Extreme longevity in trees: live slow, die old?

We have examined the extreme longevity displayed by trees in relation to a theory mainly developed in animals, namely, the controversial rate of living (ROL) theory of aging which proposes that longevity is negatively correlated to metabolic rate. Plant metabolism implies respiration and photosynthesis; both are susceptible to negatively impact longevity. The relationship between longevity and metabolism was studied in leaves and stems of several species with the aim of challenging the ROL theory in trees. Leaf and stem life spans were found to be highly correlated to metabolism (R(2) = 0.97), and stems displayed a much lower metabolism than leaves. Analysis of covariance (ANCOVA), with metabolism as the covariate, revealed no difference between mean leaf and stem life spans, which would appear to conform to the expectations of the ROL theory. Consequently, the extremely high longevity of trees may be explained by the lower metabolism displayed by the stems. These results clearly reflect different energy allocation and energy expenditure rate strategies between leaves and stems, which may result in different senescence rates (and life spans) in these organs. They also suggest that, in contrast to animals, the ROL theory of aging may apply to woody plants at the organ level, thereby opening a promising new line of research to guide future studies on plant senescence.