- 11/5/2025
Talk at keep it wild event, Cederberg
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00:00This is the missing talk yesterday in the event Keeping It Wild in Cedarburg.
00:04I will talk about the enigmatic ecology of cedar seen through three rings.
00:09This research is part of a project that is ongoing now that's called Paleo Cedar.
00:16It's a collaboration with colleagues from the University of Stellenbosch, David Drew, Anton Kunike, and Ed Febrey-Dawi-Burger from Sustainable Cedar.
00:25And also with colleagues in France, Spain, the Witt University.
00:33I will start by giving a historical context of cedar touring research.
00:39Then talk about the decline and this elusive ecology of cedar that we don't understand why the species is declining.
00:48And then I will finish with a natural continuation to the previous research, which is what we are doing in this Paleo Cedar project.
00:58And I will show you the advance of two questions that we get.
01:03So first is when the trees grow and then what is the environmental signal stored in the tree rings of cedars?
01:10The Cape region is one of the five Mediterranean biomes in the planet and it's the smallest Mediterranean region and it's all the area around the Southern Cape area.
01:24The Mediterranean climate of the region is shaped by the convergence of two currents, the Aguilas current and also the Benguela current, which is a cold ocean current that cools the climate and also contributes to the formation of this Mediterranean climate here in the tip of Africa.
01:47This is a view of a cedar around 1972, when a group of dendrochronologists from the Tucson touring lab in the U.S. went to visit the region.
02:00They were doing a survey of species suitable for touring analysis and climate reconstruction in the Southern Hemisphere.
02:09So they went to South America, they went to Australia, and they also went to Cedarburg in Southern Africa.
02:15Then they were received by Fred Kruger, who was a forester at the time.
02:24He took them to different places in the Cape region, and they sampled intensively the area of Cedarburg.
02:32But there is one particular site there that was important, and it's Dievoss near Langloof.
02:40The analysis of this site led to the first dated touring chronology in Southern Africa.
02:48They found that the rings are annual, but they have a poor correlation with rainfall.
02:57So this is the first view of tourings, and in the publication they show this figure where we see the ring boundaries,
03:06but we also see several fluctuations that look like false rings.
03:12And here they show, for instance, in the ring of 1786, they show a band, a resin band.
03:22So that is probably related to some fire.
03:25And the year after, in the spring in 1787, they find a frost ring here.
03:35So there was a very cold spring that killed the cells of the wood.
03:41This is how the time series of each tree, the growth of each tree that they sample.
03:47They sample here around 30 trees, and so they sample several young trees that were around 100, 150 years old.
03:59And then they sample the oldest tree was this one here below that has 356 years, and it was a living tree.
04:10And then they sample also dead trees, dead wood here that is the one that reached the oldest rings.
04:17And here the chronology, what we call, this is the average of all these trees for each year,
04:24start in 1564 and finish in 1976.
04:30By that time, this is how the Cedar Forest looked.
04:35Then, a few decades later, we saw this.
04:40There is a massive mortality and decline of the species,
04:43and it occurred in all over the distribution of the species.
04:47The same thing in other places, and the trees have disappeared.
04:53But by then, we're already showing signs of mortality or decline.
04:57And another feature here is that the grassy vegetation also increased in the present compared with 1976.
05:06And to understand why Cedar can be dying, here's where, in February, he conducted a research around the 2000s.
05:17And that was then published in 2007, where he used water tracer isotopes, one isotope that we call oxygen.
05:29And this isotope allow us to, it's like a marker, so allow us to differentiate between different water sources.
05:40And if we measure it in the leaf of the tree, we can know if it's drinking water from the rain or surface water or deep water.
05:50In the paper, not only he, they found that radial growth is not related to precipitation.
05:57They monitored it with band dendrometers and found no correlation with precipitation.
06:04And the other important result here was that Cedars depend on deep water.
06:11So I call it rock moisture.
06:13So this is the water that seeps through the rocks and it's stored there over the summer
06:19and is the water that the trees use to sustain growth.
06:22By around 2000, 2003, this was the view of Cedars in the research site that Ed chose, which is in Belvedad.
