Whilst we have much to learn about how trees affect climate, the trees themselves are also extremely useful for examining our recent climate history. By looking at how much a tree’s rings grow year on year, you can get a good idea of how hot the summer periods were in that part of the world in that year. Cambridge University’s Ulf Büntgen has been tapping into just such a resource, and I went to meet him at his lab…

Ulf - There are different treeing parameters, there are different tree species, there are also different sites. All this has to be taken into account and then depending on our question, we have to basically adapt the methods, the archive, and the approach. So let's start from a simple example. We are interested in reconstructing past temperature variations. So all these trees, they record something that happens during their growing season, which is roughly the summer from spring into autumn. We have to go to trees that grow under conditions where summer temperatures or changes in summer temperatures are becoming most relevant for the growth. So the northern treeline in Siberia or in Scandinavia where small changes in summer temperature are already reflected in the annual growth rings. These trees would function almost like a meteorological thermometer for us. If it's a little bit warmer during these summer months or weeks, we would expect a wider ring. We also would expect the higher wood density. So how dense is the wood in a certain year? And with density we actually mean the ratio between cell size, that is a lumen, and cell wall thickness. And then another parameter beside ring widths and wood density would be wood anatomical parameter. So we can find really fine grain, go into a single year and then look at the cells and they maybe show us some anatomical features that also refer to a frost event or an insect affiliation. So things that are really happening within one of these growing seasons and are reflected or basically are reflected in anatomical anomalies.

Will - When you get this information, when you have a look at the rings that have grown on a tree year on year, and you see the difference in width. And if you get, say, a wider one after a narrow one, you go, 'we must have had higher temperatures this summer.' Are you able to discern the difference in degrees? How good is your resolution?

Ulf - The first important thing is that it's never the absolute width, but it's a relative change from year to year. So we would measure these different parameters, ring width, density and so on for hundreds of trees for hundreds or thousands of years, right? And then we build tree ring chronologies. Each of the trees that we consider will show slightly different growth variations because each tree is an individual and influenced by different things. If we merge the information, if we average the information from many hundreds of trees, we cancel out the individual 'noise,' so these individual growth variations. And we emphasise the common signal. So then we already get a very strong time series of what this forest basically tells us and what we need. Now that is the answer to your question. We need an overlapping period to instrumental measurements of temperature. So we know, for instance, for the last a hundred or 150 years, in some places, like in England, we even have longer meteorological records, right? And we need the overlap between the proxy and the measured temperatures. And in this period of overlap, which is usually the 20th century, we are building a model. And then we can transfer wide narrow, wide narrow into degree Celsius or millimetre precipitation or a drought index. And then we actually see what is worse that we are taking out of the trees.

Will - A massive advantage, as you've already said, is the fact that you have access to so many samples so close to each other. Because say for instance, you could only look at one tree and it may have had a blight one year, that affected growth. The ability to then look at 400 other trees and go, 'no, that was actually pretty good growth that year' seemingly gives you a big leg up over say drilling a hole in the Antarctic ice sheet.

Ulf - Yes, that's correct. And obviously the different climate proxy archives or records like ice cores, tree rings, they all have their advantages and disadvantages. So ideally the paleoclimatologist at the end wants to create something that we call a multiproxy record or approach, where we basically take the benefits from each of the records and disregard where they are less good.

Trees are currently coming under increasing attacks from parasites and pathogens. Perhaps the most renowned case is that of dutch elm disease, a fungal disease spread by bark beetles. Since its arrival in the UK in the 1960s, the disease has killed tens of millions of trees, and there is no cure. So, what causes this disease , and others like it, to be so destructive? And is there anything we can do about it? Leigh Greenwood is the director of The Nature Conservancy Forest Pest and Pathogen Program…

Leigh - So when we talk about the different pests that affect trees, sometimes it's easiest to understand the ones that we can physically see. So sometimes we think a lot about the beetles because you can see a beetle, you know what it is, you know how it works. But oftentimes the beetles themselves aren't the primary way that the damage happens. They are spreading a fungus in many, many cases, and that beetle fungus relationship is actually what kills the tree. And these diseases can be fungal, they can be bacterial, they can be other kingdoms. They often clog up the circulatory system of the tree. So what happens is that they disrupt the sap movement in a very, very critical way. And so the circulation system of the whole tree fails and that's why they can cause such severe damage.

Will - Do we know what is boosting these? Are they becoming more common or are we becoming better at monitoring them And that's why they seem more common? Or are cases actually on the rise?

Leigh - Cases are actually on the rise because nearly all invasive forest pests and pathogens are being moved due to human activity, including international trade tourism, immigration, and human caused changes.

Will - Is there a climate angle to this as well? Because as the climate warms, we are seeing the redistribution of many animal species outside of their regular habitat distributions and trees can't move. So it seems like they're going to be first affected if a new wave of a certain bug or beetle comes into town.

Leigh - Yeah, there's several different important climate aspects here that are really interacting when a tree is at the edge of its sort of comfort zone. So if it's at the southern border of where that tree's ecosystem keeps it healthy and thriving, then as climate change pushes that southern border into a hotter and drier state in the soil or the air, that tree will stress and stress. And both native and non-native pests will have much more dramatic effects on it because the defences that that tree could mount are probably so busy with the other stressors that that becomes a much more vulnerable tree. Another element is that because global trade is driving a lot of this problem, when you trade from areas that have a well matched environment, such as a hot and wet forest to a different hot and wet forest, you're much more likely to move a forest pest and pathogen that could survive at a new place. But when you're trading from somewhere that's hot and wet to somewhere that's cold and icy, it's basically not going to happen. When you add climate change on top of that idea, more places are getting hot, so more places match and the pests and pathogens can match up when they go to the new place.

Will - As with every disease, it feels like an ounce of prevention is worth a pound of cure. So is there anything that can be done to prevent these outbreaks before they get worse?

Leigh - Absolutely. There's two different important elements to prevention, one of which is really strong rules and adherence to the existing rules of international trade and travel. So when you see a sign that says, you know, don't bring any fresh fruit on this trip to a beautiful tropical island, take that seriously. Don't do it. It's not so they can steal your fruit. It's to prevent the movement of some sort of, you know, agricultural pest. But then beyond that, if you see an unusual pattern of damage or even tree death in your neighbourhood, you should absolutely tell somebody because sometimes these things, they're seen first by somebody who's just really familiar with their street and their trees and their neighbourhood, and they know they've never seen an insect like this or they've never seen a tree die in this very odd way. And reporting that newness, that moment of sort of, wait, I've really never seen a tree look like that before, is how with the vast majority of these forest pests and pathogens get reported to the authorities

Will - And anything could be done for the unfortunate populations that are already suffering their blights?

Leigh - That's such a great question because in many cases, the horses have left the barns. When we look at the forest pests and pathogens that we know the names of that have been here for a long time, some of them do not have strong management for existing trees. And for those we have to look to the future of selective breeding. If you find two trees that are doing very well, when all the other trees around them are suffering from these blights and fungi or beetles, you can actually take them into a tree nursery and breed them and then multiply that effort out across the whole ecosystem and create a generation or more generations of improved trees that have resistance. We have several different types of trees that we are creating improved tree programs for, some of which are showing very good success right now, some of which are still in their infancy. We have to look really strategically into the future. If we cannot fix it today, what will make it better in the future?