According to the rate of living theory of aging, the longevity of living organisms should be negatively correlated with body metabolic rate. For the organisms having the same body size, the metabolic rate is usually greater in favorable habitats than in poor-quality sites, as demonstrated in many previous studies. Therefore, it is expected that organisms would live longer in environments with low-resource availability than in their counterparts. Specifically, we hypothesize that plant branches or twigs would live shorter in sun-lit microhabitats than in shaded or partial-shaded ones. This hypothesis is consistent with the recently established leaf and wood economic spectrums, in which leaf longevity is positively associated with leaf mass per area but negatively with leaf nitrogen content and photosynthetic capacity that often characterize favorable sites. A similar hypothesis has also been interspecifically tested to be true at whole-individual level of tree species, where long-lived species are often associated with low respiration rates.
In order to test the above hypothesis, we in this study examined the effects of light level on branch longevity and on the relationship between crown shape and the longevity for an evergreen species (Osmanthus fragrans) and a deciduous species (Metasequoia glyptostroboides) in Nanjing, southeast China. We measured plant size (height and diameter at breast height), crown depth (i.e. vertical crown length) that was obtained by plant height minus under branch height, crown profile that was calculated as the ratio of crown depth to crown width, and relative crown width that was defined as the ratio of crown width to plant height; we also determined the longevity of shed branches by bud scales for plants (with similar size) grown in different light conditions (under full sun light/unshaded, partial shaded, fully shaded).
In both species, branch longevity was found to be significantly greater for plants living in the fully-shaded environments than for those grown in open sites; the longevity increased with increasing shading level, consistent with the theoretical prediction. Crown depth and crown profile increased, but relative crown width decreased with increasing shading level in both species, i.e. shading tended to result in narrow and deep plant crowns. In addition, branch longevity was positively correlated with crown depth and crown profile but negatively with relative crown width in both species, and branch longevity was positively related to relative crown depth in O. fragrans, not in M. glyptostroboides. The possible underlying mechanism is that shading might have increased the level of apical dominance but decreased the self-shading level of crown interior (as reflected by increased crown profile and decreased crown width), which potentially led to low metabolic rates.
These results collectively suggest that the morphological responses of plant crowns to light may largely account for the variation in branch longevity under different shading levels. However, the current study did not address the importance of life form to plant metabolic rates and organism longevity despite two different species being investigated. Future studies need to examine branch biomass allocation, leaf photosynthetic capacity and respiration rates to fully understand the relationship between branch longevity and habitat quality for species differing in life forms.