BEIJING -- Researchers from China and the United States have found what makes ginkgo trees live for more than 1,000 years, China Science Daily reported.

Researchers from Beijing Forestry University, the University of North Texas and some other Chinese research institutions reported in the US journal Proceedings of National Academy of Sciences that they analyzed 34 ginkgo trees and divided them into three groups according to their ages.

Compared with adult trees, the ancient trees, aging from 193 to 667 years, have thinner cambium cell layers. Cambium cells are layers of stem cells between the internal wood and external bark differentiating into other tissues.

The old trees have a higher level of IAA, a plant growth regulator and growth-stimulating hormone, and a lower level of ABA, a growth-inhibiting hormone.

However, the average cross-sectional area of trees at breast height in the ancient trees group has been increasing with aging which shows the continuous cambial cells division.

Little difference was found when it comes to their ability to photosynthesize, grow leaves, germinate seeds or resist disease compared to younger trees, showing that the ancient trees group are actually in a healthy and mature state.

Lin Jinxing of Beijing Forestry University and an author of the study said that trees usually die from outside stress like pests. They found disease resistance-associated genes retained high expression in old trees, along with genes associated with the synthesis of protective compounds, showing that the old trees do not lose their ability to defend against outside stressors.

The researchers concluded that the longevity of ginkgo trees is a balance between the growing and aging processes, rather than being regulated by a single gene.