06:33And we see that there are dying trees here and we know everything's of fire so far or recent fire.
06:42And so he conducted some update of some surveys that are permanent forest plots established in the 70s.
06:51Here is a view of a tree dying here without any evidence of direct fire.
06:57But then there were other sites where fires have recently burned young regenerating trees.
07:04And this can still be seen in the nearby site in Drukh in 2024.
07:10These trees that die very slowly.
07:13So here is a view of one of the instruments that Ed used, band dendrometers to measure the growing girt of the trees.
07:25And he installed a meteorological station as well there.
07:28Here is his main result.
07:31Was that so he measured the rain monthly here and the growth also measured monthly for a group of trees in green.
07:41And we see that there is a clear trend.
07:45So the trees on average grew around three millimeters radially.
07:49But there is no clear seasonality.
07:52And in some cases here we see that the growing increase in winter.
07:58But I think this is related to the fact that when the bark wets by the rain, it expands.
08:06So there is this noise with this type of instrument.
08:10Then 14 years later, the latest paper published on cedar tree rings is by a colleague and project member in Belgium.
08:20And with Ed as well.
08:21Where they took these samples that were collected by LaMarche in another store in the US, in the Turing lab.
08:30And analyzed the carbon and oxygen isotopes in the tree rings for a period of 30 years.
08:37They measure also density.
08:39And made a new analysis with climate because until now we don't find a good climate signal for these cedars.
08:53So they thought that by measuring other proxies, what we call, we could find a better signal.
09:00And they found that minimum density, which is related to the spring growth, wood density, the minimum wood density in the spring is related to the size of the cell.
09:12So if the growth is fast, then there is less density.
09:16So in a way, the spring growth here was correlated with October-November precipitation.
09:23And the signal here is the correlation at this point where the chronology is against gridded climate data of all the region.
09:33Basically, the precipitation over all the Mediterranean region of the Cape area is correlated with the growth of cedars here.
09:43But the relationship was not that strong as to do a climate reconstruction.
09:49But they used just the samples that were stored in the lab and that were collected in 1976.
09:56So we don't know what has happened after 1976.
10:00Here is the situation today.
10:01So if we plot all in the map, all the Turing site samples in the world, which are the pink crosses, we see that in Southern Africa there is only one dot and that's Diebos.
10:15And until recently, this site has not been updated since 1976.
10:22This motivated us to visit the region in October 2023, where we met Dawi and one of his collaborators in sustainable cedar.
10:38We went to recognize the forest, what's the state, and also to talk with Monique in Cape Nature to see if we would have the chance to get permissions to work with cedar.
10:53After that visit, we brought the paleocedar project where we seek to quantify the ecological resilience and the climate refuge decline of the species.
11:03And we hypothesize that the species is resilient to changes in climate up to a point, but that the refuge has shrunk mainly because of increased fire intensity and frequency.
11:19To answer these questions, we define four work packages or objectives.
11:27The first one is to conduct a truing sampling and analysis.
11:32And the second is to monitor the environment and the growth trees with dendrometers.
11:38And the third one is to do what we call wood growth monitoring, which is a sampling that we do every month in some trees with a small tool to get a small piece of wood and study the cells, how the wood is developing.
11:56With that, we want to train a model to try to model the growth of cedar.
12:01The final objective is to do a comprehensive historical analysis of the climate refuge and of the fire regime in Cedarburg to understand how the physiology of the trees is responding and also to try to disentangle it from the effect of changes in fire.
12:23For the analysis, we are using tools that I call physiological dendocrinology, which is the use of multiple lines of evidence from the same brain.
12:38So we use these automatic dendrometers to monitor tree growth every 15 minutes.
12:44And at the same time, we take these samples, this microcoding sample where we see the developing wood through time.
12:52And that tells us about the biomass growth process.
12:55And then in the same range, we analyze the isotopes, stable isotopes, which is carbon and oxygen, which are these chemical markers that tell us about how the tree is using the water and what type of water it's using.
13:10It's what Ed used before.
13:12And also we measure with the dendrometers, the formation of the root rings to see how the trees are locating growth to the stem and the roots.
