Effects of above- and belowground competition on growth and survival of rain forest tree seedlings

Because reducing aboveground competition for photosynthetically active radiation (PAR) increases growth and survival rates of tropical rain forest seedlings, belowground competition for nutrients is often assumed to be of little or no importance. We tested this assumption. We measured the growth, over one year, of seedlings of Aspidosperma carapanauba (shade tolerator) and Dinizia excelsa (light demander) transplanted into understory (high aboveground competition; 1% incoming photosynthetic photon flux density [PPFD]) and single treefall gap areas (lower aboveground competition; 6% PPFD in centers of gaps) without and with trenches cut around plots (high and low belowground competition). Over the wetter six months, location in gaps significantly increased relative height growth rates by 320% and 570%, and relative leaf production rates by 190% and 280%, while trenching significantly increased height by 330% and 520%, and leaf production by 170% and 260%, for Aspidosperma and Dinizia, respectively, with significant interactions between treatments Trenching approximately doubled height growth in the understory. Leaf loss rates were unaffected by either treatment but were 3.7 times higher in Dinizia compared with Aspidosperma. Aspidosperma could persist in the understory, as leaf production and loss were balanced at 0.60% PPFD, whereas Dinizia could not, as it required 2.4% PPFD. Reducing either aboveground competition or belowground competition allowed Dinizia to persist as leaf production was increased above unchanging leaf loss rates. Gap and trenching treatments both significantly reduced mortality rates in Dinizia. Location in gaps significantly reduced mortality rates in Aspidosperma. Both species significantly increased allocation to leaves and significantly decreased allocation to roots in response to trenching. The increased growth in the wet season caused by trenching, and increased foliar nitrogen and phosphorus amount, showed that both species were probably competing with adult trees for nutrients as well as PAR. We deduced that phosphorus did not limit growth; however, we could not deduce which nutrient did limit growth. Given that 60-70% of the world's tropical rain forests occur on low-fertility oxisols, intense adult-juvenile competition both aboveground and belowground may be the norm, not the exception.