Studies of evolutionary plant senescence use both demographic and evolutionary approaches to understand why the deleterious phenomenon of senescence persists in populations. Evaluating plant senescence within a demographic and evolutionary context is a relatively new approach (Watkinson, 1992; Roach, 1993; Pedersen, 1999), and there are many studies that will need to be done before we will have a clear understanding of the processes that have shaped life histories at the latest ages. Some guidelines and issues that should be considered in future studies are discussed below.
There is currently a paucity of high quality data on age-specific demography from plant populations. Despite the fact that the demography of plants is comparatively easy, few studies have followed a population of individuals for their entire life cycle, particularly for species which live more than a few years (reviewed by Watkinson, 1992; Roach, 1993). Geographically and taxonomically, studies that have been done, to date, have barely begun to sample the diversity of the plant kingdom (Franco and Silvertown, 1990). The studies which have been done have used small sample sizes, and there have been very few attempts to estimate the variance in age-specific life history traits.
One of the limitations of gerontological studies, in all species, has been sample size (Finch, 1990). Survivorship curves based on small sample sizes can provide estimates of life expectancy at birth, but it is not possible to estimate mortality rates late in life because so few individuals remain alive at older ages. Moreover, in field experiments there is a high rate of random mortality affecting survival at all ages. With a large sample size, it will be possible to attain a more accurate understanding of the inherent changes in mortality at different ages.
It is important to emphasize that data on age-specific mortality, not just life span data, are needed to understand the evolution of senescence. Plant species show a wide range of variation in their life spans, ranging from a few weeks, for some ephemeral annuals, to over 1,000 years for many conifer species. The presence of senescence cannot be inferred from life span measures for several reasons. First, evidence for senescence is derived from a change in the shape of the mortality curve for the population. Data on the maximum longevity of a species give the endpoints for the curve, but tell us nothing about a change in the shape of the curve with age (Bell, 1992). Secondly, two species can show different longevities due to differences in their annual mortality rates irrespective of any differences in senescence (Partridge and Barton, 1996). Thus, more complete demographic data on age-specific mortality are critical.
As it has been defined here, senescence refers to an increase in adult mortality with age. Theoretical studies have shown that factors other than senescence may cause mortality to increase with age (Abrams, 1993; Blarer et al., 1995, McNamara and Houston, 1996).
Specifically, the theoretical expectation is that an optimized life history may show an increase in mortality due to an increased reproductive effort late in life. For plant species with determinate growth, in other words a fixed size at sexual maturity, reproductive output is expected to decline with age, but for some plant species, which increase in size with age, an increase in late reproduction may be observed. The change in age-specific reproductive effort, and its consequences for mortality rates has not been experimentally evaluated. Furthermore, selection acts jointly on survival and reproduction. Consequently, future studies with complete data on age-specific mortality and age-specific fecundity may demonstrate that some combination of mortality and fecundity will be a better measure, to detect the presence of aging in a population and to compare rates of aging, than mortality measures alone (Partridge and Barton, 1993,1996). Plants are good experimental organisms to test these ideas, because age-specific reproduction is relatively easy to quantify.
The comparative biology of different species is one of the most useful approaches in evolutionary biology. There are a wide variety of growth forms, life spans, and mortality patterns within the plant kingdom and species have traditionally been classified either by their longevity, annual, biennial, perennial, or by their number of reproductive episodes, monocarpic, polycarpic. With more complete data on a wider range of species, perhaps we can look forward to the time when a thorough phylogenetic analysis of the evolution of whole plant senescence can be considered.
Finally, it is hoped that future research will begin to make a link between physiological and evolutionary approaches to plant senescence. Perhaps then, we will be able to make a bridge between our understanding of the physiological processes which occur within an individual, how the processes of physiological senescence change with age, and how they affect life history traits and the evolution of senescence.
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