Are Millennial Trees Immortal?

The idea of becoming immortal has always been extremely attractive to humans. Long-lived trees have received special attention in the study of ageing, as a sort of mirror in which we could potentially see ourselves reflected in our efforts to achieve greater longevity (Figure 1). During the last decade, it has been established that senescence is not universal. This is particularly clear in modular organisms with indeterminate growth, such as trees, which build upon repeated modules and prevent the wear and tear of ageing to the extreme that organismal senescence cannot be gauged in nature [1,2]. This supports the hypothesis of Finch, who proposed that species with the capacity to compartmentalize risk may escape senescence [3]. A recent study adds considerably to this idea by showing that, despite the vascular cambium from ginkgo trees (Ginkgo biloba) displaying age-related reductions in the expression of genes related to cell division, expansion, and differentiation, measurements of photosynthesis and seed germination rates indicate that old trees are still in a healthy, mature state, and senescence is not manifested at the organismal level; at least not within the age range considered (15- to 667-year-old trees) [4]. This finding is especially important because it is from the first study in long-lived trees to measure age-related changes in the physiology of the vascular cambium (stem cells that produce secondary xylem and secondary phloem). Moreover, it adds to the general idea that organismal senescence is not universal within the plant kingdom, at least it is not in long-lived trees growing in their natural habitat. The study is also particularly important because, through the use of a number of different approaches (including transcriptomics, hormonal profiling, and measurements of both leaf and whole-plant physiology), it adds novel information about the physiology of cambial stem cells; which, from a very reductionist point of view, are the essence of life in plants (Figure 2).

How Can Millennial Trees Reach Extreme Longevities?

A combination of resistance to disease, a preference for sites away from competition with others, and retaining the capacity for vegetative growth (sprouting from embedded buds near the base of the trunk), has been observed previously in ginkgoes and other millennial trees [5,6]. This, in addition to the recently identified molecular mechanism by which cambial cells retain growth potential while defences are enhanced, are key determinants of longevity [4]. Furthermore, several tree species show complex mosaicism in crown development that is strongly species specific and becomes particularly complex at old ages, helping them increase longevity [5]. Indeed, the design of the plant body as a mosaic or modular structure, allows the organism to virtually escape from senescence occurring in their modules [6., 7., 8.]. In this respect it should be noted that trees (with a plant body based on a trunk) attain much longer maximum lifespans than perennial herbs [1], thus underlining the tremendous success that this strategy has had during evolution in plants. A huge capacity for retaining vegetative growth at old ages, despite mechanical constraints, appears to be essential for attaining extreme longevity [9]. Indeed, the capacity of the vascular cambium to resume growth each year allows life to be built around a sea of necromass, making trees exceptional long-lived organisms at the limits of life and death (the percentage of living biomass in an old mature tree can be below 5%). Does this mean, however, that these long-lived trees are immortal?

Are Millennial Trees Immortal?

