Putting seedlings on the map: Trade-offs in demographic rates between ontogenetic size classes in five tropical forests

Abstract

All species must partition resources among the processes that underly growth, survival, and reproduction. The resulting demographic trade-offs constrain the range of viable life-history strategies and are hypothesized to promote local coexistence. Tropical forests pose ideal systems to study demographic trade-offs as they have a high diversity of coexisting tree species whose life-history strategies tend to align along two orthogonal axes of variation: a growth–survival trade-off that separates species with fast growth from species with high survival and a stature–recruitment trade-off that separates species that achieve large stature from species with high recruitment. As these trade-offs have typically been explored for trees ≥1 cm dbh, it is unclear how species' growth and survival during earliest seedling stages are related to the trade-offs for trees ≥1 cm dbh. Here, we used principal components and correlation analyses to (1) determine the main demographic trade-offs among seed-to-seedling transition rates and growth and survival rates from the seedling to overstory size classes of 1188 tree species from large-scale forest dynamics plots in Panama, Puerto Rico, Ecuador, Taiwan, and Malaysia and (2) quantify the predictive power of maximum dbh, wood density, seed mass, and specific leaf area for species' position along these demographic trade-off gradients. In four out of five forests, the growth–survival trade-off was the most important demographic trade-off and encompassed growth and survival of both seedlings and trees ≥1 cm dbh. The second most important trade-off separated species with relatively fast growth and high survival at the seedling stage from species with relatively fast growth and high survival ≥1 cm dbh. The relationship between seed-to-seedling transition rates and these two trade-off aces differed between sites. All four traits were significant predictors for species' position along the two trade-off gradients, albeit with varying importance. We concluded that, after accounting for the species' position along the growth–survival trade-off, tree species tend to trade off growth and survival at the seedling with later life stages. This ontogenetic trade-off offers a mechanistic explanation for the stature–recruitment trade-off that constitutes an additional ontogenetic dimension of life-history variation in species-rich ecosystems.