Gaps and root trenching increase tree seedling growth in Panamanian semi-evergreen forest

Although competition between plants is nearly universal in vegetation, we know relatively little about belowground competition and how it interacts with aboveground competition in tropical forests, and almost nothing about such interactions on soils of intermediate fertility in sites with a moderate dry season, despite the fact that such forests are extensive. We investigated this over one year in a Panamanian tropical semi-evergreen rain forest, using tree seedlings (Simarouba amara, Gustavia superba, Tachigali versicolor, and Aspidosperma cruenta; least to most shade tolerant), experimental gaps, and trenching. Gaps increased growth and decreased mortality; growth increases were: Simarouba 684% (increase in height relative growth rate), Gustavia 411%, Aspidosperma 364%, and Tachigali 324%. Trenching in gaps increased growth in three species (Simarouba 49%, Gustavia 63%, and Aspidosperma 38%) but had very small effects in the understory; trenching did not affect mortality. We infer that trenching caused increased growth due to increased nutrients in the wet season, and increased water and/or nutrients in the dry season. Thus, across the tropics, in all but the wettest sites with fertile soils, seedlings of many species will be limited by belowground competition, at least in gaps. This is similar to the pervasive importance of belowground competition in temperate forests.