13:24For the study, we selected two sites.
13:27One is in Druuk, near the house of Dawi, and also Diebos, which is the site that the colleague from the Tucson Laboratory sample in 1976.
13:42In May, we visit the sites to start to do the first sampling campaign.
13:49And we went with Ed to visit one of his sites.
13:54And here, while reflecting under the shade of a nice cedar tree, Ed summarized what we seek for this Paleo-Cedar project.
14:02We visit Diebos in the 24th of May, and then in Druuk, we started to install the dendrometers, which is anchored in the stem of a tree and then in the same tree, one in the root.
14:19And this is how the instrument looks, and they measure the fluctuations in size here in this in this part of the of the instrument.
14:29We covered the dendrometers with a with a small hat to avoid the effect of the rain, wetting the bark and then fooling us with these changes in bark hydration.
14:43We took samples with with an increment border, so this is five millimeter diameter core where we can see the rings of the cedars.
14:55We got help from our local partner for sustainable cedar, the burger family with the with the hard task of coring and we tag every tree with a permanent tag and also geotag to follow them through time.
15:14And this is the view finally how the rings of a cedar look like. This is a first tree sample in Diebos. This is a very important site for us.
15:26So we we we see here the cells. So this direction of growth, the growth start here and finish here in a year.
15:34So we see the first thing is we see very clear boundaries, very clearly defined rings here in in this part in this tree.
15:41And we see in the wood very big cells with small cell walls here in the spring.
15:50So there is a lot of space in the wood, less dense and then that would be the spring wood and then that transition toward the summer wood where the cells, the size reduce and also the the cell walls become thicker until the end of the ring here.
16:07So these two type of wood are visible in these rings.
16:13And then here we also obtain at the time the first snapshot of wood formation with this micro course.
16:22So when we were there in May 2024, this is the sample. Here we see the wood, the xylem is dyed pink.
16:30So the pink is the mature wood and and the here there is one part that is bustling, which are cells that are forming.
16:42And we were here in in May. So the ring was still forming.
16:46We don't we did not know if the ring of 2023 that start in the spring of 2023 has finished it or the growth just stopped during the summer and then resume in the early autumn.
17:01Now here is in more detail of the same sample. So we see here the cells.
17:05So we see the spring wood, the transition towards the the the summer wood and then we got to a point where the cells are very small and very thick walls and those are the mature cells.
17:16But then there was this band here with maturing cells.
17:21So they were first they divide and then they start to fill the walls and the flat cells here is the cambium.
17:29So those are the the mother cells. It seems that there is a main growing period, but then there is also a second growing period after the end of the summer.
17:40That is what we could see with the first snapshot. And then when we go and see a sample, a wood sample of three rings, we see that there are these bands in some of the years.
17:58So the the growth direction is from left to right. So and here we see that what we saw in that snapshot basically explains what occurs in in several years.
18:09So first we have the the nice spring wood as the summer comes, the cells become thicker and smaller and then sometimes the growth stop and then it continues toward the beginning of the autumn for for a bit.
18:24And then in the winter is it stops. So this this dynamic seems to be a very common in in Druuk.
18:33As a bonus is a view of rooibos bush with anatomy and we see that the rings in the rooibos are very also clearly visible.
18:44So this this plant has around five or six years old. Our second line of evidence are the dendrometers.
18:52Typically, in a dendrometer record, we see variability that is reversible and superposed on seasonal trend.
19:02So what we see here is that the seasonal trend is the growth in wood of the tree and part of bark as well.
19:13And the reversible variability is due to the hydrological pulse.
19:19So it has to do with swelling and shrinking of the tree due to hydration.
19:25So the tree shrinks when it dehydrates and swells when it hydrates.
19:32There is a diurnal pulse here, for instance, that we see that the small variability and also weakly variability.
19:39So this is how the hydrological pulse can be seen in trees.
19:46Then if we look at the root of the same tree, we see that the hydrological pulse is much smaller.
19:53So the variability diurnal or weakly variabilities are much smaller and there are mainly steps in growth.