Negligible or even negative senescence (i.e., virtually unaltered or reduced mortality with increasing age, respectively) occurs in several long-lived plants in their natural habitat [1., 2., 3.], but this does not mean they are immortal. Long life at the organismal level implies an increase in the maximum lifespan of any given species, and efficient mechanisms underlying an increased maximum lifespan (such as those reported in ginkgoes [4]) It manifests in many forms: when inflamed, it becomes red, tender, hot and painful; urination becomes either difficult, or painful; frequent urination, pain in groin (where thighs join with the body), low back pain or fever are some other typical symptoms of the disease. buy cheap cialis take a look at the drugstore here For the young women’s cialis buy online anemia, some of them do not even speak about it to anyone and have just been used to this problem. Everyone faces some or the other kind of blood cell problem Have hearing problems Have hemophilia or some kind of bleeding problem Have NAION (non-arteritic anterior ischemic optic neuropathy) in selected persons using these drugs in the post-promoting (outside of clinical trials) setting.Most, but not all, uk generic viagra of these clients had underlying anatomic or vascular probability issues for progress of NAION unrelated to PDE5 use, together. The process of baby making seems much pretty easier but for these couples or in reality the process viagra soft 50mg is very much costly. allow for the occurrence of very old individuals. However, these mechanisms cannot provide infinite protection against the wear and tear of ageing. Indeed, individuals which reach very old ages in natural populations are always an exception rather than the rule. The number of trees that reach ages close to the maximum lifespan of their species in any given environment can usually be counted on the fingers of one hand (www.rmtrr.org/oldlist.htm) [5,10]. Most studies that address senescence patterns in long-lived plants growing in their natural habitat fail to provide any robust statistical analysis of the oldest individuals, as there are very few of them (note that [4] is no exception to this). The maximum lifespan is far from the average age of individuals in natural tree populations, and this is why previous studies have shown that several plant species show negligible or even negative senescence. There is no doubt that indeterminate growth helps to increase the maximum lifespan of these species and seems to be a key factor in the apparent absence of senescence in their natural habitat. However, this does not mean that trees show no senescence symptoms in the oldest cohorts that could still be alive. In other words, the difficulties in gauging organismal senescence in the field, due to very limited sample sizes at the oldest ages, does not mean that millennial trees are not subject to the wear and tear of ageing; and even less does it mean that they are immortal (immortality can only be attained through clonal reproduction or through the germ line, see [9]).

Is Ageing a Sort of Stress?

The trade-offs between reproduction, stress, and survival, and the distinctions between semelparous (single reproductive episode before death) and iteroparous (multiple reproductive episodes before death) life cycles, have long been recognized as key issues in the study of life histories. Semelparity and iteroparity should be understood within a continuum of different modes of parity, which differ by the degree to which they disperse or concentrate reproductive effort over time [11]. However, iteroparity and a long life in long-lived tree species can only be achieved by investing in reproduction and overcoming stress, which may have an unavoidable cost on survival. However, from humans, for whom determinate growth and telomere shortening may play a major role in ageing [12], to perennials (such as ginkgoes or bristlecone pines), which show extreme longevity and may virtually escape from mutation accumulation in meristematic stem cells [13,14], there is a world of difference. It is highly probably that physiological senescence occurs in all organisms, but that the limited human lifespan prevents us from properly gauging it in long-lived trees in nature, in real time.The recent findings in ginkgoes provide, however, some useful new insight from which we can indirectly infer responses to this question. Despite a limited sample size (n = 3 per age group), the study shows that ginkgoes display age-related reductions in the contents of auxin (from 20- to 200-year-old plants) and increases in abscisic acid (ABA) (from 200- to 600-year-old plants) in cambial cells. These findings fit in with reduced cell division and expansion capacity, as well as increased activation of the biosynthesis of secondary metabolites for defence [4]. Although still not approaching their maximum lifespan (ginkgoes can live for over 1000 years), the study shows that the oldest individuals sampled (667 years) experience higher physiological stress than the younger ones. Indeed, these hormonal changes are related to the construction of long-distance vessel conduits that progressively decrease in diameter with tree age, which after centuries will inevitably and ultimately lead to plant death due to mechanical constraints. Although the timescale on which these trees survive with this sort of stress response is difficult for us to grasp, we can infer from the study that physiological senescence might ultimately occur. This is because the stress associated with ageing, although impacting very slowly on the physiology of the tree, will never cease over time and mechanical constraints can ultimately lead to collapse and death of the organism.

Concluding Remarks

In conclusion, although long-lived trees possess sophisticated mechanisms for attaining long life, and their senescence cannot be properly gauged in nature (because of very limited sample sizes at the oldest ages, and our intrinsic limitation of following trees for several centuries in longitudinal studies), millennial trees are indeed subjected to the wear and tear of ageing: they are not immortal. Ageing, as part of life, can indeed be considered a sort of stress from which neither we, nor any organisms, can ultimately escape. Indeterminate growth, mosaicism, plasticity, stress tolerance, and other features, allow long-lived trees to survive much longer. As the timescale is at least one order of magnitude higher in millennial trees than in humans, the sort of stress imposed by ageing in long-lived trees might almost be imperceptible for us, although it still occurs.