Correlates of Seed Size Variation: A Comparison Among Five Temperate Floras

Abstract

1 Five temperate floras were studied to assess to what extent seed size correlations with other plant attributes are consistent across floras. The floras were from three continents: Australia (semiarid woodlands of western New South Wales, arid woodlands of Central Australia, and the Sydney region), North America (Indiana Dunes) and Europe (Sheffield region, UK). The plant attributes used were growth form, plant height, perenniality and dispersal mode. We used general linear models to consider not only the primary correlations between seed size and each other attribute, but also the overlap patterns among correlations to determine if each correlation could be interpreted as a secondary effect via a third variable. 2 Plant height and growth form were consistently correlated with the largest proportion of log seed mass variation (up to 37% in Central Australia). Although there was strong overlap in the amount of log seed mass variation explained by the two attributes (6-22%), each could explain small but significant variation after the other in all floras. The strong association between growth form/plant height and seed size was found not only among unassisted and wind-adapted species, but also among species dispersed by other means. 3 In all floras, dispersal mode was also able to account for significant variation in log seed mass independently of growth form and plant height. The association between plant perenniality and seed size could be explained as a secondary correlation of growth form and plant height with both seed size and perenniality. 4 There were significant differences in log seed mass among the five floras. However, seed size ranged over at least five orders of magnitude in each flora. Differences between floras could account for relatively little (4%) of the variation in seed size between species, compared to the attributes growth form (20%), plant height (20%) and dispersal mode (29%), despite the quite different soils and climates of the five floras. This suggests that seed size is more strongly associated with other plant attributes than with the environmental conditions for establishment. It appears that within any one community, plants have found a diversity of possible solutions to the problems of seedling establishment, resulting in a wide range of log seed mass.

Factors that shape seed mass evolution

“…The idea that changes in seed mass are predominantly driven by changes in growth form is consistent with the fact that plant size is the strongest correlate of seed mass across present-day species (9). It is also consistent with the fact that 9 of the 10 largest divergences in seed mass in the history of plants were associated with a divergence in growth form (5).…”

Root traits explain different foraging strategies between resprouting life histories

“…For the remaining variables (ratios, number of tips per depth interval, branching angle and branch frequency) root dry weight was not included in the analyses. An alternative covariate could have been seed mass as it is correlated with plant size and seedling survival (Leishman et al 1995;Moles and Westoby 2004); however, resprouters and nonresprouters do not differ in seed mass when a wide range of species are considered (Pausas & Verdú 2005). In our data set non-resprouters tend to have smaller seeds than resprouters (P = 0.011; seed size data at species level only).…”

The repeated evolution of large seeds on islands

“…Therefore, a germinating seedling on an island is more likely to be adjacent to a conspecific, leading to greater levels of intraspecific competition [13,14]. Larger seeds are more competitive than small seeds, all else being equal [2,4,5]. Therefore, higher levels of intraspecific competition on islands may also select for increases in seed size.…”