20:01So these are pulses of cells that are making up the seasonal growth of the tree.
20:05So first thing we see here that the growth is not continuous.
20:09It occurs through pulses.
20:11In order to interpret the hydrological pulse of the trees, we have to measure the environmental variability.
20:18And one of the most important factors here is soil moisture.
20:21We measure it at 10 centimeters, the gray shading here, and at 40 centimeters depth, which is the black line.
20:29So what we see here is that when there are rains, there are these pulses in soil moisture at all depth.
20:38Every time there is a rain event, when the tree is growing, there is a pulse in hydration and there is a pulse in growth as well.
20:48So the rainfall events are very important for the dynamic of the tree and also for the development of cells.
20:58The other factor that is important is the dryness of the atmosphere.
21:03And we also measure it at the same site every half an hour.
21:09This is related to temperature monthly.
21:12And we see that when there are days, sunny days or very hot days, particularly here in the summer, with high values of dryness in the atmosphere, the trees shrink.
21:24So if we put it all together, the trees swell and grow when there is rainfall and the trees shrink when the tree dehydrates due to very dry atmosphere.
21:42So the tree transpires a lot and then it dehydrates.
21:45Now we see how is the behavior of other trees.
21:49This is for Diavos. The thick lines are individual trees.
21:53So we have eight trees instrumented.
21:55The fastest growing trees reach to around two millimeters in radial growth in the growing season.
22:02The growth start in October.
22:06So mainly start to the tree, probably in September, October, it start to form cells and then it go very fast over the spring and tend to slow down a bit towards the summer
22:18as the soil dries.
22:20Also, we see here some roots.
22:22The thin lines are roots of the respective trees.
22:27And we see that for some trees, the roots really follow the water state of the soil.
22:32So, for instance, the green tree here is the tree number five.
22:36The root has shrunk over the growing season.
22:41So we cannot distinguish there how much it has grown because the dehydration of the tissue is bigger than the new cells that it might have added.
22:52And another interesting feature here is that small rain events over the summer, the rainy period, usually when it's driest,
23:01it cause a really disproportionate effect in tree growth, similar to more stronger rain events in the spring.
23:12So it's summer here produce every drop of rain, produce a big response in the cedar.
23:18This is in Die Vos and then we have also another site, which is in Druc, which is a drier site somehow.
23:26And we see here that the trees follow the same pattern.
23:31So start to grow in here more clearly in September, September, early October.
23:40The growth goes very fast up to the end of December.
23:45And then we see that somehow the trees start to shrink.
23:49And that means that probably here new cells are added and then the trees start to experience physiological drought.
23:58And another interesting feature here is that in March, there was no rain apparent here.
24:09But we saw in all the trees, in the stems and the roots, very, very strong hydration response.
24:16And then the trees continue dehydrating after that response.
24:21And we analyzed the data and it turns out that this is related to dew.
24:28So in these weeks, there was abundant dew during the night.
24:34And even though he went to download the data, he saw that there was a lot of dew in the morning on the trail.
24:42So this response is revealing that trees are able to use dew precipitation to maintain growth and hydration.
24:54So this is a new result that we could not anticipate.
25:00And probably it's also related to the rocks where the trees are.
25:04Probably the rocks are, they cool down strongly in the night.
25:08And then there is a lot of dew deposition on the surface and they funnel water toward the root of the trees.
25:17With these results, now we can make a conceptual model of how the seeders are growing.
25:23On the background, there is one ring where we see the spring wood here, very, very big cells.
25:33And then there is a transition toward the summer wood with smaller cells and thicker walls.
25:41And then if we measure them in the dendrometer, we see that almost half of the growth occur in the spring.
25:47And then the other half is in the summer.
25:51So that transition from spring wood to summer wood is related to also the soil moisture.
25:59So the summer would start to form when the soil start to become drier and when there is no water value for the trees in the surface.
26:11And then we see these pulses in the late summer and early autumn where growth again resumes with the first rains and eventually due precipitation here might contribute also.
26:28And then the growth stop in probably around June when it's too cold and then it starts in the next spring.
