cycle is likely to give positive feedback on climate change. That means we need to more to cut the emission because forest capacity to take CO2 are getting less and less as climate change intensifies. 

Medha Chaturvedi
And what has been your biggest challenge in this research so far? 

Prof. Zhihong Xu
I like to verify this technology globally, at least in Europe, in the US. That means we get a global pattern of flood, drought, wildfires in the past 50 to 100 years. And by doing that, we can also understand regionally, locally, how the climate change impact on climate extremes. And subsequently, that will be very useful for disaster management, such as flood management, wildfire management, and drought management. And to minimize the loss of life and the property, but also adapt to the changing climate.

Medha Chaturvedi
Professor Xu, do you hope that your research will influence public awareness and policy regarding forest conservation and climate change mitigation? And what are your goals for this? 

Prof. Zhihong Xu
I think the understanding, the pattern of climate extremes, particularly drought, flood and wildfires have a significant impact on ecosystems and biodiversity and also carbon sequestration. That is likely to give a much better understanding and helping the modelers to project the future climate with the real data and with empirical data, not just the modeling by itself.

Medha Chaturvedi (16:19.246)
And you are the editor in chief of Journal of Soils and Sediments. Do you think that in the near future you will be hosting a collection or a special issue which is based on climate mitigation strategies for future, basically future proofing ourselves and working with this particular theme of science in climate mitigation strategies? 

Prof. Zhihong Xu
Well, I think as the research progress, we are actually planning one of the special issues of Journal of Soil and Sediments, which particularly looks at the wildfire impact on soil and sediments at regional and global level, I think the special issue will be open to the call for the submission that would be closed by the first October 2024. And hopefully in three to six months time, we have a special issue coming out in next year which is just the right time to celebrate the 25th anniversary of June of Soil and Sediment. 

Medha Chaturvedi
Wow, that's a really big landmark. Congratulations. Professor Xu, since you are not only a researcher but also an educator, what advice do you have for aspiring scientists or for young early career researchers who are pursuing research in environmental science and climate mitigation strategies?

Prof. Zhihong Xu
Well, my advice to early career researchers and the student is while the climate change are happening and faster, but we need to understand them and also understand how to adapt management and the risk of the climate change in extreme, but also understand the urgency of climate change because it is getting more intense of the flood and the drought and also get more frequent. So we really need to get to the bottom of the climate change. That means that we really need greenhouse gas emission and the changing the life pattern and make sure that we live on the renewable, clean energy.

Medha Chaturvedi (18:35.534)
All right. And my final question to you would be in what ways does your work contribute to achieving sustainable development goals related to climate action and life on land? And how does that impact our day to day life? 

Prof. Xhihong Xu
Well, as I mentioned just earlier, the sustainable development requires a sustainable energy and renewable energy because energy is one of the key things driving the modern society, but also one of the key side effect of energy or of the past fossil fuel is also if we don't manage properly, it can also lead to adverse impact of the technology revolution. So that means that by learning the past, we can also doing better and also make sure that the energy has positive rather than negative impact on the society and on the industry development. 

Medha Chaturvedi
That is really interesting to hear, Professor Xu. The past is the key to our future. Thank you very much for joining me today. And I hope that we can interact once again to see how your research progresses and where it leads us in terms of developing better climate mitigation strategies. Thank you, Professor Xu. Thank you very much. 

Well, there you have it. She liked it and she put a ring on it. By she, I of course mean the climate conditions of the past and by ring, well, it's the tree rings. Since we can't go back in time to learn about past climates, scientists are now relying on natural sources of climate data such as these tree rings. And what makes us even more exciting is that these tree rings are so plainly visible for everyone to see.

They not only indicate the age of a tree, but also tell the story of the climate conditions that that tree has withstood. What has been is the key to understanding what could be. I hope that this conversation has led you down a path of curiosity, because remember, the conversation doesn't have to end here. Dig deeper, ask questions, and engage further with this tremendous research.

Medha Chaturvedi (21:02.51)
Subscribe now to MySediments exactly wherever you get your podcasts. This is your host Modha Chaturvedi signing off and until next time, Rock on!