Mathematical Biology Seminar Helene Muller-Landau University of Minnesota and Smithsonian Tropical Research Institute 3:05PM, Wednesday, September 22, 2010 LCB 225 Rethinking interspecific variation in seed size and wood density Abstract: Seed size and wood density both vary widely among coexisting plant species, and both are correlated with succesional status. Seed size variation has long been explained in terms of a tradeoff between high seedling competitive ability of large seeds and higher seed production associated with small seeds, with shade-tolerants benefitting more from the former and pioneers from the latter. But empirical evidence suggests that the key advantage of large seeds is greater tolerance of stresses such as shade or drought that are present in some but not all regeneration sites - not competitive ability in all sites. I present a simple, analytically tractable model of species coexistence in heterogeneous habitats through a tolerance-fecundity tradeoff, which enables stable coexistence of large numbers of species. Under this mechanism, the more tolerant species win all the more stressful regeneration sites and some of those that are less stressful, while the more fecund species win most but not all of the less stressful sites. Similarly, high wood density has been thought to provide higher strength, but at higher cost, an investment that pays off for long-lived but not for short-lived species. However, a fat trunk of low-density wood can provide higher strength at lower cost than a thin trunk of low-density wood, so low wood density has the advantage in both strength and economy of construction. We hypothesize that the countervailing advantage of high wood density is lower maintenance costs due to lower trunk surface area, as surface area correlates with maintenance respiration. Both the advantages of being able to recruit in stressful environments and the lower lifetime maintenance costs would be particularly important to long-lived, shade-tolerant trees, and could explain why they tend to have these traits.