26:38As a result, in a seeder ring, we have two periods.
26:42One period with fast growing in the spring when rain still occurs.
26:48And then we have a second period, which is the summer wood, where it occurs mainly over the rainless period.
26:56So with this conceptual understanding of how a seeder grows, we can start to understand the climate signal that's stored in the rings of seeder trees.
27:08In order to do that, we compare the available to rain chronology here in Diavos with the hydrological record of river runoff in the nearest station in the Doreen catchment.
27:22So it's the catchment that drains the cedar bird.
27:26Here is the spring runoff since 1924 until 2023.
27:34And this is average over October, December.
27:38And then the black line is a reconstructed runoff over the common period.
27:44And we are able to reconstruct 54% of the variability of the observations.
27:51So this is an excellent spring hydrological signal.
27:57Normally, in true runoff reconstruction, we explain between 30 and 50% of the observed variability.
28:04And here we reconstruct 54%.
28:06So this is very good.
28:08And also, if we verify our calibration over the period before 1940, we are able to reconstruct around 40% of the variance still.
28:20So this is a very, very good result for a climate reconstruction.
28:25Here is the entire climate reconstruction.
28:28This is a landmark result from cedar two rings.
28:33And so we have now reconstruction of river runoff that start in 1564.
28:39And if we put it together with the observation, we can reach the present.
28:45So we see here that there are periods of very high runoff, particularly at the end of 1700 and in the earlier part of the record as well.
28:59But to visualise better the variability, we can standardise the record.
29:04And so in blue is the wet years and in red are the dry years.
29:11And we can really see that the period around 1790s was very wet.
29:17And again, around 1690s is also very wet.
29:21And overall, we tend to see a much wetter condition before 1800 than after.
29:30We see that the recent drought has been an unusually intense dry spell in the context of the past 400 years.
29:39We can now also interpret the history of the region on the light of this hydrological record.
29:46Here we can see that by 1652, when the Europeans arrived to the Cape region, it was indeed a wet period.
29:58And then the first farm was established near Clangwilliam at the end of a dry period.
30:07Clangwilliam was founded right after the wet spell here.
30:13And also it's interesting to note that the arrival of the first settlers in the Clangwilliam region around Siderberg, it occurred in a very, very dry decade as well.
30:26We can also validate this reconstruction with available historical records for the region.
30:32Here, there is an example of a recent study by a project partner, Stephan Grapp, where they used historical records to reconstruct 18 years of the climate at the end of the 1700s.
30:45And they find an extreme weather and climate variability in this period.
30:50Interestingly, they find that the longest recorded wet spell in the whole historical record of the last 200 years occurred in 1787, which is the wettest year in the reconstruction.
31:07Here, if we integrate over a longer time period, here is a decadal variability in the reconstructed runoff.
31:15We can see that this period was very, very, very wet.
31:20Indeed, it's the longest and more intense wet period.
31:24We can see that there around 1800, there was a shift over more drier conditions and with less frequent and less intense wet spells compared with the previous centuries.
31:38Finally, to finish, I would like to highlight some key message.
31:43And the first one is that growth peaks in spring and early summer.
31:49It slows or stops in the midsummer, but it continues for a period in the spring.
31:56So we have a sort of bimodal growth season.
32:00Also, by tapping dew, the seeders might power this secondary phase of growing late summer.
32:09So this hidden precipitation might be an important water source for the trees.
32:14We have also overturned a 50 year old scientific paradigm of lack of climate signal in Cedar tree rings.
32:21And we demonstrate that cedars are a very precise river gauge for the whole Cedar area and for the whole biome indeed.
32:31And finally, this reconstruction shows that there was a shift towards drier conditions and reduced growth in this biome since 1800 onwards.
32:43More important insights should come as we analyze the samples.
32:48Finally, I look forward to return to Cedar bear again anytime this year or early next year.
32:55I would like to finish thanking Anton Kunicki and Dowie Burger for all their help.
33:00Because without them, all this work would not have been possible.
33:06And I look forward to being back and work with them in Cedar bear again.
33:12Thank you very much.
33:14Thank you very much.
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