These survivors in ancient forests worldwide “are a great model of stress tolerance and resilience. Specifically, these are regarded as an exception regarding the respective species at a populational scale and are models that help us better understand the importance of interindividual variability in adaptative processes”, notes Sergi Munné-Bosch, professor at the Department of Evolutionary Biology, Ecology and Environmental Sciences.
Slow-growing and stress tolerant trees
In the world of plants, great longevities are usually linked to tolerance and resilience to stress (extreme temperatures, lack of nutrients, droughts, etc.), slow growth and regenerative potential. Regarding the oldest trees in the planet, this efficient eco-physiological response regarding external factors joins a modular growth pattern and great ability to regenerate to efficiently resume plant growth in case of damage during their life cycle.
Modular growth cycles of millennial trees revolve around the trunk, “and this provides them with a greater robustness and ability to survive for more years. The trunk is made of 99% of dead tissue, and the xylem –a series of vases in the vascular tissue- is also dead. The living tissues that make the phloem –conducting vase of the sap- and vascular cambium are protected by the tree bark”, notes Munné-Bosch.
Herbs and bushes are also perennial plants that can last long. Aged over 300, the Borderea pyrenaica –an endemic plant in the Pyrenees- is the herbaceous plant with the longest longevity described to date. In order to survive, this plant keeps its growth period cycles on a tuber.
Longevity and senescence in millennial trees
The ability to survive in perennial plants is a scientific reference to study the mechanisms related to longevity and senescence. Specifically, senescence is a programmed cell death process that has a biological function in the plant (removing cell compounds, mobilizing nutrients in the plant, etc.).
In long-lived perennial plants, the physiological senescence could take place, but it is not seen in the natural environment. In this case, the potential longevity is so extraordinary that the plant usually dies due to external factors before we can see any physiological damage related to ageing (negligible senescence).
“This is easy to see in millennial trees. Chances of dying for any organism -regardless of how tolerant and resilient to stress they are- increase with time. It is very difficult for any organism to survive to external threats for millennia”, notes Sergi Munné-Bosch.
Over the years, structural limitations are the main cause of functional decay of long-lived plant species. “A tree can reach its maximum height depending on its genome and the environmental conditions of its natural habitat. Later, it can increase its longevity through the production of new branches and their regeneration when damaged. But everything has its limits. When the vascular tissue connecting belowground with aboveground tissues is sufficiently damage and fail, the plant will however inevitably die”, concludes the researcher.