Root carbohydrate storage in temperate and tropical forest tree seedlings: Implications for species coexistence

With the goal of better understanding the functional significance of intra- and interspecific variation in plant resource allocation, its implications for species growth and survival and hence for community composition, I carried out experiments with tree seedlings from both temperate and tropical forest communities. In Michigan, seedlings of eight common northern temperate species were grown in a greenhouse under two levels of light and nitrogen availability, spanning the range of variation found in northern Michigan forests. In the tropics, I transplanted ∼3,000 seedlings of five common dry tropical forest tree species into natural gradients of light and soil phosphorus (P) availability in Palo Verde National Park, Costa Rica. Light availability in the plots ranged from ∼4% to ∼40% full sun, and phosphorus availability, from <1 to ∼150 mg P/kg soil. In both the tropical and temperate experiments, seedlings were harvested at intervals throughout the experiment and their root morphology and allocation to root total nonstructural carbohydrate (TNC) storage measured. Across species and biomes, stored root reserves accounted for 8--60% of root dry mass. TNC increased with increased light availability and with decreased nutrient availability. In both temperate and tropical species, root TNC clearly drives changes in root mass ratio (RMR) while resource-driven changes in structural root mass are absent in most species and weak in others. Hence, my results suggest that the increased RMR associated with lower nutrient availability---usually interpreted as an increased allocation towards nutrient capture---is unlikely to lead to increased nutrient uptake since it is due largely to increased storage. In Michigan, root carbohydrate storage was negatively correlated with seedling growth rates and positively correlated with increased survival of field seedlings. In the tropics, there was a positive correlation between root TNC growth in the high soil P site and a negative correlation in the low P site. Contrary to expectations, survival and root TNC were not positively linked in the tropics. There was an intersepcific trade-off between species growth under high resource availability and survival under low resource availability in the high soil P site but no trade-off in the low soil P site. The competitive hierarchy of species changed with both light and soil resource levels, and suggested resource gradient partitioning among regenerating seedlings of these dry forest species.