General Discussion of Factors Regulating the Timing of Top Senescence in Other Species

Remarkably little information exists regarding the causes and consequences of variation in the timing of senescence of the annual shoots of perennial plants. Brown et al. (1985) appear to be the first to report variation in the timing of top senescence for a native plant species in situ. In an analysis of within and between species variation in vegetative phenology for two co-occurring clonal forest herbs, Aster acuminatus and Clintonia borealis, they found a strong positive correlation between the timing of what they called ramet death (and which is equivalent to our definition of endogenously controlled top shoot senescence) and ramet size, in both species. The correlation was stronger in A. acuminatus than in

C. boreali. However, variation in the timing of shoot senescence was less among ramets of A. acuminatus than among those of C. borealis. They suggested that the stronger correlations noted in A. acuminatus reflected the smaller amount of clonal integration in that species [see Hutchings and Barkham (1976) and Suzuki and Hutchings (1997) for discussions of clonal integration and storage]. Brown et al. (1985) also were the first to note the difficulties of distinguishing between cause and effect in the correlation between ramet size and senescence timing. That is, do large ramets senesce later because they are larger or are they larger because of their phenological characteristics? They suggested that a positive feedback system that includes reproductive success, like that reported by us for mayapple, was likely to be involved.

Additional data come from work on other perennial understory herbs of the deciduous forest floor. A study of sex change in two Arisaema species reports delayed senescence in fruit bearing as opposed to male or non-fruit bearing female plants (Richardson, 1999). Lapointe (pers. commun.) also reports delay in top shoot senescence of fruit bearing versus vegetative shoots of Erythronium americanum and Trillium erectum, suggesting that ephemeral clonal herbs of the northeast US deciduous forest understory behave in similar ways.

In sum, the study of causes of variation in the timing of top senescence of perennial shoots promises to provide insight into plant resource dynamics and the evolution of phenological traits. Consistent patterns of variation are identified, and their causal bases explored.

0 0

Post a comment