You know what's wild?

Sometimes, cells don't need to be alive in order to do their job. In some kinds of tissues, cells can be completely deceased and yet still dutifully working to keep the organism alive.

This is notable in plants, where trees take it to the extreme. Trees don't mind being full of dead cells -- it is their way. But archaeologists?

Archaeologists hate it. Finding an ancient piece of wood can frustrate an archaeologist to no end. Here’s how trees pull off living dead status, and exactly what those archaeologists are so annoyed about. [INTRO] Trees contain a surprising amount of dead tissue.

To some extent, most plants do. All higher plants have a dead tissue called xylem used to transport water around the plant. Xylem is basically just plant plumbing.

It's made throughout a plant's life when long rows of special double-walled cells grow and then die, leaving their skeletons behind for the plant to use as a straw to move water and minerals through the plant. Trees and other woody plants have perfected the use of xylem. Trees are tall, and they need a way to get nutrients from the roots to the crown where the leaves and reproductive parts live.

At some point in tree evolution, woody plants like trees discovered it was easiest to just make a superhighway system out of dead tubes to transport their juices from their bottoms to their tops. In addition, xylem provides mechanical support to the big heavy tree. Xylem in the trunk provides scaffolding to keep the whole plant from drooping over.

The live cells in a tree are concentrated in the outermost portion of the trunk, called the sapwood. The xylem there contains living parenchymal cells whose business it is to do things like help the tree grow, heal wounds, and store nutrients. But, in the case of most trees, the center of the trunk -- the heartwood -- is formed as the tree grows, and all the xylem in there is compressed and dead as a doornail.

However, heartwood is incredibly strong and resilient, so it keeps the tree standing. Trees have figured it all out! They're not just surviving, they're thriving!

But first, since all science needs funding, let’s take a quick break. This SciShow video is supported by the Presidents of Science: McLaren Stanley and Charlie Stanley! This month marks one year of gracious and benevolent contributions by the Stanleys!

That’s a dozen SciShow videos funded by these incredible patrons! Our presidents certainly keep SciShow running. And thank you to every SciShow patron, from our presidents to our two dollar per month Curiosity Investigators.

If you wanna join the cabinet yourself, head to patreon.com/SciShow. Great for trees, right? Yet archaeologists take major umbrage with this whole approach.

They're vexed to the point that they've officially named the Tree Way Of Doing Things the old wood problem. I know. Pretty scathing.

But this is why archaeologists are beefing with trees. Archaeology deals heavily in carbon dating to create an accurate picture of when things happened long ago. Carbon dating can tell us how long ago an organism died.

Carbon dating works because our planet has lots of the normal vanilla flavor of carbon, called carbon-12. And it also has a fair amount of another flavor, called carbon-14. These are both isotopes of carbon.

Carbon-12 has six neutrons and six protons in its atomic nucleus. Carbon-14 still has six protons, so it’s still carbon, but it has two extra neutrons along for the ride. Carbon-12's six neutrons make it stable, but carbon-14's 8 neutrons make it prone to radioactive decay.

Earth's atmosphere has plenty of carbon-14, which living things are constantly absorbing into their tissue. However, this absorption immediately stops after death, and the carbon-14 begins to gradually decay into other atoms at a known rate. Scientists can figure out how long something has been dead by measuring how much carbon-14 remains in an organic object, up to as many as 60,000 years ago.

You might see where I'm going with this. If you carbon date heartwood from a tree that some Neolithic lumberjack cut down to build a house, you might get a number that's hundreds of years older than the artifact or site you're trying to date – because the heartwood died before the tree did. The tree was alive and happily growing for a couple hundred years, but its heartwood wasn’t taking on any more carbon-14, so the ratio’s off.

For archaeologists looking for clues about what folks were up to millennia ago, they’re most likely to stumble on wood preserved in old fire pits. Charcoal is often well-preserved because it's resistant to microbial breakdown, so it's a very reliable  source of material to date. Except that the old wood problem can make it really difficult to tell when the fire actually burned.

For two reasons. One is that you don’t really know when the wood was cut down relative to when it was burned. But the other is because  so much of a tree is dead.

If the charcoal you’re dating was sapwood, it’s probably fine. But if it’s heartwood, it was dead well before it was burned. This means that exactly when the fire pit you're looking at was last active is anybody's guess, but you’re most likely to get an estimate that’s a bit too old.

Now that archaeologists have worked out that this is a problem, they try to find literally any other non-wood organic object to date if they possibly can. But sometimes, wood's the only option. And in fairness, not all wood is bad.

Some shorter-lived tree species yield more accurate carbon dating results, and wood decays faster in some environments, so it's less of a problem for archaeologists working in those parts of the world. For this reason, it's been  difficult for archaeologists to get a handle on how big a problem wood actually presents to their discipline. For example, the guy who first flagged the old wood problem in the 1980s was working in southern Arizona, where there's a lot of wood  in archaeological sites because it doesn't decay as quickly as in damp regions.

But he saw it as a universal problem for archaeologists, no matter the environment they're working in, due to the variety of tree  species in temperate environments that are resistant to decay. For a few decades, researchers were debating over whether to strike wood  charcoal radiocarbon data from the record completely. However, recent papers have approached the topic with a little more equanimity, like the 2019 study from Korea that found little difference between the ages of wood and soybeans from the same sites – suggesting old wood doesn't seem to pose a big problem for Korean archaeology.

And there are little tricks to getting around the problem too, like selecting smaller-diameter wood like twigs when possible,. Or using dendrochronology, a technique that matches unique tree ring patterns with known climate or historical events. In the end, archaeologists  still have to work all that out.

So this is all to say, when it comes to wood, archaeologists can sometimes be hundreds of years off in their dates. Which probably has them  feeling kind of dead inside. Like a tree. [ OUTRO ]