We all get old, but the hands of the clock move a lot more slowly for some than others. Certain trees, for example, manage to live for thousands of years, making biologists question what is going on at a cellular level. A study of one species known for its remarkable lifespan has provided a few answers, although anyone hoping for the secret of such longevity for ourselves probably should not hold their breath.
Ginkgo biloba trees are not the longest-lived species on Earth, a claim contested among Bristlecone pines, quaking aspen, and more. They have, however, been recorded as living well past their second millennium, impressive enough for most of us. Professor Richard Dixon of the University of North Texas collected samples from Ginkgo trees aged 15 to 667 to see how they do it. After all, besides their individual survival, Ginkgo trees have graced the planet for at least 270 million years, and you don't live that long without learning a trick or two.
In some ways, the surprising thing is how little Dixon's trees changed between their first and seventh century. Leaf area, photosynthetic efficiency, and rates of seed germination were unaltered by time.
On the other hand the rings laid down by the young trees were wide, narrowing rapidly over the first two centuries. Declining ring width was reflected in fewer cell layers added each year between the bark and the established wood. After age 200 the downward trend continued, but much more slowly. Moreover, since each newly formed ring was wrapped around a wider core the trees continued to add similar areas of wood at chest height each year.
Other aspects measured, such as cell division and expansion, declined with age, but never seem to stop. The trees just keep on adding to themselves, albeit at an ever-slower rate. Senescence, the deterioration of function in an aging individual, was not detectable tree-wide Dixon discovered, although of course individual leaves were struck by it all the time.
Moreover, the Ginkgo trees maintained the expression of genes associated with resistance to threats, the plant equivalent of an immune system. Perhaps not coincidentally, Ginkgo trees are particularly rich in these.
The findings, reported in Proceedings of the National Academy of Sciences, fit with earlier external observations of the trees. Old trees start adding wood later in the spring and stop earlier in the fall, but they still grow every year. Moreover, in a year with good weather, even a very old tree can put on something of a growth spurt.
Other species of long-lived trees have not been studied in the same way, so it's possible some have developed different survival mechanisms.