Perennials and seed size

While the main Tradeoff Debate is concerned with the putative negative correlation between plant longevity and yearly seed yield, those familiar with grassland perennial species might be forgiven for suspecting that perennials generally also have smaller seeds than annuals. Certainly, domesticated grains--currently all annuals--have much larger seeds than wild herbaceous perennials. The annual ancestors of some of the annual grains also have relatively large seeds.

However, when hundreds of plant species are compared, perenniality is only a weak predictor of a species' seed size:

• "the difference between the means for annual and perennial herbaceous autotrophs is small enough that herbaceous plants will be treated as a single group for many purposes." [Baker, 1972]
• "In each of the four floras [databases of plants from three continents] , plant perenniality was not associated with significant seed size variation after the addition of plant height or growth form." [Leishman et al., 1988]

Height is a robust predictor of seed size (tall species, in general, produce larger seeds), as is plant form (trees and vines generally have larger seeds than forbs and graminoids), according to Leishman et al. (1988). Perenniality is only a weak predictor...and it is the perennial habit that is associated with larger seed size.

• "Seed size was significantly associated with plant perenniality in the five floras (Fig. 4), with seeds of annual species generally smaller than seeds of perennials." [Leishman et al., p. 525]
• Baker (1972) found significant differences (p<0.001)>

• Mean seed weight of 664 native annual herbs: 5.76

  Mean seed weight of 940 native perennial herbs: 6.35

  Mean seed weight of 441 native shrubs: 7.55

  Mean seed weight of 94 native trees: 9.60

Sources:

Leishman, M. R., Westoby, M., Jurado, E. (1995) Correlates of seed size variation: a comparison among five temperate floras. Journal of Ecology. 83: 517-530

H. G. Baker (1972) Seed Weight in Relation to Environmental Conditions in California. Ecology, Vol. 53, No. 6 (Nov., 1972), pp. 997-1010